Algora, A

Cano-Ott, D

Courtin, S

Dessagne, Philippe

Fraile-Prieto, L M

Gadea, A

García-Borge, M J

Gelletly, W

Jungclaus, A

Maréchal, F

Miehé, C

Nácher, E

Perrot, F

Poirier, E

Rubio, Berta

Taín, J L

Tengblad, O

CERN Document Server

Proposal to the ISOLDE and Neutron Time of Flight CommitteeStudies of the Beta-Decay of Sr nuclei on and near the N=Z line with aTotal Absorption Gamma Ray SpectrometerA. Algora1, M.J.G. Borge2, D. Cano-Ott3, S. Courtin4, Ph. Dessagne4, L- M. Fraile5, A. Gadea6, W.Gelletly7, A. Jungclaus2, F. Marechal4, Ch. Miehe4, E. Nacher1, F. Perrot4, E. Poirier4,B. Rubio1, J.L. Tain1and O. Tengblad21. IFIC, CSIC-University of Valencia, Spain2. IEM, CSIC Madrid, Spain3. CIEMAT, Madrid, Spain4. IReS, Strasbourg, France5. ISOLDE, CERN, Switzerland6. INFN, Legnaro, Italy7. University of Surrey, Guildford, UKSpokespersons: Ph. Dessagne and B. RubioContact: G. Le ScornetAbstractIn the framework of the investigation of the shapes of the ground states of the parent nucleus, wepropose to carry out measurements of the complete Gamow-Teller strength distribution for the76 80Srisotopes, with a new Total Absorption gamma Spectrometer installed on a new beam line. The resultswill be compared with theoretical calculations based on the mean eld approach. A brief report onthe IS370 experiment on72 75Kr decay, which was recently performed at ISOLDE, will be given andthe performance of the sum spectrometer will be presented.1EUROPEAN ORGANIZATION FOR NUCLEAR RESEARCHCERN/INTC 2001-024INTC/P-14420 August 20011 Introduction1.1 Physical contextCurrently, the only experimental access to nuclear levels out of the Yrast sequence in nuclei far fromstability is provided by - and isomeric-decay. As a result the study of -decay makes a major contri-bution to our understanding of highly unstable nuclei. In addition, a very clear advantage of studyingthe -decay process is the close relationship between the wave functions of the parent state and thestates populated in the daughter nucleus which makes it amenable to theoretical calculation. In thisparticular proposal we concentrate on measurements of Gamow-Teller transition probabilities whichare very important if we are to understand nuclear structure and astrophysical processes. The pro-posed measurements are planned in the N=Z region where phenomena such as shape coexistence andn-p pairing play an important role.In this context the rst theoretical developments dedicated to the neutron decient nuclei along theN=Z line taking into account deformation and pairing correlations were carried out by Hamamotoand collaborators [1,2]. According to them, the GT process carries invaluable information on the signof the nuclear deformation. Large dierences appear in the calculated GT strengths - total intensityand energy distribution - depending on the shape of the parent nucleus. Of special interest in thisrespect are the N = Z nuclei in the A=70-80 mass region, where an oblate to prolate transition ispredicted, and for which various deformation amplitudes have already been inferred from experimentalresults [3,4]. More recently, extensive studies have been carried out on the decay of the even- andodd-mass nuclei by P. Sarriguren, E. Moya de Guerra and coworkers, using the self consistent HF plusRPA method with dierent types of Skyrme interaction, stressing the role of deformation, residualinteractions, pairing and RPA correlations on the GT strength distributions [5,6,7].In this spirit, high resolution measurements of the +  EC decay of two even-even, N=Z isotopes72Kr and76Sr and searches for their delayed proton emission have been performed at the ISOLDEPSB facility. An indication of prolate shape for the76Sr ground state was obtained [8]. No indicationfor the expected oblate deformation of72Kr [8] was obtained. The experimental results revealed thelimitations inherent in using semiconductor detectors to measure the gamma-ray intensities.We have carried out experiments on the Kr isotopes using a dierent experimental technique [9] whichwe shall describe below, and we will propose the next series of measurements focussed on the Sr nucleiin chapter 2.1.2 Description of the technique and report on the rst experiment carried outwith the new deviceThe idea of determining nuclear shapes which we have outlined above, relies not only on the precisedetermination of the total Gamow-Teller strength accessible in the