Measurement Of Fast Neutron Spectra Research Articles

Measurement of fast neutron spectrum in the radial channel of VR-1 research reactor

Having knowledge of the spectrum of fast neutrons in the irradiation position of a reactor plays an important role in many experimental settings. If well described, it can be used as a reference neutron field for validating and testing measuring devices or detectors. This paper compares model calculations with measurements of the neutron spectrum in the research reactor VR-1. The fast neutron spectrum was measured in the radial channel of VR-1 reactor using a stilbene scintillation detector and the NGA-01 measuring device. The NGA-01 is a two-parameter spectrometric system for neutron and gamma radiation fields. The calculations were performed in a critical mode of the MCNP6 code using ENDF/B-VII.1 nuclear data library. Generally, a good agreement was achieved. However, a slight discrepancy in the neutron spectrum between the calculated and measured values was found in the 2–3 MeV region. In other regions, the differences between calculated and experimental values are comparable with uncertainties. Since the detector is distanced from the core, the neutron beam can be assumed parallel and therefore possible issues with stilbene anisotropy are eliminated. Thus, this experiment can be used for validation of a reactor leakage spectrum and nuclear cross sections in the energy region above 6 MeV, which is of interest for the nuclear data section of International Atomic Energy Agency.

The polysiloxane-based scintillator for measurements of fast neutron spectra in nuclear physics experiments

Next generation radioactive ion beam (RIB) facilities among which the SPES facility in construction at INFN-LNL require selective detection apparatuses of high resolution and efficiency as well as high granularity and then the highest covering of the solid angle. On this line the fast neutron detection still represents an uneasy task. Among the organic scintillators, the new polysiloxane-based scintillators have appealing features for measurements of neutron spectra in RIBs applications. In this work a prototype of polysiloxane-based scintillator was fully tested to gamma, alfa and neutron radiation. The scintillator calibration was studied by comparing more than one method and the pulse shape discrimination capabilities were appropriately assessed. Three fast neutron spectra were produced at the INFN-LSN tandem accelerator by means of the 7Li(p,n)7Be reaction at different energies of the incident protons. The measurements of the produced neutron yield together with theoretical estimations based on FLUKA simulations allowed first measurements of the detection efficiency. The fast response, non-toxicity, non-flammability, cheapness and high radiation resistance together with good efficiency and pulse shape capabilities make these scintillators an interesting detector for fast neutron spectroscopy in applications with RIBs.

NEW VALIDATION OF SI CROSS-SECTION USING SILICA SAND

Silicon cross-sections, being important for a criticality safety of final spent fuel disposals, were recently reevaluated within the IAEA INDEN project. Similarly, the thermal Scattering Law matrix for silicon dioxide, which is also important for criticality safety, was also reevaluated in the ENDF/B-VIII.0 library. Due to these reasons, a series of validation experiments with silica sand were performed at the LR-0 reactor. This paper describes these validation experiments, which used two different amounts of silica sand placed in the core. The first part of validation was carried out as critical experiments in order to benefit from the suitability of integral experiments for validation. The second part -fast neutron spectrum measurement in the sand -was performed in order to obtain knowledge of its characteristics and its agreement with a calculation. The results showed significant improvement of the Thermal Scattering Law matrix for silicon dioxide available in the ENDF/B-VIII.0 library. They also showed that the new INDEN evaluation of silicon cross-sections, together with its description in the ENDF/B-VIII.0 gives disagreement rate closest to the experiments carried out without silica sand insertions. The spectrum measurement showed that the calculations of fast neutron spectra in the sand show only slight differences between different evaluations of silicon cross-sections. However, the fast neutron spectrum is not dependent on the Thermal Scattering Law. The calculated spectra show relatively good agreement with the measurement.

Determination of fast neutron spectrum using recoil-proton track imaging method

High sensitivity and good neutron/gamma discrimination, as well as online determination, are goals that researchers are pursuing for fast neutron spectrum measurements (FNSM). For this purpose, we are attempting a novel approach based on the recoil-proton track imaging (RPTI) method to do FNSM. The RPTI neutron spectrometer has been designed and built, and it mainly consists of a polyethylene foil for converting neutrons to protons, a gas scintillation chamber for generating photons along the recoil proton track, and a scientific camera system for recording the track photons in real time. Proof-of-principle experiments have been performed with the RPTI neutron spectrometer at a DT neutron generator at China Institute of Atomic Energy. As a result, numerous recoil proton track images at the single-particle level were successfully acquired and revealed in real time. Also, the proton tracks can be clearly distinguished from the background in the track images, indicating good signal-to-background ratio of the system. Particularly, both the proton ranges and the Bragg curve peak positions can be extracted online from the track images, which can lead to online generation of DT neutron spectra. These results show promising performance of the novel fast neutron spectrometer, which will bring significant advantages in various applications.

