The isoscalar giant dipole resonance (ISGDR), which is the lowest-energy overtone mode of the isoscalar giant resonances, has been studied in some medium-heavy and heavy nuclei in coincidence measurements. The observation of the direct nucleon decay channels significantly helped to enhance giant resonance strengths with respect to the underlying background and continuum, which gave an indication for the existence of a new mode with L = 2 character, assumingly the overtone of the ISGQR

Csatlos, M

Csige, L

Davids, B

de Huu, MA

Garg, U

Gulyas, J

Harakeh, MN

Hunyadi, M

Krasznahorkay, A

Sohler, D

van den Berg, AM

Wortche, HJ

The isoscalar giant dipole resonance (ISGDR), which is the lowest-energy overtone mode of the isoscalar giant resonances, has been studied in some medium-heavy and heavy nuclei in coincidence measurements. The observation of the direct nucleon decay channels significantly helped to enhance giant resonance strengths with respect to the underlying background and continuum, which gave an indication for the existence of a new mode with L = 2 character, assumingly the overtone of the ISGQR.</p

ARTS repository - University of Groningen

   University of GroningenOvertones of isoscalar giant resonances studied in direct particle decay measurementsHunyadi, M; van den Berg, AM; Csatlos, M; Csige, L; Davids, B; Garg, U; Gulyas, J; Harakeh,MN; de Huu, MA; Krasznahorkay, APublished in:Acta Physica Polonica BIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2005Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Hunyadi, M., van den Berg, AM., Csatlos, M., Csige, L., Davids, B., Garg, U., Gulyas, J., Harakeh, MN., deHuu, MA., Krasznahorkay, A., Sohler, D., & Wortche, HJ. (2005). Overtones of isoscalar giant resonancesstudied in direct particle decay measurements. Acta Physica Polonica B, 36(4), 1115-1119.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 07-06-2022Vol. 36 (2005) ACTA PHYSICA POLONICA B No 4OVERTONES OF ISOSCALAR GIANT RESONANCESSTUDIED IN DIRECT PARTICLE DECAYMEASUREMENTS∗M. Hunyadia, A.M. van den Bergb, M. Csatlósa, L. CsigeaB. Davidsb, U. Gargc, J. Gulyása, M.N. Harakehb, M.A. de HuubA. Krasznahorkaya, D. Sohlera, and H.J. WörtchebaInst. of Nuclear Research of the Hung. Acad. of Sciences, Debrecen, HungarybKernfysisch Versneller Instituut, Groningen, The NetherlandscUniversity of Notre Dame, Notre Dame, USA(Received November 30, 2004)The isoscalar giant dipole resonance (ISGDR), which is the lowest-energy overtone mode of the isoscalar giant resonances, has been studied insome medium-heavy and heavy nuclei in coincidence measurements. Theobservation of the direct nucleon decay channels significantly helped to en-hance giant resonance strengths with respect to the underlying backgroundand continuum, which gave an indication for the existence of a new modewith L=2 character, assumingly the overtone of the ISGQR.PACS numbers: 24.30.Cz, 25.55.Ci, 23.50.+zThe extension of experimental studies of the isoscalar giant resonances(GRs) toward the investigations of overtone modes was mainly motivatedby the possibility of learning about the formation and damping mechanismof GRs over a wide excitation energy region. As the overtone modes areexcited by the second-order term of the transition operators, they are weakresponses to external fields, and this in combination with large oscillationfrequencies and moderately fragmented doorway-state structures results indamped, broad strength distributions [1, 2]. The observation of such weakstructures above an overwhelming background and continuum requires spe-cial technique with high selectivity for excitations of single-particle charac-ter, which effectively disentangles GR strengths from the continuum.∗ Presented at the XXXIX Zakopane School of Physics — International Symposium“Atomic Nuclei at Extreme Values of Temperature, Spin and Isospin”, Zakopane,Poland, August 31–September 5, 2004.(1115)1116 M. Hunyadi et al.The application of coincidence techniques offered a clean solution to ob-tain a better definition of GR strengths with respect to previous single-armmeasurements [3] (and Refs. therein). On the other hand, a coincidenceexperiment, determining partial branching ratios of the direct-particle (neu-tron and proton) decay, is the only way to study the microscopic structureof GRs in medium-heavy and heavy nuclei.The 3~ω isoscalar giant dipole resonance (ISGDR), being the lowest-energy isoscalar overtone mode, has been studied in coincidence experimentsobserving decay channels by neutron emission of 90Zr, 116Sn and 208Pb [4],and by proton emission of 208Pb [5]. The experiments were performed atthe AGOR superconducting cyclotron facility of KVI using (α,α′) reactionat Eα=200 MeV. The α′ particles were momentum analyzed by the Big-BiteSpectrometer (BBS) in conjunction with the EuroSuperNova detector sys-tem. In the neutron decay experiment the kinetic energy of the neutronswas measured by the time-of-flight technique. In the proton decay experi-ment the protons were detected by an array of 5-mm thick Si(Li)-detectors.Further details of the experimental setup can be found in Refs. [5, 6].Ex (MeV)Strength (arb. units)90Zr(α,α´n)D-O-S0<Ef.s.<6 MeVa)Ex (MeV)116Sn(α,α´n)D-O-S0<Ef.s.<6 MeVb)Ex (MeV)208Pb(α,α´n)D-O-S0<Ef.s.<6 MeVc)0120 30 20 30 20 30Fig. 1. L=1 strength distributions generated by the difference-of-spectra techniquefrom the double-differential cross sections gated on the direct neutron decay of (a)90Zr, (b) 116Sn and (c) 208Pb.In order to disentangle the dipole strength from other multipolarities,angular distributions of the scattered α′ particles were measured in angularintervals, which comprise both enhancement and suppression of the dipolestrength, and allows the use of the difference-of-spectra technique. The gateon the direct decay channels, which was determined in the final-state energyspectra after taking into account the contribution of the statistical decay,strongly suppressed the underlying instrumental background and nuclearcontinuum. Although, in the neutron decay experiment the moderate en-ergy resolution, the partial separation of the direct and statistical decaysOvertones of Isoscalar Giant Resonances Studied in . . . 1117and the uncertainty of the random coincidence correction lead to difficultiesin the definition of the ISGDR structures. In Fig. 1(a)–(c) a significantconcentration of the dipole strength was observed. The parameters of thestrength distributions at the expected ISGDR energies were determined byGaussian fit and listed in Table I for comparison with the results of single-arm experiments of Refs. [7–9]. A satisfactory agreement was obtained forthe excitation energies, but the deviations of the resonance widths are stillremarkable and needs deeper analysis of the evaluation methods applied.TABLE IComparison of parameters obtained for the ISGDR.This work Single-arm experimentsEx Γ Ex ΓTarget (MeV) (MeV) (MeV) (MeV)90Zr 25.8(0.8)n 4.5(1.9)n 26.7(0.5)a 8.8(1.0)a26.9(0.7)b 12.0(1.5)b116Sn 24.0(1.5)n 5.3(1.7)n 25.5(0.6)c 12.0(0.6)c25.4(0.5)b 15.7(2.3)b208Pb 20.9(1.0)n 5.7(2.3)n 22.2(0.3)c 9.39(0.35)c22.1(0.3)p 3.8(0.8)p 22.7(0.2)b 11.9(0.4)bnn-decay exp., p p-decay exp.; a Ref. [7], b Ref. [8], c Ref. [9].Most of the problems of the neutron decay experiment were not presentin the case of the proton decay experiment, since the high Coulomb barrierhindered the statistical decay, and created especially clean conditions for theobservation of GR strength. Fig. 2(a)–(b) shows the double-differential crossEx (MeV)d2σdΩ-1dE-1 (mb·sr-1·MeV-1)ISGDRHEOR(ISGQR2)Ex (MeV)208Pb(α,α'p)1.5o< Θα' <3.0o1<Ef.s.<2 MeV208Pb(α,α'p)1.5o< Θα' <3.0o0<Ef.s.<1 MeVb)a)ISGDRHEOR (ISGQR2)00.20.420 25 30 20 25 30Fig. 2. Double-differential cross sections as a function of Ex gated on the directproton decay of 208Pb to Ef.s. regions in207Tl indicated in the panels.1118 M. Hunyadi et al.sections for two final-state energy regions, in which three GR components ofthe structure were recognized. While the ISGDR bump was easily identifiedbased on the analysis of the angular distributions (see Fig. 3(a)), the bumpat higher excitation energy most likely carried a quadrupole character (seeFig. 3(b)), but L = 3 and L = 4 multipolarities were not excluded. Recentlyperformed continuum-RPA calculations on isoscalar overtone GRs predicteda broad quadrupole structure around Ex = 30.5 MeV corresponding to the4~ω overtone mode of the ISGQR (→ ISGQR2) [2].Θα´ (deg.)dσ/dΩ (mb/sr)208Pb(α,α´p)0<Ef.s.<2 MeVISGDR bumpa)Θα´ (deg.)dσ/dΩ (mb/sr)208Pb(α,α´p)0<Ef.s.<2 MeVISGQR2(?) bumpb)2 4 60.00.20.40.60.81.02 4 60.00.20.40.60.81.0Fig. 3. Angular distributions of the (a) ISGDR and (b) ISGQR2(?). The theoreticalcurves were calculated by DWBA, and were scaled to fit the data points. In panel(a) L=1 curve (solid), in panel (b) L=2 curve (solid), L = 3 curve (dashed) andL = 4 curve (dotted) are shown.In conclusion, we have performed the first coincidence experiments toinvestigate the direct neutron and proton decay channels of the ISGDR andpossibly of other overtone resonances. The application of the direct parti-cle decay measurements helped to find a better definition of GR strengths,which may also lead to the discovery of new GR modes, like the ISGQR2.Systematic decay studies of isoscalar GRs at high excitation energies wouldyield the ultimate evidence for overtone modes other than the ISGDR.We acknowledge the Marie Curie Fellowship program of the EC (HPMF-CT-2000-00663), the transnational access program of the EC (HPRI-CT-1999-00109) and the Hungarian OTKA Foundation (T038404 and D34587).Overtones of Isoscalar Giant Resonances Studied in . . . 1119REFERENCES[1] G. Colò, Nucl. Phys. A731c, 15 (2004).[2] M. Gorelik et al., Phys. Rev. C69, 054322 (2004).[3] U. Garg, Nucl. Phys. A731c, 3 (2004).[4] M. Hunyadi et al., (prepared for submission).[5] M. Hunyadi et al., Phys. Lett. B576, 253 (2003).[6] M. Hunyadi et al., Nucl. Phys. A731c, 49 (2004).[7] D.H. Youngblood et al, Phys. Rev. C69, 054312 (2004).[8] M. Uchida et al., Phys. Rev. C69, 051301(R) (2004).[9] D.H. Youngblood et al., Phys. Rev. C69, 034315 (2004).

