Differential cross sections of the ${}^{16}\mathrm{O}(n,\alpha )$ reaction at neutron energies from 3.8 to 15 MeV

The importance of studying the ${}^{16}\mathrm{O}(n,\alpha )$ reaction is motivated by multiple nuclear applications which rely on an accurate nuclear reaction data library for oxygen. Discrepancies between past experimental data on the ${}^{16}\mathrm{O}(n,\alpha ){}^{13}\mathrm{C}$ reaction and its time-reverse ${}^{13}\mathrm{C}(\alpha ,n){}^{16}\mathrm{O}$ reaction have led to various different nuclear data evaluations. We have measured ${}^{16}\mathrm{O}(n,\alpha )$ reaction cross sections using the LENZ instrument with the unmoderated white neutron source at LANSCE. Results from 2016/2017 data are discussed and used to benchmark the mcnp and geant simulations of the LENZ experimental setup. We report partial differential cross sections of ${}^{16}\mathrm{O}(n,{\alpha }_0)$ at $E_n=3.8–15$ MeV and ${}^{16}\mathrm{O}(n,{\alpha }_1+{\alpha }_2+{\alpha }_3)$ at $E_n=9–15$ MeV, based on the new measurement in 2021. The resonances that we observed are in good agreement with the levels in ${}^{17}\mathrm{O}$ that were previously measured. The LENZ cross sections are in better agreement with the ENDF/B-VIII.0 evaluation than the with reduced cross section found in ENDF/B-VII.1 up to 6 MeV. However, the current results for ${}^{16}\mathrm{O}(n,{\alpha }_0){}^{13}\mathrm{C}_{\mathrm{g}.\mathrm{s}.}$ appear to be in the best agreement over the entire energy range with that of JENDL/AN-2005 (ENDF/B-VI.0).

Figure 1

Current status of available evaluations for the ${}^{16}\mathrm{O}(n,\alpha )$ reaction. JEFF3.3 and JENDL4.0 are very similar to ENDF/B-VI.8, so they are omitted in this comparison.

Figure 2

Subset of available experimental data performed on the ${}^{16}\mathrm{O}(n,\alpha )$ reaction (top) and the reverse ${}^{13}\mathrm{C}(\alpha ,n)$ reaction (bottom) retrieved from EXFOR [11]. The number in the parenthesis of the legend corresponds to the EXFOR subentry number. The Walton1957 data were taken from Ref. [13].

Figure 3

LENZ instrument with DSSD detectors positioned to detect outgoing $\alpha$ particles at forward angles in the 2016 experimental setup.

Figure 4

Picture of ${\mathrm{Ta}}_2{\mathrm{O}}_5$ targets that were manufactured by the anodizing technique on 30 $\mu \text{m}$ thick Ta backings, except the $V=225$ V one on a 3 $\mu \text{m}$ thick Ta, to reduce beam-induced backgrounds. The voltages shown on each picture are the anodization voltages, corresponding to different optical colors.

Figure 5

$\gamma$-ray spectrum of the ${}^{16}\mathrm{O}(p,\gamma ){}^{17}\mathrm{F}$ reaction obtained with a HPGe detector using the anodized ${\mathrm{Ta}}_2{\mathrm{O}}_5$ target. The peak was obtained by detecting $E_{\gamma }$ = 1.04 MeV and the arrow in the width represented the target thickness of 400 nm, which was used in 2016/2017 measurement campaigns. Details of the method is discussed in Sect. 2b.

Figure 6

Diagram to describe the angle determination in DSSDs for the experimental configuration of the 2016 and 2017 data sets. The beam spot size was 2 cm in diameter, therefore the actual angle per each annular segmentation in DSSDs ranges from ${\theta }_1$ minimum angle to ${\theta }_2$ maximum angle, instead of the nominal angle $\theta$.

