Measurements have been made of the gamma ray branching ratios for the decay of the lowest isobaric analog states of Ge71 and Kr81. These states could be populated by high-energy neutrinos in gallium- and bromine-based detectors. Although these states are unbound to particle decay, all channels with significant penetrabilities are isospin forbidden, so that gamma decay is a possibility. Our results indicate that these states decay mainly by neutron emission and therefore contribute very little to detection sensitivities for Ga71 and Br81

Champagne, A.E.

Dodge, G.E.

Kouzes, R.T.

Lowry, M.M.

McDonald, A.B.

Roberson, M.W.

Carolina Digital Repository

PHYSICAL REVIEW C VOLUME 38, NUMBER 2 AUGUST 1988Gamma decays of isobaric analog states relevant to neutrino detectionA. E. Champagne, G. E. Dodge, R. T. Kouzes, M. M. Lowry,A. B.McDonald, and M. W. RobersonDepartment of Physics, Princeton University, Princeton, New Jersey 08544(Received 7 March 1988)Measurements have been made of the gamma ray branching ratios for the decay of the lowest iso-baric analog states of 'Ge and 'Kr. These states could be populated by high-energy neutrinos ingallium- and bromine-based detectors. Although these states are unbound to particle decay, allchannels with significant penetrabilities are isospin forbidden, so that gamma decay is a possibility.Our results indicate that these states decay mainly by neutron emission and therefore contributevery little to detection sensitivities for "Ga and "Br.INTRODUCTIONThe reported' discrepancy between the measured andpredicted flux of solar neutrinos has spurred the develop-ment of new radiochemical neutrino detectors using galli-um ' or bromine. ' Gallium-based detectors arepresently under construction and are primarily sensitiveto the pp solar neutrinos. In contrast, a bromine detectoris suited for detection of Be and B neutrinos and awaitsthe development of an effective single-atom countingscheme.The isobaric analog states (IAS) of the ground states ofGa, 'Ga and 'Br (corresponding to excited states inGe, 'Ge, and 'Kr, respectively) can be populated viainverse Fermi beta decay transitions induced by thehigh-energy neutrinos produced by the p decay of sB inthe sun. Because of the large overlap of the ground statewith its IAS, these transitions have large cross sections.Therefore, if the IAS subsequently gamma decays, thischannel can contribute to the measured flux of neutrinos.Such a gamma decay of the IAS could increase the 'Geproduction rate by about 2% [1.9 solar neutrino unit(SNU) out of 133.7 SNU in the standard solar model ]and the 'Kr production rate by about 6% (1.7 SNU outof 27.8 SNU in the standard solar model ). The impacton the 'Ge production could be more significant if the ppneutrino flux is suppressed by the Mikheyev-Srnirnov-Wolfenstein effect. ' The spin part of the M1 gammatransition strengths from the IAS to lower-lying levels isproportional to the Gamow-Teller matrix elements be-tween the gallium or bromine ground states and the low-lying states in germanium and krypton. Thus, if the IASdecays by gamma emission, a measurement of these M1strengths would be a check of these matrix elementswhich are otherwise determined from (p, n) data. "' Inaddition, for the detection of high-energy neutrinos pro-duced by supernova events, the IAS could make asignificant contribution to the detection rate, if it has asignificant gamma decay branch.The IAS in 'Ge (at an excitation energy ofE„=8.932(13) MeV)"' is unbound to proton decay (by0.66 MeV) and alpha decay (by 4.49 MeV). The IAS in'Kr (at E„=9.717(15) MeV)' is unbound to proton de-cay (by 0.61 MeV) and alpha decay (by 4.19 MeV). Thepenetrabilities