Measurement of the 14N ⁡(n,p) 14C cross section at the CERN n_TOF facility from subthermal energy to 800 keV

PHYSICAL REVIEW C 107, 064617 (2023) Measurement of the 14N(n,p)14Ccross section at the CERN n_TOF facility from subthermal energy to 800 keV Pablo Torres-Sánchez ,1,*Javier Praena,1Ignacio Porras,1Marta Sabaté-Gilarte,2,3Claudia Lederer-Woods,4Oliver Aberle,2 Victor Alcayne,5Simone Amaducci,6,7Józef Andrzejewski,8Laurent Audouin,9Vicente Bécares,5Victor Babiano-Suarez,10 Michael Bacak,2,11,12 Massimo Barbagallo,2,13 František Beˇ cvᡠr,14 Giorgio Bellia,6,7Eric Berthoumieux,12 Jon Billowes,15 Damir Bosnar,16 Adam Brown,17 Maurizio Busso,18,19 Manuel Caamaño,20 Luis Caballero,10 Francisco Calviño,21 Marco Calviani,2Daniel Cano-Ott,5Adria Casanovas,21 Francesco Cerutti,2Yonghao Chen,9Enrico Chiaveri,2,15,3 Nicola Colonna,13 Guillem Cortés,21 Miguel Cortés-Giraldo,3Luigi Cosentino,6Sergio Cristallo,18,22 Lucia-Anna Damone,13,23 Maria Diakaki,24,2Mirco Dietz,4César Domingo-Pardo,10 Rugard Dressler,25 Emmeric Dupont,12 Ignacio Durán,20 Zinovia Eleme,26 Beatriz Fernández-Domínguez,20 Alfredo Ferrari,2Francisco Javier Ferrer,27,3Paolo Finocchiaro,6 Valter Furman,28 Kathrin Göbel,29 Ruchi Garg,4Aleksandra Gawlik-Rami˛ega,8Benoit Geslot,30 Simone Gilardoni,2 Tudor Glodariu,31,†Isabel Gonçalves,32 Enrique González-Romero,5Carlos Guerrero,3Frank Gunsing,12 Hideo Harada,33 Stephan Heinitz,25 Jan Heyse,34 David Jenkins,17 Erwin Jericha,11 Franz Käppeler,35,†Yacine Kadi,2Atsushi Kimura,33 Niko Kivel,25 Michael Kokkoris,24 Yury Kopatch,28 Milan Krtiˇ cka,14 Deniz Kurtulgil,29 Ion Ladarescu,10 Helmut Leeb,11 Jorge Lerendegui-Marco,3Sergio Lo Meo,36,37 Sarah-Jane Lonsdale,4Daniela Macina,2Alice Manna,37,38 Trinitario Martínez,5 Alessandro Masi,2Cristian Massimi,37,38 Pierfrancesco Mastinu,39 Mario Mastromarco,2Francesca Matteucci,40,41 Emilio-Andrea Maugeri,25 Annamaria Mazzone,13,42 Emilio Mendoza,5Alberto Mengoni,36 Veatriki Michalopoulou,24 Paolo Maria Milazzo,40 Federica Mingrone,2Agatino Musumarra,6,7Alexandru Negret,31 Ralf Nolte,43 Francisco Ogállar,1 Andreea Oprea,31 Nikolas Patronis,26 Andreas Pavlik,44 Jarosław Perkowski,8Luciano Persanti,13,18,22 José-Manuel Quesada,3 Désirée Radeck,43 Diego Ramos-Doval,9Thomas Rauscher,45,46 René Reifarth,29 Dimitri Rochman,25 Carlo Rubbia,2 Alok Saxena,47 Peter Schillebeeckx,34 Dorothea Schumann,25 Gavin Smith,15 Nikolay Sosnin,15 Athanasios Stamatopoulos,24 Giuseppe Tagliente,13 José Tain,10 Zeynep Talip,25 Ariel Tarifeño-Saldivia,21 Laurent Tassan-Got,2,24,9Andrea Tsinganis,2 Jiri Ulrich,25 Sebastian Urlass,2,48 Stanislav Valenta,14 Gianni Vannini,37,38 Vincenzo Variale,13 Pedro Vaz,32 Alberto Ventura,37 Vasilis Vlachoudis,2Rosa Vlastou,24 Anton Wallner,49 PhilipJohn Woods,4Tobias Wright,15 and Petar Žugec16 (The n_TOF Collaboration)‡ 1University of Granada, Granada, Spain 2European Organization for Nuclear Research (CERN), Geneva, Switzerland 3Universidad de Sevilla, Seville, Spain 4School of Physics and Astronomy, University of Edinburgh, Edinburgh, United Kingdom 5Centro de Investigaciones Energéticas Medioambientales y Tecnológicas (CIEMAT), Madrid, Spain 6INFN Laboratori Nazionali del Sud, Catania, Italy 7Dipartimento di Fisica e Astronomia, Università di Catania, Catania, Italy 8University of Lodz, Lodz, Poland 9Institut de Physique Nucléaire, CNRS-IN2P3, Université Paris-Sud, Université Paris-Saclay, F-91406 Orsay Cedex, France 10Instituto de Física Corpuscular, CSIC - Universidad de Valencia, Valencia, Spain 11TU Wien, Atominstitut, Stadionallee 2, 1020 Wien, Austria 12CEA Irfu, Université Paris-Saclay, F-91191 Gif-sur-Yvette, France 13Istituto Nazionale di Fisica Nucleare, Sezione di Bari, Bari, Italy 14Charles University, Prague, Czech Republic 15University of Manchester, Manchester, United Kingdom 16Department of Physics, Faculty of Science, University of Zagreb, Zagreb, Croatia 17University of York, York, United Kingdom 18Istituto Nazionale di Fisica Nucleare, Sezione di Perugia, Perugia, Italy 19Dipartimento di Fisica e Geologia, Università di Perugia, Perugia, Italy 20University of Santiago de Compostela, Santiago de Compostela, Spain 21Universitat Politècnica de Catalunya, Barcelona, Spain 22Istituto Nazionale di Astrofisica - Osservatorio Astronomico di Teramo, Teramo, Italy 23Dipartimento Interateneo di Fisica, Università degli Studi di Bari, Bari, Italy *[email protected] †Deceased. ‡www.cern.ch/ntof Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. 