Multipolarity of the2−→1−, ground-state transition inBi210via multivariable angular correlation analysis

Abstract

The multipolarity of the main transition leading to the ground state in 210 Bi was investigated using the angular correlations of γ rays. The analyzed γ -coincidence data were obtained from the 209 Bi(n,γ ) 210 Bi experiment performed at Institut Laue-Langevin Grenoble at the PF1B cold-neutron facility. The EXILL (EXOGAM at the ILL) multidetector array, consisting of 16 high-purity germanium detectors, was used to detect γ transitions. The mixing ratio of the 320-keV γ ray was defined by minimizing a multivariable χ 2 function constructed from the coefficients of angular correlation functions for seven pairs of strong transitions in 210 Bi. As a result, the almost pure M1 multipolarity of the 320-keV γ ray was obtained, with an E2 admixture of less than 0.6% only (95% confidence limit). Based on this multipolarity the neutron-capture cross section leading to the ground state in 210 Bi, that decays in turn to radiotoxic 210 Po, was determined to be within the limits 21.3(9) and 21.5(9) mb. This result is important for nuclear reactor applications.

Reduction in the uncertainty of the neutron-capture cross section of 210Bi: Impact of a precise multipolarity measurement of the 2− → 1− main ground-state transition

“…M1/E2 multipolarity was adopted for the 2 − → 1 − 320-keV line defined by an analysis based on minimization of a multivariable χ 2 cost function [9] and the intensities reported in [12]. The δ 320 = 0.076, corresponding to an upper limit of 0.6% admixture of E2, defines the lower limit of σ gs , i.e., 21.3 mb.…”

“…The δ 320 = −3.04 value would imply a significant (90%) admixture of E2 multipolarity, which, in consequence, would result in a lower value of the neutron-capture cross section to the ground state in 210 Bi (comparing to this value calculated assuming pure M1 multipolarity of the 320-keV γ ray). However, as discussed in [9] with respect to the measurement of the half-life of the 320-keV state (T 1/2 = 7.5(14) ps [11]), one can conclude that the δ 320 = −3.04 solution is highly unlikely (the typical T 1/2 values for E2 in the neighboring nuclei would be much longer). Therefore, for the recalculation of the neutron-capture cross section to the ground state in 210 Bi, we adopted only the value δ 320 = 0.05(2), which confirms almost pure M1 multipolarity of the 320-keV transition.…”

“…(4) was solved by minimizing the cost function χ 2 , using the Downhill Simplex algorithm (also known as Nelder-Mead method) [10]. The minimization algorithm determined 3 lowest minima, which gives three sets of mixing ratios for investigated transitions, with very similar values of χ 2 [9]. For those three minima, only two different values of δ 320 : 0.05(2) and −3.04(13) were found.…”

“…However, to find the δ k values in this case one must take into account other pairs of transitions being in coincidence with the 674-320-keV cascade, and use the multivariable minimization method outlined in the next section. A detailed description of this procedure can be found in [9].…”

The γ-ray spectroscopy studies of low-spin structures in 210Bi and 206Tl using cold neutron capture reactions

“…In this paper, we report preliminary results of the γ-coincidence measurements of 210 Bi and 206 Tl, performed at the Institut Laue-Langevin (Grenoble, France) by employing the thermal neutron capture reaction and the two multidetector germanium arrays: a) EXILL in the 210 Bi case [9,10] and, b) the new FIPPS array for 206 Tl.…”