Neutron resonance spectroscopy:La139

Abstract

Results of high resolution neutron time of flight spectroscopy measurements on natural lanthanum (99.9% ' La) are presented. Plots of o, vs E are given to 76 keV. Values of Ep and gl "arepresented for all 150 ' La levels observed to 26 keV. About 50 of these levels are probably p levels, based on Bayes' theorem and the minimum gl " for detection vs E. Intermediate structure seems to be present in Sp = -Qgl e/n8'. From 6 to 18 keV, 10~Se= 0.50. Two strong levels at 3 and 3.5 keV, and several strong levels in the 18.5 to 26 keV region, boost Sp to a value 10 Sp= 0.76 for the full 26 keV range. A standard analysis that ignores intermediate structure and uses a single channel Porter-Thomas fit to the entire gI""population yields 10 Sp = 0.76 + 0.13, Dp = 208 + 10 eV, and 10 S 1 = (0.6+p, 2 ). NUCLEAR REACTIONS La(n, total), E =3 eV -76 keV; measured o&(E) to 76 keV; deduced resonance Ep g I' Dp Sp S~.

TheLa139(n,γ) cross section: Key for the onset of theTerlizzi1, Abbondanno2, Aerts3 et al. 2007Phys. Rev. C

“…The spin assignment of each level is carefully checked by comparing the fits obtained with different assignments indicated in the ENDF/B-VI.8, JEFF-3.1, and JENDL-3.3 libraries [30]. For many levels, especially when n γ , the best fit is found by adopting the transmission value as the neutron width, whereas in the few cases with γ n , the γ width is fixed to the average value (Table V) the results of the transmission measurement [8]. The remaining differences might be attributed to the fact that several p-wave levels (in this work, but especially in Hacken et al [8]) are missing and that the neutron width does not always dominate the total width of the resonance but fluctuates following a Porter-Thomas distribution.…”

“…For many levels, especially when n γ , the best fit is found by adopting the transmission value as the neutron width, whereas in the few cases with γ n , the γ width is fixed to the average value (Table V) the results of the transmission measurement [8]. The remaining differences might be attributed to the fact that several p-wave levels (in this work, but especially in Hacken et al [8]) are missing and that the neutron width does not always dominate the total width of the resonance but fluctuates following a Porter-Thomas distribution. In some cases, the capture channel may therefore provide a sizable contribution to the total cross section, while in other cases, this contribution is negligible.…”

“…The comparison with the evaluated cross sections from the JENDL, ENDF, and JEF nuclear data libraries [30] shows that the n TOF results agree best with JENDL in terms of orbital angular momentum assignments and of general resonance properties. The better agreement could be due to the fact that the JENDL evaluation takes all previous measurements [6][7][8] into account. Nevertheless, the capture cross sections measured at n TOF are lower than the JENDL data, by about 10% in the first part of the neutron energy spectrum (<1 keV) and by 3% on average between 5 and 9 keV.…”

“…Time-of-flight (TOF) measurements of the 139 La(n, γ ) cross section have been performed at Oak Ridge [6] and JAERI [7], and a transmission measurement was reported from the Columbia University synchrotron [8]. Recently, several activation measurements [9][10][11] have provided evidence for substantial systematic uncertainties in the previous TOF data.…”

Neutron resonance parameters of bromine-79 and bromine-81.

“…Intermediate structures similar to the case of 81Br seem to exist in the neutron closed shell nuclei, 87Rb (17), 1ssLa (18), 141Pr(19) and Pb i~o t o p i s (~~) ( '~) .…”