PHYSICAL REVIEW C VOLUME 56, NUMBER 2 AUGUST 1997 71 b decay of the proton-rich Tz521/2 nucleus, Kr M. Oinonen,1 A. Jokinen,2 J. A¨ ysto¨,1,2 P. Baumann,3 F. Didierjean,3,* A. Honkanen,1 A. Huck,3 M. Huyse,4 A. Knipper,3 G. Marguier,5 Yu. Novikov,6 A. Popov,6 M. Ramdhane,3 D. M. Seliverstov,6 P. Van Duppen,4 G. Walter,3 and the ISOLDE Collaboration7 1Department of Physics, University of Jyva¨skyla¨, P.O. Box 35, FIN-40351, Jyva¨skyla¨, Finland 2CERN, PPE Division, CH-1211 Geneva 23, Switzerland 3CRN, CNRS-IN2P3, Universite´ Louis Pasteur, B.P. 28, F-67037 Strasbourg, France 4Instituut voor Kern- en Stralingsfysica, University of Leuven, Celestijnenlaan 200 D, B-3001 Leuven, Belgium 5IPN, CNRS-IN2P3, Universite´ Claude Bernard, F-69622 Villeurbanne, France 6St. Petersburg Nuclear Physics Institute, Gatchina, 188350 St. Petersburg, Russia 7CERN, CH-1211 Geneva 23, Switzerland ~Received 25 October 1996! 71 71 b decay of the Tz521/2 nuclide Kr has been studied at the ISOLDE PSB Facility at CERN. Kr ions were produced in spallation reactions in a Nb foil using the 1 GeV proton beam and studied by means of b-delayed proton, b- and g-ray spectroscopy. The half-life and the b-decay energy of 71Kr were determined using the decay of protons and positrons. These results: T1/2510063 ms and QEC510.1460.32 MeV and the first observation of the b branch to the 207 keV level in 71Br makes the extension of the systematics of Gamow-Teller matrix elements of mirror nuclei up to A571 possible. The Gamow-Teller strength of the same magnitude as that of the fp-shell mirror nuclei is observed for the ground-state transition. @S0556-2813~97!03408-0# PACS number~s!: 23.40.Hc, 21.10.Tg, 23.50.1z, 27.50.1e I. INTRODUCTION ments on these fast-decaying nuclei produced in low yields. b-decay half-lives have been measured for 61Ga, 63Ge, and 65 Studies of neutron-deficient nuclei above A;60 are im- As and their main decay mode could be characterized to be portant for understanding the evolution of nuclear for sys- fast b decay @10#. More detailed information was obtained 67 71 75 tems with a nearly equal number of protons and neutrons @1#. for the decays of Se, Kr, and Sr in studies performed Due to the increasing importance of the Coulomb interaction, at ISOLDE and GANIL. At ISOLDE, b-delayed g-ray spec- troscopy has been exploited for 67Se @11# and detection of these nuclei are only weakly bound or even unbound, and high-energy b particles for 71Kr @12#. Recently, projectile consequently small effects due to the proton-neutron interac- fragmentation studies of 78Kr at GANIL allowed the obser- tion, the shell structure, and the deformation become essen- vation of weak b-delayed proton branches in the cases of tial in determining the nuclear properties. These properties 67Se, 71Kr, and 75Sr @13# and induced also some discrepan- are also of prime importance for a deeper understanding of cies with the previous measurements. In the GANIL study, the rp process above Ni @2#. the half-life for 71Kr was determined to be 6418/25ms Pairs of mirror nuclei with T51/2 provide an ideal labo- using the detection of b-delayed protons. This value dis- ratory for probing the evolution of fine details of nuclear agrees significantly with the previous result 9769 ms, where only -particle detection was used to obtain the half-life properties. Qualitatively, the decays of Tz521/2 nuclei are b characterized by fast combined Fermi and Gamow-Teller @12#. Short half-lives for these decays, in which the transition ~GT! decays and short half-lives (;100 ms) due to the high to the analog state should be predominant, indicate enhanced transitions that are difficult to account for theoretically. decay energies determined by the well-defined Coulomb- Therefore, to obtain relevant information on Gamow-Teller energy differences. These nuclei have been studied with high 59 59 matrix elements involved in these decays one has to use both enough precision only up to Zn @3–5#. Above Zn experi- b-delayed proton and g detection techniques. The present 71 mental information is incomplete. Nucleon stability of Tz5 experiment on Kr, performed using the General Purpose 21/2 nuclei has been studied mainly via fragmentation reac- Separator of the ISOLDE Facility located at the CERN PS tions. Particle stability, i.e., T1/2.100 ns, has been con- Booster @14#, was undertaken with these requirements in firmed up to 75Sr with two exceptions, namely 69Br and mind. In addition, the experiment had as another goal the 73Rb @6–8#. Above strontium, particle stability has been ob- search for the radioactive decays of other, lighter isotopes of served for 87Ru, 89Rh, and 91Pd @9#. Despite the success in Kr. We report here only on the 71Kr decay; information ob- identifying these exotic nuclei, progress in the study of their tained on 69Kr and 70Kr will be presented separately. It decay and structure has been slow. This is mainly due to the should be mentioned that b-delayed proton emitters along 77 experimental difficulties in performing high-accuracy experi- the Tz511/2 line have been studied in detail up to Sr @15–17#, providing an excellent data set for comparison with the Tz521/2 nuclei, especially concerning the statistical *Present address: Eurisys, Strasbourg-Lingolsheim, France. features of the b-delayed proton decay. 