Further Study on Y Transitions in 140Ce

Abstract

A photoline spectrum from the γ rays of energies greater than 200 keV following the decay of 40-hour 140La has been studied using a high resolution double focusing ß-ray spectrometer. The photoelectrons are produced from an uranium radiator of thickness 2.9 mg/cm2. The resulting momentum resolution was 0.3 → 0.8% depending on energy. In addition to γ rays previously reported, we have observed 17 new γ transitions in the energy region of about one MeV to 2.53 MeV. The relative γ-ray intensities have been determined from the external conversion spectrum applying the corrections for the angular dependence of the photo-electric process. Gamma energies and intensities higher than 2.53 MeV have been investigated by means of a scintillation spectrometer using a RiDL-transistorized 400 channel analyzer. A complete set of energy sums was computed in order to survey the possibilities of cascade crossover combinations. Thirteen direct ground state transitions of energies 204.87, 241.95, 993.12, 1597.48, 1973.38, 2026.37, 2350.23, 2371.11, 2410.33, 2530.03, 2900, 3100 and 3471 keV were observed. A few modifications on the previously reported level scheme of 140Ce are proposed after the addition of the 17 new γ rays. Conversion coefficients were obtained by normalizing the ratios of conversion and photon intensities with respect to the 487 keV-E2 transition in 140Ce. From the known internal conversion coefficient of this transition, the absolute K-internal conversion coefficients of the transitions 200.08, 241.95, 273.69, 337.50, 374.81, 400.53 and 875.30 keV were determined to confirm their multipolarities and support spin and parity assignments of the levels 242, 617, 1260, 1323, 2184, 2515, 2585 and 3390 keV. The absolute K-internal conversion coefficients of the 542.43, 641.41, 752.62, 927.41, 980.19, 1016.20, 1040.31 and 1054.65 keV transitions were measured for the first time. The multipolarity assignments of these transitions were studied on the basis of the conversion coefficients. These measurements together with the previously known assigned levels establish the spins and parities 3-, 0+, 2+, 4+, 2+, and 2+ for the levels 205, 1889, 1973, 2084, 2350 and 2530 keV, respectively. Spin and parity assignments of 1+ and 4- are proposed for the 2900 and 3100 keV excited levels in 140Ce respectively.