High-spin bands in117,119I and118Xe

Abstract

Excited states in 11vI, 118Xe and 119I were populated in the reactions l~ 160 and 1~176~2C. The groundstate band in ~SXe was observed to its (18 +) member and the 11/2 bands in 117I and 119I to their (35/2-) and (43/2-) members, respectively.The structure of the bands in the I isotopes is interpreted as a rch11/2 proton coupled to an even-even band structure in Xe, and the band crossings observed in 11SXe and 119I are interpreted in the Cranked Shell Model framework as AB, of vh~ 1/2.Recently there have been many attempts to reach very neutron-deficient nuclides in the tin region, i.e. nuclei with Z~50 and 100<A < 132, and to study their excited structure. Thus one has studied e.g. light tin isotopes [I] the production of l~ and 11~[2], band structures in even-A t16-122Te [3] and intruder bands in odd-A I isotopes [4]. In these cases the main interest has been focussed on high-spin bands, and generally spin values of 15-20h have been reached. The main obstacle for studies in this region is the complexity of the reaction, since protons, neutrons and alpha particles are emitted from the compound nucleus with comparable probabilities. See, for example, Table2 of Gizonet al. [5], where six comparable cross sections were determined in the reaction ~2C+ l~ at E~ab=49MeV. In the present work we attempt to reach high-spin bands in nuclei around mass number A ~ 118. Figure 1 shows the region in question, and as is seen, the nuclei reached are situated in a very neutrondeficient part of the nuclear chart. ~60+1~ and 12C+1~176 using ~60 beams from the Tandem van de Graaff facility of the Brookhaven National Laboratory and 12C beams from the 225cm cyclotron of the Research Institute of Physics in Stockholm. Metallic targets enriched to ~95~o were used. Excitation functions, 7-ray angular distributions and 7-I' coincidences were recorded. The product identification gave no problems in this case since the low-spin parts of the bands had been identified earlier, both in ~lSXe [8] and in 117'1~~ [9]. The level schemes obtained are presented in Fig. 2, where the yrast cascade of 12~ (from Ref.3) is included for comparison (cf. below). The spin assignments are based on the angular distributions, where all transitions are of quadrupole character. The ordering of the transitions is made on intensity basis and it is for most cases unambiguous. In 11SXe the 775 keV transitions is a doublet and thus the ordering of the 765 and (upper) 775 keV transitions might be reversed. Therefore the 14 § is drawn dashed. However, additional analysis favours the present assignment. The lowest-lying states in the odd ! isotopes, with

Band structure of118Xe

“…These nuclei exhibit rich spectra of both collective and quasiparticle excitation nature, and provide fruitful testing grounds for detailed theoretical descriptions such as the cranked shell model [1], and the interacting boson model [2]. For the excited states in 118Xe, the yrast band up to (20 +) state [7,8], a negative parity band up to (21-) state and a few levels in 7 band [9] have been reported. For the excited states in 118Xe, the yrast band up to (20 +) state [7,8], a negative parity band up to (21-) state and a few levels in 7 band [9] have been reported.…”