Study of low-lying excited states in $^{139}$La via the $\left(n,n^{\prime}\gamma\right)$ reaction*

An observable result of Pauli-blocking in atomic nuclei [1] is a reduction in the transition rates between equivalent phonon excitations in odd-A nuclei and even-even nuclei. In particular, studies of low-lying $J^{\pi}=1^-$ states in $^{141}$Pr [2,3], imply that the $B(E1: 1^- \rightarrow 0_1^+)$ values associated with decays from states with $[[2^+ \otimes 3^-]\otimes particle]_{J^{\pi}=1^-}$ configurations are $\sim 52-83\%$ of the $B(E1: 1^- \rightarrow 0_1^+)$ value associated with decay from the $[2^+ \otimes 3^-]_{J^{\pi}=1^-}$ state in $^{140}$Ce. This presentation will focus on lifetimes deduced from an angular-distribution measurement of $^{139}$La, via the $\left(n,n^{\prime}\gamma\right)$ reaction with E$_n = 2.0$ MeV, and a comparative interpretation of Pauli-blocking in $^{139}$La and $^{141}$Pr will be drawn.\\ $[1]$ V.G.~Soloviev, Theory~of~Atomic~Nuclei:~Quasiparticles~and~Phonons, IOP~Publishing~(Bristol,~United Kingdom), (1992)\\ $[2]$ M.~Scheck~et~al., Phys.~Rev.~C~\textbf{75}, 044313 (2007)\\ $[3]$ M.~Scheck~et~al., submitted to Phys.~Rev.~C