Quasi-monoenergetic Neutron Beams Research Articles

Fast neutrons at LNL Legnaro

In this contribution we describe NEPIR, the fast-neutron irradiation facility under construction at the 70 MeV cyclotron SPES facility of the INFN laboratory of Legnaro (LNL). NEPIR will be constructed in stages, according to the available funds. The initial configuration, based on a thick Be neutron production target, will be operational in 2022; it will be used for shielding studies against fast neutrons for space applications and to investigate neutron-induced Single Event Effects (SEE) in microelectronic devices and systems. In its final configuration NEPIR will have two target systems: one will deliver a Quasi Mono-energetic Neutron (QMN) beam, of general interest, with an adjustable energy peak in the 20–70 MeV range; the second target will deliver a specialized continuous energy neutron beam for studying the effects of fast neutrons produced in cosmic ray air-showers in electronic devices and systems. We review the use of NEPIR to characterize the sensitivity of electronics, describe the neutron production targets and the facility layout. In closing we describe ways, presently under investigation, to use the 15 MV XTU Tandem of LNL to produce nearly monochromatic fast neutrons that would complement the QMN system by allowing one to probe for SEE below 20 MeV.

Experimental study of the Ho165(n,2n) reaction: Cross section measurements for the population of the Ho164 ground state and isomeric state from the threshold up to 20 MeV

In the present work the $^{165}\mathrm{Ho}(n,2n)$ reaction was studied experimentally and theoretically for the population of the ground state (${J}^{\ensuremath{\pi}}={1}^{+}$) of the $^{164}\mathrm{Ho}$ product nucleus, as well as for the population of its isomeric state (${E}_{ex}=139.8$ keV, ${J}^{\ensuremath{\pi}}={6}^{\ensuremath{-}}$). The cross sections of both the ground and isomeric state channels were measured at energies near the reaction threshold (${E}_{th}=8.04$ MeV), at 10.1, 10.4, and 10.7 MeV, as well as at energies higher than 17 MeV, at 17.1, 18.1, 19.0, and 19.6 MeV. The adopted method was the activation technique relative to the $^{197}\mathrm{Au}(n,2n)^{196}\mathrm{Au}$ and $^{27}\mathrm{Al}(n,\ensuremath{\alpha})^{24}\mathrm{Na}$ reference reactions. The quasi-monoenergetic neutron beams for the near threshold energies were produced via the $^{2}\mathrm{H}(d,n)^{3}\mathrm{He}$ reaction, while for the neutron beams above 17 MeV the $^{3}\mathrm{H}(d,n)^{4}\mathrm{He}$ reaction was utilized. In both cases the deuteron beams were delivered by the 5.5 MV Tandem Van de Graaff accelerator of the Institute of Nuclear and Particle Physics at N.C.S.R. ``Demokritos''. Additional to the experimental study, calculations were performed using the talys code (v. 1.9). The experimental data of the present work along with the previous measurements were compared with the talys results so as to investigate the reproduction of the experimental data for the different level density models of the code.

Characterization of stilbene’s scintillation anisotropy for recoil protons between 0.56 and 10 MeV

The scintillation anisotropy of the single-crystal organic scintillator trans-stilbene was characterized for recoil protons between 0.56 and 10 MeV. The light output and pulse shape anisotropies were measured at 11 distinct recoil proton energies for over 168 recoil proton trajectories relative to the crystal axes. The measurements were performed using a neutron scatter kinematic measurement system and quasi-monoenergetic neutron beams produced by the tandem Van de Graaff accelerator at Triangle Universities Nuclear Laboratory (TUNL). The extensive recoil proton directional coverage enables interpolation over both energy and direction to form a complete response function of stilbene’s scintillation anisotropy for the measured energy range.

