Abstract A long counter detector was manufactured by the Institute of Advanced Studies (IEAV) and was characterised in the neutron low scattering room at Brazilian National Ionising Radiation Metrology Laboratory (LNMRI/IRD) to deploy a secondary Standard for neutron fluence. The effective centre was measured experimentally with 252Cf+D2O, 252Cf, 241AmBe and 238PuBe neutron sources, having average energies from 0.55 to 4.16 MeV. The experimental arrangement and detector construction were carefully reproduced in Monte Carlo simulations, and the computational results were found to be in good agreement with those from experiment. INTRODUCTION In order to perform a reliable neutron fluence measurement, considering the difficulties of indirectly detecting this particle, it is necessary to improve methods and tools for measurements that provide results with the smallest associated uncertainty. The long counter (LC), when carefully characterised, is a suitable device for determination of fluence reference values, due to relatively flat response over a significant range of neutron energies(1, 2). This detector is historically used by reference laboratories, such as the National Physical Laboratory (NPL) or the Institute for Radiation Protection and Nuclear Safety (IRSN)(1–5). The LC characterisation starts with the determination of the effective centre as its position varies with neutron energy. This is due to its geometry composed of a cylindrical thermal neutron detector surrounded by moderator materials(4). The Brazilian National Standard Reference Laboratory for neutron fluence is the Neutron Metrology Laboratory (LN) part of the National Laboratory of Metrology of Ionising Radiation (LNMRI/IRD). Among its equipment, the LN has a LC with home made geometry, derived from the De Pangher one. This device in question was last used to measure neutron fluence in the late 1970s(6). Thus, with the intending to expand the traceability network of standards neutron fluence, this device was calibrated to act as a secondary standard instrument. Measurements were performed at the LN facilities in four neutron energy distribution produced by radionuclide sources: 241Am–Be, 252Cf, 252Cf+D2O and 238Pu–Be. The neutron sources used in this study were calibrated in the primary standardisation manganese sulphate bath system (MBS) which also belongs to LNMRI(7). Relative uncertainties in calculated emission rate range from 0.75 to 0.9% (coverage factor k = 1). The calculations using the Monte Carlo radiation transport code MCNPX(8) have been combined with the measurements to determine the effective centre in each neutron field. Long counter design and characteristics The LC characterised in this study has a cylindrical geometry of 35.0 cm length and 44.0 cm diameter. Its physical principle is based on the detection of slow neutrons in a cylindrical proportional gas LC filled with BF3 at 0.8 bars (nominal) surrounded by moderator materials. The detector, manufactured by Centronic Ltd, is 26.5 cm long with a 4.68 cm outside diameter and an active length of about 19 cm. The inner material is composed of high-density polyethylene and on the outer layer; there is a 9.9 cm thick boron loaded paraffin ring. A 0.2 cm thick cadmium coating was added to the front face to minimise direct detection of incident thermal neutrons. These structures are contained in a cylindrical aluminium case. The LC used was assembled by Institute for Advanced Studies (IEAV)(9). LC geometry and the internal structures are shown in Figure 1. Figure 1. View largeDownload slide View of LC structures considered for the MCNPX simulations. Figure 1. View largeDownload slide View of LC structures considered for the MCNPX simulations. Evaluation of the effective centres The effective centre has to be determined for this type of device. It is the distance from the front face of the LC to the calibration point to be considered to determine the distance between the neutron source and the LC for neutron fluence measurements. In general, its position does not correspond to the geometric centre, due to the moderating structures, being dependent on the incident neutron energy. Therefore, it must be well-characterised for each neutron field where the LC will be used. The method used in