Qecwindow, but even more onthe strength distribution as a function of excitation energy in the daughter nucleus. The precisemeasurement of this distribution is a dicult task, which cannot be achieved using semiconductorGe detectors alone. The reason is that the BGTis derived from the feeding to the individual levels(assuming half life and Qecknown), and the feeding is measured as the balance of the intensity ofthe gamma-rays feeding and de-exciting each level. Therefore, any missing gamma-ray distorts thederived BGTdistribution (we talk here about the -delayed gamma decay, the pert of the strengthproceeding through . -delayed particles will be dicussed later).To overcome this problem we have designed and installed a new Total Absorption gamma Spectrometer\Lucrecia" on a new beamline at ISOLDE. The main idea is two fold, rst to use a device sensitiveto the -feeding rather than to the individual gamma-rays, second to achieve 100% total eciencyfor each gamma cascade produced in the decay. A device with these characteristics has been usedsuccessfully at the GSI On-line Mass-Separator [10] where the Gamow-Teller resonance near100Sn[11] and146Gd [12] has been observed. The new spectrometer consists of a very large, cylindrical2NaI mono-crystal (38 cm diameter, 38 cm long) with a hole of 7.5 cm diameter perpendicular to thesymmetry axis. It was manufactured by Saint Gobain Crystals and Detectors (see Fig. 1). The lightis collected by eight ve-inch photomultiplier tubes. The crystal acts as a sum spectrometer detectingall the gammas emitted in a cascade after the beta or the EC process. The stability of the PMT signalis continuously monitored by a Light-Emitting Diode. Two ancillary detectors are coupled to the NaIcrystal. They consist of a Ge telescope with a small planar detector in front with 400 eV energyresolution at 6 keV to measure the characteristic X-rays, and a thin plastic detector to detect the Esignal left by the positrons. Both detectors are used in coincidence with the NaI crystal allowing usto separate the -decay from the EC process. The crystal and the ancillary detectors are mountedon movable tables and are coupled to a tape transport system operated in vacuum. The detectorsare housed within layers of neutron and gamma shielding made of boron polyethylene (10cm), lead (5cm), copper (2 cm) and aluminum (2 cm).The NaI crystal has a measured energy resolution of 5.2 % at 1.33 MeV, very good linearity in energyand a high total eciency (evaluated to be 90 % at 5 MeV).In June 2001 we carried out the rst experiment with \Lucrecia". We measured the -decay of mass72 to 75 krypton isotopes in carrying out the approved experiment IS370 which was based on thephysics outlined in the introduction. The experiment was performed with a Niobium cold plasma ionsource mounted on the High Resolution Separator. The transmission of the ions was, after 40 metresof beam line, of the order of 60%.The raw spectra clearly show previously unobserved strength at high excitation energy. It should benoted at this point that the process of data analysis is far from trivial. Although the total eciencyof the detector is very high for single gamma rays, the photopeak eciency for a gamma cascade canbe small and depends on the multiplicity of the cascade. In other words we face the so called Ill-posedinverse problem which consists of extracting the -feedings out of a continuum spectrum involving theresponse function of the spectrometer. A method of solving this problem has been developed over thelast few years [13] and successfully applied to experiments performed at GSI. However the responseof the spectrometer to the dierent particles, betas and gammas, depends strongly on the individualapparatus and has to be treated specically. We are now making an eort in this direction with theMonte-Carlo simulation code GEANT 4 which has proved to give a much better description of thepenetration of the betas in the crystal than GEANT 3.Preliminary results on the study of the krypton decays, together with a description of the sum spec-trometer \Lucrecia", will be presented to the panel.The case of the Sr isotopes presented in the next chapter is intimately linked to the previous proposalon the Kr nuclei and it is important to study them under the same conditions. Thus the presentproposal can be viewed as a continuation of the proposal IS370. The newly determined responsefunction of the spectrometer