2469-9985/2023/107(6)/064617(15) 064617-1 Published by the American Physical Society PABLO TORRES-SÁNCHEZ et al. PHYSICAL REVIEW C 107, 064617 (2023) 24National Technical University of Athens, Athens, Greece 25Paul Scherrer Institut (PSI), Villigen, Switzerland 26University of Ioannina, Ioannina, Greece 27Centro Nacional de Aceleradores (CNA), Seville, Spain 28Joint Institute for Nuclear Research (JINR), Dubna, Russia 29Goethe University Frankfurt, Frankfurt-am-Main, Germany 30CEA Cadarache, DES, Saint-Paul-les-Durance 13108, France 31Horia Hulubei National Institute of Physics and Nuclear Engineering, Bucharest-Magurele, Romania 32Instituto Superior Técnico, Lisbon, Portugal 33Japan Atomic Energy Agency (JAEA), Tokai-Mura, Japan 34Joint Research Centre (JRC), European Commission, Geel, Belgium 35Karlsruhe Institute of Technology, Campus North, IKP, 76021 Karlsruhe, Germany 36Agenzia Nazionale per le Nuove Tecnologie (ENEA), Bologna, Italy 37Istituto Nazionale di Fisica Nucleare, Sezione di Bologna, Bologna, Italy 38Dipartimento di Fisica e Astronomia, Università di Bologna, Bologna, Italy 39Istituto Nazionale di Fisica Nucleare, Sezione di Legnaro, Italy 40Istituto Nazionale di Fisica Nucleare, Sezione di Trieste, Trieste, Italy 41Dipartimento di Astronomia, Università di Trieste, Trieste, Italy 42Consiglio Nazionale delle Ricerche, Bari, Italy 43Physikalisch-Technische Bundesanstalt (PTB), Bundesallee 100, 38116 Braunschweig, Germany 44Faculty of Physics, University of Vienna, Vienna, Austria 45Department of Physics, University of Basel, Basel, Switzerland 46Centre for Astrophysics Research, University of Hertfordshire, Hatfield, United Kingdom 47Bhabha Atomic Research Centre (BARC), Mumbai, India 48Helmholtz-Zentrum Dresden-Rossendorf, Dresden, Germany 49Australian National University, Canberra, Australia (Received 15 December 2022; accepted 22 May 2023; published 29 June 2023) Background: The 14N(n,p)14Creaction is of interest in neutron capture therapy, where nitrogen-related dose is the main component due to low-energy neutrons, and in astrophysics, where 14Nacts as a neutron poison in the sprocess. 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 14N(n,p)14Ccross 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. 10B(n,α)7Li and 235U(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 ±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/venergy 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 14N(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 =5tokT =100 keV. DOI: 10.1103/PhysRevC.107.064617 I. INTRODUCTION The 14N(n,p)14Creaction plays a key role in many fields. It is relevant in nuclear astrophysics because it is a significant neutron poison in the s-process nucleosynthesis [1]. Also, this reaction is an important proton feeder in the production of fluorine, via the chain of reactions 18O(p,α)15N(α, γ )19F. 19Fis a useful tracer of the physical conditions in stellar interiors, since it can be easily destroyed by proton or αreactions, hence any production site needs also to enable 19Fto escape from the stellar interiors as it is observed with high abundance in asymptotic giant branch (AGB) star atmospheres [2]. In radiotherapy, the dose in healthy tissues is a limiting factor. Specifically, in boron neutron capture therapy (BNCT), the 064617-2 MEASUREMENT OF THE 14N(n,p)14C… PHYSICAL REVIEW C 107, 064617 (2023) 14N(n,p)14Creaction constitutes the main contribution to the dose in healthy tissue due to thermal neutrons [3], therefore it is crucial for determining the delivered dose in a BNCT treatment [4]. In addition, the International Commission on Radiation Units and Measurements (ICRU) recommends that the delivered dose should have less than 5% deviation from the prescribed dose [5]. At present, nuclear data are under review and upgrade for the new IAEA Technical Document (TECDOC) on BNCT [6]. An accurate 14N(n,p)14Ccross section is one of most important data to be considered there. The 14N(n,p)14Creaction has been measured several times, though all of these measurements have been focused on a specific energy region: thermal energy (25.3 meV), 1/v region (up to a few tens of keV), integral measurements in the keV range (25–200 keV), or resonance region (above 400 keV). Thus, there is no single measurement connecting the thermal energy or the 1/vregion with the resonance region. The first resonance is above 450 keV. Despite the number of experiments, there are some issues that need to be clarified in the different energy ranges. At thermal energy, there are several measurements [7–13] with values from 1.7 to 2.0 b. The last two provided 1.93 ± 0.05 b [12] and 1.868 ±0.006 b [13], and they are higher than the ENDF/B-VIII.0 and JEFF-3.3 evaluations (1.8271 b) [14]. In BNCT, the disparities between measurements and evaluations in involved reactions can lead to differences up to 15% in the thermal neutron dose estimation in human tissues and 3–4% in total dose in no-tumor tissues (including γand boron-induced doses). Thus, a reduction of the uncertainty in the 14N(n,p) reaction leads to more accurate treatments, approaching ICRU recommendations [5]. The thermal value is also relevant in astrophysics. For instance, it has been shown that the thermal value impacts considerably the Maxwellian averaged cross section (MACS) for (n,p) reactions [15]. The only differential measurement available in the 1/vregion is by Koehler et al. [16], covering the range from 61 meV to 35 keV. It was normalized by extrapolation to the thermal value in the Nuclear Data Compilation by