Two-proton multiplets in theN=82 nucleus 62 144 Sm82 from in-beamγ-measurements

Abstract

Abstract:In a high sensitivity (a,2n~,) in-beam study of 144Sms2, with the K6lner Wiixfel OSIRIS "f-detector array including a Compton polaximeter, we have identified the complete two-proton multiplets ~rd~-72, lrd-15/~g~/2,-1 ~rg~-/~, lrhll/2d~-/~ and the 9-to 4-members of the 7rhll./~ g7/2 multiplet.PAC." 21.30+y; 21.60.Cs; 27.60.+j Nuclear two-body residual interactions are basic input information for shell model analyses. They are determined in experiment from the excitation energies of the respective two-nucleon levels that in general however lie above the yrast line and are difficult to excite in compound-evaporation reactions. The N=82 144Sm nucleus, with two protons less than the 146Gd shell model core, can however be reached through the (a,2n) reaction. At low beam energy, this reaction transfers little angular momentum to the compound nucleus, and therefore can populate levels well above the yrast line, including the sought-after two proton multiplets.Our (a,2n7) measurements included v-ray excitation functions, which gave 24.5 MeV as a favorable beam energy, singles v-ray angular distributions, and 7"r-coincidences with the W/irfel detector array equipped with five Compton-suppressed Ge-deteetors and a high-sensitivity Compton polarimeter consisting of 5 coaxial Ge-deteetors. The 6 • 107 double coincidence events gave the v-detection sensitivity of 5• -4 of the (a,2n) channel at 1.5 MeV v-ray energy, and accordingly ~44Sm levels fed with >2 ~ could firmly be identified in the data. The highest observed state is the known 5.361 MeV 13-level [1]; below it we identified 86 excited 144Sm states where 72 had not been observed in previous compoundevaporation in-beam v-measurements [1,2].In Fig.1 we show the 144Sm energy levels observed in the present (a,2n) experiments up to 3.7 MeV excitation, which is the region where five low-lying twoproton multiplets should occur. These include the three positive-parity two-hole multiplets involving the rrda/2 and a'gv/2 orbitals, and also the odd-parity multiplets where one proton is lifted into the h11/2 shell. The spin-parity assignments are firm for most of the levels (cf. Fig. 1) and derive largely from the present data, but pertinent literature [3 to 6] has also been considered.The configuration assignments for the above five multiplets are quite clear and based on their expected excitation energies and on the observed Vdecay branchings; more detailed arguments will be presented later [7]. Here we will only comment on some of the states observed in the region of interest which belong to other configurations. The 2.423MeV firm 2 + state must prominently be rSl/2ds-/2 and corresponds [8] to the 146Gd first excited 2 + state at 1.972 MeV. The assignment for the 2.883 MeV 4 + level is ~rd3/2ds/2. These two states are lowered through the attractive ~ = v = 0 particle-hole interaction in the aligned coupling, and they are the only even-parity proton particle-hole states that should occur so low in energy.The two levels slightly above 3.3 MeV are assigned from resonant ...