Nonstatistical fluctuations in the ${}^{35}\mathrm{Cl}(n,p){}^{35}\mathrm{S}$ reaction cross section at fast-neutron energies from 0.6 to 6 MeV

The lack of experimental data on the ${}^{35}\mathrm{Cl}(n,p){}^{35}\mathrm{S}$ reaction above 100 keV has led to nuclear data evaluations that are relatively unconstrained at fast neutron energies. As a result, efforts to explore, develop, and potentially certify next generation reactor designs that incorporate chloride salts as a coolant material have been hindered. In this paper, we report partial cross section data for the ${}^{35}\mathrm{Cl}(n,p){}^{35}\mathrm{S}$ and ${}^{35}\mathrm{Cl}(n,\alpha ){}^{32}\mathrm{P}$ reactions at incident neutron energies between 0.6 MeV and 6 MeV. The measurement was performed using the pulsed beam of neutrons at the unmoderated WNR spallation neutron source at the Los Alamos Neutron Science Center, with the outgoing charged particles detected by the LENZ experimental setup, consisting of annular silicon detectors. Nonstatistical fluctuations in the ${}^{35}\mathrm{Cl}(n,p_0)$ cross section were observed up to around 3 MeV, and the magnitude of the cross section was systematically lower than all available data evaluations at energies above 1 MeV. Modifications to the ENDF/B-VIII.0 data evaluation are suggested to better reproduce the energy averaged experimental data.

Figure 1

The ${}^{35}\mathrm{Cl}(n,p){}^{35}\mathrm{S}$ reaction cross section, highlighting the lack of experimental data between 100 keV and 14 MeV. Recent data from Ref. [12] show a dramatic reduction in the cross section relative to all available evaluations and suggests that a resolved resonance approach may be necessary as high as 3 MeV.

Figure 2

Sketch of experimental setup 3, as described in Table 1, consisting of annular silicon detectors and a NaCl target. The direction of the beam defines the $+z$ axis. The target is supported by a tantalum sheet with the same dimensions as an S1-type silicon detector.

Figure 3

The detected charged particle energy of the outgoing proton or $\alpha$ particle, and the corresponding neutron time-of-flight, is used to reconstruct the $Q$ value of the reaction channel.

Figure 4

(a) Experimental data for all ($n,z$) reactions on a brass target (65% Cu, 35% Zn) as compared to (b) an MCNP simulation using the unmodified ENDF/B-VIII.0 as an input and (c) an MCNP simulation using the newly evaluated energy spectra [28], based on ENDF/B-VIII.0 cross sections, as an input. In the unmodified ENDF/B-VIII.0, there is no outgoing charged particle spectrum for the $\alpha$ particles from ${}^{64}\mathrm{Zn}(n,\alpha )$.

Figure 5

(a) Comparison between brass backing data and the modified MCNP simulation and, for the same energy range, (b) the NaCl data with fit ($n,p$) and ($n,\alpha$) signals for the first few excited states of ${}^{35}\mathrm{S}$ and ${}^{32}\mathrm{P}$. The components of the simulated spectrum are shown in (a) for ($n,p$) reactions on brass (orange dashed line) and ($n,\alpha$) reactions on brass (magenta thin solid line).

Figure 6

The ${}^{63}\mathrm{Cu}(n,p_{0,1,2}){}^{63}\mathrm{Ni}$ contribution to the brass backing data shows good agreement with the ENDF/B-VIII.0 cross section evaluation and previous measurements [29] and is used as a consistency check for the beam normalization in this work. The calculation of the partial cross section contribution to the total cross section was calculated using the Hauser-Feshbach code coh3 [30].

Figure 7

Top: A comparison of past experimental data with the current ENDF/B-VIII.0 evaluation. The ENDF/B-VIII.0 library also includes partial cross sections for populating different excited states of ${}^{32}\mathrm{P}$. Bottom: Experimental data from this work, summing the yield for the ($n,{\alpha }_0$) and ($n,{\alpha }_1$) reaction channels, is in good agreement with the current ENDF/B-VIII.0 library.

Figure 8

Background subtracted excitation energy spectrum at forward angles. The ($n,p_0$) reaction channel is identified by its narrow profile at the expected energy.

Figure 9

Angular distributions for different reaction channels at various energies compared with calculations from Ref. [28]. The $\alpha$ channel is scaled by a factor of two for clarity.

Figure 10

($n,p$) reaction channels leading to higher excited states in ${}^{35}\mathrm{S}$ are observed at higher incident neutron energies.

Figure 11

Partial angle-integrated cross sections in 500 keV wide bins are shown in comparison to the ENDF/B-VIII.0 evaluated partial cross sections for ($n,p$) reactions up to (a) the first two excited states of ${}^{35}\mathrm{S}$ and (b) up to ($n,p_5$) The results in this work show a significant reduction relative to the ENDF/B-VIII.0 cross section for the ($n,p_0$) reaction channel between 1 and 5 MeV and a slight reduction for the excited states.

Figure 12

Differential cross section data for populating the ground state of ${}^{35}\mathrm{S}$ for different flight paths, flight path lengths, and laboratory angles. The dominant resonance structures are observed in all cases with consistent energies.

Figure 13

Bottom: The partial angle-integrated cross section for populating the ground state of ${}^{35}\mathrm{S}$. The data are reasonably well bounded, albeit with significant fluctuations, by the calculated results adopted from Ref. [36], and from the statistical calculation employing a modified Kunieda potential from this work. For comparison, the top panel shows the resonance structures in the ${}^{35}\mathrm{Cl}(n$,total) spectrum of ENDF/B-VIII.0. The resonances that we observe in the ($n,p$) data are clearly the sum of multiple narrower resonances that appear to have analogues in the ($n$,total) data but are not strongly represented in the ($n,p$) evaluation below 1 MeV.

Figure 14

(a) Calculated total cross sections for ${}^{35}\mathrm{Cl}$ with the Koning-Delaroche spherical optical potential and Kunieda coupled-channel potential are compared with the evaluated cross section of ENDF/B-VIII.0 (histogram) and the experimental data of Foster and Glasgow [41] and Fasoli et al. [42]. The experimental data are for natural chlorine. (b) Comparisons with modified coupled-channels potential. In the resonance region, the evaluated total cross sections are group-averaged for better visibility.

Figure 15

Comparisons of calculated and experimental ${}^{35}\mathrm{Cl}(n,p_x)$ cross sections, where $x=0$ and 1 in (a) and $x=2$, 3, 4, and 5 in (b). The comparison between the calculated and experimental ${}^{35}\mathrm{Cl}(n,{\alpha }_0+{\alpha }_1)$ cross section is also shown in (a).

Figure 16

Calculated total ($n,p$) cross sections with the Koning-Delaroche spherical optical potential (dashed curve), the Kunieda deformed optical potential (dotted curve), and the modified Kunieda potential (dot-dashed curve), compared with the evaluated cross section given in ENDF/B-VIII.0. The ENDF cross sections below 1.2 MeV are reconstructed from the resolved resonance parameters, and group-averaged.