Ajzenberg-Selove, at 1.83 ±0.03 b [17]. In a second measurement, Koehler et al. confirmed the assumed value at thermal energy [18], obtaining the same results as Gledenov et al. [11]. Between 35 and 150 keV, there are no differential measurements, and there are only derived data from the inverse-reaction measurement by Gibbons et al. [19]. In the astrophysical range several integral measurements have been carried out [1,20–23]. For all of them there is a good agreement at 25 keV, except for those by Brehm et al. [20]. However, some differences arise in the measurements above 25 keV. Shima et al. [23] observed a reduction in the cross sections from 1.67 mb at 35.8 keV to 1.19 mb at 67.1 keV, while Gledenov et al. [21] saw a mostly flat behavior between 24.5 and 144 keV with values oscillating between 2.04 and 2.08 mb. The last measurement by Wallner et al. [1] encountered a reduction above 25 keV, with values of 0.88 and 0.90 mb at 123 and 178 keV, respectively, in clear disagreement with Gledenov et al. [21]. In the resonance region the most important measurements were carried out by Johnson et al. [24] and Morgan [25]. Johnson et al. measured the cross section from 150 keV to 2.15 MeV, using neutrons from a lithium target bombarded by protons with an energy spread of 5 keV [24]. Later, Morgan [25] using time of flight measured above 450 keV with better resolution, although he did not provide an analysis of the resonance parameters. Morgan is the reference for the evaluations in the resonance region. More recently, based on the comparison of the aforementioned cross section determined at 123 and 178 keV (at the low-energy tail of the first resonance at 492.7 keV) with the ENDF data in this region, Wallner et al. suggested a factor of about 3.3 lower strength for this resonance. The ENDF/B-VIII.0 evaluation does not provide resonance parameters for this reaction, and refers to the compilation by Ajzenberg-Selove [17]. Information on the resonance parameters could be also obtained via other reactions that lead to the same compound nucleus [26]. Specifically, the inverse 14C(p,n)14Nreaction, whose threshold allows the observation of the lowest neutron resonances, has been used to provide information on their Jπ[19,27–29]. Other reactions have thresholds above these states of 15N[17]. Some measurements found an anisotropy in the first resonance [28] (indicating J>1 2), while others did not [19]. The neutron polarization measurement by Niecke et al. [29] reported a positive parity for this state. Contrarily, negative parity was assigned in another measurement [27] and in other compilations [17,30]. To summarize, although the 14N(n,p)14Ccross- section has been measured several times in different energy ranges, many open questions remain. In order to solve the discrepancies found in the 14N(n,p)14Creaction, a measurement covering the range from thermal neutron energy to the first two resonances was carried out at the Experimental Area 2 of the n_TOF facility at CERN. The neutron beam at this experimental area presents an excellent compromise between high neutron flux, in particular at thermal energy, and energy resolution. The present measurement has covered by far the largest neutron energy range, and, in particular, for the first time has connected the thermal, 1/v, and resonance regions. This has allowed an accurate analysis of the resonances, providing new information on the cross section in different energy ranges. This measurement is a part of the scientific program of the n_TOF Collaboration, including a series of experiments aiming at studying reactions of relevant interest in nuclear astrophysics and in medical physics [31]. II. EXPERIMENTAL SETUP A. The n_TOF neutron beam The experiment was performed at the Experimental Area 2 (EAR-2) of the n_TOF facility at CERN, located around 19.5 m from the neutron production target in the vertical direction. The neutrons are generated by the 20 GeV/cproton beam from the CERN Proton Synchrotron (PS) impinging onto a lead target. The technical features of the facility and the characteristics of the neutron beam are described in detail in Refs. [32,33]. 064617-3 PABLO TORRES-SÁNCHEZ et al. PHYSICAL REVIEW C 107, 064617 (2023) FIG. 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. B. Detectors and data acquisition The experimental setup for charged particle detection consisted of a couple of separate systems that worked in parallel. The first of them (upstream) was based on a stack of micromesh gaseous structure detectors (MicroMegas), while the second (downstream) used double-sided silicon strip detectors (DSSSD). The MicroMegas detectors are based on microbulk technology, for which the low mass, robustness, and neutron and γtransparency allow the use of several detectors along the beam with minimal perturbation. This allows a geometrical efficiency close to 50% when one of the two reaction products is detected [34]. This type of detector has been used thoroughly in n_TOF fission and charged-particle emission