0556-2813/97/56~2!/745~8!/$10.0056 745 © 1997 The American Physical Society 746 M. OINONEN et al. 56 II. EXPERIMENTAL INFORMATION Efficiency calibrations for the g and X detectors were made using standard 152Eu, 56Co, and 133Ba sources. Effi- Short-lived Kr isotopes were produced in spallation reac- ciency calibration for the b scintillator was determined by tions by using the pulsed 1 GeV proton beam from the PS using the b-gated and singles spectra of protons following Booster at CERN and mass separated in the ISOLDE facility the b decay of 33Ar produced in large enough yields on line. @14#. Very clean conditions for producing Kr beams were The efficiency of the proton telescope was measured to be obtained with a Nb foil target connected to the plasma ion 6.261.9% from the 73Kr decay, whose bg and bp branches source via a cooled transfer line. The average proton-beam are well known @19#. Energy calibration for the proton tele- intensity in this experiment was 2.1 mA. The beam consisted scope was obtained using an external a source which in- of subsequent short-duration proton pulses with a 2.4 ms cluded 239Pu, 241Am, and 244Cm. Two additional calibration length and spacing of an integer multiple of 1.2 s between points were obtained from the proton spectrum of 33Ar, in the pulses. Production rates for the 73Kr and 72Kr isotopes which the 2096 and 3167.6 keV proton peaks were clearly and the short-lived 71Kr were 7.63104, 1.53103, and observed. The energy resolution of the telescope for protons 1.7 atoms/mC, respectively. The production rate for 71Kr was determined to be about 30 keV. was two times higher than in the previous experiment at the The internal energy calibration of the b detector was SC-ISOLDE facility @12#. made using the previously mentioned g-ray sources and as a The isobarically pure Kr beam was implanted into an high-energy calibration point the 6.130 MeV g ray from a aluminized-Mylar tape, which was tilted to 45° with respect 13O(a,n)16O* source. The error due to the calibration to the beam axis. This angle allowed undisturbed b-delayed around 10 MeV was less than 20 keV. In addition, the proton detection by a special detector telescope system fac- calibration was tested by measuring the known b-decay 73 ing the point of implantation. The transport tape was moved end-point energy for the Kr decay. at every tenth proton pulse, 800 ms after the pulse impact, to reduce the background due to long-lived activities. The III. RESULTS beam-on period for the radioactive-ion beam following the A. b-delayed g decay proton pulse was set to 250 ms. This short time associated with the selectivity of the target-ion source for noble gases g spectra in coincidence with b’s and gated with the and the periodic movement of the tape reduced the back- short-lived ~30–800 ms! and the long-lived part ~801–4800 ground remarkably. The only contaminants present in the ms! of the time spectrum, respectively, are shown in Fig. 1. beam were the long-lived activities 71As and 71Zn. Two lines of interest for 71Kr are seen at 198 and 207 keV; The detection setup consisted of a gas-Si detector tele- they are short lived and their relative intensity is in agree- scope for protons, a 70% coaxial, and a 20-mm-thick planar ment with those observed in an in-beam study on 71Br by HPGe detectors for g and x rays, and a telescope detector for Arrison et al. @20#. Hence, they are associated with the b high-energy b’s, all in close geometry around the implanta- decay of 71Kr, and deexcite the 207 keV level in 71Br. No tion position. Low-energy b-delayed protons could be iden- other g lines related with the decay of 71Kr could be identi- tified with the specially designed gas-silicon detector tele- fied. The level scheme of 71Br is given in @20#, including the scope @18#, in which the gas detector has an effective cascade of the 198 keV and ~unobserved! 9 keV g rays, 2 p thickness or only 70 mg/cm , consisting of a deexciting the 207 keV level, i.e., EX5207, J 2 2 p 2 p 2 45-mg/cm -thick polypropylene entrance window and an 5(3/2) EX59, J 5(1/2 ) g.s., J 5(5/2) .