· 50 · IMP & HIRFL Annual Report 2015

2 - 2 Lifetime Measurements of the Low-lying Excited

States in 87Zr and 87Nb

Qiang Yunhua, Liu Minliang, Zhou Xiaohong, Wang Jianguo, Wang Kailong, Lv Bingfeng, Zheng Yong,

Gan Zaiguo, Zhang Zhiyuan, Ma Long, Yang Huabin, Yu Lin, Jiang Jian and Sun Mingdao

Lifetime measurements of low-lying excited states in 87Zr and 87Nb have been performed via β−γ and γ−γ

coincidences. The 124 MeV 32S beam was delivered from the Sector-Focusing Cyclotron (SFC) of the Heavy Ion R-

Fig. 1 Setup of the present experiment.

esearch Facility in Lanzhou (HIRFL). The parent nu-

clei 87Nb and 87Mo were produced by the respective

reactions 58Ni (32S, 3p) and 58Ni (32S, 2p1n), at a beam

energy 100 MeV through the 8 µm Al degrader. A self-

supported 400 µg/cm2 58Ni foil was employed as the

target. After evaporation residues were separated from

the projectile beams by the gas-filled recoil separator

SHANS[1], the nuclei 87Zr and 87Nb can be obtained

with a higher purity and then were implanted into a

300 µm silicon detector (see Fig. 1). As shown in the

figure, the emitted β+ particles and γ rays following β+

decay were detected by a 3 µm plastic scintillator at 0◦,

and two LaBr3 and two high purity germanium (HPGe)

detectors at 90◦ to the beam axis, respectively; the ave-rage distance from the front face of the detectors to the center of the silicon detector was approximately 10 cm

Characteristic γ rays were detected by the HPGe detectors to identify the 87Zr and 87Nb nuclei. When the plastic

scintillator and at least one LaBr3 detectors were fired in the same time, the time and energy signals from the

detectors were recorded as an event. The energy calibration was made by using 207Bi sources for the plastic

scintillator detector and 137Cs, 133Ba and 152Eu sources for the LaBr3 and HPGe detectors.

Fig. 2 Preliminary mean lifetimes for the low-lying states at 267 and 400 keV in 87Nb and at

200 and 470 keV in 87Zr from the present work.

The lifetimes of the levels at 200.9 and 470.3 keV in 87Zr and at 266.9 and 400.7 keV in 87Nb have been

extracted by fitting the time spectra using the exponential-folded Gaussians plus a horizontal background[2−4]. The

2015 IMP & HIRFL Annual Report · 51 ·

preliminary mean lifetimes were shown in Fig. 2. It is worth noting that the lifetimes of the levels at 470.3 keV

in 87Zr and at 266.9 and 400.7 keV in 87Nb are firstly measured. However, both β−γ and γ−γ coincidences give

a mean lifetime τ=771±8.5 ps for the 200.9 keV level in 87Zr, which is very different from the published value

τ=2.44±0.1 ns in Refs. [5, 6]. The reasons would be checked in the forthcoming work.

References

[1] Z. Y. Zhang, L Ma, Z. G Gan, et al., Nucl. Instr. and Meth. B, 317(2013)315.

[2] H. Mach, R. L Gill, M Moszyki, et al., Nucl. Instr. and. Meth., A, 280 (1989)49.

[3] M. Moszynski, H. Mach, Nucl. Instr. and. Meth, A, 277(1989)407.

[4] H. Mach, M. Moszyki, R. F. Casten, et al., Phys. Rev. Lett., 63(1989)143.

[5] S. E. Arnell, S. Sjoberg, O. Skeppstedt, et al., Z. Phys., A, 289 (1978)89.

[6] http://www.nndc.bnl.gov/ensdf/.

2 - 3 Lifetime Measurement of the First Excited State in 37S

Wang Kailong, Liu Minliang, Wang Jianguo, Qiang Yunhua, Gao Bingshui, Zheng Yong,

Lv Bingfeng, Zhang Zhiyuan, Zhang Wenqiang, Zhou Xiaohong, Wang Jiansong, Yang Yanyun,

Bai Zhen, Ma Peng, Jin Shilun, Ma Junbing, Duan Fangfang, Zhang Xing and Jiao Lei

We have found that the excited energy from the ground state with ν1f7/2 configuration to the first excited state

with ν2p3/2 configuration in 37S is obviously lower compared with that in nearby N=21 isotones 35Si, 39Ar and41Ca (see Fig. 1, The insert indicates the relevant single-particle orbits of proton and neutron). This interesting

phenomenon may result from the collapse of N=28 closure, which gives the sufficient valence space for collective

deformation which can been revealed from the B(E2) value. Therefore, to obtain the B(E2), we perform the

experiments for measuring the lifetime of the first excited state in 37S.

Fig. 1 Systematics of the energy of the first excited statefor N=21 isotones[1].

Fig. 2 Schematic lay-out of the experimental setups.

The experiment was carried out at the Radioactive Ion Beam Line at the Heavy Ion Research Facility in

Lanzhou (HIRFL-RIBLL1)[2]. The primary beam of 40Ar at 70 AMeV was supplied to impinge on a 9Be target

with a thickness of 987 µm. The second beam of 37P at 39 AMeV and with a intensity about 900 pps and a purity

of about 80% was separated and purified by RIBLL1. A time of flight (TOF) detector with a flight path of 17 m

was used for the particle identification for the secondary beam. The schematic layout of the detection system is

shown in Fig. 2. The telescope detector system, which consists of a 140 µm thick double-sided silicon strip detector

(DSSD) and a 1 mm thick single silicon detector(SSD), was used to identify the 37P particles. The SSD was also

used as a β-decay deposition of 37P. The β− particles of parent nuclei 37P and γ rays of daughter nuclei 37S were

detected by plastic scintillator (PS) and LaBr3 detectors, respectively. The 646-keV characteristic γ-ray of 37S was

identified by the high energy resolution HPGe detector. The lifetime of the first excited state in 37S was measured

by β−γ fast timing coincidence technique[3,4].