Giant dipole resonance photofission and photoneutron reactions in ${}^{238}\mathrm{U}$ and ${}^{232}\mathrm{Th}$

New measurements of photofission and photoneutron reactions on ${}^{238}\mathrm{U}$ and ${}^{232}\mathrm{Th}$ in the giant dipole resonance (GDR) energy region were performed at the laser Compton-scattering $\gamma$-ray source of the NewSUBARU synchrotron radiation facility using a high-and-flat efficiency moderated ${}^3\mathrm{He}$ detection array. The neutron-multiplicity sorting of high-multiplicity fission neutron coincidence events was performed using a dedicated energy dependent, multiple firing statistical treatment. The photoneutron reactions $(\gamma ,in)$ with $i=1–3$ and photofission reaction $(\gamma ,F)$ were discriminated by considering a Gaussian distribution of prompt-fission neutron (PFN) multiplicities predicted by the theory of evaporation in sequential neutron emission from excited fission fragments. We report experimental $(\gamma ,n)$, $(\gamma ,2n)$, $(\gamma ,3n)$, and $(\gamma ,F)$ cross sections, average energies of PFNs and of $(\gamma ,in)$ photoneutrons, as well as the mean number of PFNs per fission and the width of the PFN multiplicity distribution. Based on these primary experimental results and combined with reasonable assumptions, we extract also the first- and second-chance fission contributions. The new experimental results are compared with statistical-model calculations performed with the empire 3.2 Malta and talys 1.964 codes on the present data and with prompt fission emission calculations obtained with the Los Alamos model in the frame of the most probable fragmentation approach with and without sequential emission.

Figure 1

(a) LCS $\gamma$-ray beam line BL01 and the GACKO experimental hutch at the NewSUBARU synchrotron radiation facility. (b) Diagram showing the ${}^3\mathrm{He}$ counters layout in the flat efficiency neutron detector (FED).

Figure 2

The spectral distributions of the 40 incident LCS $\gamma$-ray beams used in the present experiment, as obtained by Monte Carlo simulations with the eliLaBr code [32, 33, 34]. The energy spread in full width at half maximum varies between 120 and 750 keV for the 5.87 and the 20.14 MeV LCS $\gamma$-ray beams, respectively. The blue band shows the energy spectra uncertainty. Each curve is offset along the vertical axis by 1 arbitrary unit for clarity. The inset shows the magnified spectrum for the 14.08 MeV maximum energy beam. The 410 keV FWHM is shown by the red arrow.

Figure 3

(a) Neutron detection efficiency obtained by mcnp simulations for Maxwell (black) and neutron evaporation spectra (blue) compared with the experimental ${}^{252}\mathrm{Cf}$ calibration result. (b) The average neutron energy as function of the inner-ring/outer-rings detection efficiency ratio.

Figure 4

Arrival-time distributions of neutrons recorded by the FED in the photoneutron and photofission reactions on ${}^{238}\mathrm{U}$ at 18.67 MeV. The experimental neutron counts recorded during beam on (black) and beam off (blue) are displayed for (a) onefold, (b) twofold, (c) threefold, (d) fourfold, (e) fivefold, (f) sixfold, and (g) sevenfold neutron events. The red lines show least-squares fits to the experimental distributions obtained using a sum of two exponentials and a constant background component.

Figure 5

Same as Fig. 4, where the experimental neutron counts recorded by the inner ring counters (left column) and by the counters in the two outer rings (right column) are displayed for (a,b) onefold, (c,d) twofold, (e,f) threefold, and (g,h) four- to sevenfold neutron events.

Figure 6

Experimental ($N_i$, empty black dots) and best fit ($N_i^{\text{MF}}$, blue crosses) $i$-fold cross sections, defined in Eq. (1) as the number of $i$-fold neutron events recorded per incident photon and target nucleus, for the photoneutron and photofission reactions on ${}^{238}\mathrm{U}$ (a)–(g) and ${}^{232}\mathrm{Th}$ (h)–(n). The empty red dots show the total subtracted background contribution $N_1$ from the aluminum container and from the oxygen nuclei in the ${\mathrm{U}}_3{\mathrm{O}}_8$ and the ${\mathrm{ThO}}_2$ molecules. The error bars are statistical only.

