Monoenergetic Neutron Fields Research Articles

Spectral flux of the p-7Li(C) Q-M neutron source measured by proton recoil telescope

The cyclotron-based fast neutron source at NPI produces mono-energetic neutron fields up to 35 MeV neutron energy using the p + 7Li(carbon backing) reactions. To be applied for activation cross-section measurements, not only the intensity of neutron peak, but also the contribution of low-energy continuum in the spectra must be well determined. Simulations of the spectral flux from present source at a position of irradiated samples were performed using CYRIC TOF-data validated in the present work against LA150h by calculations with the transport Monte Carlo code MCNPX.Simulated spectra were tested by absolute measurements using a proton-recoil telescope technique. The recoil-proton spectrometer consisted of a shielded scattering chamber with polyethylene and carbon radiators and the ΔE1-ΔE2-E telescope of silicon-surface detectors located to the neutron beam axis at 45° in the laboratory system. Si-detectors were handled by usual data acquisition system. Dead-time – and pulse-overlap losses of events were determined from the count rate of pulse generator registered during duty cycle of accelerator operation. The proton beam charge and data were taken in the list mode for later replay and analysis.The calculations for 7 Li(p,n) and 12 C(p,n) reactions reasonably reproduce CYRIC TOF neutron source spectra. The influence of neutron source set-up (proton beam dimensions, 7 Li-foil, carbon stopper, cooling medium, target support/chamber and the geometry-arrangement of irradiated sample) on the spectral flux is discussed in details.

Performance of fluorescence nuclear track detectors in mono-energetic and broad spectrum neutron fields

Properties of fluorescent nuclear track detectors (FNTDs) made of Al 2O 3:C, Mg crystals are investigated after irradiation in mono-energetic and broad spectrum neutron fields. Two types of neutron converters were investigated, high density polyethylene and natural LiF. Imaging in fluorescence contrast is performed using laser scanning confocal detection. Individual tracks induced by recoil protons, alpha particles and tritium ions are identified and counted. Two approaches to median neutron energy estimation using track density as a function of scanning depth and the ratio of track densities for two different converters are discussed.

Novel recoil nuclei detectors to qualify the AMANDE facility as a standard for mono-energetic neutron fields

The AMANDE facility at IRSN-Cadarache produces mono-energetic neutron fields from 2 keV to 20 MeV with metrological quality. To be considered as a standard facility, characteristics of neutron field i.e. fluence distribution must be well known by a device using absolute measurements. The development of new detector systems allowing a direct measurement of neutron energy and fluence has started in 2006. Using the proton recoil telescope principle with the goal of increase the efficiency, two systems with full localization are studied. A proton recoil telescope using CMOS sensor (CMOS-RPT) is studied for measurements at high energies and the helium 4 gaseous μ-time projection chamber (μ-TPC 4He) will be dedicated to the lowest energies. Simulations of the two systems were performed with the transport Monte Carlo code MCNPX, to choose the components and the geometry, to optimize the efficiency and detection limits of both devices or to estimate performances expected. First preliminary measurements realised in 2008 demonstrated the proof of principle of these novel detectors for neutron metrology.

Neutron fluence spectrometry using disk activation

A simple and robust detector for spectrometry of environmental neutrons has been developed. The technique is based on neutron activation of a series of different metal disks followed by low-level gamma-ray spectrometry of the activated disks and subsequent neutron spectrum unfolding. The technique is similar to foil activation but here the applied neutron fluence rates are much lower than usually in the case of foil activation. The detector has been tested in quasi mono-energetic neutron fields with fluence rates in the order of 1000–10000 cm −2 s −1, where the obtained spectra showed good agreement with spectra measured using a Bonner sphere spectrometer. The detector has also been tested using an AmBe source and at a neutron fluence rate of about 40 cm −2 s −1, again, a good agreement with the assumed spectrum was achieved.

Calibration procedure for a neutron monitor at energies below 20 MeV

A liquid scintillation detector aimed for neutron energy and fluence measurements in the energy region below 20MeV has been calibrated using monoenergetic and white spectrum neutron fields. Careful measurements of the proton light output function and the response matrix have been performed allowing for the application of unfolding techniques using existing codes. The response matrix is used to characterize monoenergetic neutron fields produced by the T(d,n) reaction at low deuteron energies.