English

Overtones of isoscalar giant resonances studied in direct particle decay measurements

Proceedings - University of Groningen

   University of GroningenOvertones of isoscalar giant resonances studied in direct particle decay measurementsHunyadi, M; van den Berg, AM; Csatlos, M; Csige, L; Davids, B; Garg, U; Gulyas, J; Harakeh,MN; de Huu, MA; Krasznahorkay, APublished in:Acta Physica Polonica BIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2005Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Hunyadi, M., van den Berg, AM., Csatlos, M., Csige, L., Davids, B., Garg, U., Gulyas, J., Harakeh, MN., deHuu, MA., Krasznahorkay, A., Sohler, D., & Wortche, HJ. (2005). Overtones of isoscalar giant resonancesstudied in direct particle decay measurements. Acta Physica Polonica B, 36(4), 1115-1119.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 31-10-2023Vol. 36 (2005) ACTA PHYSICA POLONICA B No 4OVERTONES OF ISOSCALAR GIANT RESONANCESSTUDIED IN DIRECT PARTICLE DECAYMEASUREMENTS∗M. Hunyadia, A.M. van den Bergb, M. Csatlósa, L. CsigeaB. Davidsb, U. Gargc, J. Gulyása, M.N. Harakehb, M.A. de HuubA. Krasznahorkaya, D. Sohlera, and H.J. WörtchebaInst. of Nuclear Research of the Hung. Acad. of Sciences, Debrecen, HungarybKernfysisch Versneller Instituut, Groningen, The NetherlandscUniversity of Notre Dame, Notre Dame, USA(Received November 30, 2004)The isoscalar giant dipole resonance (ISGDR), which is the lowest-energy overtone mode of the isoscalar giant resonances, has been studied insome medium-heavy and heavy nuclei in coincidence measurements. Theobservation of the direct nucleon decay channels significantly helped to en-hance giant resonance strengths with respect to the underlying backgroundand continuum, which gave an indication for the existence of a new modewith L=2 character, assumingly the overtone of the ISGQR.PACS numbers: 24.30.Cz, 25.55.Ci, 23.50.+zThe extension of experimental studies of the isoscalar giant resonances(GRs) toward the investigations of overtone modes was mainly motivatedby the possibility of learning about the formation and damping mechanismof GRs over a wide excitation energy region. As the overtone modes areexcited by the second-order term of the transition operators, they are weakresponses to external fields, and this in combination with large oscillationfrequencies and moderately fragmented doorway-state structures results indamped, broad strength distributions [1, 2]. The observation of such weakstructures above an overwhelming background and continuum requires spe-cial technique with high selectivity for excitations of single-particle charac-ter, which effectively disentangles GR strengths from the continuum.∗ Presented at the XXXIX Zakopane School of Physics — International Symposium“Atomic Nuclei at Extreme Values of Temperature, Spin and Isospin”, Zakopane,Poland, August 31–September 5, 2004.(1115)1116 M. Hunyadi et al.The application of coincidence techniques offered a clean solution to ob-tain a better definition of GR strengths with respect to previous single-armmeasurements [3] (and Refs. therein). On the other hand, a coincidenceexperiment, determining partial branching ratios of the direct-particle (neu-tron and proton) decay, is the only way to study the microscopic structureof GRs in medium-heavy and heavy nuclei.The 3~ω isoscalar giant dipole resonance (ISGDR), being the lowest-energy isoscalar overtone mode, has been studied in coincidence experimentsobserving decay channels by neutron emission of 90Zr, 116Sn and 208Pb [4],and by proton emission of 208Pb [5]. The experiments were performed atthe AGOR superconducting cyclotron facility of KVI using (α,α′) reactionat Eα=200 MeV. The α′ particles were momentum analyzed by the Big-BiteSpectrometer (BBS) in conjunction with the EuroSuperNova detector sys-tem. In the neutron decay experiment the kinetic energy of the neutronswas measured by the time-of-flight technique. In the proton decay experi-ment the protons were detected by an array of 5-mm thick Si(Li)-detectors.Further details of the experimental setup can be found in Refs. [5, 6].Ex (MeV)Strength (arb. units)90Zr(α,α´n)D-O-S0<Ef.s.<6 MeVa)Ex (MeV)116Sn(α,α´n)D-O-S0<Ef.s.<6 MeVb)Ex (MeV)208Pb(α,α´n)D-O-S0<Ef.s.<6 MeVc)0120 30 20 30 20 30Fig. 1. L=1 strength distributions generated by the difference-of-spectra techniquefrom the double-differential cross sections gated on the direct neutron decay of (a)90Zr, (b) 116Sn and (c) 208Pb.In order to disentangle the dipole strength from other multipolarities,angular distributions of the scattered α′ particles were measured in angularintervals, which comprise both enhancement and suppression of the dipolestrength, and allows the use of the difference-of-spectra technique. The gateon the direct decay channels, which was determined in the final-state energyspectra after taking into account the contribution of the statistical decay,strongly suppressed the underlying instrumental background and nuclearcontinuum. Although, in the neutron decay experiment the moderate en-ergy resolution, the partial separation of the direct and statistical decaysOvertones of Isoscalar Giant Resonances Studied in . . . 1117and the uncertainty of the random coincidence correction lead to difficultiesin the definition of the ISGDR structures. In Fig. 1(a)–(c) a significantconcentration of the dipole strength was observed. The parameters of thestrength distributions at the expected ISGDR energies were determined byGaussian fit and listed in Table I for comparison with the results of single-arm experiments of Refs. [7–9]. A satisfactory agreement was obtained forthe excitation energies, but the deviations of the resonance widths are stillremarkable and needs deeper analysis of the evaluation methods applied.TABLE IComparison of parameters obtained for the ISGDR.This work Single-arm experimentsEx Γ Ex ΓTarget (MeV) (MeV) (MeV) (MeV)90Zr 25.8(0.8)n 4.5(1.9)n 26.7(0.5)a 8.8(1.0)a26.9(0.7)b 12.0(1.5)b116Sn 24.0(1.5)n 5.3(1.7)n 25.5(0.6)c 12.0(0.6)c25.4(0.5)b 15.7(2.3)b208Pb 20.9(1.0)n 5.7(2.3)n 22.2(0.3)c 9.39(0.35)c22.1(0.3)p 3.8(0.8)p 22.7(0.2)b 11.9(0.4)bnn-decay exp., p p-decay exp.; a Ref. [7], b Ref. [8], c Ref. [9].Most of the problems of the neutron decay experiment were not presentin the case of the proton decay experiment, since the high Coulomb barrierhindered the statistical decay, and created especially clean conditions for theobservation of GR strength. Fig. 2(a)–(b) shows the double-differential crossEx (MeV)d2σdΩ-1dE-1 (mb·sr-1·MeV-1)ISGDRHEOR(ISGQR2)Ex (MeV)208Pb(α,α'p)1.5o< Θα' <3.0o1<Ef.s.<2 MeV208Pb(α,α'p)1.5o< Θα' <3.0o0<Ef.s.<1 MeVb)a)ISGDRHEOR (ISGQR2)00.20.420 25 30 20 25 30Fig. 2. Double-differential cross sections as a function of Ex gated on the directproton decay of 208Pb to Ef.s. regions in207Tl indicated in the panels.1118 M. Hunyadi et al.sections for two final-state energy regions, in which three GR components ofthe structure were recognized. While the ISGDR bump was easily identifiedbased on the analysis of the angular distributions (see Fig. 3(a)), the bumpat higher excitation energy most likely carried a quadrupole character (seeFig. 3(b)), but L = 3 and L = 4 multipolarities were not excluded. Recentlyperformed continuum-RPA calculations on isoscalar overtone GRs predicteda broad quadrupole structure around Ex = 30.5 MeV corresponding to the4~ω overtone mode of the ISGQR (→ ISGQR2) [2].Θα´ (deg.)dσ/dΩ (mb/sr)208Pb(α,α´p)0<Ef.s.<2 MeVISGDR bumpa)Θα´ (deg.)dσ/dΩ (mb/sr)208Pb(α,α´p)0<Ef.s.<2 MeVISGQR2(?) bumpb)2 4 60.00.20.40.60.81.02 4 60.00.20.40.60.81.0Fig. 3. Angular distributions of the (a) ISGDR and (b) ISGQR2(?). The theoreticalcurves were calculated by DWBA, and were scaled to fit the data points. In panel(a) L=1 curve (solid), in panel (b) L=2 curve (solid), L = 3 curve (dashed) andL = 4 curve (dotted) are shown.In conclusion, we have performed the first coincidence experiments toinvestigate the direct neutron and proton decay channels of the ISGDR andpossibly of other overtone resonances. The application of the direct parti-cle decay measurements helped to find a better definition of GR strengths,which may also lead to the discovery of new GR modes, like the ISGQR2.Systematic decay studies of isoscalar GRs at high excitation energies wouldyield the ultimate evidence for overtone modes other than the ISGDR.We acknowledge the Marie Curie Fellowship program of the EC (HPMF-CT-2000-00663), the transnational access program of the EC (HPRI-CT-1999-00109) and the Hungarian OTKA Foundation (T038404 and D34587).Overtones of Isoscalar Giant Resonances Studied in . . . 1119REFERENCES[1] G. Colò, Nucl. Phys. A731c, 15 (2004).[2] M. Gorelik et al., Phys. Rev. C69, 054322 (2004).[3] U. Garg, Nucl. Phys. A731c, 3 (2004).[4] M. Hunyadi et al., (prepared for submission).[5] M. Hunyadi et al., Phys. Lett. B576, 253 (2003).[6] M. Hunyadi et al., Nucl. Phys. A731c, 49 (2004).[7] D.H. Youngblood et al, Phys. Rev. C69, 054312 (2004).[8] M. Uchida et al., Phys. Rev. C69, 051301(R) (2004).[9] D.H. Youngblood et al., Phys. Rev. C69, 034315 (2004).

