Near-barrier photofission in ${}^{232}\mathrm{Th}$ and ${}^{238}\mathrm{U}$

A study of photofission of ${}^{232}\mathrm{Th}$ and ${}^{238}\mathrm{U}$ was performed by using quasimonoenergetic, linearly polarized $\gamma$-ray beams from the High Intensity $\gamma$-ray Source at Triangle Universities Nuclear Laboratory. The prompt-photofission neutron polarization asymmetries, neutron multiplicities, and the photofission cross sections were measured in the near-barrier energy range of 4.3 to 6.0 MeV. This data set constitutes the lowest energy measurements of those observables to date using quasimonoenergetic photons. Large polarization asymmetries are observed in both nuclei, consistent with the $E1$ excitation as observed by another measurement of this kind made in a higher-energy range. Previous experimental evidence of a deep third minimum in the ${}^{238}\mathrm{U}$ fission barrier has been identified as an accelerator-induced background.

Figure 1

A triple-humped interpretation of the ${}^{238}\mathrm{U}$ fission barrier based on barrier parameters from Ref. [4]. The barrier parameters were determined by reproducing the experimentally measured photofission reaction cross section with statistical model calculations.

Figure 2

Illustration of the experimental geometry. The target was mounted in a vacuum pipe inserted through the center of the INVS neutron detector. Borated polyethylene neutron shielding surrounded the detector. An upstream scintillating paddle monitored the $\gamma$-ray beam flux, while a downstream HPGe could be moved into the beam to measure the energy spectrum.

Figure 3

Diagram of the INVS detector used in this work (not to scale).

Figure 4

PSD plot from a PC tube for a $\gamma$-ray beam incident on a ${}^{232}\mathrm{Th}$ target, with a neutron cut region.

Figure 5

Comparison of experimental data and a Monte Carlo simulation of neutron time distributions in the INVS detector for the ${}^{238}\mathrm{U}(\gamma ,f)$ reaction at $E_{\gamma }=5.1$ MeV.

Figure 6

Neutron multiplicity rates, normalized to 1 Hz, from backgrounds caused by cosmic-ray-induced neutrons, bremsstrahlung contamination of the $\gamma$-ray beam inducing ${}^{\text{nat}}\mathrm{Pb}(\gamma ,xn)$ reaction neutrons, and Compton-scattered $\gamma$ rays inducing $\text{D}(\gamma ,n)$ reaction neutrons in the polyethylene moderator of the INVS detector.

Figure 7

$\mathrm{HI}\gamma \mathrm{S}$ beam-induced background neutron count rates observed with a ${}^{\text{nat}}\mathrm{Pb}$ target, normalized to the flux of the primary $\gamma$-ray beam.

Figure 8

Detected neutron yields and asymmetry in the inner and outer rings of PC tubes in the INVS detector for $E_{\gamma }=5.6$ MeV on a ${}^{238}\mathrm{U}$ target. The outer ring counts have been multiplied by a factor of 1.3 to appear on the same scale as the inner ring. Statistical error bars are smaller than the markers.

Figure 9

Simulated correlation between the detected asymmetry $b_d$ and emitted neutron polarization asymmetry $b_n$ for the ${}^{238}\mathrm{U}$ and ${}^{232}\mathrm{Th}$ photofission neutrons.

Figure 10

Photofission neutron polarization asymmetries for (a) ${}^{232}\mathrm{Th}$ and (b) ${}^{238}\mathrm{U}$. The error bars reflect the fit error from Eq. (3).

Figure 11

Photofission neutron polarization asymmetries in (a) ${}^{232}\mathrm{Th}$ and (b) ${}^{238}\mathrm{U}$ for neutrons with $E_n>1.5$ MeV, compared with the data of Mueller et al. [19].

Figure 12

Measured mean photofission neutron multiplicities for (a) ${}^{232}\mathrm{Th}$ and (b) ${}^{238}\mathrm{U}$ compared with the data of Caldwell et al. [39], Findlay et al. [40], the ENDF/B-VII.1 evaluation [41], and the empirical model of Lengyel et al. [42].

Figure 13

Measured spreads of the prompt-neutron multiplicity distributions for (a) ${}^{232}\mathrm{Th}$ and (b) ${}^{238}\mathrm{U}$, compared with the data of Caldwell et al. [39].

Figure 14

The measured photofission cross section of ${}^{232}\mathrm{Th}$ (red circles) compared with data obtained by using bremsstrahlung $\gamma$-ray beams (various gray markers) [15, 40, 43, 44, 45], data obtained by using quasimonoenergetic $\gamma$-ray beams (green triangles, blue squares) [16, 46], and the ENDF/B-VIII.0 evaluation (dashed line) [47]. Vertical error bars represent statistical uncertainty and horizontal bars represent the energy resolution of the $\gamma$-ray beams.

Figure 15

Measured photofission cross section of ${}^{238}\mathrm{U}$ (red circles) compared with data obtained by using bremsstrahlung $\gamma$-ray beams (various gray markers) [43, 45, 54, 55, 56], data obtained by using quasimonoenergetic $\gamma$-ray beams (purple crosses, blue stars, orange triangles, blue crosses, pink diamonds, green triangles, blue squares) [4, 46, 49, 51, 52, 53, 57], and the ENDF/B-VIII.0 evaluation (dashed line) [47]. Vertical error bars represent statistical uncertainty and horizontal bars represent the energy resolution of the $\gamma$-ray beams.

Figure 16

${}^{238}\mathrm{U}$ photofission-cross-section results with and without the $\mathrm{HI}\gamma \mathrm{S} \gamma$-ray beam bremsstrahlung background subtracted, compared with the data of Csige et al. [4] and the ENDF/B-VIII.0 evaluation [47]. Photofission-cross-section calculations from Csige et al. for double- and triple-humped fission barrier fits to their data are also shown.