Study of the 165Ho(n,2n)164Ho reaction at near threshold energies

The present work concerns the preliminary analysis for the study of the (n,2n) reaction channel of the 165Ho isotope at near threshold energies: 10.1, 10.4 and 10.7 MeV (Eth=8.04 MeV). The cross sections for the population of both the ground state (Jπ=1+) and the isomeric state (Jπ=6-, Eex=139.8 keV) of the 164Ho product-nucleus were measured at the afore mentioned energies via the activation technique relative to the 27Al(n,α)24Na and 197Au(n,2n)196Au reactions. The quasi-monoenergetic neutron beams were produced through the 2H(d,n)3He reaction in the 5.5 MV Tandem Van de Graaf accelerator of N.C.S.R ''Demokritos''. The preliminary experimental results are compared with theoretical predictions based on TALYS code.

Study of the 232Th(n,f) Cross section at NCSR ‘Demokritos’ using Micromegas Detectors

The accurate knowledge of neutron-induced fission cross sections in actinides, is of great importance when it comes to the design of fast nuclear reactors, as well as accelerator driven systems. Specifically for the 232Th(n,f) case, the existing experimental datasets are quite discrepant in both the low and high energy MeV regions, thus leading to poor evaluations, a fact that in turn implies the need for more accurate measurements.In the present work, the total cross section of the 232Th(n,f) reaction has been measured relative to the 235U(n,f) and 238U(n,f) ones, at incident energies of 7.2, 8.4, 9.9 MeV and 14.8, 16.5, 17.8 MeV utilizing the 2H(d,n) and 3H(d,n) reactions respectively, which generally yield quasi-monoenergetic neutron beams. The experiments were performed at the 5.5 MV Tandem accelerator laboratory of N.C.S.R. “Demokritos”, using a Micromegas detector assembly and an ultra thin ThO2 target, especially prepared for fission measurements at n_ToF, CERN during its first phase of operations, using the painting technique. The masses of all actinide samples were determined via α-spectroscopy. The produced fission yields along with the results obtained from activation foils were studied in parallel, using both the NeusDesc [1] and MCNP5 [2] codes, taking into consideration competing nuclear reactions (e.g. deuteron break up), along with neutron elastic and inelastic scattering with the beam line, detector housing and experimental hall materials. Since the 232Th(n,f) reaction has a relatively low energy threshold and can thus be affected by parasitic neutrons originating from a variety of sources, the thorough characterization of the neutron flux impinging on the targets is a prerequisite for accurate cross-section measurements, especially in the absence of time-of-flight capabilities. Additional Monte-Carlo simulations were also performed coupling both GEF [3] and FLUKA [4] codes for the determination of the detection efficiency.

The measurement of gamma rays from neutron-oxygen interactions

Neutrino neutral-current elastic interaction by atmospheric neutrinos is one of the main background in Supernova relic neutrino (SRN) search at Super-Kamiokande with Gadolinium. Understanding of gamma-ray production via neutron interaction on oxygen is important for the study of neutrino neutral-current elastic interactions. A measurement of gamma-rays production from such reactions was performed by using a 30 MeV quasi-mono energetic neutron beam in the Research Center for Nuclear Physics, Osaka University. In this proceedings the current status and future prospects of this mesurement are presented.

Comparing fission-product yields from photon-induced fission of 240Pu and neutron-induced fission of 239Pu as a test of the Bohr hypothesis in nuclear fission

Fission product yields (FPYs) are a uniquely sensitive probe of the fission process, with well established dependence on the species of nucleus undergoing fission, its excitation energy and spin. Thus FPYs are well suited for testing Bohr’s hypothesis in the context of nuclear fission, which states that the decay of a compound nucleus with a given excitation energy, spin and parity is independent of its formation. Using FPYs, we have performed a new highprecision test of the combined effects of the entrance channel, spin and parity on the fission process from two of the most commonly used particles to induce fission neutrons and photons. The 239 Pu(n,f) reaction at En = 4.6 MeV and the 240 Pu(γ,f) reaction at Eγ = 11.2 MeV were used to produce a 240 Pu∗ compound nucleus with the same excitation energy. The FPYs from these two reactions were measured using quasimonoenergetic neutron beams from the TUNL’s FN tandem Van de Graaff accelerator and quasimonenergetic photon beams from the High Intensity γ-ray Source (HIγS) facility. The FPYs from these two reactions are compared quantitatively for the first time.