University of Groningen Research Database

  
 University of Groningen
Overtones of isoscalar giant resonances studied in direct particle decay measurements
Hunyadi, M; van den Berg, AM; Csatlos, M; Csige, L; Davids, B; Garg, U; Gulyas, J; Harakeh,
MN; de Huu, MA; Krasznahorkay, A
Published in:
Acta Physica Polonica B
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from
it. Please check the document version below.
Document Version
Publisher's PDF, also known as Version of record
Publication date:
2005
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Hunyadi, M., van den Berg, AM., Csatlos, M., Csige, L., Davids, B., Garg, U., ... Wortche, HJ. (2005).
Overtones of isoscalar giant resonances studied in direct particle decay measurements. Acta Physica
Polonica B, 36(4), 1115-1119.
Copyright
Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the
author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).
Take-down policy
If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately
and investigate your claim.
Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the
number of authors shown on this cover page is limited to 10 maximum.
Download date: 21-05-2019
Vol. 36 (2005) ACTA PHYSICA POLONICA B No 4
OVERTONES OF ISOSCALAR GIANT RESONANCES
STUDIED IN DIRECT PARTICLE DECAY
MEASUREMENTS∗
M. Hunyadia, A.M. van den Bergb, M. Csatlósa, L. Csigea
B. Davidsb, U. Gargc, J. Gulyása, M.N. Harakehb, M.A. de Huub
A. Krasznahorkaya, D. Sohlera, and H.J. Wörtcheb
aInst. of Nuclear Research of the Hung. Acad. of Sciences, Debrecen, Hungary
bKernfysisch Versneller Instituut, Groningen, The Netherlands
cUniversity of Notre Dame, Notre Dame, USA
(Received November 30, 2004)
The isoscalar giant dipole resonance (ISGDR), which is the lowest-
energy overtone mode of the isoscalar giant resonances, has been studied in
some medium-heavy and heavy nuclei in coincidence measurements. The
observation of the direct nucleon decay channels significantly helped to en-
hance giant resonance strengths with respect to the underlying background
and continuum, which gave an indication for the existence of a new mode
with L=2 character, assumingly the overtone of the ISGQR.
PACS numbers: 24.30.Cz, 25.55.Ci, 23.50.+z
The extension of experimental studies of the isoscalar giant resonances
(GRs) toward the investigations of overtone modes was mainly motivated
by the possibility of learning about the formation and damping mechanism
of GRs over a wide excitation energy region. As the overtone modes are
excited by the second-order term of the transition operators, they are weak
responses to external fields, and this in combination with large oscillation
frequencies and moderately fragmented doorway-state structures results in
damped, broad strength distributions [1, 2]. The observation of such weak
structures above an overwhelming background and continuum requires spe-
cial technique with high selectivity for excitations of single-particle charac-
ter, which effectively disentangles GR strengths from the continuum.
∗ Presented at the XXXIX Zakopane School of Physics — International Symposium
“Atomic Nuclei at Extreme Values of Temperature, Spin and Isospin”, Zakopane,
Poland, August 31–September 5, 2004.
(1115)
1116 M. Hunyadi et al.
The application of coincidence techniques offered a clean solution to ob-
tain a better definition of GR strengths with respect to previous single-arm
measurements [3] (and Refs. therein). On the other hand, a coincidence
experiment, determining partial branching ratios of the direct-particle (neu-
tron and proton) decay, is the only way to study the microscopic structure
of GRs in medium-heavy and heavy nuclei.
The 3~ω isoscalar giant dipole resonance (ISGDR), being the lowest-
energy isoscalar overtone mode, has been studied in coincidence experiments
observing decay channels by neutron emission of 90Zr, 116Sn and 208Pb [4],
and by proton emission of 208Pb [5]. The experiments were performed at
the AGOR superconducting cyclotron facility of KVI using (α,α′) reaction
at Eα=200 MeV. The α
′ particles were momentum analyzed by the Big-Bite
Spectrometer (BBS) in conjunction with the EuroSuperNova detector sys-
tem. In the neutron decay experiment the kinetic energy of the neutrons
was measured by the time-of-flight technique. In the proton decay experi-
ment the protons were detected by an array of 5-mm thick Si(Li)-detectors.
Further details of the experimental setup can be found in Refs. [5, 6].
Ex (MeV)
St
re
ng
th
 (a
rb.
 un
its
)
90Zr(a ,a ´n)
D-O-S
0<Ef.s.<6 MeV
a)
Ex (MeV)
116Sn(a ,a ´n)
D-O-S
0<Ef.s.<6 MeV
b)
Ex (MeV)
208Pb(a ,a ´n)
D-O-S
0<Ef.s.<6 MeV
c)
0
1
20 30 20 30 20 30
Fig. 1. L=1 strength distributions generated by the difference-of-spectra technique
from the double-differential cross sections gated on the direct neutron decay of (a)
90Zr, (b) 116Sn and (c) 208Pb.
In order to disentangle the dipole strength from other multipolarities,
angular distributions of the scattered α′ particles were measured in angular
intervals, which comprise both enhancement and suppression of the dipole
strength, and allows the use of the difference-of-spectra technique. The gate
on the direct decay channels, which was determined in the final-state energy
spectra after taking into account the contribution of the statistical decay,
strongly suppressed the underlying instrumental background and nuclear
continuum. Although, in the neutron decay experiment the moderate en-
ergy resolution, the partial separation of the direct and statistical decays
Overtones of Isoscalar Giant Resonances Studied in . . . 1117
and the uncertainty of the random coincidence correction lead to difficulties
in the definition of the ISGDR structures. In Fig. 1(a)–(c) a significant
concentration of the dipole strength was observed. The parameters of the
strength distributions at the expected ISGDR energies were determined by
Gaussian fit and listed in Table I for comparison with the results of single-
arm experiments of Refs. [7–9]. A satisfactory agreement was obtained for
the excitation energies, but the deviations of the resonance widths are still
remarkable and needs deeper analysis of the evaluation methods applied.
TABLE I
Comparison of parameters obtained for the ISGDR.
This work Single-arm experiments
Ex Γ Ex Γ
Target (MeV) (MeV) (MeV) (MeV)
90Zr 25.8(0.8)n 4.5(1.9)n 26.7(0.5)a 8.8(1.0)a
26.9(0.7)b 12.0(1.5)b
116Sn 24.0(1.5)n 5.3(1.7)n 25.5(0.6)c 12.0(0.6)c
25.4(0.5)b 15.7(2.3)b
208Pb 20.9(1.0)n 5.7(2.3)n 22.2(0.3)c 9.39(0.35)c
22.1(0.3)p 3.8(0.8)p 22.7(0.2)b 11.9(0.4)b
n
n-decay exp., p p-decay exp.; a Ref. [7], b Ref. [8], c Ref. [9].
Most of the problems of the neutron decay experiment were not present
in the case of the proton decay experiment, since the high Coulomb barrier
hindered the statistical decay, and created especially clean conditions for the
observation of GR strength. Fig. 2(a)–(b) shows the double-differential cross
Ex (MeV)
d2
s
dW
-
1 d
E-
1  
(m
b·s
r-1
·
M
eV
-
1 )
ISGDR
HEOR
(ISGQR2)
Ex (MeV)
208Pb(a ,a 'p)
1.5o< Q
a '
 <3.0o
1<Ef.s.<2 MeV
208Pb(a ,a 'p)
1.5o< Q
a '
 <3.0o
0<Ef.s.<1 MeV
b)a)
ISGDR
HEOR (ISGQR2)
0
0.2
0.4
20 25 30 20 25 30
Fig. 2. Double-differential cross sections as a function of Ex gated on the direct
proton decay of 208Pb to Ef.s. regions in
207Tl indicated in the panels.
1118 M. Hunyadi et al.
sections for two final-state energy regions, in which three GR components of
the structure were recognized. While the ISGDR bump was easily identified
based on the analysis of the angular distributions (see Fig. 3(a)), the bump
at higher excitation energy most likely carried a quadrupole character (see
Fig. 3(b)), but L = 3 and L = 4 multipolarities were not excluded. Recently
performed continuum-RPA calculations on isoscalar overtone GRs predicted
a broad quadrupole structure around Ex = 30.5 MeV corresponding to the
4~ω overtone mode of the ISGQR (→ ISGQR2) [2].
Q
a ´
 (deg.)
ds
/d
W
 
(m
b/s
r)
208Pb(a ,a ´p)
0<Ef.s.<2 MeV
ISGDR bump
a)
Q
a ´
 (deg.)
ds
/d
W
 
(m
b/s
r)
208Pb(a ,a ´p)
0<Ef.s.<2 MeV
ISGQR2(?) bump
b)
2 4 6
0.0
0.2
0.4
0.6
0.8
1.0
2 4 6
0.0
0.2
0.4
0.6
0.8
1.0
Fig. 3. Angular distributions of the (a) ISGDR and (b) ISGQR2(?). The theoretical
curves were calculated by DWBA, and were scaled to fit the data points. In panel
(a) L=1 curve (solid), in panel (b) L=2 curve (solid), L = 3 curve (dashed) and
L = 4 curve (dotted) are shown.
In conclusion, we have performed the first coincidence experiments to
investigate the direct neutron and proton decay channels of the ISGDR and
possibly of other overtone resonances. The application of the direct parti-
cle decay measurements helped to find a better definition of GR strengths,
which may also lead to the discovery of new GR modes, like the ISGQR2.
Systematic decay studies of isoscalar GRs at high excitation energies would
yield the ultimate evidence for overtone modes other than the ISGDR.
We acknowledge the Marie Curie Fellowship program of the EC (HPMF-
CT-2000-00663), the transnational access program of the EC (HPRI-CT-
1999-00109) and the Hungarian OTKA Foundation (T038404 and D34587).
Overtones of Isoscalar Giant Resonances Studied in . . . 1119
REFERENCES
[1] G. Colò, Nucl. Phys. A731c, 15 (2004).
[2] M. Gorelik et al., Phys. Rev. C69, 054322 (2004).
[3] U. Garg, Nucl. Phys. A731c, 3 (2004).
[4] M. Hunyadi et al., (prepared for submission).
[5] M. Hunyadi et al., Phys. Lett. B576, 253 (2003).
[6] M. Hunyadi et al., Nucl. Phys. A731c, 49 (2004).
[7] D.H. Youngblood et al, Phys. Rev. C69, 054312 (2004).
[8] M. Uchida et al., Phys. Rev. C69, 051301(R) (2004).
[9] D.H. Youngblood et al., Phys. Rev. C69, 034315 (2004).