measurements, including the neutron flux determination [33,35]. A stack of MicroMegas detectors (9.5 cm in diameter) was mounted in a common reaction chamber as shown in Fig. 1, and operated with a gas mixture of 90% Ar and 10% CF4. The beam-spot size at the position of the first sample was 1.91 cm (FWHM). The 235Uand 10Bsamples were placed in the forward direction, the two nitrogen (adenine) samples were placed in a back-to-back configuration, and two KCl samples, which were used for a separate measurement (not analyzed in this work), were placed in the same configuration as the adenine samples. Additional measurements, replacing the adenine samples with dummy samples (Al foils as in the substrate of the adenine samples), were carried out in order to determine the background and reduce systematic uncertainties. The DSSSD are silicon detectors that provide position sensitivity and allow background subtraction by means of front-rear strip coincidence analysis. W1 Model DC Strip Detectors with 16 ×16 strips from Micron Semiconductors (Design W D/S Type 2M/2M. Rrg No A-4366 and Ceramic substrate Number A-4331) were used [36]. These detectors have to be placed off-beam where a lower beam-related background is found, at the cost of a lower geometrical efficiency. The DSSSD allow one to extend the measured neutron energy range toward high energy and also analyze the angular distribution of the emitted protons. The setup, shown in Fig. 2, consisted of a couple of detectors, one facing the adenine sample (top) and the other facing the KCl sample (bottom), which was used for a separate measurement as in the MicroMegas FIG. 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. setup. The center of the detector is at 5.7 cm from the center of the sample. The angular range covered by the strips of the detector is 48◦–98◦. The beam-spot size at the sample position was 2.14 cm (FWHM). The bottom panel of Fig. 2shows a simulation of the protons emitted from the sample after the neutron capture reaction. Protons from the (n,p) reaction impinging on the DSSSD deposit all their energy inside the silicon layer. Conversely, those protons emitted from the adenine sample in the backward direction (likewise for the KCl sample) were absorbed by the Al backings. These samples were replaced temporarily at the beginning and end of the measurement with 10Bsamples for normalization purposes, and also at the end with dummy samples (Al foils) in order to determine the overall background, following the same strategy as with the MicroMegas setup. The experimental setup was aligned to the nominal beam position. The real position and spatial profile of the beam were checked by the use of Gafchromic foils. These are radiosensitive films that contain a dye that changes color when exposed to ionizing radiation, providing high resolution of the beam profile distribution. The foils were placed at the position of interest. After irradiation, data was processed through digital scanning. This information was also adopted for corrections in the efficiency related to the beam-to-sample intersection. The detector signals were acquired by the standard n_TOF data acquisition system (DAQ), based on SPDevices ADQ412DC-3G cards of 2 GS/s maximum sampling rate, 12 bits resolution, and 175 MB on-board memory [37]. The special features of these cards ensure the collection of data for 064617-4 MEASUREMENT OF THE 14N(n,p)14C… PHYSICAL REVIEW C 107, 064617 (2023) a time of flight (TOF) corresponding to neutron energies well below the thermal energy. The signal induced by the prompt γ-flash generated in the interaction of the 20 GeV/c proton beam with the lead target was used as a reference signal to determine the time of flight of the neutrons. C. Samples preparation and characterization In this measurement, one 235U, one 10B, and two 14N samples were used for the MicroMegas setup. For the DSSSD setup, one 10Band one 14Nsample were used. All the sample deposits were 9 cm in diameter in the case of MicroMegas and 5 cm in diameter in the case of DSSSD, which is larger than the neutron beam. The 235Usample, enriched to 99.934%, was prepared with the electrodeposition method and an areal density of 0.1176 ±0.0005 mg/cm2onto 30 µm of Al. The boron samples, with a thickness of 20 nm (MicroMegas) and 25 nm (DSSSD), were made of 10B4C on Al by the sputtering method. The nitrogen samples were made of adenine (C5H5N5) and prepared by thermal evaporation onto Al at CERN. Adenine was used due to a large nitrogen content. At the same time, carbon has a low neutron cross section, and neutron capture on hydrogen produces only γradiation that is not detected by the MicroMegas detectors. In addition, the proton recoils from elastic scattering of neutrons have lower energy than those from the 14N(n,p) reaction and can be identified and thus filtered out. The adenine samples were characterized via Rutherford backscattering (RBS) of H+at 0.85 MeV at the Centro Nacional de Aceleradores (Spain), where previous works showed excellent