Measurement of the ${}^{14}\mathrm{N}(n,p){}^{14}\mathrm{C}$ cross section at the CERN n_TOF facility from subthermal energy to 800 keV

Background: The ${}^{14}\mathrm{N}(n,p){}^{14}\mathrm{C}$ reaction is of interest in neutron capture therapy, where nitrogen-related dose is the main component due to low-energy neutrons, and in astrophysics, where ${}^{14}\mathrm{N}$ acts as a neutron poison in the $s$ process. Several discrepancies remain between the existing data obtained in partial energy ranges: thermal energy, keV region, and resonance region.

Purpose: We aim to measure the ${}^{14}\mathrm{N}(n,p){}^{14}\mathrm{C}$ cross section from thermal to the resonance region in a single measurement for the first time, including characterization of the first resonances, and provide calculations of Maxwellian averaged cross sections (MACS).

Method: We apply the time-of-flight technique at Experimental Area 2 (EAR-2) of the neutron time-of-flight (n_TOF) facility at CERN. ${}^{10}\mathrm{B}(\mathrm{n},\alpha ){}^7\mathrm{Li}$ and ${}^{235}\mathrm{U}(n,f)$ reactions are used as references. Two detection systems are run simultaneously, one on beam and another off beam. Resonances are described with the $R$-matrix code sammy.

Results: The cross section was measured from subthermal energy to 800 keV, resolving the first two resonances (at 492.7 and 644 keV). A thermal cross section was obtained ($1.809\pm 0.045$ b) that is lower than the two most recent measurements by slightly more than one standard deviation, but in line with the ENDF/B-VIII.0 and JEFF-3.3 evaluations. A 1/$v$ energy dependence of the cross section was confirmed up to tens of keV neutron energy. The low energy tail of the first resonance at 492.7 keV is lower than suggested by evaluated values, while the overall resonance strength agrees with evaluations.

Conclusions: Our measurement has allowed determination of the ${}^{14}\mathrm{N}(n,p)$ cross section over a wide energy range for the first time. We have obtained cross sections with high accuracy (2.5%) from subthermal energy to 800 keV and used these data to calculate the MACS for $kT=5$ to $kT=100$ keV.

Figure 1

The MicroMegas setup, with the six targets and six MicroMegas detectors used in the measurement. Adenine and KCl samples were placed in back-to-back pairs.

Figure 2

Top: The DSSSD setup, with the two samples in the center on Al backings (adenine facing upwards and KCl downwards) and two DSSSDs, one facing each sample. Bottom: Simulation to extract the count distribution and detector efficiency considering isotropic proton emission. The position of the adenine sample and the DSSSD are indicated, as well as the incoming neutron beam which serves as the reference direction. The color code corresponds to the proton track density per primary in the simulation.

Figure 3

Example of a single RBS measurement. The results of the simnra fit with detailed contributions from each isotope are also shown with dashed lines.

Figure 4

Mass density radial distribution for the forward adenine sample in the MicroMegas setup. The horizontal error bars match the size of the probe (3 mm). The quadratic fit is shown with a dashed line.

Figure 5

2D histogram of deposited energy vs time of flight for the MicroMegas detector in forward emission from ${}^{10}\mathrm{B}$. The lines indicate the energy-dependent range applied to select the $\alpha$ particles used in the analysis. The two regions corresponding to the detection of $\alpha$ particles and ${}^7\mathrm{Li}$ are clearly distinguished above and below the bottom red line, respectively. Pileup events can also be observed above the regular signals from $\alpha$ particles. The behavior at low TOF is related to the effect of the $\gamma$ flash.

Figure 6

Energy deposition spectra for seven TOF intervals at the MicroMegas detector facing the ${}^{235}\mathrm{U}$ sample. The two-bump structure corresponds to the fission fragments' asymmetric masses. The curves were normalized to the same integral between 2 and 18 and indicate that the fraction of fission fragments lost below the threshold is independent of the neutron energy.

Figure 7

Energy deposition spectra for four neutron energy intervals in the MicroMegas detector facing the forward adenine sample. The adenine spectra were normalized to the same integral between 3.7 and 10. The background spectra were normalized to the adenine ones using the total proton intensity from the PS pulses.

Figure 8

2D histogram of deposited energy (signal area) vs TOF for the DSSSD facing the adenine sample. The spectra correspond only to the LI pulses. The red lines indicate the region to select the protons from the ${}^{14}\mathrm{N}(n,p)$ reaction used in the analysis. Signals below 0.3 in signal area are not shown as they are strongly dominated by the electronic noise. The signals corresponding to the first two resonances can be clearly identified around 2000 ns. The huge number of events at short TOF (lower than $3\times {10}^3$ ns) and lower signal area than the protons from the ${}^{14}\mathrm{N}(n,p)$ reaction are also protons from recoils of elastic scattering of neutrons off hydrogen present in adenine; its structure is related to the EAR-2 spectral flux, which presents several dips in this energy range. These are caused by absorption in Al or Fe present in the target and pipes. The bottom panel is a zoom in the low TOF range.

Figure 9

Measured ${}^{235}\mathrm{U}$ TOF spectra with MicroMegas in the low energy and resonance region (blue points for LI pulses, black points for HI pulses), compared to the ENDF/B-VIII.0 evaluation (red line) convoluted with the n_TOF EAR-2 resolution function by means of the n_TOF Transport Code. Small differences observed in some valleys are likely from improper values in ENDF/B-VIII.0; compare this database with, e.g., JEFF-3.3.

Figure 10

Measured ${}^{235}\mathrm{U}/{}^{10}\mathrm{B}$ yield ratio for HI (black) and LI (blue) PS proton pulses, compared to the ENDF/B-VIII.0 evaluation (red) folded with the n_TOF EAR-2 resolution function.

Figure 11

Upper panel: Simulated detection efficiency of the MicroMegas detectors. Lower panel: Simulated detection efficiency of the DSSSD. Note the different vertical scales for Micromegas and DSSSD.

Figure 12

${}^{14}\mathrm{N}(n,p)$ yield/areal density ($Y/N$) from the experiment. Data are shown at 10 bins per decade below 300 keV and 100 bins per decade above 300 keV. MicroMegas results are separated for forward and backward samples, and also in HI and LI PS proton pulses.

Figure 13

Experimental counts at the horizontal strips of the DSSSD (black), compared with the simulated distribution of counts for protons emitted isotropically ($J=1/2$, red) and other resonance spins. The nominal central angle subtended by strip 1 corresponds to an angle of $97.2{}^{\circ }$and strip 16 corresponds to an angle of $49.5{}^{\circ }$. Data are normalized so that the total number of counts (from all active strips) is 1. Error bars of the experimental data correspond to statistical uncertainty.

Figure 14

The yield data corrected for the areal density ($Y/N$) are shown in black. The red line is the sammy fit of the ($Y/N$) data, the blue line is the cross section at 300 K deduced from parameters obtained with sammy. The integral measurements by Wallner et al. at 25, 123, and 178 keV are shown in magenta. The ENDF/B-VIII.0 evaluation is included (in green). The upper panel shows the full range covered by this measurement, and the lower panel shows a detail of the resonance region.

Figure 15

${}^{14}\mathrm{N}(n,p){}^{14}\mathrm{C}$ thermal values. Values from ENDF/B-VIII.0 and JEFF-3.3 (blue), JENDL-5 (green), and Atlas of Neutron Resonances (magenta, labeled ATLAS) [30] are indicated with the dashed lines, with semitransparent bands corresponding to uncertainty. Experimental values are marked with squares; our value is shown in red.