this study is based on the decrease of the counting rate with the distance, r, to a point source following a law, in vacuum, in 1/(r + r0)2 with r0 the effective centre distance from the LC front face. To the extent that we consider the magnitude of the all neutron-scattering effects, including neutron-scattering by the air, walls, floor and ceiling of the LC irradiation room, some correction factors must be included in the equation. In general, the results can be determined by transport calculations or by measurements with correction for effect due to source anisotropy and to the contribution of scattered neutrons with distance(10). The effective centre position can be determined by the following equation. Mt(r)=K(r+r0)2.e−Σ¯(r).×F1(θ).(a.(r2)+b.(r)+c) (1) where Mt(r) represents the total measured count rates corrected from the background and dead time. r is the reference distance from LC front face and K is the source-detector characteristic constant. The correction factors a.(r2)+b.(r)+c model represent neutron scattering by the air and by the room. The scattered neutron evaluation in this study was calculated. To perform the shadow-cone method, a significant set of cones, not available at LNMRI, would have been required. Dimensions of the low scattering room are 15.2 m length, 7.6 m width and 4.5 m high, the sources axis being at 3.2 m from the ground. The data calculated by simulation will be discussed later. The proposed method includes linear attenuation coefficient air attenuation ( Σ¯)(11) and F1(θ) anisotropy correction factor. MEASUREMENTS The LC effective centre measurements started with the characterisation of the radiation fields established by the radionuclide sources. These sources were characterised considering their spectra, anisotropies and emission rates. Source spectrum The spectra measurements were performed at LN low scattering facility using a Bonner Spheres System with a LiI(Eu) central counter. The measurements were performed with the naked counter and six polyethylene spheres with diameter ranging from 2′ to 12′. The count rates were analysed by INSPECTOR multichannel analyser from Canberra. Figure 2 shows the neutron fluence energy distributions, in relatively good agreement with the energy distribution given by ISO 8529-1 standard. The mean energy of these distributions are of 0.54 ± 0.03 MeV, 2.06 ± 0.10 MeV, 4.19 ± 0.05 MeV and 3.76 ± 0.19 MeV, respectively, for the heavy water moderated 252Cf, 252Cf, 241Am–Be and 238Pu–Be sources. Figure 2. View largeDownload slide Spectra from ISO 8529-1 252Cf+D2O, 252Cf, 241Am–Be and spectra from sources LN facility. Spectra are represented as plots of E·BE (on a linear scale) versus the neutron energy, En (on a logarithmic scale). Where BE (spectral source strength) are the ordinate values derived as BE = ∆B/∆E = Bi/(Ei+1− Ei). They have been calculated by En.BE = Bi/ln(Ei+1− Ei)(10). Figure 2. View largeDownload slide Spectra from ISO 8529-1 252Cf+D2O, 252Cf, 241Am–Be and spectra from sources LN facility. Spectra are represented as plots of E·BE (on a linear scale) versus the neutron energy, En (on a logarithmic scale). Where BE (spectral source strength) are the ordinate values derived as BE = ∆B/∆E = Bi/(Ei+1− Ei). They have been calculated by En.BE = Bi/ln(Ei+1− Ei)(10). Neutron emission rate The emission rates of the sources were measured in the reference laboratory manganese sulphate bath system LNMRI. This bath participated successfully to the CCRI(III)-K9 international comparison(7). The emission rates values are shown in Table 1. Table 1. Emission rate values for LN neutron source obtained by means of the MSB. Source Emission rate, Q (106 n s−1) 252Cf+D2O 36.059 ± 0.075 252Cf 38.268 ± 0.075 238Pu–Be 81.326 ± 0.085 241Am–Be 10.561 ± 0.079 Source Emission rate, Q (106 n s−1) 252Cf+D2O 36.059 ± 0.075 252Cf 38.268 ± 0.075 238Pu–Be 81.326 ± 0.085 241Am–Be 10.561 ± 0.079 Table 1. Emission rate values for LN neutron source obtained by means of the MSB. Source Emission rate, Q (106 n s−1) 252Cf+D2O 36.059 ± 0.075 252Cf 38.268 ± 0.075 238Pu–Be 81.326 ± 0.085 241Am–Be 10.561 ± 0.079 Source Emission rate, Q (106 n s−1) 252Cf+D2O 36.059 ± 0.075 252Cf 38.268 ± 0.075 238Pu–Be 81.326 ± 0.085 241Am–Be 10.561 ± 0.079 Anisotropy The measurements were