Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 47 (2014)

Beta-delayed proton emission of 73 Sr and effective lifetime of therp-process waiting point 72 Kr in X-ray bursts environment
L. Sinclair,∗1 G. Lorusso,∗2 P. Davies,∗1 J. Wu,∗3 Z.Y. Xu,∗4 S. Nishimura,∗2 P. Doornenbal,∗2P.-A. Söderström,∗2 F. Browne,∗5 D. Lubos,∗6 Z. Patel,∗7 S. Rice,∗7 H. Baba,∗2 A. Estrade,∗8 Y. Fang,∗9J. Henderson,∗1 T. Isobe,∗2 D. Jenkins,∗1 S. Kubono,∗2 Z. Li,∗3 I. Nishizuka,∗10 H. Sakurai,∗2 P. Schury,∗2T. Sumikama,∗10 R. Wadsworth,∗1 H. Watanabe,∗2 V. Werner,∗11 and the EURICA collaboration
The nuclide 72 Kr is a potential waiting pointof the astrophysical rp-process. However, its lifetime under conditions of X-ray bursts may significantly be reduced by the two-proton capture reaction
72
Kr(p,γ)73 Rb(p,γ)74 Sr. The rate of this reaction ishighly sensitive to the characteristics of the low-energy
states of the intermediate nucleus 73 Rb and, in particular, to the proton-separation energy, Sp , of 73 Rb. No
constraints from direct measurements exist, resulting
in significant uncertainties in calculations of astrophysical interest.
Nuclear structure in the 70–80 mass-number regionis particularly interesting. Rapid shape changes, shapecoexistence, and np-pairing effects are all expected.
This is a challenging region from a theoretical perspective and little experimental data exists for nuclei
beyond the N=Z line. This experiment affords an opportunity to access nuclei both at and beyond the N=Z
line in this mass-number region.
The nuclei of interest were produced by fragmentation of a 345 MeV/u 124 Xe primary beam colliding with a 9 Be target. The beam intensity ranged
from 30–35 pnA. The secondary beam purification andidentification was performed using the BigRIPS fragment separator. The fragments of interest were unambiguously identified, and their subsequent decays
were recorded using the WAS3ABi silicon stopper in
conjunction with EURICA1 . Implantations were correlated with their subsequent β-decays on the basis ofposition and time, enabling measurement of half-lives
and β-delayed γ rays.
Two experimental settings were used to accessproton-rich isotopes around 73 Sr (as shown in theparticle-identification (PID) plot in Fig. 1 with 73 Srhighlighted). As a first setting, BigRIPS was set for
maximum transmission of 73 Sr, the number of implanted nuclei predicted by LISE++ was 2500 and therequested beam time was 2.5 days. A second setting

was optimized for the transmission of 74 Sr, and thenumber of implanted nuclei predicted was 8000 for
0.5 days. The actual total beam time was 45 h for 73 Srand 9 h for 74 Sr. The number of implanted 73 Sr and74
Sr were 186 and 590, respectively. The discrepancies observed between the expected counts and actual
counts is due to the actual production cross-section being lower than that predicted by LISE++ calculations.
3973105

Sr

38
37104

Z

36

103

35
34102

3332

103130
1.9

1.95

2

2.05
A/Q

2.1

2.15

2.2

1Fig. 1. PID plot of nuclei transmitted through the fragment
separator BigRIPS from the 73 Sr setting.

Upon the experiment’s completion, an energy calibration of WAS3ABi was carried out using 482 and972 keV conversion electrons emitted from a 207 Bisource.
The β + -decay half-lives of nuclei 73 Sr, 74 Sr and 76 Y
have been measured. The accuracy of these measurements were verified by extracting the half-lives of previously studied nuclei and comparing with the literature
values.
The isotopes 69 Br, 72 Rb and 73 Rb were observed inBigRIPS, for which there is evidence of implantationand decay events in WAS3ABi. Future analyses will
focus on these events, as well as the low-lying structure
populated through β + -decay of exotic nuclei produced
around 73 Sr. A search for new isomeric states, such as
the one found in 70 Se, is also being carried out.
This large set of data will provide new half-lives, direct input for rp-process calculations, and new insights
into the structure of nuclei in this region.

∗1Department of Physics, University of YorkRIKEN Nishina Center∗3 School of Physics, Peking University∗4 Department of Physics, University of Tokyo∗5 CEM, University of Brighton∗6 Technische Universität München∗7 Department of Physics, University of Surrey∗8 School of Physics and Astronomy, University of Edinburgh∗9 Department of Physics, Osaka University∗10 Department of Physics, Tohoku University∗11 Department of Physics, Yale University∗2References
1) P.-A. Söderström et al.: Nucl. Instr. and Meth. B317,649-652, (2013)- 12 -