Study of the distribution of partial radiative widths for the Ta(n,)Ta reaction

河村 しほり*; 遠藤 駿典   ; 岩本 修  ; 岩本 信之  ; 木村 敦   ; 北口 雅暁*; 中村 詔司  ; 奥平 琢也*; Rovira Leveroni, G.  ; 清水 裕彦*

Kawamura, Shiori*; Endo, Shunsuke; Iwamoto, Osamu; Iwamoto, Nobuyuki; Kimura, Atsushi; Kitaguchi, Masaaki*; Nakamura, Shoji; Okudaira, Takuya*; Rovira Leveroni, G.; Shimizu, Hirohiko*

Random Matrix Theory (RMT) has been proposed to deal with the statistics of eigenvalues and eigenfunctions of complex many-body quantum systems. For the Gaussian orthogonal ensemble (GOE) of matrices, RMT is expected to correctly describe fluctuation properties of nuclear levels at relatively high excitations, such as near the neutron threshold. The reduced partial width amplitudes, which determine the strength of the coupling between a resonance and a particular channel, have a Gaussian probability distribution centered at zero, while the reduced partial widths (the squares of the partial width amplitudes) have a chi-square distribution with one degree of freedom: the Porter-Thomas distribution. The fluctuation properties of partial radiative widths have been measured and studied for many nuclides. For Ta, the results reported by Wasson et al. and Stelts et al. are inconsistent, and its fluctuation properties of widths are not precisely understood. One possible reason for this disagreement may arise from imprecise determination of the spins of the resonances. We therefore planned to determine the spins of the resonances from measurements and examine the statistical fluctuations of the partial radiative widths based on the results of spins. In this study, as a first step, we measured the gamma-rays emitted by neutron capture reactions of Ta at J-PARC MLF ANNRI (BL04) and confirmed the fluctuations of the radiative widths. Using the spins of the resonances listed in the nuclear data library, JENDL-5, the present result was consistent with that of Stelts et al. which deviated from the Porter-Thomas distribution.