Measurement of fast neutron spectra from the interaction of 20 MeV protons with thick Be and C targets using CR-39 detector

Fast neutron spectral yield from the interaction of 20MeV protons with thick Be and C targets are measured using CR-39 detectors. An image analysing program (autoTRAK_n) dedicated for neutron spectrum generation and dose estimation is developed based on the analysis of greylevel variations within each track and is used in the present study. The most important parameters for neutron spectrometry, i.e., track depth (length) and angle of all neutron-induced recoil tracks are determined using autoTRAK_n. The methodology is tested successfully to reproduce the spectra of some standard neutron sources which was reported earlier. In this study, autoTRAK_n is applied to generate neutron spectra from the above mentioned reactions, i.e., 9Be(p, n), 12C(p, n) and the neutron yields (total number of neutrons per projectile) for both the reactions are determined by folding the track density with the detector response. The dose equivalents and H∗(10)-to-fluence ratios are also estimated. All these quantities are automatically obtained from the program. The neutron yield and the dose equivalent for 9Be(p, n) reaction are found to be about 4 times higher than that for 12C(p, n) reaction. The present methodology for neutron spectrum generation is found to be simple and effective and does not involve the complex spectrum unfolding procedures.

Acquisition of fast neutron spectra upon possible disruptions of the accelerator beam

The technique for time-of-flight measurements of fast neutron spectra upon possible disruptions of the accelerator beam is presented. The measurement procedure is corrected, the algorithm of functioning of the measuring system is refined, and a version of the hardware based on this algorithm is considered. Results of the study are discussed.

Computer studies of detection efficiency of fast-neutron spectrum based on PADC using the Monte Carlo method

A FORTRAN code for simulation of SSNTD detection efficiency is designed using the Monte Carlo method in this work. The theoretical basis for the fast-neutron spectrum measurement based on the assembly of PADC 1 1 Poly-allyl diglycol carbonate (PADC) sometimes referred to by the trade-name CR-39. detector+radiator+degrader is described. The main principle of this technique is that, the recoiled protons produced in the radiator by fast-neutrons can be slowed down in the degrader and only protons of energies within a limited range can be detected by PADC (proton energy within the PADC detection energy window). Therefore with a set of different-thickness degraders one can measure the fast-neutron spectrum. Studies show that the program can give reasonable results.

A small spherical NE213 scintillation detector for use in in-assembly fast neutron spectrum measurements

A small spherical 14 mm diameter NE213 scintillation spectrometer has been developed. Its linearity, gain stability and pulse shape discrimination were examined through experiments using a 14 MeV neutron field. The pulse responses were analyzed by a Monte Carlo code and compared with the measured ones. The detector proposed by us is superior in in-assembly measurements.

A miniaturised tissue-equivalent organic liquid scintillator for fast-neutron spectrometry

The design of a tissue-equivalent fast-neutron spectrometer based on a miniature NE213 organic liquid scintillator cell is described. The neutron-gamma discrimination, the relative monoenergetic neutron response and the angular response have been studied. The detector efficiency was measured with a calibrated isotopic neutron source. The limitations of the spectrometer for clinical measurements of fast-neutron spectra by distinguishing between fast-neutron and gamma-ray events with a static discriminator and by unfolding the spectra with a differential code are summarised.

Utilization of (n, α) and (n, p) reactions for the determination of the low level fast neutron flux of a critical facility

Threshold detectors which utilize the reactions, 58Ni(n, p) 58Co, 54Fe(n, p) 54Mn, 46Ti(n, p) 46Sc, 27Al(n, α) 24Na, 24Mg(n, p) 24Na and 48Ti(n, p) 48Sc were introduced for fast neutron spectrum measurement in the JMTRC.

High Rfsolution Fast Neutron Spectrometry without Time-of-Flight

Performance tests of a spectrometer tube of the type developed by Cuttler and Shalev show that the measurement of fast neutron spectra with this device can be made with an energy resolution previously obtainable only in large time-of-flight facilities. In preliminary tests, resolutions of 16.4 keV for thermal neutrons and 30.9 keV for 1-MeV neutrons were obtained. A broad-window pulse-shape discrimination (PSD) system is used to remove fram pulse-height distributions most of the continua due to 3He-recoil events, noise, and wall effect. Use of PSD improved the energy resolution to 12.9 keV for thermal neutrons and 29.2 keV for l-MeV neutrons. The detector is a viable tool for neutron research at nominally equipped accelerator laboratories.

A high-resolution cylindrical proportional counter for neutron-spectrum measurements in a fast critical assembly

Compact proportional counters filled with hydrogen and with methane, respectively, have been constructed for measurements of fast-neutron spectra in a fast critical assembly. Field tubes made by metallizing a ceramic-seal surface are satisfactory to remove recoil events in the volume defined by the tubes. The best resolution of the hydrogen-filled counter is 3.8% at a gas multiplication 17.6. The total number of electrons collected before saturation in gas multiplication occurs is about 2 × 10 5 when initial ionizations are highly localized or the direction of a recoil-proton track is nearly perpendicular to the anode wire. A value of 14 MeV is recommended to the product of energy loss and gas multiplication as a critical value for the proportional mode of operation. This value is about one seventh as large as the values reported previously.