Figure 7

Average energies in monochromatic approximation of photoneutron and PFNs for (a)–(d) ${}^{238}\mathrm{U}$ and (e)–(h) ${}^{232}\mathrm{Th}$: average energies of neutrons recorded in $i$-fold coincidence events (experimental: empty black dots; best fit: blue crosses), and multiple firing neutron multiplicity sorting results for the average energies $E_{\gamma ,xn}$ of $(\gamma ,xn)$ photoneutrons and $E_{\gamma ,F}$ of PFNs (full black dots). The error bars for the average energies of $i$-fold coincidence events represent the statistical component and a $3\%$ systematic component accounting for the uncertainty in the neutron detection efficiency calibration.

Figure 8

Present (left) ${}^{238}\mathrm{U}$ and (right) ${}^{232}\mathrm{Th}$ results before (open symbols) and after energy unfolding (full symbols): (a) $(\gamma ,n)$, (b) $(\gamma ,2n)$ and $(\gamma ,3n)$, and (c) $(\gamma ,F)$ cross sections, (d) mean PFN numbers per fission, (e) $(\gamma ,n)$, $(\gamma ,2n)$, and $(\gamma ,3n)$ average photoneutron energies, and (f) average PFN energies. The error bars account for the statistical uncertainty and for the uncertainties in the neutron detection efficiency, photon flux, target thickness, and incident photon spectra.

Figure 9

Dependence on incident photon energy for the $\sigma$ spread of PFN multiplicity distributions in the photofission reactions on (a) ${}^{238}\mathrm{U}$ and (b) ${}^{232}\mathrm{Th}$. Present results (full black dots) are compared with LA model predictions [40] (full blue lines), recent $\mathrm{HI}\gamma \mathrm{S}$ LCS $\gamma$-ray beam data of Silano and Karwowski [45] (full purple diamonds), and Livermore positron in-flight annihilation data [20] (full red dots). A linear fit to the present data is shown by the full black line.

Figure 10

Dependence on incident photon energy for the mean PFN multiplicities in the photofission reactions on (a) ${}^{238}\mathrm{U}$ and (b) ${}^{232}\mathrm{Th}$. The present results (full black dots) are compared with recent $\mathrm{HI}\gamma \mathrm{S}$ LCS $\gamma$-ray beam data of Silano and Karwowski [45] (full purple diamonds), Livermore positron in flight annihilation data [20] (full red dots), capture $\gamma$-ray data [50] (brown empty squares), bremsstrahlung data of Ref. [51] (gray open triangles), and indirect determinations from bremsstrahlung studies of fission product yield distributions of Refs. [52, 53, 54, 55, 56, 57] (half full blue diamonds). The green dashed-dotted lines show the systematic linear dependences deduced from neutron-induced fission experiments by Soleilhac et al. [16] and used by the Saclay group [19] in the data reduction. The full blue lines are LA model predictions [40]. The IAEA PD 2019 [23] and the ENDF/B-VIII.0 [58] evaluations are shown in dotted black and respectively solid red lines.

Figure 11

Average energies of neutrons emitted in photon induced reactions on (a) ${}^{238}\mathrm{U}$ and (b) ${}^{232}\mathrm{Th}$. Average energy $E_{\gamma ,n}$ of neutrons emitted in $(\gamma ,n)$ reactions (green dots), $E_{\gamma ,2n}$ for $(\gamma ,2n)$ neutrons (blue triangles), and $E_{\gamma ,3n}$ for $(\gamma ,3n)$ neutrons (black triangles) are compared with results of present empire statistical model calculations (solid lines in corresponding color for each reaction). Present PFN energies $E_{\gamma ,F}$ (black dots) are compared to the ENDF/B-VIII.0 evaluation [58] (dashed blue lines) and LA model predictions [40] (red lines), and, for ${}^{238}\mathrm{U}$, also to the results of Caldwell et al. [20] (empty red dots).

Figure 12

Present cross sections (black dots) for the (a) $(\gamma ,n)$, (b) $(\gamma ,2n)$ and $(\gamma ,3n)$, and (c) $(\gamma ,F)$ reactions on (upper row) ${}^{238}\mathrm{U}$ and (lower row) ${}^{232}\mathrm{Th}$ compared with existing data obtained with positron in flight annihilation beams at Saclay [19] (blue full diamonds), Livermore [20] (red full dots), Giessen [59] (cyan empty dots), and Moscow [60] (cyan empty square), bremsstrahlung beams [61, 62] (gray symbols), bremsstrahlung monochromators [63, 64, 65] (orange symbols), and capture $\gamma$ rays [50, 66, 67] (brown symbols). Present empire and talys statistical model calculations and the ${}^{238}\mathrm{U}$ ones of Ref. [11] are shown by the black, green, and red solid lines, respectively.