will be valid only in so far as nothing is changed in the present geometry.2 Proposed experimentAs mentioned in the introduction, the onset of deformation as well as the existence of shape coexistenceare two of the attractive aspects of studying N=Z nuclei. Dierent kinds of ground state deformationare predicted for nuclei which dier only slightly in the number of protons and neutrons. In general,mean eld calculations of the energy surface predict a second minimum only a few hundred keV awayand with a dierent deformation. Which of the two states becomes the ground state depends on thedetails of the calculation and the eective interaction.Experimentally one has evidence of shape coexistence in the case of the74Kr [14] and72Kr [15]isotopes where a second 0+isomeric state has been observed in fragmentation reactions. Howeversuch experiments do not determine the shape of the isomer or the ground state; the assignments areentirely based on the theoretical predictions. For the Sr cases no shape isomerism has been observedso far, but an indication of prolate deformation has been extracted from the -decay of76Sr [8].3NaI(Tl)PM’s                                 Plastic ScintillatorRadioactive SourceLight GuideBeam PipeGe−CoaxialGe−PlanarNaI(Tl)NaI(Tl)Tape transport system (schematic view)Figure 1: Experimental device.Theoretically there are several relevant studies of NZ nuclei. In particular, for the Sr isotopescalculations have been carried out using several approaches: both relativistic and non-relativisticframeworks with a Skyrme SIII force [16]; in the Dirac-Hartree-Bogoliubov (DHB) approximation[17]; with the nite-range droplet macroscopic model and the folded-Yukawa single particle microscopicmodel of Moller and Nix [18] who predict prolate deformation parameter for the ground states of thenon-spherical nuclei. These calculations do not always arrive at the same conclusions, for instancein refs [17] and [18] dierent shapes are predicted for the ground state of78Sr. The N=Z=3876Srnucleus is particularly interesting since it has the largest ground state deformation known in the region[19].A completely independent method of determining the sign of the deformation is based on the studyof the beta strength distribution. The basic idea is that the single particle deformed orbitals involvedin the  process may lie at dierent energies depending on the deformation. This may lead to sizeabledierences in the BGTdistribution.Figure 2 shows the results of P. Sarriguren et al. [7]. It shows the BGTdistribution obtained fromHF-BCS-QRPA calculations using the SG2 Skyrme force. It is very clear that the dierence in thestrength distribution between the prolate and the oblate shape solutions increases as one moves fromthe Ge to Sr nuclei. Meanwhile the authors of this publication are calculating the odd cases wheresimilar dierences are expected [20]. Consequently the Sr chain seems to be an ideal testing groundfor the method of determining the shape of the g.s from the BGTdistribution.There is another factor which helps to make these cases cleaner in comparison with the Kr isotopes; thetwo potential energy minima are better separated here. Consequently, one expects smaller admixturesin the strontium ground state wave functions. In turn this implies that we might need the Sr resultsin order to interpret the Kr measurements where strong admixtures are predicted [21].We should also mention that the part of the strength which proceeds via particle emission has alreadybeen measured for the76;77Sr cases [8,22]. However a further experiment, which combines the particledetection and the high eciency of the NaI spectrometer, will be the subject of a future proposal.43 MeasurementsIt is proposed to measure the decay of the76 80Sr isotopes using the the newly installed TotalAbsorption gamma-ray Spectrometer \Lucrecia". For the isotopes of interest, the main features oftheir decay are quoted in Table 1.ISOTOPE Half life (s) Sp(keV) Qec(keV)76Sr 7.89 3526 (17) 6090 (300)#77Sr 9.0 3137 (14) 6856 (150)78Sr 159 4054 (12) 3761 (11)79Sr 135 3924 (10) 5320 (11)80Sr 6378 5020 (8) 1868 (11)Table 1. General features of the decay of Sr isotopes.The separated parent activity produced by ISOLDE will either be caught on tape at the centre of theTAgS [in the case of the very short-lived isotopes] or will be carried to a remote counting positionby the tape. By requiring coincidences with signals from