Cross sections for the 44Ti(α, p)47V reaction which significantly affects the yield of 44Ti in supernovae were measured in the energy range 5.7MeV ≤ Ec.m. ≤ 9 MeV, using a beam of radioactive 44Ti. The cross sections and the deduced astrophysical reaction rates are larger than the results from theoretical calculations by about a factor of 2. The implications of this increase in the reaction rate for the search of supernovae using space-based gamma detectors are discussed

Ahmad, I.

Borasi, F.

Bowers, D.L.

Brumwell, F.

Caggiano, J.

Davids, C.N.

Greene, J.P.

Harss, B.

Heinz, A.

Henderson, D.

Janssens, R.V.F.

Jiang, C.L.

McMichael, G.

Nolen, J.

Pardo, R.C.

Paul, M.

Rehm, K.E.

Schiffer, J.P.

Segel, R.E.

Seweryniak, D.

Siemssen, R.H.

Sonzogni, A.A.

Truran, J.W.

Uusitalo, J.

Wiedenhöver, I.

Carolina Digital Repository

VOLUME 84, NUMBER 8 P H Y S I C A L R E V I E W L E T T E R S 21 FEBRUARY 2000The 44Tia, p Reaction and its Implication on the 44Ti Yield in SupernovaeA. A. Sonzogni,1 K. E. Rehm,1 I. Ahmad,1 F. Borasi,2 D. L. Bowers,1 F. Brumwell,1 J. Caggiano,1 C. N. Davids,1J. P. Greene,1 B. Harss,1 A. Heinz,1 D. Henderson,1 R. V. F. Janssens,1 C. L. Jiang,1 G. McMichael,1 J. Nolen,1R. C. Pardo,1 M. Paul,3 J. P. Schiffer,1 R. E. Segel,2 D. Seweryniak,1 R. H. Siemssen,1 J. W. Truran,4 J. Uusitalo,1I. Wiedenhöver,1 and B. Zabransky11Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 604392Northwestern University, Evanston, Illinois 602083Hebrew University, Jerusalem, Israel4Department of Astronomy and Astrophysics, University of Chicago, 5640 South Ellis Avenue, Chicago, Illinois 60637(Received 15 October 1999)Cross sections for the 44Tia, p47V reaction which significantly affects the yield of 44Ti in supernovaewere measured in the energy range 5.7 MeV # Ec.m. # 9 MeV, using a beam of radioactive 44Ti. Thecross sections and the deduced astrophysical reaction rates are larger than the results from theoreticalcalculations by about a factor of 2. The implications of this increase in the reaction rate for the searchof supernovae using space-based gamma detectors are discussed.PACS numbers: 26.30.+k, 25.60.Dz, 25.60.Je, 26.50.+xThe nucleus of 44Ti is of great importance in nuclear as-trophysics [1,2]. Nuclei of 44Ti are produced in the laststages of a supernova event in the so-called alpha-richfreeze-out [1]. The recent observation of g rays asso-ciated with the decay of 44Ti by the COMPTEL spacetelescope from the Cassiopeia A [3] and (possibly) fromthe Vela supernova remnants [4] not only confirmed theseideas but also demonstrated that the 44Ti gamma afterglowcan be used to locate individual supernova remnants. Itis expected that in the near future much scientific activ-ity will be centered around 44Ti. With the recent pre-cise measurements of the 44Ti half-life (59.0 6 0.6 yr [5],60.3 6 1.3 yr [6], 62 6 2 yr [7], and 60.7 6 1.2 yr [8])and with the launch of the next-generation space-basedgamma-ray observatory INTEGRAL [9] (scheduled for2001), new supernova remnants are likely to be discov-ered and the total mass of 44Ti ejected in the supernovaevents will be determined.The use of the 44Ti afterglow of supernovae for distanceor age determination depends critically on the amount of44Ti produced in the explosion which is governed by asubtle interplay between the nuclear reactions that produceit and those that destroy it. This subject was studied indetail for type II supernovae by The et al. [10]. Fromthese calculations, it was concluded that the amount of44Ti produced in a supernova depends strongly on the crosssection of the 44Tia, p47V reaction.Since the 44Tia, p47V reaction destroys 44Ti, an in-crease in its rate will decrease the final amount of 44Tiand vice versa. Because of the relatively high value ofthe Coulomb barrier for the 4He 1 44Ti system, the influ-ence of this reaction is felt at