possibilities for sample characterization [38]. The RBS spectra were analyzed using the SIMNRA package [39]. In the SIMNRA simulations the Rutherford cross section for the scattering of H+in Al was used. For C, N, and H, the evaluated cross section data from the IBANDL database were used [40]. Considering the dimension of the samples (5 and 9 cm in diameter) and of the H+beam spot (3 mm) used for the characterization of the samples, several points were analyzed for each sample to provide a picture of the homogeneity of the thickness. The samples were scanned from the edges to the center in three different directions. In order to perform an accurate and precise determination of the number of atoms of 14N, a few points outside the area coated with adenine were also analyzed by RBS. This allowed the determination of any possible contamination of the substrate, reducing the free parameters of the SIMNRA fit of the data. The overall thickness of adenine is revealed both by the presence of the peak and the reduction in energy of the edge from the Al substrate. Figure 3 shows an example SIMNRA fit to the data. The mass density for each measured point was determined with an uncertainty of 1–2%. We found a smooth reduction of the adenine mass density from the center to the edges. Within uncertainties, the same mass density was found for points at the same distance from the center. Figure 4reveals a parabolic pattern in the mass density, which was attained through the equation m=m0−ar2. FIG. 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. The parameter ais a measure of the mass distribution throughout the sample, quantified as the curvature of the quadratic fit, while m0is the mass density at the center of the samples. The total mass is computed as the integral of the mass density given by the above mentioned formula. The mass of each sample was determined with an uncertainty better than 1.5%. Table Isummarizes the results for all the samples. III. DATA ANALYSIS The digitized signals from MicroMegas detectors and DSSSD were reconstructed offline by means of a pulse shape analysis routine described in Ref. [41], from which information was extracted on the amplitude, area, timing, and other features of the signals. The analysis was done separately for high intensity (HI) and low intensity (LI) proton pulses from the PS accelerator complex. High intensity pulses (around 7×1012 protons per bunch) allow larger statistics, but suffer from a very intense γflash. On the other hand, low intensity pulses (around 3.5×1012 protons per bunch) profit from a reduced γflash which allows a better signal identification, especially for higher neutron energies. The use of LI pulses 10 12 14 16 18 20 22 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 Mass density (10-6 at/barn) Distance from center (cm) Experiment Quadratic fit FIG. 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. 064617-5 PABLO TORRES-SÁNCHEZ et al. PHYSICAL REVIEW C 107, 064617 (2023) TABLE I. Results of the characterization of the mass density for all the adenine samples. Total number of atoms Sample m0(atoms/b) a[atoms/(b cm2)] (1020 atoms) DSSSD 6.88 ×10−67.24 ×10−81.306 ±0.020 Backward MicroMegas 1.382 ×10−52.18 ×10−78.79 ±0.11 Forward MicroMegas 1.958 ×10−53.91 ×10−79.94 ±0.12 thus extends the energy range of the measurement in the high energy region, at the expense of a smaller neutron flux and hence lower statistics. Figure 5shows a two-dimensional (2D) histogram of the area of the signals versus their TOF for the MicroMegas detector facing the 10Bsample, where signals from αparticles are discriminated from the other reaction products, electronic noise, and pileup events. The αparticles from the two reaction channels—the decay to the ground state (n,α 0) and to the first excited state (n,α 1γ)—can be observed around 2.0 and 1.7 in area, respectively. Some regions with low number of counts are observed, especially below 104ns, which correspond to dips in the neutron flux caused by Al in the target and beam pipes. Similarly, a threshold in energy deposition was used to separate the fission fragments from the signals from αdecay and electronic noise for the detector facing the 235Usample, asshowninFig.6. The figure shows the projection of the signal areas on the vertical axis (of a plot similar to Fig. 5) for several TOF ranges. Individual curves were normalized to prove that the distribution is the same at all energies, hence the correction due to the fraction of fission events lost below the threshold can be considered to be the same at all TOF values. The residual background within the selection thresholds was measured by means of dummy samples. The relative background at all the neutron energies was found to be well below 10−3for the 235Uand 10Bsamples, therefore it was neglected in the analysis. FIG. 5. 