performed with the LC at the LN low scattering facility. The neutron sources, partly due to their cylindrical geometry, always present an anisotropic neutron emission requiring to determine a related correction factor. Its emission is commonly assumed symmetric about the cylinder axis. Thus, the measurements have been performed only as a function of polar angle θ between the LC and the symmetry axis. Rotation steps of 10° in the range of 0°–180° were used for polar angle and the distance of 2 m between LC and the source. The calculations were based on Bardell studies(12). The F1(θ) anisotropy correction factors are found in Table 2. Table 2. F1(θ) anisotropy correction factors. Source F(θ) 252Cf 1.065 ± 0.005 238Pu–Be 1.043 ± 0.003 241Am–Be 1.027 ± 0.002 Source F(θ) 252Cf 1.065 ± 0.005 238Pu–Be 1.043 ± 0.003 241Am–Be 1.027 ± 0.002 Table 2. F1(θ) anisotropy correction factors. Source F(θ) 252Cf 1.065 ± 0.005 238Pu–Be 1.043 ± 0.003 241Am–Be 1.027 ± 0.002 Source F(θ) 252Cf 1.065 ± 0.005 238Pu–Be 1.043 ± 0.003 241Am–Be 1.027 ± 0.002 Effective centre The irradiations with the LC were made at the LN low scattering facility. Measurements of counting rates were made at 30 distances, from 30 up to 320 cm, between the source to the LC front face covered with a cadmium shield as illustrated in Figure 3. A least square unweighted fit, with Origin 8.0, was applied to the 30 points using Equation 1, with K and r0 as free parameters. These fits with an example given in Figure 4, have been used to determine the effective centre position r0 for each neutron source. Figure 3. View largeDownload slide LC frontal view in LN low scattering room. The distance is measured to the cadmium shield. Figure 3. View largeDownload slide LC frontal view in LN low scattering room. The distance is measured to the cadmium shield. Figure 4. View largeDownload slide The LC count rate measurements at several distance for 252Cf neutron source. Figure 4. View largeDownload slide The LC count rate measurements at several distance for 252Cf neutron source. CALCULATIONS Calculation of the effective centre position in the LC The effective centre was also calculated with the MCNPX code requiring a detailed description of LC geometry and their materials The LC-source arrangement was reproduced in a vacuum and the reference spectra ISO 8529 were used and simulated as point sources. The responses of the LC were calculated at several distances from the neutron source ranging from 30 to 400 cm. The FM14 (1 261 107) tally multiplier card is used to give the total alpha production, through the 10B(n,α)7Li reaction rates, in the BF3 active volume (264.65 cm3) per starting particle history at each source. The number of histories is the same for each distance corresponding to 109. Thus, the effective centres are analysed using the same least squares fitting method as described experimentally. Determination of the correction factors for neutron scattering The scattered neutron evaluation was performed by modelling in detail the Neutron Low Scattering Laboratory, the LC and shadow cone between the two. The source is positioned in the centre of the room 3.2 m above the ground. The shadow cone and the LC are aligned on the longitudinal axis with the source. The tally F5 was used to calculate the average neutron flux over the detector tube. A set of outputs is obtained by simulating, at several distances in a vacuum and in the air. The correction parameters a, b and c are obtained from a polynomial fit of the simulated data. These fits, shown in Figure 5, have been used to determine the scattering neutron correction factors. The corrections factors determined for the 241Am–Be source were arbitrarily also used for the 238Pu–Be, considering the close average mean energy of their energy distribution. Figure 5. View largeDownload slide Polynomial fitting to determine the scattering neutron correction factors obtained from Monte Carlo simulation for LN facility. Figure 5. View largeDownload slide Polynomial fitting to determine the scattering neutron correction factors obtained from Monte Carlo simulation for LN facility. RESULTS AND CONCLUSION The effective centre of the LC was experimentally determined and calculated for the energy distribution of the 252Cf+D2O, 252Cf, 241Am–Be and 