High-energy response of the PRESCILA and WENDI-II neutron rem meters

WENDI-II was designed at the Los Alamos National Laboratory (LANL) specifically as a wide-range rem meter, suitable for applications at particle accelerators, with response extension to 5 GeV. PRESCILA was also designed at LANL, mainly as a lightweight alternative to traditional rem meters, but has shown excellent response characteristics above 20 MeV. This Note summarises measurements performed over a span of 4 y to characterise the high-energy neutron response (>20 MeV) of these meters to several hundred million electron volts. High-energy quasi-monoenergetic beams utilised as part of this study were produced by the cyclotron facilities at the Université Catholique de Louvain (33 and 60 MeV) and the T. Svedberg Laboratory ( 46, 95, 143 and 173 MeV). In addition, measurements were also conducted at the Los Alamos Neutron Science Center, 800 MeV spallation neutron source, in broad energy fields with an average energy of 345 MeV. For the sake of completeness, data collected between 2.5 and 19 MeV in monoenergetic neutron fields at the German Physikalisch-Technische Bundesanstalt (PTB) facility are also included in this study.

Response of semiconductor-type electronic personal dosemeter to high-energy neutrons

In order to improve an active type dosemeter for protection dosimetry against high-energy neutrons, characteristic response of an electronic personal dosemeter consisting of Si semiconductor detector covered with polyethylene radiator has been checked in monoenergetic neutron fields of 14.8 and 65 MeV. Pulse height distributions have been measured for several different radiator thicknesses. An analytical estimation of distributions in energy dissipated in depletion layer of Si detector has been carried out under simplified conditions. A good agreement was confirmed between both distributions in the shape and its variation with the radiator thickness and the neutron energy. The energy dependence of the dosemeter response has also been discussed. It is suggested that the energy dependence could be compensated by introducing the two-window technique with different discrimination levels on the pulse height.

A monitor for neutron flux measurements up to 20 MeV

A liquid scintillation detector aimed for neutron energy and fluence measurements in the energy region <20 MeV has been calibrated using monoenergetic and white spectrum neutron fields. Careful measurements of the proton light output function and the response matrix have been performed allowing for the application of unfolding techniques using existing codes. The response matrix is used to characterise monoenergetic neutron fields produced by the T(d,n) at a low-energy deuteron accelerator installed at the Swedish Defense Research Agency (FOI).

Performance of the UAB and the INFN-LNF Bonner sphere spectrometers in quasi monoenergetic neutron fields

In the framework of collaboration between the Universidad Autónoma de Barcelona and the INFN Frascati National Laboratories, an experimental Bonner Sphere neutron spectrometry exercise has been performed in the 2.5 MeV and 14.2 MeV quasi monoenergetic neutron beams of the ENEA Fast Neutron Generator. The neutron spectra at given distances from the accelerator target have been determined, taking advantage of the new unfolding FRUIT code, recently developed by the LNF group. The results show a good coherence between the two spectrometers, and between the measured and simulated data.

Feasibility study of extremity dosemeter based on polyallyldiglycolcarbonate (CR-39) for neutron exposure

In nuclear facilities, some activities such as reprocessing, recycling and production of bare fuel rods expose the workers to mixed neutron-photon fields. For several workplaces, particularly in glove boxes, some workers expose their hands to mixed fields. The mastery of the photon extremity dosimetry is relatively good, whereas the neutron dosimetry still raises difficulties. In this context, the Institute for Radiological Protection and Nuclear Safety (IRSN) has proposed a study on a passive neutron extremity dosemeter based on chemically etched CR-39 (PADC: polyallyldiglycolcarbonate), named PN-3, already used in routine practice for whole body dosimetry. This dosemeter is a chip of plastic sensitive to recoil protons. The chemical etching process amplifies the size of the impact. The reading system for tracks counting is composed of a microscope, a video camera and an image analyser. This system is combined with the dose evaluation algorithm. The performance of the dosemeter PN-3 has been largely studied and proved by several laboratories in terms of passive individual neutron dosemeter which is used in routine production by different companies. This study focuses on the sensitivity of the extremity dosemeter, as well as its performance in the function of the level of the neutron energy. The dosemeter was exposed to monoenergetic neutron fields in laboratory conditions and to mixed fields in glove boxes at workplaces.