Measurement of the234U(n,f) cross section in the energy range between 14.8 and 17.8 MeV using Micromegas detectors

The234U(n,f) reaction cross section was measured for three neutron energies 14.8, 16.5 and 17.8 MeV, relative to the238U(n,f) reference reaction. The in-beam measurements were carried out by using a set-up based on Micromegas detectors, while the quasi-monoenergetic neutron beams were produced by means of the3H(d,n)4He reaction at the 5.5MV Tandem T11/25 Accelerator Laboratory of the National Center for Scientific Research "Demokritos" in Athens (Greece). Additionaly,α-spectroscopy measurements were performed in order to determine the active mass of the samples and the corresponding impurities. In order to estimate the fission-fragment detection efficiency, Monte Carlo simulations were carried out using the GEF and FLUKA codes. Furthermore, simulations were also performed by coupling the NeuSDesc and MCNP5 codes for the determination of the neutron energy distribution in all the irradiated samples and the results were used in order to correct for the contribution of low energy parasitic neutrons in the fission yield. The final cross section data are presented, along with the methodology adopted for the treatment of the parasitic neutrons.

Measurement of the 236U(n,f) cross section at fast neutron energies with Micromegas Detectors

In the present work, the measurement of the 236U(n,f) cross section was performed, with reference to the 238U(n,f) reaction. The measurements took place at the neutron beam facility of the National Centre for Scientific Research "Demokritos" (Greece) and the quasi-monoenergetic neutron beams were produced via the 2H(d,n)3He reaction in the energy range 4–10 MeV. Five actinide targets (two 236U, two 238U and one 235U) and the corresponding Micromegas detectors for the detection of the fission fragments were used. Detailed Monte Carlo simulations were performed, on one hand for the study of the neutron flux and energy distribution at the position of each target, and on the other hand for the study of the energy deposition of the fission fragments in the active volume of the detector. The mass and homogeneity of the actinide targets were experimentally determined via alpha spectroscopy and the Rutherford Backscattering Spectrometry, respectively. The experimental procedure, the analysis, the methodology implemented to correct for the presence of parasitic neutrons and the cross section results will be presented and discussed.

Study of the 191Ir(n,2n)190Ir cross section

The 191Ir(n,2n)190Ir cross section was measured by using of the activation technique at four neutron energies in the range 10.0-11.3 MeV. The quasimonoenergetic neutron beam was produced via the 2H(d,n)3He reaction at the 5.5 MV Tandem Van de Graaff accelerator of NCSR ”Demokritos”. After the irradiations the induced activity of the samples was measured through a 56% relative efficiency HPGe detector. The cross section for the population of the second high spin (11−) isomeric state was measured along with the sum of the reaction cross section populating both the ground (4−) and the first isomeric state (1−). The experimental data will be combared to theoretical calculations in order to deduce information for the spin dependence of the level density parameters.

Validating the Bohr hypothesis: Comparing fission-product yields from photon-induced fission of 240Pu and neutron-induced fission of Pu

The Bohr hypothesis, one of the most fundamental assumptions in nuclear fission theory, states that the decay of a compound nucleus with a given excitation energy, spin and parity is independent of its formation. Using fission product yields (FPYs) as a sensitive probe, we have performed new high precision test of the combined effects of the entrance channel, spin and parity on the fission process. Two different reactions were used in a self-consistent manner to produce a compound 240Pu nucleus with the same excitation energy: neutron induced fission of 239Pu at En = 4.6 MeV and photon-induced fission of 240Pu at Eγ = 11.2 MeV. The FPYs from these two reactions were measured using quasimonoenergetic neutron beams from the TUNL's FN tandem Van de Graaff accelerator and quasimonenergetic photon beams from the High Intensity γ-ray Source (HlγS) facility. The first results comparing the FPYs from these two reactions will be presented. Implications for validating the Bohr hypothesis will be discussed.