of these decays are negligibly small. Theseisobaric analog states are also unbound to neutron decay(by 1.84 MeV in 'Kr and 1.54 MeV in 'Ge), but sincethe decay is isospin forbidden, the branching ratio isdifficult to estimate. The IAS in Ge (at E„=7.00(5)MeV)' is only unbound to isospin forbidden alpha decay(by 3.39 MeV). The 39 h Ge, produced from the 60%abundant Ga, may be of interest for the detection ofhigh-energy neutrinos, for which the IAS transition willdominate the cross section.We have measured the branching ratios for gamma de-cay of the IAS in 'Ge and 'Kr using the ( He, ty ) reac-tion. Outgoing tritons were detected at H~,b —0' in the fo-cal plane of a quadrupole-dipole-dipole-dipole (QDDD)magnetic spectrometer. Gamma rays were measured incoincidence with these tritons using a 12.7 crn by 10.2 cmNaI crystal placed at about 30 cm from the target and atan angle of 125' with respect to the beam, as shown inFig. 1. Since the IAS have J =—'„ the angular distributionof this garnrna decay can have a maxirnurn complexitygiven by the Legendre polynomial, P2(cos8). Becausethe NaI detector was placed near a zero of Pz(cos8), itmeasured the total decay strength.7lgeThe 'Ga( He, t) 'Ge reaction was studied using a 29.9MeV He beam from the Princeton University AVF cy-clotron and a target of 99.8% enriched 'Ga of about 500pg/cm thickness. A spectrum of tritons from the popu-lation of the IAS in 'Ge is shown in Fig. 2(a).Six parameter data was recorded on magnetic tape forall events triggered by a particle in the QDDD focalplane detector. Triton particle identification was ob-tained from the focal plane detector system, tritonmomentum was determined by the focal plane position,and the energy of coincident gamma rays was determinedby the NaI detector. One time-to-amplitude converter(TAC) was started by the NaI detector and stopped by asignal from the cyclotron radio frequency generator("RF-TAC"). This parameter provides a prompt coin-cidence constraint on the data to eliminate neutron and38 900 1988 The American Physical Society38 GAMMA DECAYS OF ISOBARIC ANALOG STATES RELEVANT . . ~ 901l~45' ~I rTRITONDETECTORLTIPOLEBEAMSTOPTRI TONS~QUADRUPOLERGFTNol 7DETECTORCYCLOTRONBEAMrandom backgrounds. A second TAC was started by thesignal from the QDDD focal plane scintillator and wasstopped by the Nal detector ("Reals-TAC"). This pa-rameter provides a constraint on true versus randomcoincidences between the two detectors. The beamcurrent was adjusted to provide a constant 80 kHz rate inthe NaI detector.Gamma ray spectra were accumulated with gates onthe IAS peak in the triton spectrum, the prompt peak inthe "RF-TAC" and the "Reals-TAC". Figure 3(a) showsthe gamma rays in coincidence with the "real" events inthe "Reals-TAC", and Fig. 3(b) shows the gamma rays incoincidence with the "random" events in the "Reals-TAC".To calibrate the system, two reactions were used,Al( He, d) Si and Al( He, t) Si as seen in Fig. 4. The1.78 MeV 2+ state in Si is strongly populated and has a100% gamma branch to the ground state providing agamma line shape and pileup measurement. The ob-FIG. 1. Schematic of the experimental setup showing thebeam stop inside the first dipole, the triton trajectory to the fo-cal plane and the NaI position.(a)12000-9000-M1-x'DO 6000-(o)'Go (3He, t) Ge30 MeV0'lAS8,932 MeV8070-60-~ 50-1-X0 40-030- I 04 MeV'Go( He, ty) 'GeREALS3000-20-16000.Position Gote forI (Gamma CalncidenceeCHANNEL(b)90.80.70-Gamma EnergyGa( He, ty) GeRANDOMS12000-M1-Oo 8000-818r (3He, t)8 Kr29.8 MeV30'lAS(9.7I7 MeV60-50-X& 40-C330-20-4000- 10-II I ~ ~pe m lllkQ~J~r~m a. I WALaLLlam aa ma ~ a m a a a aGamma EnergyI ICHANNELFIG. 