Investigation of neutron-deficient tungsten isotopes168?174W

Abstract

The decays of 168-174 w were observed in the reactions of 36Ar and 4~ beams with enriched targets of 136BaF2 and 138BaF2" The measured half-lives are 53 (2) s, 76 (6) s, 2.42(4) min, 2.38(4) rain, 6.6(9) min, 6.3(4) rain, and 35.3(5) rain, respectively. Decay schemes were derived from the 7-and X-ray coincidence data. Spin assignments of ground-and excited states are discussed within experimental level systematics and compared with the results of microscopic-macroscopic model calculations. Excitation energies of modeled one-quasiparticle states were found in reasonable agreement with the experimental data. Allowed unhindered beta decay from the 5/2-[523] to the 7/2-[523] configuration was identified in lVlW and this transition is also proposed for the decays of the even-even isotopes 168, 1vow. PACS: 27.70. + q for half-life measurements. Coincidence data from a standard fast-slow coincidence set-up connecting the 7-and X-ray detector were recorded in list-mode on magnetic tape. Each coincidence event was also tagged with the time elapsed since the last tape movement. Identification of the isotopes ~7-,TS wThe isotopes 167-171W were identified by the measurement of excitation functions of new 7-rays and comparison with those of the ~33Cs(S6Ar, xn)169-XTa reactions ( Fig. 1 a). The evaporation of 3 <_ x < 6 neutrons yielded the well known ?-rays 396, 211, 163, and 159 keV from ~6a-~~176 decays. Gamma rays following the decays of tungsten isotopes were selected by requiring coincidence with tantalum X-rays. Excitation functions of tungsten 7-rays using a ~36Ba target are given in Fig. lb. The comparison in Fig. 1 of the measured yields for 94.4, 178.5, and 169.5 keV ?-rays gives the assignment of these lines to the decays of the nuclei 167W [4], ~68W, and 169W, respectively. Accordingly, the y-rays with energies 316.2 and 184.2 keV, which were observed in 36Ar on ~38Ba reactn .4-r-._d K. oq o~ >-a) 133[s (36Ar,xn)169-XTa 211 keV :~=Ta 163 keV ~5Ta / 10 ~ .... ~6s~ \

Revision of the decay data of166?170Re, including new isomers167m,169mRe

“…lb was measured after the bombardment of enriched 136Ba with a6Ar ions at 193 MeV energy. The three strongest e-lines at 4.74, 4.56 and 4.40MeV origin from the decays of 166,167, 168W, respectively I- [4][5][6]. At this irradiation energy, we note that the contaminations from elements with Z < 74 are weaker and that no e-rays with energies above 4.74 MeV were detected in Fig.…”

“…The rhenium isomers are then interpreted as M4 (A K =4) isomers. If due to a large decoupling parameter the 5/2-member of the 1/2-[541] band would be the lowest-lying state, as it is the case in many nuclei of this region [5,30], we were faced with E2 (A K =4) isomers. Although hindered, however, the life-times of such isomers are probably much shorter than those observed in our experiment.…”

“…In order to optimize the production of a specific isotope in the fusion-evaporation reaction, additional tantalum degrader foils were used in front of the target to vary the mean reaction energy (always given for the middle of the target) in the range of ~ 175 to ~ 204 MeV. Excitation functions were measured in this energy range for different deexcitation channels, especially for the well-known tungsten isotopes E2, 4,5] produced via 141pr(32S, pxn) reactions.…”

“…5. We show that the production ratio of 179keV v-rays (168W [5]) and of 363 keV v-rays (t68Re) is almost constant in the beam energy range between 175 and 204 MeV, covered by up to 5.8 mg/cm 2 Ta degrader. The production ratio of 5.26 MeV e-rays and 363 keV v-rays, however, increases rapidly for higher irradiation energies.…”

“…The element assignment is based on xa3Cs(a6Ar, xn)169-~Ta [2,[4][5][6], 136Ba(36Ar, xn)172-xw, and 135Ba(36Ar, xn) 171 -~W cross-bombardments, in which no isotopes of the element rhenium can be produced.…”