2D histogram of deposited energy vs time of flight for the MicroMegas detector in forward emission from 10B. The lines indicate the energy-dependent range applied to select the αparticles used in the analysis. The two regions corresponding to the detection of αparticles and 7Li are clearly distinguished above and below the bottom red line, respectively. Pileup events can also be observed above the regular signals from αparticles. The behavior at low TOF is related to the effect of the γflash. In the case of the adenine samples, the emitted proton has a lower energy than the αand 7Li from 10Bcapture, which makes it more complicated to separate their signals from the low signal area noise and background. Fittings of the count distribution along the whole neutron energy range were performed, in order to estimate the fraction of rejected counts below the selection threshold. Figure 7shows the counts from the forward adenine sample and the dummy sample used to estimate the background for several neutron energy ranges. The signals from protons between 3.7 and 10 in area can be clearly distinguished above the background. Note an increase of the noise (signals with area lower than about 3) in the detector at higher neutron energies. This noise limits the precision of the background subtraction at neutron energies above 130 keV. Note also the slight increase in energy deposition at higher neutron energies, due to kinematics. For the analysis of measurement with DSSSD, only coincidence signals from the front and rear strips in the detector were considered in order to reduce part of the background signals appearing at high neutron energies. A time and signal area coincidence rejection was performed. Figure 8shows a 2D histogram of the signal area versus TOF for the DSSSD facing the adenine sample. The use of only coincident signals has significant impact, especially at TOF corresponding to the first two resonances, where we already see impact of the γflash. Only signals within a neutron-energy-dependent signal area region, enclosed in red lines in Fig. 8, were considered in the analysis. A small background was subtracted using collected data from dummy samples (Al backings). Nonetheless, there 0 5 10 15 20 25 30 Signal Area (arb. units) 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 Counts (arb. units) ns 4 - 10 3 3.16x10 ns 4 - 3.16x10 4 10 ns 5 - 10 4 3.16x10 ns 5 - 3.16x10 5 10 ns 6 - 10 5 3.16x10 ns 6 - 3.16x10 6 10 ns 7 - 10 6 3.16x10 FIG. 6. Energy deposition spectra for seven TOF intervals at the MicroMegas detector facing the 235Usample. 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. 064617-6 MEASUREMENT OF THE 14N(n,p)14C… PHYSICAL REVIEW C 107, 064617 (2023) 0 5 10 15 20 25 Signal Area (arb. units) 6− 10 5− 10 4− 10 3− 10 2− 10 1− 10 1 10 2 10 Normalized Counts Adenine 2-20 keV Dummy (Al) 2-20 keV Adenine 20-200 eV Dummy (Al) 20-200 eV Adenine 0.2-2 eV Dummy (Al) 0.2-2 eV Adenine 8-20 meV Dummy (Al) 8-20 meV FIG. 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. was an additional small background contribution that cannot be easily subtracted, which was considered in the R-matrix (SAMMY) analysis of the data. Pileup events are negligible for all neutron energies due to the low counting rate, given the off-beam location of the DSSSD. A small amount of boron contamination is observed at large TOF from7Li and αsignals. They have higher signal area than the protons (around 1.5 and 2.8 in area in the plot, respectively), and hence do not affect the analysis. As mentioned above, the spectra from LI proton pulses allow analysis up to a higher energy as the background is lower than for the HI ones. Both HI and LI data were used below 300 keV and only LI data were use above that energy, resulting in lower collected statistics in the resonance region. A. Energy calibration The TOF-to-energy calibration was performed according to the method described in Ref. [42]. The experimental TOF yield was compared to the simulations from the n_TOF Transport Code, which includes ENDF/B-VIII.0 cross section evaluation with the effect of the n_TOF EAR-2 resolution function. This comparison is shown for the low energy resonances of 235Uin Fig. 9. There is an overall good reproduction in the whole range, except in the dips between some of the resonances, which had already been spotted as disagreements between the last ENDF/B-VIII.0 and JEFF-3.3 evaluations. The n_TOF EAR-2 resolution function is dependent on the type of pulses (e.g., by the proton pulse width, 7 ns for HI and 14 ns for LI) and thus the corresponding resolution function data were used in each case. The extracted effective flight path was 19.39 m for the 235Usample position in the MicroMegas chamber. This corresponds to the geometrical distance of the experimental apparatus from the surface of the Pb spallation target. This flight path was adjusted for the subsequent samples according to their position inside the Micromegas chamber. For the DSSSD setup, the flight path was 19.75 m FIG. 