International key comparison of neutron fluence measurements in mono-energetic neutron fields: CCRI(III)-K10

CCRI Section III (neutron measurements) conducted a unique key comparison of neutron fluence measurements in mono-energetic neutron fields. In contrast to former comparisons, here the fluence measurements were performed with the participants' instruments in the same neutron fields at the PTB accelerator facility. Seven laboratories—the CIAE (China), IRMM (EC), NMIJ (Japan), NIST (USA), NPL (UK), PTB (Germany) and the VNIIM (Russia)—employed their primary standard reference methods or transfer instruments carefully calibrated against their primary standards, to determine the fluence of 0.144 MeV, 1.2 MeV, 5.0 MeV and 14.8 MeV neutrons and reported calibration coefficients for a selected neutron monitor and each neutron energy with a detailed uncertainty budget for the measurements.The key comparison reference values (KCRV) were finally evaluated as the weighted mean values of the neutron fluence at 1 m distance from the target in vacuum per neutron monitor count. The uncertainties of each KCRV amounted to about 1%. The degree of equivalence (DoE), defined as the deviation of the result reported by the laboratories for each energy from the corresponding KCRV, and the associated expanded uncertainty are also reported. The deviations between the results of two laboratories each with the corresponding expanded uncertainties complete the documentation of the degrees of equivalence.Main text.To reach the main text of this paper, click on Final Report. Note that this text is that which appears in Appendix B of the BIPM key comparison database kcdb.bipm.org/.The final report has been peer-reviewed and approved for publication by the CCRI Section III, according to the provisions of the CIPM Mutual Recognition Arrangement (MRA).

(n,p) reactions contribution in spherical proton recoil proportional counter responses

Proton recoil proportional counters are used in several metrological institutes as reference neutron spectrometers within the energy range from a few tenth of keV up to a few MeV. When such counters are used to determine the low energy range of neutron fields with energies above 5 MeV, protons generated by (n,p) reactions within the stainless steel counter walls will add their contribution to the proton recoil process. In this work, this contribution has been estimated in the calculated response of the counter, using MCNPX Monte Carlo code. The validation of the calculated response has been performed at several AMANDE mono-energetic neutron fields.

Determination of the full response function of personal neutron dosemeters

The response of neutron dosemeters may be determined directly from measurements, provided a sufficiently large number of measurements in monoenergetic neutron fields covering the entire energy range of interest is available. In practice this is not feasible due to the lack of monoenergetic neutron fields in the thermal and intermediate energy region (i.e. energies<24 keV). To deal with this difficulty, we have developed a method which can take into account additional information about the response of the dosemeter. Our analysis makes use of two types of data, measurements made using monoenergetic neutron beams and measurements made in neutron fields with broad energy distributions. The dosemeter responses are described using a parametrised model, based on a minimum of assumptions: that they should fit the data within experimental uncertainties, and that they should remain close to a simple interpolation of the monoenergetic and thermal neutron field data.

Improved characterisation of the HPA PADC neutron personal dosemeter

The fast neutron energy dependence of response of the HPA PADC neutron personal dosemeter has been measured from 144 keV to 19 MeV using monoenergetic neutron fields. Below 144 keV the relative energy and angle dependence of response have been determined using MCNP-4C2. New data from the SIGMA field at Cadarache, France, have been used to determine the appropriate scaling factor for the calculated response to thermal and intermediate energy neutrons. These newly determined response characteristics of the dosemeter are discussed with respect to its performance in the EVIDOS workplace field irradiations.

A tissue equivalent proportional counter filled with a mixture of tissue equivalent gas and 3He for neutron monitoring

Tissue equivalent proportional counters (TEPC) allow the measurement of the ambient dose equivalent H(*)(10) in mixed fields. IRSN has been studying the design and the response of a TEPC in terms of neutron H(*)(10). First, a cylindrical counter was filled with propane gas at a low pressure. H(*)(10) measured in monoenergetic neutron fields underestimated the reference (>50%) at low energies (< 500 keV). A small amount of (3)He was then added to the gas in order to increase the response. The underestimation observed decreased but the results (> 40%) were not totally complying with the objectives (< 20%). Finally the choice was made to improve the analysis of the microdosimetric spectra y.d(y) in order to identify the energy of the incident neutrons. The analysis allows a better estimate of H(*)(10). The aim of this article is to describe the TEPC and the effect of these methods of optimisation.