Dissertations of the University of Groningen

   University of GroningenOvertones of isoscalar giant resonances studied in direct particle decay measurementsHunyadi, M; van den Berg, AM; Csatlos, M; Csige, L; Davids, B; Garg, U; Gulyas, J; Harakeh,MN; de Huu, MA; Krasznahorkay, APublished in:Acta Physica Polonica BIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2005Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Hunyadi, M., van den Berg, AM., Csatlos, M., Csige, L., Davids, B., Garg, U., Gulyas, J., Harakeh, MN., deHuu, MA., Krasznahorkay, A., Sohler, D., & Wortche, HJ. (2005). Overtones of isoscalar giant resonancesstudied in direct particle decay measurements. Acta Physica Polonica B, 36(4), 1115-1119.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 04-06-2022Vol. 36 (2005) ACTA PHYSICA POLONICA B No 4OVERTONES OF ISOSCALAR GIANT RESONANCESSTUDIED IN DIRECT PARTICLE DECAYMEASUREMENTS∗M. Hunyadia, A.M. van den Bergb, M. Csatlósa, L. CsigeaB. Davidsb, U. Gargc, J. Gulyása, M.N. Harakehb, M.A. de HuubA. Krasznahorkaya, D. Sohlera, and H.J. WörtchebaInst. of Nuclear Research of the Hung. Acad. of Sciences, Debrecen, HungarybKernfysisch Versneller Instituut, Groningen, The NetherlandscUniversity of Notre Dame, Notre Dame, USA(Received November 30, 2004)The isoscalar giant dipole resonance (ISGDR), which is the lowest-energy overtone mode of the isoscalar giant resonances, has been studied insome medium-heavy and heavy nuclei in coincidence measurements. Theobservation of the direct nucleon decay channels significantly helped to en-hance giant resonance strengths with respect to the underlying backgroundand continuum, which gave an indication for the existence of a new modewith L=2 character, assumingly the overtone of the ISGQR.PACS numbers: 24.30.Cz, 25.55.Ci, 23.50.+zThe extension of experimental studies of the isoscalar giant resonances(GRs) toward the investigations of overtone modes was mainly motivatedby the possibility of learning about the formation and damping mechanismof GRs over a wide excitation energy region. As the overtone modes areexcited by the second-order term of the transition operators, they are weakresponses to external fields, and this in combination with large oscillationfrequencies and moderately fragmented doorway-state structures results indamped, broad strength distributions [1, 2]. The observation of such weakstructures above an overwhelming background and continuum requires spe-cial technique with high selectivity for excitations of single-particle charac-ter, which effectively disentangles GR strengths from the continuum.∗ Presented at the XXXIX Zakopane School of Physics — International Symposium“Atomic Nuclei at Extreme Values of Temperature, Spin and Isospin”, Zakopane,Poland, August 31–September 5, 2004.(1115)1116 M. Hunyadi et al.The application of coincidence techniques offered a clean solution to ob-tain a better definition of GR strengths with respect to previous single-armmeasurements [3] (and Refs. therein). On the other hand, a coincidenceexperiment, determining partial branching ratios of the direct-particle (neu-tron and proton) decay, is the only way to study the microscopic structureof GRs in medium-heavy and heavy nuclei.The 3~ω isoscalar giant dipole resonance (ISGDR), being the lowest-energy isoscalar overtone mode, has been studied in coincidence experimentsobserving decay channels by neutron emission of 90Zr, 116Sn and 208Pb [4],and by proton emission of 208Pb [5]. The experiments were performed atthe AGOR superconducting cyclotron facility of KVI using (α,α′) reactionat Eα=200 MeV. The α′ particles were momentum analyzed by the Big-BiteSpectrometer (BBS) in conjunction with the EuroSuperNova detector sys-tem. In the neutron decay experiment the kinetic energy of the neutronswas measured by the time-of-flight technique. In the proton decay experi-ment the protons were detected by an array of 5-mm thick Si(Li)-detectors.Further details of the experimental setup can be found in Refs. [5, 6].Ex (MeV)Strength (arb. units)90Zr(α,α´n)D-O-S0<Ef.s.<6 MeVa)Ex (MeV)116Sn(α,α´n)D-O-S0<Ef.s.<6 MeVb)Ex (MeV)208Pb(α,α´n)D-O-S0<Ef.s.<6 MeVc)0120 30 20 30 20 30Fig. 1. L=1 strength distributions generated by the difference-of-spectra techniquefrom the double-differential cross sections gated on the direct neutron decay of (a)90Zr, (b) 116Sn and (c) 208Pb.In order to disentangle the dipole strength from other multipolarities,angular distributions of the scattered α′ particles were measured in angularintervals, which comprise both enhancement and suppression of the dipolestrength, and allows the use of the difference-of-spectra technique. The gateon the direct decay channels, which was determined in the final-state energyspectra after taking into account the contribution of the statistical decay,strongly suppressed the underlying instrumental background and nuclearcontinuum. Although, in the neutron decay experiment the moderate en-ergy resolution, the partial separation of the direct and statistical decaysOvertones of Isoscalar Giant Resonances Studied in . . . 1117and the uncertainty of the random coincidence correction lead to difficultiesin the definition of the ISGDR structures. In Fig. 1(a)–(c) a significantconcentration of the dipole strength was observed. The parameters of thestrength distributions at the expected ISGDR energies were determined byGaussian fit and listed in Table I for comparison with the results of single-arm experiments of Refs. [7–9]. A satisfactory agreement was obtained forthe excitation energies, but the deviations of the resonance widths are stillremarkable and needs deeper analysis of the evaluation methods applied.TABLE IComparison of parameters obtained for the ISGDR.This work Single-arm experimentsEx Γ Ex ΓTarget (MeV) (MeV) (MeV) (MeV)90Zr 25.8(0.8)n 4.5(1.9)n 26.7(0.5)a 8.8(1.0)a26.9(0.7)b 12.0(1.5)b116Sn 24.0(1.5)n 5.3(1.7)n 25.5(0.6)c 12.0(0.6)c25.4(0.5)b 15.7(2.3)b208Pb 20.9(1.0)n 5.7(2.3)n 22.2(0.3)c 9.39(0.35)c22.1(0.3)p 3.8(0.8)p 22.7(0.2)b 11.9(0.4)bnn-decay exp., p p-decay exp.; a Ref. [7], b Ref. [8], c Ref. [9].Most of the problems of the neutron decay experiment were not presentin the case of the proton decay experiment, since the high Coulomb barrierhindered the statistical decay, and created especially clean conditions for theobservation of GR strength. Fig. 2(a)–(b) shows the double-differential crossEx (MeV)d2σdΩ-1dE-1 (mb·sr-1·MeV-1)ISGDRHEOR(ISGQR2)Ex (MeV)208Pb(α,α'p)1.5o< Θα' <3.0o1<Ef.s.<2 MeV208Pb(α,α'p)1.5o< Θα' <3.0o0<Ef.s.<1 MeVb)a)ISGDRHEOR (ISGQR2)00.20.420 25 30 20 25 30Fig. 2. Double-differential cross sections as a function of Ex gated on the directproton decay of 208Pb to Ef.s. regions in207Tl indicated in the panels.1118 M. Hunyadi et al.sections for two final-state energy regions, in which three GR components ofthe structure were recognized. While the ISGDR bump was easily identifiedbased on the analysis of the angular distributions (see Fig. 3(a)), the bumpat higher excitation energy most likely carried a quadrupole character (seeFig. 3(b)), but L = 3 and L = 4 multipolarities were not excluded. Recentlyperformed continuum-RPA calculations on isoscalar overtone GRs predicteda broad quadrupole structure around Ex = 30.5 MeV corresponding to the4~ω overtone mode of the ISGQR (→ ISGQR2) [2].Θα´ (deg.)dσ/dΩ (mb/sr)208Pb(α,α´p)0<Ef.s.<2 MeVISGDR bumpa)Θα´ (deg.)dσ/dΩ (mb/sr)208Pb(α,α´p)0<Ef.s.<2 MeVISGQR2(?) bumpb)2 4 60.00.20.40.60.81.02 4 60.00.20.40.60.81.0Fig. 3. Angular distributions of the (a) ISGDR and (b) ISGQR2(?). The theoreticalcurves were calculated by DWBA, and were scaled to fit the data points. In panel(a) L=1 curve (solid), in panel (b) L=2 curve (solid), L = 3 curve (dashed) andL = 4 curve (dotted) are shown.In conclusion, we have performed the first coincidence experiments toinvestigate the direct neutron and proton decay channels of the ISGDR andpossibly of other overtone resonances. The application of the direct parti-cle decay measurements helped to find a better definition of GR strengths,which may also lead to the discovery of new GR modes, like the ISGQR2.Systematic decay studies of isoscalar GRs at high excitation energies wouldyield the ultimate evidence for overtone modes other than the ISGDR.We acknowledge the Marie Curie Fellowship program of the EC (HPMF-CT-2000-00663), the transnational access program of the EC (HPRI-CT-1999-00109) and the Hungarian OTKA Foundation (T038404 and D34587).Overtones of Isoscalar Giant Resonances Studied in . . . 1119REFERENCES[1] G. Colò, Nucl. Phys. A731c, 15 (2004).[2] M. Gorelik et al., Phys. Rev. C69, 054322 (2004).[3] U. Garg, Nucl. Phys. A731c, 3 (2004).[4] M. Hunyadi et al., (prepared for submission).[5] M. Hunyadi et al., Phys. Lett. B576, 253 (2003).[6] M. Hunyadi et al., Nucl. Phys. A731c, 49 (2004).[7] D.H. Youngblood et al, Phys. Rev. C69, 054312 (2004).[8] M. Uchida et al., Phys. Rev. C69, 051301(R) (2004).[9] D.H. Youngblood et al., Phys. Rev. C69, 034315 (2004).