2. Triton spectra obtained for the IAS in (a)'Ga( He, t) 'Ge reaction and (b) 'Br( He, t) 'Kr reaction.FIG. 3. (a) Spectrum of gamma rays in coincidence with tri-tons population the IAS in "Ge and the reals in the TAC's. (b)Spectrum of gamma rays in coincidence with the IAS in "Geand the randoms in the TAC's.A. E. CHAMPAGNE et al. 38served peak to total for the 1.78 MeV state is 0.329(4)compared to the expected value' of 0.35.The 0.78 MeV —,'+ state in Si also has a 100% branchto the ground state which is isotropic and gives anefficiency measurement. The gamma rays in coincidencewith these states are shown in Fig. 5. The observed peakto total for the 0.78 MeV states is 0.52(5) compared to theexpected value' of 0.51. The overall coincidenceefficiency was found to be 0.37(4)% by comparing thenumber of counts in the 0.78 MeV triton peak with thenumber of counts in the gamma photopeak. This corn-pares to the expected efficiency of 0.37% as calculated forour geometry.The 'Ge IAS is energetically allowed to neutron de-cay to the ground state and the first two excited states ofGe, or it may simply gamma decay to lower states in'Ge most probably by emitting high-energy gamma rays.Since the 1.22 MeV second excited state of Ge decays100% to the 1.04 MeV first excited state of Ge, thegamma rays in coincidence with the tritons populatingthe 'Ge IAS will show a 1.04 MeV peak for neutron de-cay to the first two excited states of Ge, or no gammaray for neutron decay to the Ge ground state. High-energy gamma rays would be the experimental signaturefor decay to lower states in 'Ge.The 'Ge IAS gamma coincidence spectrum clearlyshows a 1.04 MeV gamma ray. Using the efficiencydetermined from the aluminum data, and correcting forbackgrounds, the branching ratio of the 'Ge IAS to astate producing a 1.04 MeV gamma is found to be53(9)%.From a pileup analysis, an upper limit can be set on thenumber of high-energy (& 2.5 MeV) gammas. Assumingthe dependence of pileup on photopeak energy is linear,using the peak shapes determined from the aluminumdata, and including a 20% systematic uncertainty for therelative rates in the NaI for the gallium and aluminummeasurements, the branching ratio for decay of the 'GeIAS by emission of a single high-energy gamma ray isfound to be 3(4)% or less than 11% for a 95% confidence8000-(o)7000-6000-5000.& 4ooo-05000.20001000.Al (~He, 4) $i27SIIII 78hlcVCO I NC I DENCEWINDOWg. s.900'800-700-600-500-X4oo-500-2002~A I (3He, 4y)28$1! 178 MeV1000- (b)900-800-PositionAI( He, 4) $i2 91004 2 86IOO-40-(b)Gamma Energy700-600-CO1-500-0C3400-i5 542 65CO I NC IDE NCEW I NDOWIo 7855.50.25-20-OAI ( H, t 7) $I500-200-I OO-2SStI78, 6 ftI I 1Triton Energy (MeV)lI1115-10. 780 kcV!5iI r ~"viue ~Ilasiiiuu «uiGamma EnergyFIG. 4. Spectra at 30 MeV and 0, indicating the coincidencewindow, for (a) Al( He, d) 'Si showing the 1.78 MeV state and(b) Al( He, t) Si showing the 0.78 MeV state.FIG. 5. Spectra of gamma rays, with randoms subtracted, incoincidence with the (a) 'Al( He, d) Si 1.78 MeV state and (b)Al( He, t) Si 0.78 MeV state.38 GAMMA DECAYS OF ISOBARIC ANALOG STATES RELEVANT. . . 903280—240 620 keV8!Br (~I-Ie, t~alKrGAMMAS in Coincidence with IAS200—Cf)I—I 60—OOI 20-I I I II.O MeV 2.0 MeVGamma EnergyFIG. 6. Spectrum of gamma rays in coincidence with the IAS in "Kr and the reals in the TAC's. The position where a 1.26 MeVgamma would appear is indicated.level. For emission of two 4.9 MeV garnmas, the limitwould be 8% for a 95% confidence level.This result implies that the 'Ge IAS does not decay byemission of high-energy gamma rays, but most likely de-cays completely by neutron emission.The measurement of the decay of the 'Kr IAS, seen inFig. 