Ⅱ-1. Nuclear Physics

RIKEN Accel. Prog. Rep. 52 (2019)

β decays of the heaviest N = Z − 1 nuclei and proton instability of97
In†J. Park,∗1,∗2 R. Krücken,∗1,∗2 D. Lubos,∗3,∗4,∗5 R. Gernhäuser,∗3 M. Lewitowicz,∗6 S. Nishimura,∗4 andH. Sakurai∗4,∗7 for the EURICA Collaboration
Table 1. Half-lives, β-decay endpoint energies, log f t values and β-delayed proton emission branching ratios of91
Pd, 95 Cd, 97 In and 99 Sn.

†∗1
∗2
∗3
∗4
∗5∗6∗7Nucleus
91
Pd95Cd97In97mIn99Sn

2T1/2 (ms)
32(3)
32(3)
28(5)
1.3–230 µs
24(4)< 0.63 MeV

0log f t
3.4(5)
3.1(5)
3.0(9)bβp (%)
3.0+1.1−0.9
4.5+1.2−1.0
1.7+1.7−0.8

14.7(36)

3.8(7)

3.9+3.4
−1.7

2

97

In

theolog [T 1/2(s)]1l = 4 (g9/2)

10230 µs

−4FRDM

In isotopes

HFB

600 ms−2

l = 1 (p 1/2)

(9/2+) ground state

0AME

−1DZ

1.3 µs−60.64 MeV

0.5QEC (MeV)
11.8(22)10.2(17)10.0(30)

S p (MeV)Interesting nuclear structure phenomena can be observed at the limits of neutron or proton binding, providing sensitive probes of shell evolution and/or collective behaviors. Studies have found the doubly magic100
Sn to be stable against proton emission, conﬁrming the robustness of the N, Z = 50 shells. Searches
for even more neutron-deﬁcient nuclei below 100 Sn resulted in a discovery of new isotopes, while also reaching the proton dripline for Rh and Ag isotopes.1)
From a decay spectroscopy experiment at the RIBF,
half-lives, β-decay endpoint energies, and β-delayed
proton emission branching ratios of N = Z − 1 nuclei 91 Pd, 95 Cd, 97 In and 99 Sn were measured with thewide-range active silicon strip stopper array for β andion detection2) (WAS3ABi). The results, summarizedin Table 1, are consistent with β decays of mirror nuclei containing a mixture of Gamow-Teller and Fermi
decay components.
The apparent stability of 97 In against proton emission was investigated. Based on the deﬁcit in the βdecay amplitude of the parent nucleus, an isomericstate in 97 In decaying to 96 Cd via one-proton emission was proposed. Although this decay branch has
not been detected directly, the observation of a γ-raytransition belonging to the β decay of 96 Cd was a supporting evidence for the proton-emitting isomer. The
missing proton events were hypothesized to have occurred in a time range between the ions’ ﬂight throughthe BigRIPS and ZeroDegree separators (∼600 ns) andthe 600-µs deadtime of WAS3ABi.
From the shell model perspective, 97m In is formedby promoting a π1p1/2 proton into the π0g9/2 orbital,
leaving an unpaired proton in the π1p1/2 orbital andresulting in a spin-parity of (1/2− ). The large spin
diﬀerence and parity change suppress a γ-ray decaybranch to the (9/2+ ) ground state. Experimental halflife limits on the isomer were converted into a hypothetical proton energy range through a theory on proton emission.3) As shown in Fig. 1, the lower angularmomentum barrier for proton emission from the π1p1/2
orbital compared to the π0g9/2 orbital would facilitate
proton emission from the isomer. This result is con-0.60.78 MeV

0.70.8Qp (MeV)HB−2KTUY

959697
A9899

Fig. 1. Left: theoretical T1/2 as a function of emitted
proton energy Qp and orbital angular momentum l.
Right: experimental proton separation energy Sp of theground state of 97 In compared with diﬀerent mass predictions.4–9)

sistent with a few mass models which predict 97 In toundergo β decay as opposed to proton emission.
References

Condensed from the article in Phys. Rev. C 97, 051301(R)(2018)
Department of Physics and Astronomy, University of British
Columbia
TRIUMF
Physik Department, Technische Universität München
RIKEN Nishina Center
Excellence Cluster Universe, Technische Universität München
Grand Accélérateur National d’Ions Lourds (GANIL)
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