University of Groningen

   University of GroningenOvertones of isoscalar giant resonances studied in direct particle decay measurementsHunyadi, M; van den Berg, AM; Csatlos, M; Csige, L; Davids, B; Garg, U; Gulyas, J; Harakeh,MN; de Huu, MA; Krasznahorkay, APublished in:Acta Physica Polonica BIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2005Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Hunyadi, M., van den Berg, AM., Csatlos, M., Csige, L., Davids, B., Garg, U., ... Wortche, HJ. (2005).Overtones of isoscalar giant resonances studied in direct particle decay measurements. Acta PhysicaPolonica B, 36(4), 1115-1119.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 12-11-2019Vol. 36 (2005) ACTA PHYSICA POLONICA B No 4OVERTONES OF ISOSCALAR GIANT RESONANCESSTUDIED IN DIRECT PARTICLE DECAYMEASUREMENTS∗M. Hunyadia, A.M. van den Bergb, M. Csatlósa, L. CsigeaB. Davidsb, U. Gargc, J. Gulyása, M.N. Harakehb, M.A. de HuubA. Krasznahorkaya, D. Sohlera, and H.J. WörtchebaInst. of Nuclear Research of the Hung. Acad. of Sciences, Debrecen, HungarybKernfysisch Versneller Instituut, Groningen, The NetherlandscUniversity of Notre Dame, Notre Dame, USA(Received November 30, 2004)The isoscalar giant dipole resonance (ISGDR), which is the lowest-energy overtone mode of the isoscalar giant resonances, has been studied insome medium-heavy and heavy nuclei in coincidence measurements. Theobservation of the direct nucleon decay channels significantly helped to en-hance giant resonance strengths with respect to the underlying backgroundand continuum, which gave an indication for the existence of a new modewith L=2 character, assumingly the overtone of the ISGQR.PACS numbers: 24.30.Cz, 25.55.Ci, 23.50.+zThe extension of experimental studies of the isoscalar giant resonances(GRs) toward the investigations of overtone modes was mainly motivatedby the possibility of learning about the formation and damping mechanismof GRs over a wide excitation energy region. As the overtone modes areexcited by the second-order term of the transition operators, they are weakresponses to external fields, and this in combination with large oscillationfrequencies and moderately fragmented doorway-state structures results indamped, broad strength distributions [1, 2]. The observation of such weakstructures above an overwhelming background and continuum requires spe-cial technique with high selectivity for excitations of single-particle charac-ter, which effectively disentangles GR strengths from the continuum.∗ Presented at the XXXIX Zakopane School of Physics — International Symposium“Atomic Nuclei at Extreme Values of Temperature, Spin and Isospin”, Zakopane,Poland, August 31–September 5, 2004.(1115)1116 M. Hunyadi et al.The application of coincidence techniques offered a clean solution to ob-tain a better definition of GR strengths with respect to previous single-armmeasurements [3] (and Refs. therein). On the other hand, a coincidenceexperiment, determining partial branching ratios of the direct-particle (neu-tron and proton) decay, is the only way to study the microscopic structureof GRs in medium-heavy and heavy nuclei.The 3~ω isoscalar giant dipole resonance (ISGDR), being the lowest-energy isoscalar overtone mode, has been studied in coincidence experimentsobserving decay channels by neutron emission of 90Zr, 116Sn and 208Pb [4],and by proton emission of 208Pb [5]. The experiments were performed atthe AGOR superconducting cyclotron facility of KVI using (α,α′) reactionat Eα=200 MeV. The α′ particles were momentum analyzed by the Big-BiteSpectrometer (BBS) in conjunction with the EuroSuperNova detector sys-tem. In the neutron decay experiment the kinetic energy of the neutronswas measured by the time-of-flight technique. In the proton decay experi-ment the protons were detected by an array of 5-mm thick Si(Li)-detectors.Further details of the experimental setup can be found in Refs. [5, 6].Ex (MeV)Strength (arb. units)90Zr(a ,a ´n)D-O-S0<Ef.s.<6 MeVa)Ex (MeV)116Sn(a ,a ´n)D-O-S0<Ef.s.<6 MeVb)Ex (MeV)208Pb(a ,a ´n)D-O-S0<Ef.s.<6 MeVc)0120 30 20 30 20 30Fig. 1. L=1 strength distributions generated by the difference-of-spectra techniquefrom the double-differential cross sections gated on the direct neutron decay of (a)90Zr, (b) 116Sn and (c) 208Pb.In order to disentangle the dipole strength from other multipolarities,angular distributions of the scattered α′ particles were measured in angularintervals, which comprise both enhancement and suppression of the dipolestrength, and allows the use of the difference-of-spectra technique. The gateon the direct decay channels, which was determined in the final-state energyspectra after taking into account the contribution of the statistical decay,strongly suppressed the underlying instrumental background and nuclearcontinuum. Although, in the neutron decay experiment the moderate en-ergy resolution, the partial separation of the direct and statistical decaysOvertones of Isoscalar Giant Resonances Studied in . . . 1117and the uncertainty of the random coincidence correction lead to difficultiesin the definition of the ISGDR structures. In Fig. 1(a)–(c) a significantconcentration of the dipole strength was observed. The parameters of thestrength distributions at the expected ISGDR energies were determined byGaussian fit and listed in Table I for comparison with the results of single-arm experiments of Refs. [7–9]. A satisfactory agreement was obtained forthe excitation energies, but the deviations of the resonance widths are stillremarkable and needs deeper analysis of the evaluation methods applied.TABLE IComparison of parameters obtained for the ISGDR.This work Single-arm experimentsEx Γ Ex ΓTarget (MeV) (MeV) (MeV) (MeV)90Zr 25.8(0.8)n 4.5(1.9)n 26.7(0.5)a 8.8(1.0)a26.9(0.7)b 12.0(1.5)b116Sn 24.0(1.5)n 5.3(1.7)n 25.5(0.6)c 12.0(0.6)c25.4(0.5)b 15.7(2.3)b208Pb 20.9(1.0)n 5.7(2.3)n 22.2(0.3)c 9.39(0.35)c22.1(0.3)p 3.8(0.8)p 22.7(0.2)b 11.9(0.4)bnn-decay exp., p p-decay exp.; a Ref. [7], b Ref. [8], c Ref. [9].Most of the problems of the neutron decay experiment were not presentin the case of the proton decay experiment, since the high Coulomb barrierhindered the statistical decay, and created especially clean conditions for theobservation of GR strength. Fig. 2(a)–(b) shows the double-differential crossEx (MeV)d2sdW-1 dE-1  (mb·sr-1·MeV-1 )ISGDRHEOR(ISGQR2)Ex (MeV)208Pb(a ,a 'p)1.5o< Qa ' <3.0o1<Ef.s.<2 MeV208Pb(a ,a 'p)1.5o< Qa ' <3.0o0<Ef.s.<1 MeVb)a)ISGDRHEOR (ISGQR2)00.20.420 25 30 20 25 30Fig. 2. Double-differential cross sections as a function of Ex gated on the directproton decay of 208Pb to Ef.s. regions in207Tl indicated in the panels.1118 M. Hunyadi et al.sections for two final-state energy regions, in which three GR components ofthe structure were recognized. While the ISGDR bump was easily identifiedbased on the analysis of the angular distributions (see Fig. 3(a)), the bumpat higher excitation energy most likely carried a quadrupole character (seeFig. 3(b)), but L = 3 and L = 4 multipolarities were not excluded. Recentlyperformed continuum-RPA calculations on isoscalar overtone GRs predicteda broad quadrupole structure around Ex = 30.5 MeV corresponding to the4~ω overtone mode of the ISGQR (→ ISGQR2) [2].Qa ´ (deg.)ds/dW (mb/sr)208Pb(a ,a ´p)0<Ef.s.<2 MeVISGDR bumpa)Qa ´ (deg.)ds/dW (mb/sr)208Pb(a ,a ´p)0<Ef.s.<2 MeVISGQR2(?) bumpb)2 4 60.00.20.40.60.81.02 4 60.00.20.40.60.81.0Fig. 3. Angular distributions of the (a) ISGDR and (b) ISGQR2(?). The theoreticalcurves were calculated by DWBA, and were scaled to fit the data points. In panel(a) L=1 curve (solid), in panel (b) L=2 curve (solid), L = 3 curve (dashed) andL = 4 curve (dotted) are shown.In conclusion, we have performed the first coincidence experiments toinvestigate the direct neutron and proton decay channels of the ISGDR andpossibly of other overtone resonances. The application of the direct parti-cle decay measurements helped to find a better definition of GR strengths,which may also lead to the discovery of new GR modes, like the ISGQR2.Systematic decay studies of isoscalar GRs at high excitation energies wouldyield the ultimate evidence for overtone modes other than the ISGDR.We acknowledge the Marie Curie Fellowship program of the EC (HPMF-CT-2000-00663), the transnational access program of the EC (HPRI-CT-1999-00109) and the Hungarian OTKA Foundation (T038404 and D34587).Overtones of Isoscalar Giant Resonances Studied in . . . 1119REFERENCES[1] G. Colò, Nucl. Phys. A731c, 15 (2004).[2] M. Gorelik et al., Phys. Rev. C69, 054322 (2004).[3] U. Garg, Nucl. Phys. A731c, 3 (2004).[4] M. Hunyadi et al., (prepared for submission).[5] M. Hunyadi et al., Phys. Lett. B576, 253 (2003).[6] M. Hunyadi et al., Nucl. Phys. A731c, 49 (2004).[7] D.H. Youngblood et al, Phys. Rev. C69, 054312 (2004).[8] M. Uchida et al., Phys. Rev. C69, 051301(R) (2004).[9] D.H. Youngblood et al., Phys. Rev. C69, 034315 (2004).

