TRIAC Progress Report - KEK

Mass, which is one of the most fundamental quantities for atomic nuclei, provides
good insight on the evolution of the nuclear structure. In addition, the mass is one of the
key parameters to build the scenarios of nucleosynthesis in universe. The mass of
160-165 163
Eu and Gd isotopes have been determined for the first time by measuring Qβ
values using a total absorption BGO detector. Among them, isotopes of 163-165 Eu were
newly observed.
The isotopes produced by proton-induced fission of 238 U of about 600 mg/cm 2
thickness were ionized by the SIS and mass-separated by the JAEA-ISOL. The isotopes
were implanted onto an aluminum-coated Mylar tape, and were periodically moved to
the center position of the total absorption BGO detector. The detector composed of twin
BGO scintillation detectors of 120 mm in diameter and 100mm in length which faced
each other with a distance of 4 mm. The solid angle of the detector was 96 %. For
measurements of 160-165 Eu, the time interval of the tape transport was set to be two or
three times longer than the half-lives of these isotopes, while, in the case of 163 Gd, the
times for collection, cooling, and measurement were set by 4, 2, and 2 minutes,
respectively, to allow the decay out of the parent nucleus 163 Eu (T1/2 = 6 s), which has a
larger Qβ value than 163 Gd.
The Fermi-Kurie plots of the total absorption spectra of 158, 159 Pm, 159, 161 Sm
160-165 163 166
Eu, Gd and Tb are shown in Fig. 3-35. Qβ values of 158, 159 Pm, 159, 161 Sm, and
166
Tb were found to be consistent with our previous experimental results. The evaluated
Qβ values given by Audi et al. [3-49] were in agreement with the present results. The
mass predictions by Duflo et al. [3-50] and Dussel et al. [3-51] were also consistent
with the experimental values.
3-4-2. Lifetime measurements of 162, 164 Gd (2 + )
The collectivity is one of useful quantities to explore the vibration of the nuclear
structure. In the case of the even-even nuclei, the transition probability from the first 2 +
state reflects the quadrupole collectivity. The lifetimes of the first 2 + states at 71.5 keV
in 162 Gd and 73.3 keV in 164 Gd were measured by means of β–γ delayed coincidence
technique for mass-separated 162 Eu and 164 Eu isotopes.
As described previously, the fission products of 162,164 Eu were ionized by SIS
embedded UCx and transported to the tape system. The tape was moved every 20 s and
9 s for 162 Gd and 164 Gd, respectively. The detection position was equipped with a
Pilot-U 60 mm × 63 mm × 1-mm thickness plastic scintillator and a 38-mm diameter ×
5-mm thickness BaF2 scintillator to detect β and γ rays, respectively.
The time interval spectra for the first 2 + state in 162 Gd and in 164 Gd were obtained
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