Gamma-Ray Directional Correlations in the Decay of EUROPIUM-154.

The nucleus of Gd('154), populated by the (beta) - decay of 8.8-yr Eu('154), lies in the so-called transitional region where deviations from the rotational model and coupling between the (beta)- and (gamma)- vibrational bands and the ground state rotational band is expected. This allows magnetic dipole (M1) admixtures in transitions from the (beta)- and (gamma)- vibrational bands and magnetic quadrupole (M2) admixtures in transitions from the octupole bands which would otherwise be pure electric quadrupole (E2) and electric dipole (E1), respectively, according to the predictions of the collective model. The most sensitive method for investigation is the study of directional correlation of gamma rays. The techniques of gamma-gamma directional correlation and some others including measurement of internal conversion coefficients have been applied previously to the study of low lying levels in Gd('154) and as a result of these investigations many features of these levels have been established. Gamma-Gamma directional correlation measurements have been made for the 723-873, 873-123, 723-(873)-123, and the 1274-123 keV cascades. Information concerning spins of the nuclear states and the M1 and M2 transition strengths is obtained. Information concerning the attenuation of the correlation in cascades involving the 123 keV level as intermediate state, due to the relatively long mean life (1.2 ns) of this state, is also obtained. The experimental arrangement includes the use of a PDP-8 digital computer for data collection and control of the experiment. Two solid state Ge(Li) detectors and a NaI(Tl) scintillation detector are used in conjunction with standard fast-slow coincidence circuits and a 4096 channels dual Analog-to-Digital Convertor (ADC). The results obtained are interpreted in the light of previous results by other investigators and in terms of appropriate nuclear models. The 723-873 keV cascade has measured values of A(,2) = -0.102 (+OR-) 0.005, A(,4) = 0.005 (+OR-) 0.006, which yield (delta)(723) = 0 and. (DIAGRAM, TABLE OR GRAPHIC OMITTED...PLEASE SEE DAI). or -0.02 (LESSTHEQ) (delta)(723) (LESSTHEQ) +0.02 and -12.8 (LESSTHEQ) (delta)(873) (LESSTHEQ) -9.5. The 723-(873)-123 keV cascade was measured to have A(,2) = -0.049 (+OR-) 0.004, A(,4) = 0.004 (+OR-) 0.005, which are consistent with (delta)(723) = 0 and 4.4 (LESSTHEQ) (VBAR)(delta)(873)(VBAR) (LESSTHEQ) 16 or -0.01 (LESSTHEQ) (delta)(723) (LESSTHEQ) +0.04 and 4.4 (LESSTHEQ) (VBAR)(delta)(873)(VBAR) (LESSTHEQ) 70. The measured values for the 873-123 keV cascade are A(,2) = -0.002 (+OR-) 0.004, A(,4) = 0.323 (+OR-) 0.001, which give -10.2 (+OR-) 0.5 for the mixing ratio in the 873 keV transition. The correlation measurements of the 1274-123 keV and the 873-123 keV cascades yield the G(,2) and G(,4) attenuation factors for correlations involving the metastable (123 keV) state as the intermediate state to be 0.80 (+OR-) 0.02 and 0.76 (+OR-) 0.02, respectively. The results of all these experiments are consistent with a spin of 2 for the 1719, 1397, 996 and 123 keV nuclear levels. The present measurements on Gd('154), in agreement with prior investigations, confirm the general conclusion that the M1 admixtures in the 2('+) (--->) 2('+) transitions in Gd('154) are consistently small. The well known discrepancies in the relative B(E2) values when compared with the rotational model are not improved by the small M1 admixtures that are observed. M1 admixtures as large as 50% are needed in order to resolve the discrepancies. Large M1 admixtures (about 80%) are observed for nuclei with mass numbers in the transition region 180 < A < 200. The transition in this region, from the strongly deformed nuclei to spherical nuclei, is a gradual one and thus a gradual variation of the properties associated with the competing modes of nuclear structure is observed in this region. There is, however, a sharp onset of deformation near A = 150. Thus, it appears that in the transitional nuclei near A = 150 (e.g., Gd('154), Sm('152)) there is at present a breakdown in the theoretical understanding of these states. One approach to the explanation of the anomalous behaviour has been the application of the pairing plus quadrupole mode (PPQ). Kumar has applied this microscopic approach to explain the branching ratios and other properties in Sm('152) and Gd('154) with considerable success. In the case of Gd('154), although the calculated band energies are too high by a factor 1.4-1.7, there is generally reasonable agreement with the experimental energies, B(E2) values, magnetic moments, E2/M1 mixing ratios, and E0 moments.