NARCIS 

   University of GroningenOvertones of isoscalar giant resonances studied in direct particle decay measurementsHunyadi, M; van den Berg, AM; Csatlos, M; Csige, L; Davids, B; Garg, U; Gulyas, J; Harakeh,MN; de Huu, MA; Krasznahorkay, APublished in:Acta Physica Polonica BIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2005Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Hunyadi, M., van den Berg, AM., Csatlos, M., Csige, L., Davids, B., Garg, U., Gulyas, J., Harakeh, MN., deHuu, MA., Krasznahorkay, A., Sohler, D., & Wortche, HJ. (2005). Overtones of isoscalar giant resonancesstudied in direct particle decay measurements. Acta Physica Polonica B, 36(4), 1115-1119.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 17-07-2023Vol. 36 (2005) ACTA PHYSICA POLONICA B No 4OVERTONES OF ISOSCALAR GIANT RESONANCESSTUDIED IN DIRECT PARTICLE DECAYMEASUREMENTS∗M. Hunyadia, A.M. van den Bergb, M. Csatlósa, L. CsigeaB. Davidsb, U. Gargc, J. Gulyása, M.N. Harakehb, M.A. de HuubA. Krasznahorkaya, D. Sohlera, and H.J. WörtchebaInst. of Nuclear Research of the Hung. Acad. of Sciences, Debrecen, HungarybKernfysisch Versneller Instituut, Groningen, The NetherlandscUniversity of Notre Dame, Notre Dame, USA(Received November 30, 2004)The isoscalar giant dipole resonance (ISGDR), which is the lowest-energy overtone mode of the isoscalar giant resonances, has been studied insome medium-heavy and heavy nuclei in coincidence measurements. Theobservation of the direct nucleon decay channels significantly helped to en-hance giant resonance strengths with respect to the underlying backgroundand continuum, which gave an indication for the existence of a new modewith L=2 character, assumingly the overtone of the ISGQR.PACS numbers: 24.30.Cz, 25.55.Ci, 23.50.+zThe extension of experimental studies of the isoscalar giant resonances(GRs) toward the investigations of overtone modes was mainly motivatedby the possibility of learning about the formation and damping mechanismof GRs over a wide excitation energy region. As the overtone modes areexcited by the second-order term of the transition operators, they are weakresponses to external fields, and this in combination with large oscillationfrequencies and moderately fragmented doorway-state structures results indamped, broad strength distributions [1, 2]. The observation of such weakstructures above an overwhelming background and continuum requires spe-cial technique with high selectivity for excitations of single-particle charac-ter, which effectively disentangles GR strengths from the continuum.∗ Presented at the XXXIX Zakopane School of Physics — International Symposium“Atomic Nuclei at Extreme Values of Temperature, Spin and Isospin”, Zakopane,Poland, August 31–September 5, 2004.(1115)1116 M. Hunyadi et al.The application of coincidence techniques offered a clean solution to ob-tain a better definition of GR strengths with respect to previous single-armmeasurements [3] (and Refs. therein). On the other hand, a coincidenceexperiment, determining partial branching ratios of the direct-particle (neu-tron and proton) decay, is the only way to study the microscopic structureof GRs in medium-heavy and heavy nuclei.The 3~ω isoscalar giant dipole resonance (ISGDR), being the lowest-energy isoscalar overtone mode, has been studied in coincidence experimentsobserving decay channels by neutron emission of 90Zr, 116Sn and 208Pb [4],and by proton emission of 208Pb [5]. The experiments were performed atthe AGOR superconducting cyclotron facility of KVI using (α,α′) reactionat Eα=200 MeV. The α′ particles were momentum analyzed by the Big-BiteSpectrometer (BBS) in conjunction with the EuroSuperNova detector sys-tem. In the neutron decay experiment the kinetic energy of the neutronswas measured by the time-of-flight technique. In the proton decay experi-ment the protons were detected by an array of 5-mm thick Si(Li)-detectors.Further details of the experimental setup can be found in Refs. [5, 6].Ex (MeV)Strength (arb. units)90Zr(α,α´n)D-O-S0<Ef.s.<6 MeVa)Ex (MeV)116Sn(α,α´n)D-O-S0<Ef.s.<6 MeVb)Ex (MeV)208Pb(α,α´n)D-O-S0<Ef.s.<6 MeVc)0120 30 20 30 20 30Fig. 1. L=1 strength distributions generated by the difference-of-spectra techniquefrom the double-differential cross sections gated on the direct neutron decay of (a)90Zr, (b) 116Sn and (c) 208Pb.In order to disentangle the dipole strength from other multipolarities,angular distributions of the scattered α′ particles were measured in angularintervals, which comprise both enhancement and suppression of the dipolestrength, and allows the use of the difference-of-spectra technique. The gateon the direct decay channels, which was determined in the final-state energyspectra after taking into account the contribution of the statistical decay,strongly suppressed the underlying instrumental background and nuclearcontinuum. Although, in the neutron decay experiment the moderate en-ergy resolution, the partial separation of the direct and statistical decaysOvertones of Isoscalar Giant Resonances Studied in . . . 1117and the uncertainty of the random coincidence correction lead to difficultiesin the definition of the ISGDR structures. In Fig. 1(a)–(c) a significantconcentration of the dipole strength was observed. The parameters of thestrength distributions at the expected ISGDR energies were determined byGaussian fit and listed in Table I for comparison with the results of single-arm experiments of Refs. [7–9]. A satisfactory agreement was obtained forthe excitation energies, but the deviations of the resonance widths are stillremarkable and needs deeper analysis of the evaluation methods applied.TABLE IComparison of parameters obtained for the ISGDR.This work Single-arm experimentsEx Γ Ex ΓTarget (MeV) (MeV) (MeV) (MeV)90Zr 25.8(0.8)n 4.5(1.9)n 26.7(0.5)a 8.8(1.0)a26.9(0.7)b 12.0(1.5)b116Sn 24.0(1.5)n 5.3(1.7)n 25.5(0.6)c 12.0(0.6)c25.4(0.5)b 15.7(2.3)b208Pb 20.9(1.0)n 5.7(2.3)n 22.2(0.3)c 9.39(0.35)c22.1(0.3)p 3.8(0.8)p 22.7(0.2)b 11.9(0.4)bnn-decay exp., p p-decay exp.; a Ref. [7], b Ref. [8], c Ref. [9].Most of the problems of the neutron decay experiment were not presentin the case of the proton decay experiment, since the high Coulomb barrierhindered the statistical decay, and created especially clean conditions for theobservation of GR strength. Fig. 2(a)–(b) shows the double-differential crossEx (MeV)d2σdΩ-1dE-1 (mb·sr-1·MeV-1)ISGDRHEOR(ISGQR2)Ex (MeV)208Pb(α,α'p)1.5o< Θα' <3.0o1<Ef.s.<2 MeV208Pb(α,α'p)1.5o< Θα' <3.0o0<Ef.s.<1 MeVb)a)ISGDRHEOR (ISGQR2)00.20.420 25 30 20 25 30Fig. 2. Double-differential cross sections as a function of Ex gated on the directproton decay of 208Pb to Ef.s. regions in207Tl indicated in the panels.1118 M. Hunyadi et al.sections for two final-state energy regions, in which three GR components ofthe structure were recognized. While the ISGDR bump was easily identifiedbased on the analysis of the angular distributions (see Fig. 3(a)), the bumpat higher excitation energy most likely carried a quadrupole character (seeFig. 3(b)), but L = 3 and L = 4 multipolarities were not excluded. Recentlyperformed continuum-RPA calculations on isoscalar overtone GRs predicteda broad quadrupole structure around Ex = 30.5 MeV corresponding to the4~ω overtone mode of the ISGQR (→ ISGQR2) [2].Θα´ (deg.)dσ/dΩ (mb/sr)208Pb(α,α´p)0<Ef.s.<2 MeVISGDR bumpa)Θα´ (deg.)dσ/dΩ (mb/sr)208Pb(α,α´p)0<Ef.s.<2 MeVISGQR2(?) bumpb)2 4 60.00.20.40.60.81.02 4 60.00.20.40.60.81.0Fig. 3. Angular distributions of the (a) ISGDR and (b) ISGQR2(?). The theoreticalcurves were calculated by DWBA, and were scaled to fit the data points. In panel(a) L=1 curve (solid), in panel (b) L=2 curve (solid), L = 3 curve (dashed) andL = 4 curve (dotted) are shown.In conclusion, we have performed the first coincidence experiments toinvestigate the direct neutron and proton decay channels of the ISGDR andpossibly of other overtone resonances. The application of the direct parti-cle decay measurements helped to find a better definition of GR strengths,which may also lead to the discovery of new GR modes, like the ISGQR2.Systematic decay studies of isoscalar GRs at high excitation energies wouldyield the ultimate evidence for overtone modes other than the ISGDR.We acknowledge the Marie Curie Fellowship program of the EC (HPMF-CT-2000-00663), the transnational access program of the EC (HPRI-CT-1999-00109) and the Hungarian OTKA Foundation (T038404 and D34587).Overtones of Isoscalar Giant Resonances Studied in . . . 1119REFERENCES[1] G. Colò, Nucl. Phys. A731c, 15 (2004).[2] M. Gorelik et al., Phys. Rev. C69, 054322 (2004).[3] U. Garg, Nucl. Phys. A731c, 3 (2004).[4] M. Hunyadi et al., (prepared for submission).[5] M. Hunyadi et al., Phys. Lett. B576, 253 (2003).[6] M. Hunyadi et al., Nucl. Phys. A731c, 49 (2004).[7] D.H. Youngblood et al, Phys. Rev. C69, 054312 (2004).[8] M. Uchida et al., Phys. Rev. C69, 051301(R) (2004).[9] D.H. Youngblood et al., Phys. Rev. C69, 034315 (2004).