temperatures of 2.2 4.2 3109 K. This temperature range corresponds to center-of-mass energies (Gamow peak values) of 4 6.1 MeV, orexcitation energies of 11.7 13.8 MeV in the compoundnucleus 48Cr. Because the proton and neutron separationenergies of 48Cr are 5.17 and 16.33 MeV, respectively,0031-90070084(8)1651(4)$15.00most of the fusion events will lead to 47V, while only afew will result in a capture leading to 48Cr. The crosssections for the 44Tia, p reaction in this energy rangeare predicted to be 0.05–300 mb [11]. In order to put thenuclear astrophysics part of 44Ti production in supernovaeon a more solid ground we have performed the first mea-surement of the 44Tia, p47V reaction.The long half-life of 44Ti allows two approaches to ameasurement of the 44Tia, p47V cross sections: a 4Hebeam bombarding a 44Ti target or a 44Ti beam bombardinga 4He target. Because of the higher energies of the reactionproducts in experiments with a 44Ti beam, which stronglyfacilitates the particle identification, it was decided to de-velop a radioactive 44Ti beam and to investigate the reac-tion in inverse kinematics, i.e., bombarding a 4He targetwith a 44Ti beam.The 44Ti material was produced via the 45Sc p, 2n re-action [12,13], using a 50 MeV, 20 mA proton beam fromthe linac injector of Argonne’s Intense Pulsed NeutronSource facility. The target, a 25 mm diam, 5 mm thickdisk of Sc, was mounted inside a water-cooled Cu holder,whose front face was made of graphite, acting both as acollimator and an energy degrader. The irradiation lasted70 h. Two weeks later, the Sc disk was removed fromthe holder and placed in front of a Ge detector. Thedominant source of gamma radiation was found to resultfrom the decay of 44Sc. Nuclei of 46Sc T12  83.8 dwere also produced by neutron capture. A 44Ti activityof 180 mCi was obtained from the g spectrum, whichrepresents 1.8 3 1016 atoms or 1.3 mg of 44Ti. Aboutfour weeks later, the 44Ti activity was chemically separatedfrom the Sc material. The method used for the chemicalseparation is similar to the ones used in Refs. [14,15]. TheSc disk was dissolved in a HNO3HCl solution and the Scthen precipitated out by the addition of HF. The solutioncontaining the Ti activity was then dried and heated. Apart of the resulting material, left as a 44TiO2 compound,© 2000 The American Physical Society 1651VOLUME 84, NUMBER 8 P H Y S I C A L R E V I E W L E T T E R S 21 FEBRUARY 2000was mixed with 50 mg of natTiO2 and placed inside a cop-per insert for a negative-ion Cs-sputter source. The 44Tiactivity from the pellet was measured to be 38 mCi.From the ion source a beam of 44TiO2 was extractedand injected into the tandem accelerator at ATLAS. Afterstripping in the terminal of the tandem, a 44Ti81 beam wasaccelerated in the linac part of ATLAS to an energy of133.5 MeV. Two guide beams were used to optimize thebeam tuning of the ATLAS accelerator for the weak 44Ti81beam: The optics of the ion source and the tandem injectorwere tuned with 48TiO2, while the superconducting linacwas tuned with 66Zn121.Argonne’s fragment mass analyzer (FMA) [16], a recoilmass separator dispersing the reaction products in its focalplane according to their mq ratios, was used to separatethe 47V reaction products emitted at small scattering angles(typically #2±) from the primary mass 44 beam particles.To further separate the 47V particles from the scatteredbeam an ionization chamber (IC) and a Si detector wereplaced behind the parallel grid avalanche counter (PGAC)at the focal plane of the FMA. The PGAC provided po-sition and timing signals, while the IC-Si detectors gaveenergy and Z information.To avoid a retuning of the linac at each energy change,which, for low intensity beams, can be quite time-consuming, Ti degrader foils ranging in thickness from0.6 to 2 mgcm2 were used. The energy measurementsof the beam were done by time of flight using