Figure 13

Present photofission cross sections (black full dots) for (a) ${}^{238}\mathrm{U}$ and (b) ${}^{232}\mathrm{Th}$ compared in the low energy region to present empire and talys statistical model calculations (black and green lines, respectively), the ${}^{238}\mathrm{U}$ ones of Ref. [11] (red line), and to existing data: recent LCS $\gamma$-ray beam data obtained at $\mathrm{HI}\gamma \mathrm{S}$ by Csige et al. [68] (blue half empty dots) using a fission chamber and by Silano and Karwowski [45] (full purple diamonds) using a moderated ${}^3\mathrm{He}$ detection array, the Saclay [19] (blue full diamonds) and Livermore [20] (red full dots) positron in flight annihilation data, bremsstrahlung data [51, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79] (gray symbols), bremsstrahlung monochromator data [63, 80] (orange symbols), and capture $\gamma$-ray data [50, 81, 82, 83, 84, 85, 86, 87, 88] (brown symbols).

Figure 14

Present photoabsorption cross sections for (a) ${}^{238}\mathrm{U}$ and (b) ${}^{232}\mathrm{Th}$ compared to existing data obtained with positron in-flight annihilation beams at Saclay [19] (blue full diamonds) and at Livermore [20] (red full dots) and to bremsstrahlung beam data [89] (gray). The SMLO curves correspond to fits to the present data using the simple modified Lorentzian function described in Ref. [93]. We also show the Sin et al. [11] photoabsorption cross section (red line) and the ENDF-VIII (solid blue lines) and PD2019 (dotted blue lines) evaluations.

Figure 15

Average energy of PFNs emitted in first-chance photofission reactions on ${}^{238}\mathrm{U}$ (black) and ${}^{232}\mathrm{Th}$ (blue) vs the mean number ${\overline{\nu }}_p$ of PFNs. Present experimental results are shown by full dots and their least-squares fit by thin solid lines. LA model predictions [40] are given in thick solid lines and the general approximation given by Terrell [90] is shown by the green dashed-dotted line.

Figure 16

Dependence with incident photon energy for the mean PFNs multiplicities in the photofission reactions on (a) ${}^{238}\mathrm{U}$ and (b) ${}^{232}\mathrm{Th}$. The present results for the total mean number ${\overline{\nu }}_p$ of PFNs per fission (black dots) and for the first chance (blue dots) and second chance contribution including the prefission neutron (red dots) are compared with the corresponding LA model predictions [40].

Figure 17

Ratios of the first-chance photofission cross section to the total photofission cross section given by the sum of first- and second-chance photofission cross sections ${\sigma }_{\gamma ,f}$ and ${\sigma }_{\gamma ,nf}$ for (a) ${}^{238}\mathrm{U}$ and (b) ${}^{232}\mathrm{Th}$. Present data (full black dots) are compared with the results of Caldwell et al. [20] (red empty dots) and to present empire (black lines), talys (green lines), and Sin et al. [11] (red line) statistical model calculations.

Figure 18

Photofission cross sections for (a) ${}^{238}\mathrm{U}$ and (b) ${}^{232}\mathrm{Th}$: total photofission cross sections ${\sigma }_{\gamma ,F}$ (black), first-chance (blue) and second-chance (red) photofission cross sections. Present measurements (full dots) are compared to experimental results of Caldwell et al. [20] (empty dots) and present empire and talys statistical model calculations (solid and dashed lines, respectively).

Figure 19

Neutron to fission branching ratio ${\mathrm{\Gamma }}_n/{\mathrm{\Gamma }}_f$ as a function of the excitation energy for (a) ${}^{238}\mathrm{U}$ and (b) ${}^{232}\mathrm{Th}$. Present data (black dots) are compared to the results of Caldwell et al. [20] (red dots) and of Veyssiere et al. [19] (blue diamonds), and to statistical model calculations: present empire (black lines) and talys (green lines) calculations and the ones of Sin et al. [11] (red line).

Figure 20

Present photoneutron cross sections (full black dots) for (a) ${}^{27}\mathrm{Al}$ and (b) ${}^{16}\mathrm{O}$ compared with positron in flight annihilation data taken at Saclay [106] (full blue diamonds), at Livermore [107, 108, 109, 110, 111] (red symbols) and at Giessen [112] (full yellow dots), and with bremsstrahlung data [113, 114] (gray symbols).