   University of GroningenOvertones of isoscalar giant resonances studied in direct particle decay measurementsHunyadi, M; van den Berg, AM; Csatlos, M; Csige, L; Davids, B; Garg, U; Gulyas, J; Harakeh,MN; de Huu, MA; Krasznahorkay, APublished in:Acta Physica Polonica BIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2005Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Hunyadi, M., van den Berg, AM., Csatlos, M., Csige, L., Davids, B., Garg, U., Gulyas, J., Harakeh, MN., deHuu, MA., Krasznahorkay, A., Sohler, D., & Wortche, HJ. (2005). Overtones of isoscalar giant resonancesstudied in direct particle decay measurements. Acta Physica Polonica B, 36(4), 1115-1119.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 09-09-2023Vol. 36 (2005) ACTA PHYSICA POLONICA B No 4OVERTONES OF ISOSCALAR GIANT RESONANCESSTUDIED IN DIRECT PARTICLE DECAYMEASUREMENTS∗M. Hunyadia, A.M. van den Bergb, M. Csatlósa, L. CsigeaB. Davidsb, U. Gargc, J. Gulyása, M.N. Harakehb, M.A. de HuubA. Krasznahorkaya, D. Sohlera, and H.J. WörtchebaInst. of Nuclear Research of the Hung. Acad. of Sciences, Debrecen, HungarybKernfysisch Versneller Instituut, Groningen, The NetherlandscUniversity of Notre Dame, Notre Dame, USA(Received November 30, 2004)The isoscalar giant dipole resonance (ISGDR), which is the lowest-energy overtone mode of the isoscalar giant resonances, has been studied insome medium-heavy and heavy nuclei in coincidence measurements. Theobservation of the direct nucleon decay channels significantly helped to en-hance giant resonance strengths with respect to the underlying backgroundand continuum, which gave an indication for the existence of a new modewith L=2 character, assumingly the overtone of the ISGQR.PACS numbers: 24.30.Cz, 25.55.Ci, 23.50.+zThe extension of experimental studies of the isoscalar giant resonances(GRs) toward the investigations of overtone modes was mainly motivatedby the possibility of learning about the formation and damping mechanismof GRs over a wide excitation energy region. As the overtone modes areexcited by the second-order term of the transition operators, they are weakresponses to external fields, and this in combination with large oscillationfrequencies and moderately fragmented doorway-state structures results indamped, broad strength distributions [1, 2]. The observation of such weakstructures above an overwhelming background and continuum requires spe-cial technique with high selectivity for excitations of single-particle charac-ter, which effectively disentangles GR strengths from the continuum.∗ Presented at the XXXIX Zakopane School of Physics — International Symposium“Atomic Nuclei at Extreme Values of Temperature, Spin and Isospin”, Zakopane,Poland, August 31–September 5, 2004.(1115)1116 M. Hunyadi et al.The application of coincidence techniques offered a clean solution to ob-tain a better definition of GR strengths with respect to previous single-armmeasurements [3] (and Refs. therein). On the other hand, a coincidenceexperiment, determining partial branching ratios of the direct-particle (neu-tron and proton) decay, is the only way to study the microscopic structureof GRs in medium-heavy and heavy nuclei.The 3~ω isoscalar giant dipole resonance (ISGDR), being the lowest-energy isoscalar overtone mode, has been studied in coincidence experimentsobserving decay channels by neutron emission of 90Zr, 116Sn and 208Pb [4],and by proton emission of 208Pb [5]. The experiments were performed atthe AGOR superconducting cyclotron facility of KVI using (α,α′) reactionat Eα=200 MeV. The α′ particles were momentum analyzed by the Big-BiteSpectrometer (BBS) in conjunction with the EuroSuperNova detector sys-tem. In the neutron decay experiment the kinetic energy of the neutronswas measured by the time-of-flight technique. In the proton decay experi-ment the protons were detected by an array of 5-mm thick Si(Li)-detectors.Further details of the experimental setup can be found in Refs. [5, 6].Ex (MeV)Strength (arb. units)90Zr(α,α´n)D-O-S0<Ef.s.<6 MeVa)Ex (MeV)116Sn(α,α´n)D-O-S0<Ef.s.<6 MeVb)Ex (MeV)208Pb(α,α´n)D-O-S0<Ef.s.<6 MeVc)0120 30 20 30 20 30Fig. 1. L=1 strength distributions generated by the difference-of-spectra techniquefrom the double-differential cross sections gated on the direct neutron decay of (a)90Zr, (b) 116Sn and (c) 208Pb.In order to disentangle the dipole strength from other multipolarities,angular distributions of the scattered α′ particles were measured in angularintervals, which comprise both enhancement and suppression of the dipolestrength, and allows the use of the difference-of-spectra technique. The gateon the direct decay channels, which was determined in the final-state energyspectra after taking into account the contribution of the statistical decay,strongly suppressed the underlying instrumental background and nuclearcontinuum. Although, in the neutron decay experiment the moderate en-ergy resolution, the partial separation of the direct and statistical decaysOvertones of Isoscalar Giant Resonances Studied in . . . 1117and the uncertainty of the random coincidence correction lead to difficultiesin the definition of the ISGDR structures. In Fig. 1(a)–(c) a significantconcentration of the dipole strength was observed. The parameters of thestrength distributions at the expected ISGDR energies were determined byGaussian fit and listed in Table I for comparison with the results of single-arm experiments of Refs. [7–9]. A satisfactory agreement was obtained forthe excitation energies, but the deviations of the resonance widths are stillremarkable and needs deeper analysis of the evaluation methods applied.TABLE IComparison of parameters obtained for the ISGDR.This work Single-arm experimentsEx Γ Ex ΓTarget (MeV) (MeV) (MeV) (MeV)90Zr 25.8(0.8)n 4.5(1.9)n 26.7(0.5)a 8.8(1.0)a26.9(0.7)b 12.0(1.5)b116Sn 24.0(1.5)n 5.3(1.7)n 25.5(0.6)c 12.0(0.6)c25.4(0.5)b 15.7(2.3)b208Pb 20.9(1.0)n 5.7(2.3)n 22.2(0.3)c 9.39(0.35)c22.1(0.3)p 3.8(0.8)p 22.7(0.2)b 11.9(0.4)bnn-decay exp., p p-decay exp.; a Ref. [7], b Ref. [8], c Ref. [9].Most of the problems of the neutron decay experiment were not presentin the case of the proton decay experiment, since the high Coulomb barrierhindered the statistical decay, and created especially clean conditions for theobservation of GR strength. Fig. 2(a)–(b) shows the double-differential crossEx (MeV)d2σdΩ-1dE-1 (mb·sr-1·MeV-1)ISGDRHEOR(ISGQR2)Ex (MeV)208Pb(α,α'p)1.5o< Θα' <3.0o1<Ef.s.<2 MeV208Pb(α,α'p)1.5o< Θα' <3.0o0<Ef.s.<1 MeVb)a)ISGDRHEOR (ISGQR2)00.20.420 25 30 20 25 30Fig. 2. Double-differential cross sections as a function of Ex gated on the directproton decay of 208Pb to Ef.s. regions in207Tl indicated in the panels.1118 M. Hunyadi et al.sections for two final-state energy regions, in which three GR components ofthe structure were recognized. While the ISGDR bump was easily identifiedbased on the analysis of the angular distributions (see Fig. 3(a)), the bumpat higher excitation energy most likely carried a quadrupole character (seeFig. 3(b)), but L = 3 and L = 4 multipolarities were not excluded. Recentlyperformed continuum-RPA calculations on isoscalar overtone GRs predicteda broad quadrupole structure around Ex = 30.5 MeV corresponding to the4~ω overtone mode of the ISGQR (→ ISGQR2) [2].Θα´ (deg.)dσ/dΩ (mb/sr)208Pb(α,α´p)0<Ef.s.<2 MeVISGDR bumpa)Θα´ (deg.)dσ/dΩ (mb/sr)208Pb(α,α´p)0<Ef.s.<2 MeVISGQR2(?) bumpb)2 4 60.00.20.40.60.81.02 4 60.00.20.40.60.81.0Fig. 3. Angular distributions of the (a) ISGDR and (b) ISGQR2(?). The theoreticalcurves were calculated by DWBA, and were scaled to fit the data points. In panel(a) L=1 curve (solid), in panel (b) L=2 curve (solid), L = 3 curve (dashed) andL = 4 curve (dotted) are shown.In conclusion, we have performed the first coincidence experiments toinvestigate the direct neutron and proton decay channels of the ISGDR andpossibly of other overtone resonances. The application of the direct parti-cle decay measurements helped to find a better definition of GR strengths,which may also lead to the discovery of new GR modes, like the ISGQR2.Systematic decay studies of isoscalar GRs at high excitation energies wouldyield the ultimate evidence for overtone modes other than the ISGDR.We acknowledge the Marie Curie Fellowship program of the EC (HPMF-CT-2000-00663), the transnational access program of the EC (HPRI-CT-1999-00109) and the Hungarian OTKA Foundation (T038404 and D34587).Overtones of Isoscalar Giant Resonances Studied in . . . 1119REFERENCES[1] G. Colò, Nucl. Phys. A731c, 15 (2004).[2] M. Gorelik et al., Phys. Rev. C69, 054322 (2004).[3] U. Garg, Nucl. Phys. A731c, 3 (2004).[4] M. Hunyadi et al., (prepared for submission).[5] M. Hunyadi et al., Phys. Lett. B576, 253 (2003).[6] M. Hunyadi et al., Nucl. Phys. A731c, 49 (2004).[7] D.H. Youngblood et al, Phys. Rev. C69, 054312 (2004).[8] M. Uchida et al., Phys. Rev. C69, 051301(R) (2004).[9] D.H. Youngblood et al., Phys. Rev. C69, 034315 (2004).

