Speaker
Description
The reason for studying the neutron capture reaction on the mono-isotopic element thulium is twofold. Its only stable isotope, $^{169}$Tm, is often used as a neutron-flux activation monitor. The neutron capture cross-section in the relevant energy range has been measured several times [1-4] in the past and more recently at CSNS [5]. While these data show rough agreement, there are significant differences. Moreover, the uncertainties are often not quoted. These two motivate a state-of-the-art measurement and analysis of the neutron capture cross-section in the keV energy range.
The neutron capture cross-section can also be calculated via the Hauser-Feshbach approach [6], for which the key ingredients are the photon-strength-functions (PSFs) and nuclear level density (NLD). These quantities can be inferred from the $\gamma$-ray spectra of s-wave resonances by comparing them to the simulated spectra.
The neutron-capture reactions on the $^{169}$Tm nuclei have been measured with the DANCE calorimeter [7,8] at LANSCE [9]. The background-corrected sum-energy and multi-step-cascade spectra were extracted for a number of strong isolated s-wave resonances. These experimental coincident $\gamma$-ray spectra are compared with their simulated counterparts using Monte-Carlo code DICEBOX [10] to obtain information about PSFs and NLD. In particular, we investigate the scissors-mode (SM) role in the M1 PSF. Previously, SM parameters of well-deformed rare-earth nuclei were obtained by several experimental techniques, see e.g. Refs. [11-13] and review [14]. They show significant differences, especially in the strength of the mode. The shape of the low-energy tail of the giant electric-dipole resonance is uncertain too. Because of these inconsistencies, additional information on PSFs in this region is of great interest.
The neutron capture cross-section is deduced from the experimental data in the usual fashion, i.e. by subtracting backgrounds, determining the neutron flux using several flux monitors, and normalizing to the standard cross-section. The analysis steps, internal consistency of our data, preliminary results on PSFs, and neutron capture cross-sections will be presented.
[1][ R. C. Block et al., Conference proceedings, “Time of Flight Methods,” Saclay, 1961.][1]
[2] J. Gibbons et al., Phys. Rev. 122, 182 (1961).
[3] R. L. Macklin et al., Nucl. Sci. Eng. 82, 143 (1982).
[4][ Y.-J. Xia et al., Chinese Nucl. Phys. 11, 75 (1989).][4]
[5] J. Ren et al., Chin. Phys. C 46, 044002 (2022).
[6] W. Hauser and H. Feshbach, Phys. Rev. 87, 366 (1952).
[7] M. Heil et al., Nucl. Instrum. Methods Phys. Res., Sect. A 459, 229 (2001).
[8] R. Reifarth et al., Nucl. Instrum. Methods Phys. Res., Sect. A 531, 530 (2004).
[9] M. Mocko and G. Muhrer, Nucl. Instrum. Methods Phys. Res., Sect. A 704, 27 (2013).
[10] F. Bečvář, Nucl. Instr. Methods A 417, 434 (1998).
[11] U. Kneissl, H. H. Pitz, and A. Zilges, Prog. Part. Nucl. Phys. 37, 349 (1996).
[12] I. Knapová et al., Phys. Rev. C 107, 044313 (2023).
[13] E. Melby et al., Phys. Rev. C 63, 044309 (2001).
[14] S. Goriely et al., Eur. Phys. J. A 55, 172 (2019).