NIST Calibration of a Neutron Spectrometer ROSPEC.

A neutron spectrometer was acquired for use in the measurement of National Institute of Standards and Technology neutron fields. The spectrometer included options for the measurement of low and high energy neutrons, for a total measurement range from 0.01 eV up to 17 MeV. The spectrometer was evaluated in calibration fields and was used to determine the neutron spectrum of an Americium-Beryllium neutron source. The calibration fields used included bare and moderated 252Cf, monoenergetic neutron fields of 2.5 MeV and 14 MeV, and a thermal-neutron beam. Using the calibration values determined in this exercise, the spectrometer gives a good approximation of the neutron spectrum, and excellent values for neutron fluence, for all NIST calibration fields. The spectrometer also measured an Americium-Beryllium neutron field in a NIST exposure facility and determined the field quite well. The spectrometer measured scattering effects in neutron spectra which previously could be determined only by calculation or integral measurements.

A Monte Carlo Program for Estimating Characteristics of Neutron Calibration Fields Using a Pelletron Accelerator

A Monte Carlo code system, MCNP-ANT, has been developed to simulate the transport of charged particles (<4MeV) and the neutron production in an accelerator neutron target as well as the neutron transport. The code system consists of a newly developed Monte Carlo code for charged particle transport and neutron production, and an existing neutron transport code, MCNP-4B. The available reactions in the code system are 2H(d,n)3He, 3H(d,n)4He, 3H(p,n)3He, 7Li(p,n)7Be and 45Sc(p,n)45Ti as main neutron sources. The calculated neutron spectra by MCNP-ANT were compared with the experimental ones in typical monoenergetic neutron fields.

Neutron and photon spectrometry in monoenergetic neutron fields

In monoenergetic neutron fields (144 keV< E n<19 MeV), spectrometry was performed for prompt photons as well as for scattered neutrons and for neutrons produced by nuclear reactions. Time-of-flight (TOF) experiments with NE213 scintillation detectors were carried out, and spectral fluences were obtained from the unfolding of the measured pulse height spectra. Doses of the prompt photons were calculated and compared with those obtained by measurements with Geiger–Müller counters (GM). The differences between both are remarkable, and they are for most fields due to the energy dependence of the GM response for high-energy photons. For the neutrons straightforward unfolding of a spectrum recorded without TOF selection and containing in its major parts also the response of neutrons with E n>8 MeV is, however, hampered due to insufficient simulation of the response functions in regions where α-particles from the 12C(n,n′)3α reaction dominate the light output.

Characterisation of spherical recoil proton proportional counters used for neutron spectrometry

The Institute for Protection and Nuclear Safety (IPSN) standard neutron detector in the energy range 60–800 keV is a spherical proportional counter of HARWELL type SP2 nominally filled with 300 kPa hydrogen. It was characterised in the monoenergetic neutron fields of PTB at the energies of 144, 250 and 565 keV, where the neutron energy and fluence were determined with the PTB reference instruments. The neutron fields produced at the same energies with the accelerator facility of Bruyères-le-Châtel were then investigated with the calibrated SP2 counter and various PTB instruments in order to determine the mean energy and the neutron fluence. The energy scale and a neutron fluence monitor were calibrated.

Calibration and Monte Carlo modelling of neutron long counters

The Monte Carlo technique has become a very powerful tool in radiation transport as full advantage is taken of enhanced cross-section data, more powerful computers and statistical techniques, together with better characterisation of neutron and photon source spectra. At the National Physical Laboratory, calculations using the Monte Carlo radiation transport code MCNP-4B have been combined with accurate measurements to characterise two long counters routinely used to standardise monoenergetic neutron fields. New and more accurate response function curves have been produced for both long counters. A novel approach using Monte Carlo methods has been developed, validated and used to model the response function of the counters and determine more accurately their effective centres, which have always been difficult to establish experimentally. Calculations and measurements agree well, especially for the De Pangher long counter for which details of the design and constructional material are well known. The sensitivity of the Monte Carlo calculations for the efficiency of the De Pangher long counter to perturbations in density and cross-section of the polyethylene used in the construction has been investigated.