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 14N(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 ×103ns) and lower signal area than the protons from the 14N(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. and it was checked by the fitting of the thermal peak and the dips in the flux observed with the reference 10Bsample. B. Neutron fluence and normalization The 14N(n,p) cross section is extracted relative to the 10B(n,α) cross section according to σN(En)=σB(En)[CN(En)−BN(En)]B(En)NBB [CB(En)−BB(En)]N(En)NNN ,(1) where CX(X≡N or B for nitrogen and boron, respectively) is the total number of counts in the detector at a given neutron energy Enand BXis that of background counts, Xis the neutron fluence, NXis the areal density of the sample (in atoms per barn), and Xis the efficiency, which accounts for the geometric efficiency and the event selection thresholds. Reference boron cross-section data were taken from Ref. [43]. 064617-7 PABLO TORRES-SÁNCHEZ et al. PHYSICAL REVIEW C 107, 064617 (2023) 5 10×25 10×35 10×4 9− 10 8− 10 7− 10 Counts per pulse 5 10×45 10×56 10 6 10×2 time of flight (ns) 9− 10 8− 10 7− 10 Counts per pulse FIG. 9. Measured 235UTOF 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. Along with this, the 235U/10Byield ratio was calculated for validation purposes. A good agreement is found, as shown in Fig. 10, comparing the ratio of the TOF count spectra to the simulations, where the different detection efficiencies and the neutron beam resolution function are considered, following Ref. [42]. The effect of neutron flux attenuation due to the relative positions of the uranium and boron samples was also considered. C. Efficiency and dead-time corrections The efficiency corrections for the Micromegas detectors were determined by detailed Monte Carlo simulations of the reaction products’ energy loss in the samples and the gas. The simulations were performed with the MCNP6.2 code [44]. In these simulations, we used the actual thickness profiles that 4 10 5 10 6 10 7 10 time of flight (ns) 2− 10 1− 10 1 10 B Yield ratio 10 U/ 235 FIG. 10. Measured 235U/10Byield 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. 0.4 0.45 0.5 0.55 0.6 10-3 10-2 10-1 100101102103104105106 Detection Efficiency MicroMegas - Forward Adenine MicroMegas - Backward Adenine 0.035 0.036 0.037 0.038 0.039 0.04 10-3 10-2 10-1 100101102103104105106 Detection Efficiency Neutron Energy (eV) DSSSD - Adenine FIG. 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. were found experimentally Energy and angular distributions of emitted charged particles are adopted from [45]forthe boron sample, and are assumed to be isotropic at the center of mass for U and for adenine samples at low energy. See Sec. IVB for the check of the distribution from N in the resonance region. The reaction events were assumed to occur uniformly along the beam direction inside the sample, and the beam profile from the n_TOF Transport Code was used. Figure 11 shows the efficiency for the forward and backward adenine samples. As is evident from the figure, the efficiency changes only weakly. The change is seen (due to kinematics) when the incident neutron energy becomes comparable to the reaction Qvalue. The efficiency at low neutron energy is different for forward and backward samples due to the different thicknesses of the samples (larger for the forward sample), which becomes relevant for proton emission near grazing angles, especially at higher energies. Furthermore, the increasing linear momentum transfer contributes to a larger emission in the forward direction and thus a higher efficiency in the forward case. For the DSSSD setup, the geometry of the detectors was implemented in the simulations, including the silicon layer thickness and the double strip features, especially regarding the interstrip spacing. The positioning and orientation of the detector with respect to the sample and the beam were checked by using an 241Am source and also the counts from the thermal neutron capture on 10B, assuming isotropic α particle emission. Furthermore, some of the strips from the DSSSD were malfunctioning and were kept out of the analysis. The simulated detection efficiency for the DSSSD adenine sample is shown in the lower panel of Fig. 11. 064617-8 MEASUREMENT OF THE 14N(n,p)14C… PHYSICAL REVIEW C 107, 064617 (2023) Dead-time corrections were computed following the nonparalyzable response model [46] assuming a fixed dead time for each detector. The dead times were estimated following two strategies. The first was based on estimating the dead time from the FWHM of the signals and the minimum time difference between consecutive pulses (a sharp cutoff observed in the time-between-pulses distribution). The second method relied on matching the total yield for low and high intensity proton pulses, showing agreement with the first method. D. Uncertainties We adopted conservative estimations of the accuracy of the results. The statistical uncertainty of the MicroMegas data was assessed with the use of a binning of 10 bins per decade (bpd), ranging from 0.1% around the thermal point to 4–5% above 10 keV. Statistical uncertainties of the DSSSD data, at 10 bpd, range from 0.4% at thermal