Speaker
Description
Energy spectra for prompt neutrons emitted from actinide targets irradiated with $\gamma$-ray beams have been measured near the ($\gamma$, n) reaction threshold by Mueller et al. [1]. Examples of measurements of the total cross sections for prompt neutron emission from photofission and neutron multiplicities as a function of the incident $\gamma$-ray beam energy are reported by [2, 3]. The most recent neutron energy and angle double differential yields for prompt neutron emission from photofission were reported by Clark et al. [4]. Their measurements were performed using $\gamma$-rays produced in the beam stop of a neutron spallation source and therefore had a broad energy spread. We are performing the first double differential yield measurements of prompt neutrons emitted from photofission induced with a monoenergetic $\gamma$-ray beam. This work is made possible by the beam features at HIGS, e.g., monoenergetic, pulsed and circularly polarized.
In this study, we aim to expand measurements of prompt photoneutron spectra to include data taken on the low and high energy sides of the Giant Dipole Region (GDR). The experimental technique is based on tagging fission events using fission chambers as active targets. Neutrons are detected with wide angle coverage in our new "Soccer Ball" array which consists of at most thirty 12.70 cm (diameter) x 5.08 cm (thick) liquid scintillators distributed around the target. The pulsed $\gamma$-ray beam at HIGS enable the neutron energy to be determined with time-of-flight measurements using the accelerator RF signal. To determine the differential neutron multiplicity, timing coincidences between the fission chambers and neutron detectors are utilized. The experimental techniques and preliminary results will be presented.
This research is supported in part by [U. S. Department of Energy Grant numbers DE-NA0003887 and DE-NA0004069, Lawrence Livermore National Security, LLC under contract number DE-AC52-07NA27344, and Los Alamos National Nuclear Security, LLC under contract 89233218CNA000001].
[1] J. M. Mueller et al. Measurement of prompt neutron polarization asymmetries in photofission of $^{235,238U, 239}$Pu, and $^{232}$Th. Phys. Rev. C 85, 014605 (2012).
[2] S. Nair et al. Fission-neutron and fragment angular distributions from threshold photofission of $^{232}$Th and $^{238}$U. J. Phys. G: Nucl. Phys. 3, 965 (1977).
[3] A. Lengyel et al. Energy dependent prompt neutron multiplicity parameterization for actinide photofission. arXiv:1801.01107 [nucl-th] (2018).
[4] S. D. Clarke et al. Measurement of the energy and multiplicity distributions of neutrons from the photofission of $^{235}$U. Phys. Rev. C 95, 064612 (2017).
[5] J. T. Caldwell et al. Experimental Determination of Photofission Neutron Multiplicities for $^{235}$U, $^{236}$U, $^{238}$U, and $^{232}$Th Using Monoenergetic Photons. Nucl. Sci. Eng. 73, 153 (1980).