theisochronous property of the FMA [16].A 4He gas target, a movable Si detector, a set of de-grader foils, and a AuC charge-reset foil after the targetwere mounted in the scattering chamber in front of theFMA. The 4He gas target consisted of a 2.2 mm long cellwith two 1.3 mgcm2 titanium metal windows, filled with600 mbar of 4He and cooled to LN2 temperature. Theareal density was 80 mgcm2. The main purpose of the Sidetector in the scattering chamber was to monitor elasticscattering from the target for beam current normalization.Before the 44Ti experiment the experimental arrange-ment was tested with stable 40Ca and 46Ti beams. For 40Caas well as for 44Ti, the a, p reaction dominates in theenergy range of interest. With the charge state distributionsfrom Ref. [17] and a transport efficiency through the FMAwhich was calculated taking the kinematics and smallangle scattering into account, the measured excitationfunction for the 40Caa, p43Sc reaction was found to bein very good agreement with the results of Ref. [18].Experiments with radioactive beams often have theadded complication that these low-intensity beams havecontaminations from neighboring isobars [19]. For 44Tithe stable isobar is 44Ca with a mass difference DMM 1.1 3 1024 which is too small to allow a separationby the beam transport system. In order to measure thecomposition of the mass 44 beam, a 2 mgcm2 Ti foilwas used to degrade its energy and the FMA was setto separate the beam particles in mq. Ti and Ca ions1652experience a different energy loss in this foil and canthus be easily identified in the focal plane of the FMA.Figure 1 shows a Si energy vs IC energy plot where thedifferent components of the beam are indicated. Fromthese measurements it was concluded that 38% of thebeam consisted of 44Ti, 54% of 44Ca, while 33S and 11Baccounted for the remaining 8%. The 33S and 11B ionsare injected as mass 60 molecules from the ion source andions with the same charge-to-mass ratio are accelerated tothe same velocity in the linear accelerator. Intensities of5 3 105 44Tis on target were observed.Plots of the time of flight through the FMA versusthe energy measured in the Si detector behind the focalplane can be seen in Fig. 2 for an incident 44Ti energy of122.1 MeV. The left panel corresponds to an empty gascell, the right one to a gas cell filled with 600 mbar ofHe. With the empty cell only mass 44 particles are ob-served, produced by scattering beam particles in the gascell’s windows or on various components in the FMA. Theplot for the full gas cell shows the mass 47 reaction prod-ucts, well separated from the scattered mass 44 particles.To measure the contributions from the reaction productsfrom the 44Ca component of the beam [mainly 47Ti and47Sc produced via the 4He44Ca, n and 4He44Ca, p reac-tions] a pure beam of 44Ca was used in the last stages ofthe experiment, repeating the measurements under identi-cal experimental conditions. Elastically scattered particleswere used to normalize the different runs to each other.The contribution of 47V produced via the 44Tia, p re-action to the total number of mass 47 reaction productsdepended on the incident energy and the charge state cho-sen for transmission through the FMA. The charge statesfor 47V were 131 at the two highest energies and 121 and111 at the two lower ones. The amount of 47V in the massFIG. 