The single-closed-shell nucleides ¹⁴⁴₆₂Sm₈₂ and ¹⁴⁶₆₄Gd₈₂

Abstract

Alpha-particle irradiation of enriched ¹⁴²Nd and ¹⁴⁴Sm targets has been used to populate states in the N = 82 nucleides ¹⁴⁴₆₂Sm and ¹⁴⁶₆₄Gd. Singles γ-ray spectra, angular distributions, γ-γ coincidence spectra as well as time distributions with respect to the α-beam bursts, have been measured at various bombarding energies. Properties of the levels in the two nucleides are compared. The existence of excited states with the spin sequence 6⁺ → 4⁺ → 2⁺ → 0⁺ is established in both of them, similarly to all the other known N = 82, even-Z isotones. The energy spacing of these states in ¹⁴⁶Gd differs from those found in the lighter isotones, which may be attributed to the subshell effect at Z = 64 (filling of both the $\text{g}{}_{\mathrm{<mtext>7</mtext><mtext>2</mtext>}}\text{and}\text{d}{}_{\mathrm{<mtext>3</mtext><mtext>2</mtext>}}$ orbitals). The 6⁺ → 4⁺ E2 transitions are found to be retarded. The systematic occurrence of the 6 ⁺ isomeric states in all the N = 82 even-Z isotones is discussed. The lowest excited state with negative parity (3⁻, 1810 keV) in ¹⁴⁴Sm is found to be very strongly populated in the γ-ray cascade following the (α, 2n) reaction. The evidence for the existence of a rather weakly populated low-lying 3 ⁻ state in ¹⁴⁶Gd (1584 keV) is discussed. A number of higher excited states with excitation energies up to about 5.3 MeV and with spin values up to 10 or 11 are established for both nucleides. In a qualitative discussion, the majority of these states can be explained as primarily two-quasiparticle configurations. Strong population of the highest excited states in both nucleides (at about 5.3 MeV) is noted. This fact is difficult to explain on the basis of simple statistical considerations of the (α, xn) reaction mechanism.