21-29 July 2022
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Photoneutron cross section measurements on $^{208}$Pb

27 Jul 2022, 08:21
Folsom ()



Adriana Ioana Gheorghe


Photoneutron reactions on $^{208}$Pb in the Giant Dipole Resonance (GDR) energy range have been investigated at the $\gamma$-ray beam line of the NewSUBARU synchrotron radiation facility in Japan. The double magic $^{208}$Pb is a benchmark case for theoretical modeling of electric dipole response in nuclei. It has been extensively investigated through photonuclear reactions as well as through inelastic hadron scattering experiments. However, there are systematic discrepancies between the total and partial ($\gamma$,n) and ($\gamma$,2n) photoneutron cross sections obtained at the Saclay and Livermore positron in flight annihilation facilities: the Saclay results [1] for the ($\gamma$,abs) and ($\gamma$,n) cross sections are systematically higher than the Livermore ones [2], while Livermore ($\gamma$,2n) cross sections overestimate the Saclay ones. We note that the evaluations adopted in the 1999 and 2019 IAEA Photonuclear Data Libraries follow the recommendation of Berman et al. [3] of lowering the Saclay data by 7% and increasing the Livermore results by 22%, although a single renormalization factor fails to simultaneously solve the inconsistencies for both the ($\gamma$,n) and ($\gamma$,2n) channels.

Making use of quasi-monochromatic laser-Compton scattering (LCS) $\gamma$-ray beams and of a novel flat-efficiency neutron detection system along with the associated direct neutron-multiplicity sorting method [4], total and partial ($\gamma$,xn) photoneutron cross sections with x = 1 to 4 have been measured for $^{208}$Pb in a broad energy range covering the neutron threshold up to 38 MeV. In agreement with preceding data, resonant structures have been observed in the low-energy part of the GDR. Taking advantage of the flat-efficiency in the neutron multiplicity sorting, we extracted also the average kinetic energies of neutrons emitted in the ($\gamma$,xn) reactions. Similar to the behavior observed in our previous $^{209}$Bi study [5], a non-statistical emission of high energy neutrons was registered starting in the vicinity of the neutron emission threshold up to ~11 MeV excitation energy.

[1] A. Veyssiere et al., Nuclear Physics A159, 561-576 (1970).
[2] R.R. Harvey et al., Phys. Rev. 136, 126 (1964).
[3] B.L. Berman et al., Phys. Rev. C 36, 1286 (1987).
[4] H. Utsunomiya et al., Nuclear Inst. and Methods in Physics Research, A 871 (2017) 135.
[5] I. Gheorghe et al., Phys. Rev. C 96, 044604 (2017), Erratum Phys. Rev. C 99, 059901(E) (2019).

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