The calibration of 6Li semiconductor sandwich spectrometers

The 6Li Semiconductor Sandwich Spectrometers which are commonly used for the measurement of fast-neutron spectra, especially in reactors, are subject to a variety of effects which, if not taken into account, can lead to significant calibration errors. The effects which are here discussed in detail are: 1. (1) the energy degradation in the sensitive layer of the spectrometer, 2. (2) the alpha-particle contribution to the “response function” used to deconvolve the lower-energy part of the neutron spectra, and 3. (3) the superposition of a high-energy contribution on the lower part of the measured spectrum in one mode of operation of the spectrometer. It will be shown that the failure to take account of these effects can in some circumstances lead to energy-calibration errors of ∼30 keV and errors of up to 50% in measured neutron spectra.

Fast-neutron spectrum measurement by a stilbene scintillation spectrometer

A stilbene scintillation spectrometer for continuous fast-neutron spectra measurement is described in this paper. Discrimination of gamma rays is achieved by utilizing differences in the shapes of scintillation pulses produced by recoiled protons and electrons. In addition, results of measurements of fast-neutron spectra of the reactor beam are given.

Measurements of Fast-Neutron Spectra in Reactor Shields

Experimental techniques previously used for measuring neutron spectra in fast critical assemblies have been applied to reactor shielding problems. Fast-neutron spectra in the energy range from 25 MeV to 1 MeV have been measured in the water shield surrounding a small thermal research reactor. Three different water thicknesses were investigated, as well as a combination of water with a 2-in.-thick slab of iron. The spectrometer system developedfor these measurements employs small proton-recoil proportional courters, together with electronic pulse-shape discrimination for the rejection of gamma-ray signals. Since this discrimination is extremely important for shielding measurements, the detectors and electronic system are described in some detail. The basic theory underlying proton-recoil measurements of neutron spectra is reviewed, as well as the techniques used to correct for errors arising due to finite detector size and irregularities in the electric field within the counters. Results of the water measurements indicate that the spectrometer is capable of measuring the absolute neutron flux as well as relative variations within the spectrum at a given point. In addition, predictions of spectral hardening with increasing water thickness are verified. The water-iron measuremerts indicate that the system is capable of measuring spectral features caused by resonances in the crossmore » section of the shield material. (auth)« less

The use of the lithium-6 semiconductor sandwich spectrometer for the measurement of fast-neutron spectra

In order to meet the stringent target accuracies for neutron spectrum measurements in fast reactor assemblies, the existing experimental methods must be refined and the sources of experimental error identified and, where possible, eliminated. The construction and properties of the lithium-6 semiconductor detector are discussed. The technique may be used to measure neutron spectra over the energy range (10 keV–6 MeV). Major sources of experimental error have been isolated, enabling improvements to be incorporated into the existing electronics system. The life-span of the detectors is severely limited by fast neutron radiation damage of the silicon diodes. The 6Li cross-section has been remeasured to an accuracy of ±4% (one standard deviation). The method of background subtraction has been tested and found to be adequate. Fast neutron spectra have been measured in three ZEBRA assemblies in the energy range (10 keV–6 MeV) and their accuracies assessed. The experimental results are compared with results from competing experimental techniques and with the theoretical predictions made using current methods and data. It is concluded that the results provide a sufficient overlap with existing techniques to give an accurate measurement of the fast-neutron spectrum above 1 MeV.

Measurement of Fast Neutron Spectrum Outside Beam Port by Li-6 Semiconductor Spectrometer

The neutron spectrum outside a beam port of the Tsing Hua Open-pool Reactor (THOR) has been obtained by using a 6LiF sandwich-type semiconductor neutron spectrometer. Only those neutrons between 0.5 and 8MeV have been shown. The spectrum falls off exponentially above 4.5 MeV. The valley in the spectra at about 3.5 MeV may be due to the elastic scattering resonance of oxygen at that energy.

Measurement of Fast Neutron Spectrum with Helium-3 Proportional Counter

Measurement of Fast Neutron Spectrum with Helium-3 Proportional Counter

Measurement of Fast Neutron Spectrum with Helium-3 Proportional Counter

Experiments have been performed with the view to applying the 3He proportional counter to differential measurements of the continuous neutron spectrum in a fast reactor. The results of measurements in the JAERI Fast Critical Facility (FCA) proved the apparatus to be useful for the measurements of the main part of the fast reactor spectrum. A specially devised electronic circuit was employed to eliminate the effect of differences in the rise-times of pulses; the energy resolution of the spectrometer was 25 keV (F.W.H.M.). The modified inverse matrix method was used to obtain the neutron spectrum from the measured pulse-height distribution, resulting in an improvement of the effective energy resolution in the resultant spectrum to about 10 keV. The applicable energy range of the spectrometer is for the time being from 20 keV to 2 MeV.

Measurement of Fast Neutron Spectrum Outside Beam Port by Li-6 Semiconductor Spectrometer

Measurement of Fast Neutron Spectrum Outside Beam Port by Li-6 Semiconductor Spectrometer