LEVEL SCHEME OF SM-153 BASED ON (N, Y), (N,E-), AND BETA-DECAY EXPERIMENTS

The Sm152(n,γ)Sm153 spectrum was measured with the Argonne bent-crystal spectrometer and with a Ge(Li) detector at the in-pile facility at the Argonne CP-5 research reactor. The low-energy bent-crystal spectrum consisted of 251 γ transitions associated with thermal-neutron capture in Sm152, with energies between 28 and 1041 keV. The γ-ray intensities were normalized to the previously established intensity of the 103-keV line in Eu153 from the β decay of Sm153. The energies and intensities of 24 other lines associated with this β decay are also given. The high-energy (n,γ) spectrum, containing 23 lines between 4.5 and 5.9 MeV, was obtained with a Ge(Li) detector. The neutron binding energy of Sm153 was found to be 5869.3±2.0 keV. The conversion-electron spectrum, measured with the high-resolution magnetic spectrometer at Munich, was used to obtain K and L conversion coefficients and corresponding multipole assignments for 37 of the low-energy γ transitions. The γ spectrum in Sm153 following β decay of Pm153 was measured with Ge(Li) and Si(Li) detectors. The source was made at Darmstadt through the Sm154(γ,p)Pm153 reaction. The (n,γ), (n,e-), and β-decay experiments were combined to develop the level scheme of Sm153, in which unique spin and parity assignments are made for 13 of the 28 levels below 750 keV. The energy (keV) and Jπ of the first 28 levels are: 0.000, 32+; 7.535, 52+; 35.843, 32-; 53.533, 72+ or (52+); 65.475, 92+ or 72+ or 52+; 90.874, 52-; 112.954, 92+ or 72± or 52±; 127.298, 32-; 174.17, 72-; 182.90, 52-; (194.65), 52± or 72+; 262.33, 72+ or (52+); (265.93), 72- or (52±); 276.71, 32+; 321.11, 32+; 356.69, 52+; 362.29, 52+; (371.04), 92 or 72; 405.46, 32-; 414.91, 1/2+ or 32+; 447.07, 52 or 72; 450.04, 52 or 72; 481.08, 32+; 524.36, 52; 630.20, 32(-); 695.83, 1/2(+) or 32(+); 734.90, 52; and 750.32, 1/2 or 32. The parentheses around a level energy or spin assignment mean that this value is less well established or is less probable if there is a choice. Of special interest is the very low-energy (7.53 keV) first excited state with Jπ=52+, which appears to be the second member of the strongly distorted ground-state rotational band. A good match between the theoretical predictions of the Nilsson model and the observed γ-ray branching ratio was obtained when nine of the eleven levels below 200 keV were assigned to a positive-parity, K=32, ground-state rotational band and two negative-parity, K=32, rotational bands with band heads at 35.84 and 127.30 keV. © 1969 The American Physical Society.