energy to 10–15% in the 10–300 keV range. Above 300 keV, the need of a better energy resolution due to the presence of resonances makes us use 100 bpd, at a cost of a higher uncertainty, close to 20% per bin. The mass of the adenine samples was measured within 1.5% accuracy for the MicroMegas samples. Although the neutron energy dependence of the 14N(n,p) cross section was determined with respect to 10Bat all neutron energies [as indicated by Eq. (1)], the absolute cross-section normalization was made using the thermal point of the 235U(n,f) reaction, given that it is a standard at that energy and the U sample mass was known more precisely, with uncertainty of 0.43%, than the B mass. The uncertainty of the cross section of 235U(nth,f) reaction is 0.23% [43]. The uncertainty related to the angular distribution of the αparticles from the 10B(n,α)7Li reaction at higher energies was considered in the calculation of the efficiency in both the MicroMegas and DSSSD setups following the accurate data in Ref. [45]. The uncertainty due to the detection efficiency includes the statistical uncertainty in the simulations of the proton transport (and αparticles for the 10B4C sample), which was reduced below 0.2%. Additional uncertainty due to the positioning and orientation of the samples and DSSSD was estimated by simulations with perturbations on the geometry to be 1.3%. The uncertainty in the correction to the selection cuts was estimated to be 1% at thermal energy and up to 5% in the keV range. The effect of the neutron beam attenuation and neutron scattering on Al windows in the n_TOF pipes, Kapton windows of the MicroMegas and DSSSD chambers, the in-beam Micromegas detectors, and upstream samples was assessed via simulations and found to be around 1%. Since DSSSD results are normalized to the MicroMegas data in the 1/vregion, additional systematic uncertainties from this detection system can be dismissed. The sources of uncertainty are summarized in Table II. IV. RESULTS A. Data from MicroMegas and DSSSD The main goal of this measurement was to obtain a consistent data set spanning from thermal to the resonance region and to provide a verification of the first two resonances TABLE II. Major sources of uncertainty (in %) of the present 14N(n,p) cross section. The uncertainties vary depending on the binning and the energy range and on the sample. Component Uncertainty (%) Sample mass 14N1.2–1.5 Normalization 0.65–3 Efficiency and selection cuts 1.8–5 Neutron Beam attenuation 1 Statistical MicroMegas 0.1–5 Statistical DSSSD 0.4–20 strengths after a recent paper [1] suggested a possible reduction by a factor of 3.3. The measurement with MicroMegas detectors covered the range from 8 meV to 80 keV with the backward sample, and up to 130 keV with the forward sample. The extended range for the forward sample is due to its higher mass and slightly higher efficiency at these energies, which leads to an improved signal-to-background ratio. The results from both samples agree with each other within uncertainties. The measurement covers the 1/vrange, including the thermal point, reproducing and extending the data by Koehler [16]. The high-quality measurement with MicroMegas detectors profits from thick samples and a large detection efficiency. The measurement with DSSSD detectors then allows extending the range from thermal to 800 keV. This allows full coverage of the astrophysical range of interest, and allows determination of the parameters of the first two observed neutron resonances. The data from DSSSD have a lower counting statistics due to a smaller thickness of the sample and lower geometrical efficiency. The DSSSD data were normalized to that of MicroMegas detectors in the range 8 meV to 100 eV, where there are data from both detection systems. Overall, this measurement spans eight orders of magnitude of neutron energy, providing for the first time a common consistent dataset for the thermal and 1/vrange, the astrophysical range, and the resonance region. Figure 12 shows the experimental yield corrected for areal density (Y/N). This quantity differs from the cross section only by the effect of the n_TOF EAR-2 resolution function. Data are presented for both MicroMegas detectors (separately for the forward and backward samples and HI and LI proton pulses) and DSSSD (merged HI and LI below 300 keV, and only LI above 300 keV). The cross section derived from this yield by means of its fitting with the R-matrix based SAMMY code [47] will be shown below. Before that, the angular distribution of the proton emission in the first resonance will be discussed. B. Angular distribution of protons from the 492.7 keV resonance Previous measurements of the 14N(n,p) reaction did not carry out any analysis of angular distribution of protons emitted from the resonances. As the DSSSD detects protons emitted only in a range of angles, the knowledge of the angular distribution of emitted protons is crucial for the crosssection determination. In practice, the DSSSD allows one 064617-9