Statistical model calculations of 72,73Ge(n,p) and 72,74Ge(n,α) reactions on natural Ge

Systematic experimental and theoretical investigations of the 72,73Ge(n,p)72,73 Ga and 72,74Ge(n,α)69,71Znm reaction cross sections are presented in the energy range from thresh- old to about 17 MeV neutron energy. The above reaction cross sections were measured from 8.8 to 11.4 MeV by using the activation method, relative to the 27Al(n,α)24Na refer- ence reaction. The quasi-monoenergetic neutron beams were produced via the 2H(d,n)3He reaction at the 5 MV VdG Tandem T11/25 accelerator of NCSR “Demokritos”. Statisti- cal model calculations using the code EMPIRE-II (version 2.19) taking into consideration pre-equilibrium emission were performed on the data measured in this work as well as on data reported in literature.

Measurement of EJ-228 plastic scintillator proton light output using a coincident neutron scatter technique

The light output function of the fast plastic scintillator EJ-228 was measured using a coincident neutron scatter measurement system and quasi-monoenergetic neutron beams produced by the tandem Van de Graaff accelerator at Triangle Universities Nuclear Laboratory. The measurement of scintillator light output using neutron scatter kinematics provides a model-independent determination of the light output function with quantifiable sources of uncertainty. Consequently, light output measurements performed using this method have a key advantage compared to other techniques: the characterization of the scintillating material depends only on the material itself and not on the size of the detector. This advantage is realized if the light output is defined as the absolute neutron light output relative to the absolute electron light output and two conditions are met: (1) the scintillator volume is fully illuminated and edge effects can be neglected during energy calibration measurements and light output measurements, and (2) the light attenuation length for scintillation photons in the material is long enough that the lowest energy interaction of interest produces a statistically significant number of scintillation photons for an interaction at any position in the scintillator volume. Commonly implemented measurement and analysis techniques for light output characterizations are explored and evidence of bias is provided for characterizations performed by analyzing the full-energy deposition edge of a light output spectrum.

Experimental study of the 197Au(n,2n) reaction cross section

In the present work, the 197Au(n,2n) reaction cross section is experimentally determined relative to the 27Al(n,α)24Na reaction at incident neutron energies of 9.0 to 10.5 MeV by means of the activation technique. The quasi-monoenergetic fast neutron beam was produced via the 2H(d,n)3He reaction at the 5.5 MV Tandem Van de Graaff accelerator at the NCSR “Demokritos” and was studied to determine the contribution of background “parasitic” neutrons using the multiple foil activation technique and the SULSA unfolding code. The cross sections for the population of the second isomeric state (12−) of 196Au and the sum of the ground (2−) and first isomeric state (5−) population cross sections were independently determined. Auxiliary Monte Carlo simulations were performed with the MCNP code.

Spectrometry and dosimetry of neutron beams produced by 7Li (p,n) reactions in the proton energy range of 3–5 MeV

Abstract Neutron spectrometry was carried out with a state of art neutron spectrometer system named ROSPEC based on multiple proportional counters, for neutron beams generated by bombarding proton beams on a 7Li target with projectile energy in the range of 3–5 MeV. Along with the neutron spectra, neutron fluence rates and ambient dose equivalent rates at the position of the detector were also measured. Neutron spectra measured by ROSPEC were compared with theoretically simulated neutron spectra. The neutron peak energy correspond to n o group of neutrons was found to be within 10%–15% of the expected peak values (theoretical values). The study also determines the neutron fluence rate and ambient dose equivalent rates per unit beam current for neutron energy below and above 50 keV for the studied proton beams energies. The characterization of the quasi mono-energetic neutron beams could be useful for the purpose of radiation protection of the users of the beam line, Boron neutron Capture Therapy (BNCT), and study of response characteristics of the neutron detectors.

