No abstract

Resonant scattering of y rays by the 6.91-and 6.95-MeV excited states of Ca 40 has been observed with a 30-cm 8 Ge(Li) detector. The reaction ¥ n (p,a)O u served as the source of the resonant y rays. The angular distributions of the 6.91-and 6.95-MeV radiations were found to be compatible only with the assignment of spin 2 to the 6.91-MeV level, and of spin 1 to the 6.95-MeV level. No evidence for cascade de-excitation of the two levels was found. If TQ/T is assumed to be unity for both levels, the observed scattering cross sections correspond to ground-state transition widths r 0 (6.91) =0.18:4=0.03 eV and r 0 (6.95) =0.47±0.06 eV.

A gamma-ray polarimeter utilizing Compton scattering as the analyzing process is described. Measurements of the polarization-direction correlations of the gamma-rays emitted in the disintegration of Sc 46 , Co 60 , Rh 106 , and Cs 134 are reported. * This work was supported in part by the joint program of the ONR and the AEC and by grants to one of us (F.M.) from the Swiss "Arbeitsgemeinschaft fiir Stipendien der Physik und Mathematik." Preliminary results were reported in Phys. Rev. 74, 1542Rev. 74, (1948 and Phys.The angular correlation of successive gamma-rays emitted by six even-even nuclei has been investigated and found to be anisotropic in every case, and of the magnitude expected theoretically. Effects of external magnetic fields and of chemical binding on the correlations are found to be smaller than the experimental uncertainty for some exploratory experiments. Interpretation of the results in terms of the nuclear states involved is in general possible by use of additional evidence such as relative transition probabilities.

Strong resonant scattering of bremsstrahlung by a level in ' Sm at 3.225-MeV excitation energy has been observed. Angular distribution and linear polarization measurements have led to a 1 assignment for this level.From a self-absorption study, a value I 0 = 220 + 30 meV has been obtained for the partial radiative width of the ground-state transition. For the known 1 states in "Sm (1.166 MeV) and '"Sm (0.921 MeV), partial widths I 0(1.166) = (5.4~0.4) meV, and I'0(0.921) = (7.4 + 1.0) meV were measured. Combining the results of bremsstrahlung measurements with those of previous resonance fluorescence experiments utilizing high-speed rotation of radioisotopes, a width I 0 = (3.1~0.4) meV was arrived at for the ground-state transition from the 1.465-MeV 1 state in '"Sm. The excitation energies of the 1 states studied in the even Sm isotopes below N = 89 are fairly close to the sums I'E(2+, ) + E(3, )]. The B(F.1)'s initially decrease as one moves away from the deformed region. %Vhile this is not unexpected, the reversal of this trend near and at the magic neutron number X = 82, as evidenced by the large B(E1) observed in "Sm and the behavior of the B(E1)*s in "" 'Nd, is somewhat surprising. NUCLEAR REACTIONS ' +'~' 5 Sm(y, y); bremsstrahlung E=1.2-3.8 MeV; measured 0'(96 ) and o'(126'), LP; deduced J, 7(, I . Enriched~4 Sm and i5 Sm targets.

The resonant scattering of electron bremsstrahlung by an enriched sample of ' Ba has been studied for photon energies of up to 5 MeV. It provided estimates of the: radiative widths for 10 levels. Based on the relative yields at scattering angles of 96' and 126, unambiguous spin assignments were made to 5 of these levels. Where feasible, the yield measurements were supplemented by self-absorption data and by linear polarization studies. For the strongest excitation in "Ba, at 3.436 MeV, the resonance fluorescence experiments led to a 1 assignment and a value I 0 = 88+22 meV for the partial width of the ground state transition. The corresponding E1 strength is approximately 1/3 of the El strengths observed for the strongest low-lying E1 transitions in the even-even N = 82 nuclei. When combined with previous observations for N )82, the result obtained for ' Ba (N = 80) indicates that the strengths of the ground state transitions from the lowest 1 states peak at à = 82. Yield information on a few levels in "Ba and " Ba was also obtained. NUCLEAR REACTIONS '3~'3 Ba(p, p) bremsstrahlung 1.68 MeV (E (5.0 MeV; measured 0 (96') and 0 (126'), self-absorption, LP; deduced g I'0 /l, J, x.Enriched Ba target, natural target.