the beta detector, the positron componentof the +-EC decay can be selected, whereas coincidences with characteristic X-rays recorded by theGe detector can be used to select the EC events. Similar measurements on the decay of the daughteractivities will be carried out in those cases where the half life dierence is not enough to measure theSr species cleanly.The analysis of the data will be carried out using the methods of analysis established in Ref. [23]including the determination of the response function of the large NaI(Tl) crystal [24] and the pulse pile-up correction [25]. It is anticipated that this should allow a reliable extraction of the BGTdistributionup to the limit imposed by the Qecwindow.4 Beam Time RequestFor the production of strontium isotopes a Nb metal target will be used with a W surface ionisationion source. To obtain pure alkali earth element beams the chemical selectivity created by partial CF4pressure in the source is essential. Our beam time request is based on production yields obtained inprevious experiments at the ISOLDE PSB facility, the run performed on the Kr isotopes and the needto record at least 5.105coincidence events in the TAgS spectrum.Accordingly:- 15 shifts are required for -decay measurements of the76 80Sr isotopes,- 3 shifts are required for data taking of the daughter activities.The total request is 18 shifts.References1) I. Hamamoto and X.Z. Zhang, Z. Phys. A353 (1995) 145.2) F. Frisk et al., Phys. Rev. C52 (1995) 2468.3) W. Gelletly, et al., Phys. Lett. B253 (1991) 287.4) P. Lievens et al., Phys. Rev. C46 (1992) 797., P. Lievens et al., Europhys. Lett. 33, 11 (1996)5) P. Sarriguren et al., Nucl. Phys. A635 (1998) 55.6) P. Sarriguren et al., Nucl. Phys. A658 (1999) 13.7) P. Sarriguren et al., Nucl. Phys. A691 (2001) 631.8) Ch. Miehe et al., Contribution to the 2nd International Conference on Exotic Nuclei and Atomic5Masses, Shanty Creek Resort, Bellaire, Michigan,USA, 23-27 June 1998.A. Jokinen and Ch. Miehe, Hyperne Interaction 129 (2000).9) P. Dessagne and B. Rubio, IS370 Isolde proposal.10) M. Karny et al., Nucl. Instr. and Meth. Phys. Res. B 126 (1997) 12.11) M. Karny et al., Nucl. Phys. A 640 (1998) 3.12) A. Algora et al., Nucl. Phys. A 654 (1999) 727c.13) D. Cano-Ott, Ph. D. thesis Valencia University (2000) and J. L. Tain at al. to be published.14) C. Chandler et al., Phys. Rev C56 (1997) R2924.15) F. Becker et al., Abstract to the Nucl. Structure at the Borderlines Conf., Lipari, Italy, May 2001.16) P. Bonche et al., Nucl. Phys. A443 (1985) 39.P. Bonche et al., Nucl. Phys. A530 (1991) 149.17) B.V. Carlson and D. Hirata, Phys. Rev. C62 (2000) 054310.18) P. Moller et al., Nucl. Data Tables 59 (1995) 185.19) C.L. Lister et al., Phys. Rev. C42 (1990) R1191.C.L. Lister et al., Phys. Rev. Lett. 59 (1987) 1270.20) P. Sarriguren at al., private comunication.21) A. Petrovici et al., Nucl. Phys. A665 (2000) 333.22) J. Giovinazzo et al., Nucl. Phys. A674 (2000) 394.J. Giovinazzo, thesis IReS-ULP 97-20.23) J. Agramunt et al., International Symposium on New Facets of Spin Giant Resonances in Nuclei.Ed. H. Sakai, H. Okamura and T. Wakasa, World Scient. (ISBN 981-02-3511-9), 1998, page 150.24) D. Cano-Ott et al., Nucl. Inst. and Meth. Phys. Res. A430 (1999) 333.25) D. Cano-Ott et al., Nucl. Inst. and Meth. Phys. Res. A430 (1999) 488.6642 P. Sarriguren et al. / Nuclear Physics A 691 (2001) 631–648Fig. 4. Gamow–Teller strength distributions [g2A/4pi ] as a function of the excitation energy of thedaughter nucleus [MeV]. The results are for the force SG2 in QRPA and for the various shapesof the isotopes 64,66,68,70Ge, 68,70,72,74Se, 72,74,76,78Kr, and 76,78,80,82Sr. Vertical lines indicateexperimental QEC values (see Table 1 for the theoretical QEC values).Figure 2: Calculated Gamow-Teller strength distributions for even Ge, Se, Kr and Sr isotopes [7].7

Studies of the $\beta$-decay of Sr nuclei on and near the N=Z line with a total absorption $\gamma$-ray spectrometer

Studies of the $\beta$ -decay of Sr nuclei on and near the N=Z line with a total absorption $\gamma$ -ray spectrometer

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Studies of the β\betaβ-decay of Sr nuclei on and near the N=Z line with a total absorption γ\gammaγ-ray spectrometer

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Studies of the $\beta$ -decay of Sr nuclei on and near the N=Z line with a total absorption $\gamma$ -ray spectrometer