Determination of the 193Ir(n, 2n) reaction cross section and correction methodology for the 191Ir(n,$ \gamma$) contamination

The cross section of the 193Ir(n, 2n)192Ir reaction has been determined by means of the activation technique, relative to the 27Al (n,$ \alpha$) and 197Au(n, 2n) reference reactions cross sections, at neutron beam energies ranging from 10 to 21 MeV. The quasi-monoenergetic neutron beams were produced at the 5.5 MV Tandem T11/25 Accelerator Laboratory of NCSR “Demokritos” via the 2H(d, n) and 3H(d, n) reactions. The induced $ \gamma$-ray activity of the irradiated target and reference foils was measured with high resolution HPGe detectors. In order to correct for the contribution of the 191Ir(n,$ \gamma$)192Ir reaction, which is open to low energy parasitic neutrons, a recently developed analysis method was implemented and it is presented in great detail. Furthermore, cross section theoretical calculations were carried out using the EMPIRE and TALYS codes over a wide energy range.

Cross section measurement of the 191Ir(n,2n)190Ir reaction

The cross section of the 191Ir(n,2n) reaction was experimentally determined relative to the 27Al(n,α)24Na reference reaction one, for incident neutron beam energies ranging from 15.3 to 20.9 MeV, by means of the activation technique. The quasi-monoenergetic neutron beams were produced at the 5.5 MV Tandem Accelerator ofNCSR "Demokritos" via the 3H(d,n)4He reaction. Following the irradiations the activity induced by the neutron beam at the targets and reference foils was measured by HPGe detectors. The cross sections for the population of the second isomeric state (m2) of 190Ir and the sum of the ground and isomeric states (g+m1+0.086 m2) were independently determined. Additionally, theoretical calculations of the above cross sections were carried out using the EMPIRE code. The details of these calculations concerning the optical model parameters, are described in the present work.

Measurement of the 58Ni(n, p)58Co and 58Ni(n, 2n)57Ni reaction cross-sections for fast neutron energies up to 18 MeV

The 58Ni(n, p) and 58Ni(n, 2n) reaction cross-sections were measured from threshold to 18MeV neutron energies using the activation and off-line $\gamma$ -ray spectroscopic technique. The quasi-monoenergetic neutron beam was generated using the 7Li(p, n) reaction. The results from the present work were compared with those of the literature and with the evaluated data from different libraries, like ENDF-B/VII.1, JENDL-4.0, CENDL-3.1, and JEFF-3.3. Nuclear model codes, like TALYS-1.9 and EMPIRE-3.2.3-Malta, were also used for a better description of the present work and literature data. The uncertainties in the measured cross-section were evaluated using the covariance analysis. The present experimental results were found to be in good agreement with those of the literature and with the evaluated data. The nuclear model code TALYS-1.9 was found successful in reproducing the experimental data for both reactions. However, the EMPIRE-3.2.3-Malta model code was found to be able to reproduce only the trend of the 58Ni(n, p) reaction excitation function. The present work provides a better insight on the comparison of both nuclear model codes. The present work is also essential for the production cross-section and the dose rate estimation of the medical isotope 58Co.

Measurement of the 236U(n,f) cross-section between 4 and 10 MeV with Micromegas detectors

In this work, the measurement of the 236U(n,f) cross-section is presented, in the energy range 4 – 10 MeV, with the use of quasi-monoenergetic neutron beams produced via the 2H(d,n)3He reaction. The measurements took place at the neutron beam facility of the National Centre for Scientific Research ‘Demokritos’. The detection of the fission fragments was achieved with Micromegas detectors placed in a fission chamber, containing both the 236U targets and the reference samples, namely 238U and 235U. The analysis of the experimental data, as well as the first results will be presented.

Determining the Energy Spectrum of Fast Neutrons in the n + 1H → n + p Reaction on the W–Be Source at the Institute for Nuclear Research, Russian Academy of Sciences

The neutron spectrum produced in the γ +9Be → 8Be + n reaction at different boundary energies of the photon spectrum is modeled in order to examine the possibility of measuring nuclear cross sections with fast quasi-monoenergetic neutron beams of the W–Be photoneutron source at the Institute for Nuclear Research. The obtained spectra are used to model the response of a hydrogen-containing detector to neutrons in the n + 1H → n + p reaction. An experimental setup for determining the energy spectrum of neutrons of the W–Be source is constructed. The spectra of secondary neutrons detected in an active hydrogen-containing target irradiated with neutrons are measured and compared to the ones modeled.