   University of GroningenOvertones of isoscalar giant resonances studied in direct particle decay measurementsHunyadi, M; van den Berg, AM; Csatlos, M; Csige, L; Davids, B; Garg, U; Gulyas, J; Harakeh,MN; de Huu, MA; Krasznahorkay, APublished in:Acta Physica Polonica BIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2005Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Hunyadi, M., van den Berg, AM., Csatlos, M., Csige, L., Davids, B., Garg, U., Gulyas, J., Harakeh, MN., deHuu, MA., Krasznahorkay, A., Sohler, D., & Wortche, HJ. (2005). Overtones of isoscalar giant resonancesstudied in direct particle decay measurements. Acta Physica Polonica B, 36(4), 1115-1119.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 03-06-2022Vol. 36 (2005) ACTA PHYSICA POLONICA B No 4OVERTONES OF ISOSCALAR GIANT RESONANCESSTUDIED IN DIRECT PARTICLE DECAYMEASUREMENTS∗M. Hunyadia, A.M. van den Bergb, M. Csatlósa, L. CsigeaB. Davidsb, U. Gargc, J. Gulyása, M.N. Harakehb, M.A. de HuubA. Krasznahorkaya, D. Sohlera, and H.J. WörtchebaInst. of Nuclear Research of the Hung. Acad. of Sciences, Debrecen, HungarybKernfysisch Versneller Instituut, Groningen, The NetherlandscUniversity of Notre Dame, Notre Dame, USA(Received November 30, 2004)The isoscalar giant dipole resonance (ISGDR), which is the lowest-energy overtone mode of the isoscalar giant resonances, has been studied insome medium-heavy and heavy nuclei in coincidence measurements. Theobservation of the direct nucleon decay channels significantly helped to en-hance giant resonance strengths with respect to the underlying backgroundand continuum, which gave an indication for the existence of a new modewith L=2 character, assumingly the overtone of the ISGQR.PACS numbers: 24.30.Cz, 25.55.Ci, 23.50.+zThe extension of experimental studies of the isoscalar giant resonances(GRs) toward the investigations of overtone modes was mainly motivatedby the possibility of learning about the formation and damping mechanismof GRs over a wide excitation energy region. As the overtone modes areexcited by the second-order term of the transition operators, they are weakresponses to external fields, and this in combination with large oscillationfrequencies and moderately fragmented doorway-state structures results indamped, broad strength distributions [1, 2]. The observation of such weakstructures above an overwhelming background and continuum requires spe-cial technique with high selectivity for excitations of single-particle charac-ter, which effectively disentangles GR strengths from the continuum.∗ Presented at the XXXIX Zakopane School of Physics — International Symposium“Atomic Nuclei at Extreme Values of Temperature, Spin and Isospin”, Zakopane,Poland, August 31–September 5, 2004.(1115)1116 M. Hunyadi et al.The application of coincidence techniques offered a clean solution to ob-tain a better definition of GR strengths with respect to previous single-armmeasurements [3] (and Refs. therein). On the other hand, a coincidenceexperiment, determining partial branching ratios of the direct-particle (neu-tron and proton) decay, is the only way to study the microscopic structureof GRs in medium-heavy and heavy nuclei.The 3~ω isoscalar giant dipole resonance (ISGDR), being the lowest-energy isoscalar overtone mode, has been studied in coincidence experimentsobserving decay channels by neutron emission of 90Zr, 116Sn and 208Pb [4],and by proton emission of 208Pb [5]. The experiments were performed atthe AGOR superconducting cyclotron facility of KVI using (α,α′) reactionat Eα=200 MeV. The α′ particles were momentum analyzed by the Big-BiteSpectrometer (BBS) in conjunction with the EuroSuperNova detector sys-tem. In the neutron decay experiment the kinetic energy of the neutronswas measured by the time-of-flight technique. In the proton decay experi-ment the protons were detected by an array of 5-mm thick Si(Li)-detectors.Further details of the experimental setup can be found in Refs. [5, 6].Ex (MeV)Strength (arb. units)90Zr(α,α´n)D-O-S0<Ef.s.<6 MeVa)Ex (MeV)116Sn(α,α´n)D-O-S0<Ef.s.<6 MeVb)Ex (MeV)208Pb(α,α´n)D-O-S0<Ef.s.<6 MeVc)0120 30 20 30 20 30Fig. 1. L=1 strength distributions generated by the difference-of-spectra techniquefrom the double-differential cross sections gated on the direct neutron decay of (a)90Zr, (b) 116Sn and (c) 208Pb.In order to disentangle the dipole strength from other multipolarities,angular distributions of the scattered α′ particles were measured in angularintervals, which comprise both enhancement and suppression of the dipolestrength, and allows the use of the difference-of-spectra technique. The gateon the direct decay channels, which was determined in the final-state energyspectra after taking into account the contribution of the statistical decay,strongly suppressed the underlying instrumental background and nuclearcontinuum. Although, in the neutron decay experiment the moderate en-ergy resolution, the partial separation of the direct and statistical decaysOvertones of Isoscalar Giant Resonances Studied in . . . 1117and the uncertainty of the random coincidence correction lead to difficultiesin the definition of the ISGDR structures. In Fig. 1(a)–(c) a significantconcentration of the dipole strength was observed. The parameters of thestrength distributions at the expected ISGDR energies were determined byGaussian fit and listed in Table I for comparison with the results of single-arm experiments of Refs. [7–9]. A satisfactory agreement was obtained forthe excitation energies, but the deviations of the resonance widths are stillremarkable and needs deeper analysis of the evaluation methods applied.TABLE IComparison of parameters obtained for the ISGDR.This work Single-arm experimentsEx Γ Ex ΓTarget (MeV) (MeV) (MeV) (MeV)90Zr 25.8(0.8)n 4.5(1.9)n 26.7(0.5)a 8.8(1.0)a26.9(0.7)b 12.0(1.5)b116Sn 24.0(1.5)n 5.3(1.7)n 25.5(0.6)c 12.0(0.6)c25.4(0.5)b 15.7(2.3)b208Pb 20.9(1.0)n 5.7(2.3)n 22.2(0.3)c 9.39(0.35)c22.1(0.3)p 3.8(0.8)p 22.7(0.2)b 11.9(0.4)bnn-decay exp., p p-decay exp.; a Ref. [7], b Ref. [8], c Ref. [9].Most of the problems of the neutron decay experiment were not presentin the case of the proton decay experiment, since the high Coulomb barrierhindered the statistical decay, and created especially clean conditions for theobservation of GR strength. Fig. 2(a)–(b) shows the double-differential crossEx (MeV)d2σdΩ-1dE-1 (mb·sr-1·MeV-1)ISGDRHEOR(ISGQR2)Ex (MeV)208Pb(α,α'p)1.5o< Θα' <3.0o1<Ef.s.<2 MeV208Pb(α,α'p)1.5o< Θα' <3.0o0<Ef.s.<1 MeVb)a)ISGDRHEOR (ISGQR2)00.20.420 25 30 20 25 30Fig. 2. Double-differential cross sections as a function of Ex gated on the directproton decay of 208Pb to Ef.s. regions in207Tl indicated in the panels.1118 M. Hunyadi et al.sections for two final-state energy regions, in which three GR components ofthe structure were recognized. While the ISGDR bump was easily identifiedbased on the analysis of the angular distributions (see Fig. 3(a)), the bumpat higher excitation energy most likely carried a quadrupole character (seeFig. 3(b)), but L = 3 and L = 4 multipolarities were not excluded. Recentlyperformed continuum-RPA calculations on isoscalar overtone GRs predicteda broad quadrupole structure around Ex = 30.5 MeV corresponding to the4~ω overtone mode of the ISGQR (→ ISGQR2) [2].Θα´ (deg.)dσ/dΩ (mb/sr)208Pb(α,α´p)0<Ef.s.<2 MeVISGDR bumpa)Θα´ (deg.)dσ/dΩ (mb/sr)208Pb(α,α´p)0<Ef.s.<2 MeVISGQR2(?) bumpb)2 4 60.00.20.40.60.81.02 4 60.00.20.40.60.81.0Fig. 3. Angular distributions of the (a) ISGDR and (b) ISGQR2(?). The theoreticalcurves were calculated by DWBA, and were scaled to fit the data points. In panel(a) L=1 curve (solid), in panel (b) L=2 curve (solid), L = 3 curve (dashed) andL = 4 curve (dotted) are shown.In conclusion, we have performed the first coincidence experiments toinvestigate the direct neutron and proton decay channels of the ISGDR andpossibly of other overtone resonances. The application of the direct parti-cle decay measurements helped to find a better definition of GR strengths,which may also lead to the discovery of new GR modes, like the ISGQR2.Systematic decay studies of isoscalar GRs at high excitation energies wouldyield the ultimate evidence for overtone modes other than the ISGDR.We acknowledge the Marie Curie Fellowship program of the EC (HPMF-CT-2000-00663), the transnational access program of the EC (HPRI-CT-1999-00109) and the Hungarian OTKA Foundation (T038404 and D34587).Overtones of Isoscalar Giant Resonances Studied in . . . 1119REFERENCES[1] G. Colò, Nucl. Phys. A731c, 15 (2004).[2] M. Gorelik et al., Phys. Rev. C69, 054322 (2004).[3] U. Garg, Nucl. Phys. A731c, 3 (2004).[4] M. Hunyadi et al., (prepared for submission).[5] M. Hunyadi et al., Phys. Lett. B576, 253 (2003).[6] M. Hunyadi et al., Nucl. Phys. A731c, 49 (2004).[7] D.H. Youngblood et al, Phys. Rev. C69, 054312 (2004).[8] M. Uchida et al., Phys. Rev. C69, 051301(R) (2004).[9] D.H. Youngblood et al., Phys. Rev. C69, 034315 (2004).

Overtones of isoscalar giant resonances studied in direct particle decay measurements

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