1. Energy loss (measured in the ionization chamber) vsresidual energy (measured in the Si detector) for particles de-tected in the focal plane of the fragment mass analyzer. Thedifferent components of the beam are indicated.VOLUME 84, NUMBER 8 P H Y S I C A L R E V I E W L E T T E R S 21 FEBRUARY 2000FIG. 2. Time of flight (measured with the parallel-grid-avalanche counter relative to the rf timing from the accelerator)vs energy (measured in the Si detector). The left panelcorresponds to an empty gas cell, the one at the right to a fullgas cell. The reaction products with mass 47 are indicated bythe solid contour line.47 gate was found to be 51%, 23%, 30%, and 48% forthe c.m. energies of 9.0, 8.0, 6.8, and 5.7, respectively. Tocorrect for the charge-state distribution of 47V detected inthe focal plane, the distributions were measured in a sepa-rate experiment using stable 51V beams in the energy rangeElab  25 110 MeV and adjusted to the same velocities.The resulting cross sections for the 44Tia, p47V reactioncan be seen in Fig. 3. The error bars include statisticaluncertainties as well as uncertainties from the 44Ca beamcontaminant, the charge state distributions, and transportefficiencies.The solid line is the result (with standard parameters)from the statistical model code SMOKER [11] which is usedFIG. 3. Measured excitation function for the 44Tia, p47V re-action. The solid line corresponds to a calculation done with thecode SMOKER (see text for details).in many astrophysical network calculations to estimate as-trophysical reaction rates that cannot be studied in the lab-oratory. At the two higher energies the agreement betweenour experiment and the theoretical prediction is excellent.However, the falloff in the cross section due to the penetra-tion of the Coulomb barrier appears to be shifted towardslower energies resulting in cross sections that are largerby about a factor of 2 compared to the SMOKER predic-tions, thus implying a larger barrier radius or diffusenessparameter. It is interesting to note that a similar behavioris found for the neighboring 48Tia, n51Cr reaction [20]where a comparison with SMOKER shows good agreementfor energies above 8 MeV, but a similar deviation at lowerenergies. Calculations with the newer code NONSMOKER[21] gave identical results.In order to extract the astrophysical reaction rates, it isnecessary to extrapolate the cross sections towards higherand lower energies. In this work, the cross sections wereobtained by interpolating between the measured data pointsin the energy range Ec.m.  5.7 9.0 MeV. For energieslower than 5.7 MeV or higher than 9.0 MeV, the crosssections were obtained by using the shape of the SMOKERcalculation renormalized to match the lowest and highestmeasured data points. The results are shown by the solidline in Fig. 4. The uncertainties in the reaction rates rangefrom 30% at the highest temperatures to 43% at T9  2.5.The reaction rates from the SMOKER calculations as wellas those by Woosley et al. [22] are given by the dashedand dot-dashed curves, respectively. These two sets oftheoretical calculations give basically the same result forthe 44Tia, p47V reaction rates, which is lower than theexperimentally determined rate by about a factor of 2. TheFIG. 4. Astrophysical reaction rates for the 44Tia, p47V re-action. The solid line was obtained from the experimental infor-mation as described in the text. The dashed line corresponds toa SMOKER calculation and the dot-dashed line to a calculationby Woosley et al. [22].1653VOLUME 84, NUMBER 8 P H Y S I C A L R E V I E W L E T T E R S 21 FEBRUARY 2000rate can be parametrized as a function of T9 by the equation(see, e.g., Ref. [10,11])NAsy  expA 1 BT9 1 CT139 1 D3 T139 1 E 3 T9 1 F 3 T539 1 G3 lnT9 , (1)with the parameters A  2847.89, B  2167.79, C 2142.3, D  21179.5, E  27.782, F  20.704 45, andG  946.48.The higher astrophysical reaction rate results in a reduc-tion of the amount of 44Ti produced in supernovae explo-sions. Using the calculations of Ref. [10], one obtains withthe higher reaction rate shown in Fig. 4 a 25% decrease inthe 44Ti yield of a type II supernova. For a measurementof the 44Ti gamma-ray flux from a supernova, this decreasein yield translates into a 15% larger distance or a 25 yr ear-lier occurrence. It should be noted that changes in otherreaction rates which have not been measured so far [e.g.,the 45V p, g