Figure 7

Measured neutron's TOF relative to the $\gamma$ flash vs the detected charged particle's energy (top) and the MCNP calculation for the 71-$\mu \text{m}$-thick DSSD detector for the 2016 LENZ setup (bottom). In the MCNP calculation, the Th-229 source rate is overlaid with an arbitrary normalization to reproduce the LENZ data.

Figure 8

MCNP simulations using the ENDF/B-VII.1 and ENDF/B-VIII.0 evaluations for the 2016 LENZ setup on the $71\text{−}\mu \mathrm{m}$-thick DSSD. Although the nominal angles were defined as ${31}^{\circ }$–${38}^{\circ }$, the simulated actual angles covered ${30}^{\circ }$–${44}^{\circ }$.

Figure 9

MCNP simulations using ENDF/B-VIII.0 for (a,c) the 2016 experimental configuration with the detector located 3.9 cm from the target and the beam size of 1 cm in radius and (b,d) the 2021 experimental configuration with the detector located 25 cm from the target and the beam size of 0.5 cm in radius. (a,b) show nominal angles for individual 16 annular segmentations as described in Fig. 6 and (c,d) show detected angles, (${\theta }_2-{\theta }_1$), of which four annular segmentations are combined into each colored group.

Figure 10

Neutron-energy resolution function in full width at half maximum (FWHM) shown in MeV. This energy resolution function was applied for evaluated cross sections to compare with both 2017 and 2021 LENZ data.

Figure 11

(a) MCNP simulation of the neutron time of flight with respect to the $\gamma$ flash vs the detected energy in DSSD using ENDF/B-VIII.0 for the 2021 LENZ configuration with the ${\mathrm{Ta}}_2{\mathrm{O}}_5$ target, (b) the 2021 LENZ data with a mylar (${\mathrm{C}}_{10}{\mathrm{H}}_8{\mathrm{O}}_4$) target, and (c) the 2021 LENZ data with the ${\mathrm{Ta}}_2{\mathrm{O}}_5$ target on a thin Ta backing substrate. The MCNP simulation and LENZ data are compared for the detector positioned 12.5 cm from the target. The relative neutron TOF in the vertical axis means the neutron TOF is with respect to the $\gamma$ flash time arriving at the oxygen target.

Figure 12

Differential cross sections of ${}^{16}\mathrm{O}(n,{\alpha }_0){}^{13}\mathrm{C}$ from the 2021 LENZ data are compared with energy-averaged ENDF/B-VIII.0 values using the energy resolution function shown in Fig. 10.

Figure 13

Angular distributions of ${}^{16}\mathrm{O}(n,{\alpha }_0){}^{13}\mathrm{C}$ from the 2021 LENZ data are compared with Robb et al. and ENDF/B-VIII.0 at similar neutron energies.

Figure 14

Top: Partial differential cross sections of ${}^{16}\mathrm{O}(n,{\alpha }_1+{\alpha }_2+{\alpha }_3)$ measured at the average angles of $15.5^{\circ }$ and ${57}^{\circ }$ in the laboratory system. Bottom: Total cross sections populating the ground state and the first three excited states in ${}^{13}\mathrm{C}$, after being multiplied by $4\pi$, are compared with ENDF/B-VIII.0. The experimental data from EXFOR for $(n,a_0)$ that define the trend of ENDF at these energies are from Refs. [58, 59, 60, 61, 62, 63]. The widths of horizontal error bars reflect FWHM of the neutron-energy resolution function shown in Fig. 10.

Figure 15

The 2021 LENZ angle-integrated cross section of ${}^{16}\mathrm{O}(n,{\alpha }_0){}^{13}\mathrm{C}$ is compared with different releases of energy-averaged ENDF/B. Here, the shapes of the angular distributions have been constrained by the high resolution $(\alpha ,n_0)$ angular distributions from Walton (filled circles, up to 5.2 MeV) [13], Prusachenko (inverted triangles, up to 7.2 MeV) [14], and deBoer (open circles, up to 6.8 MeV) [25].