2(b), used the same technique as that described forthe 'Ge IAS. The 'Br( He, t) 'Kr reaction was observedat zero degrees with a 29.8 MeV He beam on a 300pg/cm thick target of NaBr enriched to 97.8% in 'Br.The calibration of the system was again carried outwith the Al( He, d) Si and Al( He, t) Si reactions.The 'Kr IAS is energetically allowed to neutron decay tothe ground, 0.617, 1.257, 1.320, 1.437, and 1.789 MeVstates of Kr. The gamma ray cascades of the excitedstates yield gamma rays with energies of 0.617 and 1.257MeV (22% of the decay of the 1.257 MeV state), and oth-ers attributed to the unknown decay properties of the1.320 MeV state. Again, high-energy gamma rays wouldindicate direct gamma decay of the IAS to lower states inr.The 'Kr IAS gamma coincidence spectrum seen inFig. 6, shows a strong 0.62 MeV gamma ray and no indi-cation of a 1.26 MeV gamma ray (less than 1% as strongas the 0.62 MeV state). Using the efficiency (0.18),geometric factor (2.06), and peak to totals (0.57) deter-mined from theory' and confirmed by the aluminumdata, and correcting for backgrounds, the branching ratioof the 'Kr state producing a 0.62 MeV gamma ray isfound to be 1.1(2). From a pileup analysis, an upper limitcan be set on the number of high-energy (& 1.5 MeV)gamma rays. The branching ratio for the 'Kr IAS to de-cay by emission of a single high-energy gamma ray isfound to be 0(5)% or less than 10% for a 95% confidencelevel. For the emission of two 4.9 MeV gamrnas, the lim-it is 6.4% for a 95% confidence level.This measurement implies that the 'Kr IAS does notdecay by emission of high-energy gamma rays, but mostlikely decays completely by neutron emission.SUMMARYThe isobaric analog states in 'Ge and 'Br are cleanlyproduced using the ( He, t) reaction. The gamma raydetection efficiency and pileup fraction were determinedby measurements of the Al( He, d) 'Si (1.78 MeV) andAl( He, t) Si (0.78 MeV) reactions. By examining thehigh-energy portion of the gamma ray spectrum we wereable to set an upper limit of 11% for the gamma decaybranching ratio of the IAS in 'Ge and 10% for the IASin 8'Kr.Therefore, the Fermi transitions to isobaric analogstates in 'Ge and 'Kr will contribute less than 0.3 SNU(11% limit X2% contribution X133.7 SNU) and 0.2SNU (10% limit X6% contribution X27.8 SNU), re-spectively, in gallium and bromine solar neutrino detec-tors for rates calculated from the standard solar model.A measurement of the decay of the IAS in Ge is in pro-gress.This work was supported in part by the National Sci-ence Foundation.A. E. CHAMPAGNE et al. 38R. Davis, Jr., D. S. Harmer and K. C. Hoffman, Phys. Rev.Lett. 20, 1205 (1968).J. N. Bahcall and R. Davis, Jr., Science 191,264 (1976).~A. A. Pomansky, Nucl. Instrum. Methods B17,406 (1986).4W. 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Krofcheck, E. Sugarbaker, D. J. Wagner, J. Rapaport, D.Wang, J. N. Bahcall, R. C. Byrd, C. C. Foster, C. D. Good-man, C. Gaarde, D. J. Horen, T. Carey, and T. N. Taddeucci,Phys. Rev. Lett. B 189, 299 (1987).' H. Orihara, C. D. Zafiratos, S. Nishihara, K. Furukawa, M.Kabasawa, K. Maeda, K. Miura, and H. Ohnuma, Phys. Rev.Lett. 51, 1328 (1983);private communication.~4R. T. Kouzes, M. M. Lowry, and C. L. Bennett, Phys. Rev. C25, 1076 (1982).' C. M. Fou, R. W. Zurmhle, and J. M. Joyce, Nucl. Phys. A97,458 (1967)~' J. B. Marion and F. C. Young, Nuclear Reaction AnalysisGraphics and Tables (North-Holland, Amsterdam, 1968).

Gamma decays of isobaric analog states relevant to neutrino detection

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Gamma decays of isobaric analog states relevant to neutrino detection

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