reaction] could effect the 44Ti yield as well.A more detailed study of the influence of the higher rateof 44Ti destruction on searches for 44Ti decay g rays fromsupernovae remains to be done.In addition to its effects on the energetics of the lightcurve of supernova 1987A, and for searches for 44Ti de-cay g rays from galactic supernova remnants, these newexperimental results also hold important implications forgalactic nucleosynthesis. Observational determinations ofthe abundances of (elemental) titanium in the oldest (mostmetal deficient) stars in our galaxy [23] reveal that, likethe “alpha elements” oxygen, neon, magnesium, silicon,sulfur, argon, and calcium, titanium is overabundant rela-tive to iron by a factor of 	2 3. This is understood to re-flect the nucleosynthesis products of supernovae of type II,which are associated with the evolution of massive stars ofshort lifetimes [24]. The most abundant naturally occur-ring isotope of titanium, 48Ti, is formed in such supernovaeas 48Cr, under conditions which are equivalent to those giv-ing rise to the formation of 56Ni [25,26]. Preliminary cal-culations of supernova nucleosynthesis utilizing our higherrate for 44Tia, p47V indicate that a higher level of 48Crproduction may accompany oxygen burning and/or incom-plete silicon burning.We thank Thomas Rauscher for providing us with theSMOKER and NONSMOKER cross section calculations. Thiswork was supported by the U.S. Department of Energy,1654Nuclear Physics Division, under Contract No. W-31-109-ENG-38.[1] David Arnett, Supernovae and Nucleosynthesis (PrincetonUniversity Press, Princeton, NJ, 1996).[2] M. D. Leising, Nucl. Phys. A621, 71c (1997); C. Duprazet al., Astron. Astrophys. 324, 683 (1997).[3] A. F. Iyudin et al., Astron. Astrophys. 284, L1 (1994).[4] A. F. Iyudin et al., Nature (London) 396, 142 (1998).[5] I. Ahmad et al., Phys. Rev. Lett. 80, 2550 (1998).[6] J. Görres et al., Phys. Rev. Lett. 80, 2554 (1998).[7] E. B. Norman et al., Phys. Rev. C 57, 2010 (1998).[8] F. E. Wiefeldt et al., Phys. Rev. C 59, 528 (1999).[9] International Gamma-Ray Astrophysics Laboratory, http://sci.esa.int/integral/.[10] L.-S. The et al., Astrophys. J. 504, 500 (1998).[11] F.-K. Thielemann, M. Arnould, and J. W. Truran, inAdvances in Nuclear Astrophysics, edited by E. Vangioni-Flam et al. (Editions Frontieres, Gif-sur-Yvette, 1987),p. 525.[12] T. McGee et al., Nucl. Phys. A150, 11 (1970).[13] R. Ejnisman et al., Phys. Rev. C 54, 2047 (1996).[14] D. Frekers et al., Phys. Rev. C 28, 1756 (1983).[15] R. Lange et al., Nucl. Instrum. Methods Phys. Res., Sect. A423, 247 (1999).[16] C. N. Davids et al., Nucl. Instrum. Methods Phys. Res.,Sect. B 70, 358 (1992).[17] K. Shima, T. Mikumo, and H. Tawara, At. Data Nucl. DataTables 34, 357 (1986).[18] A. J. Howard et al., Astrophys. J. 188, 131 (1974).[19] K. E. Rehm et al., Phys. Rev. Lett. 80, 676 (1998).[20] H. Vonach, R. C. Haight, and G. Winkler, Phys. Rev. C 28,2278 (1983).[21] T. Rauscher, F.-K. Thielemann, and K.-L. Kratz, Phys.Rev. C 56, 1613 (1997); T. Rauscher and F.-K. Thielemann,At. Data Nucl. Data Tables (to be published).[22] S. E. Woosley et al., At. Data Nucl. Data Tables 22, 371(1978).[23] J. C. Wheeler, C. Sneden, and J. W. Truran, Annu. Rev.Astron. Astrophys. 27, 279 (1989).[24] S. E. Woosley and T. A. Weaver, Astrophys. J. Suppl. 101,181 (1995).[25] J. W. Truran, W. D. Arnett, and A. G. W. Cameron, Can.J. Phys. 45, 2315 (1967).[26] S. E. Woosley and T. A. Weaver, in Supernovae, Proceed-ings of the Les Houches Summer School, Session LIV,edited by J. Audouze et al. (Elsevier, New York, 1999),p. 63.

The 44Ti(α, p) reaction and its implication on the 44Ti yield in supernovae

http://cdr.lib.unc.edu/downloads/dj52wg43g

The 44Ti(α, p) reaction and its implication on the 44Ti yield in supernovae

Abstract

Similar works

Full text

Available Versions

Carolina Digital Repository