238Pu–Be neutron sources. The comparison between experimental and calculated values of r0 is given in Table 3. There is a relatively good agreement between the two sets of data the exception of the results for 252Cf+D2O and 238Pu–Be. In this case, the discrepant results are thought to be related to the approximations of the scattering correction factors with the 238Pu–Be source and the size representation of 252Cf+D2O source, considered a point source in Monte Carlo simulation. The final values considered for the effective centre is the non-weighted average between both experimental and calculated values, as shown in Figure 4. The results showed energy dependency as expected with effective centre distance from the LC front face increasing with neutron energy, and close to those of other type of LCs(1–5). Some improvements have however to be made. A better source geometry is needed in the calculation related to the heavy water moderated californium and measurements will be made at monoenergetic neutron fields to improve the knowledge on the variation of the position of the LC effective centre with neutron energy. Table 3. Results of the effective centre. The uncertainties are quoted with coverage factor k = 2. Source Experimental (cm) Calculated (cm) 252Cf+D2O (Em = 0.55 MeV) 7.32 ± 0.44 9.14 ± 0.74 252Cf (Em = 2.13 MeV) 10.97 ± 0.34 11.15 ± 0.78 238PuBe (Em = 3.87 MeV) 11.07 ± 0.46 13.22 ± 0.79 241AmBe (Em = 4.16 MeV) 12.99 ± 0.59 12.19 ± 0.73 Source Experimental (cm) Calculated (cm) 252Cf+D2O (Em = 0.55 MeV) 7.32 ± 0.44 9.14 ± 0.74 252Cf (Em = 2.13 MeV) 10.97 ± 0.34 11.15 ± 0.78 238PuBe (Em = 3.87 MeV) 11.07 ± 0.46 13.22 ± 0.79 241AmBe (Em = 4.16 MeV) 12.99 ± 0.59 12.19 ± 0.73 Table 3. Results of the effective centre. The uncertainties are quoted with coverage factor k = 2. Source Experimental (cm) Calculated (cm) 252Cf+D2O (Em = 0.55 MeV) 7.32 ± 0.44 9.14 ± 0.74 252Cf (Em = 2.13 MeV) 10.97 ± 0.34 11.15 ± 0.78 238PuBe (Em = 3.87 MeV) 11.07 ± 0.46 13.22 ± 0.79 241AmBe (Em = 4.16 MeV) 12.99 ± 0.59 12.19 ± 0.73 Source Experimental (cm) Calculated (cm) 252Cf+D2O (Em = 0.55 MeV) 7.32 ± 0.44 9.14 ± 0.74 252Cf (Em = 2.13 MeV) 10.97 ± 0.34 11.15 ± 0.78 238PuBe (Em = 3.87 MeV) 11.07 ± 0.46 13.22 ± 0.79 241AmBe (Em = 4.16 MeV) 12.99 ± 0.59 12.19 ± 0.73 ACKNOWLEDGEMENTS The authors would like to thank IEAV for providing engineering details of the LC detector. FUNDING This study was funded by the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq/Capes). REFERENCES 1 Tagziria , H. and Thomas , D. J. Calibration and Monte Carlo modelling of neutron long counters . Nucl. Instrum. Methods A 452 , 470 – 483 ( 2000 ). Google Scholar CrossRef Search ADS 2 Roberts , N. J. , Tagziria , H. and Thomas , D. J. Determination of the effective centres of the NPL long counters. Report DQL RN004, NPL ( 2004 ). 3 Roberts , N. J. , Thomas , D. J. , Lacoste , V. , Böttger , R. and Loeb , S. Comparison of long counter measurements of monoenergetic and radionuclide source-based neutron fluence . Radiat. Meas. 45 , 1151 – 1153 ( 2010 ). Google Scholar CrossRef Search ADS 4 Lacoste , V. and Gressier , V. Experimental characterization of the IRSN long counter for the determination of the neutron fluence reference values at the AMANDE facility . Radiat. Meas. 45 , 1254 – 1257 ( 2010 ). Google Scholar CrossRef Search ADS 5 Gressier , V. , Lacoste , V. , Martin , A. and Pepino , M. Characterization of a measurement reference standard and neutron fluence determination method in IRSN monoenergetic neutron fields . Metrologia 51 ( 5 ), 431 – 440 ( 2014 ). Google Scholar CrossRef Search ADS 6 Schuch , A. L. ‘Calibração de monitores de nêutrons com moderadores e aplicação na determinação de fatores de calibração de dosímetros de albedo’. Dissertação de M.Sc. IME, Rio de Janeiro, Brasil ( 1978 ). 7 Roberts , N. J et al. . International key comparison of measurements of neutron source emission rate (1999–2005): CCRI(III)-K9.AmBe . Metrologia 48 ( S 251 Tech. Suppl. ), 06018 ( 2011 ). Google Scholar CrossRef Search ADS 8 Pelowitz , D. B. (Ed). MCNPX User’s Manual, Version 2.7.0. Los Alamos National Laboratory Report LA-CP-11–00438. 1–645 ( 2011 ). 9 Federico , C. A. et al. . Avaliação da resposta de um contador do tipo ‘Long-Counter’ para nêutrons do 241Am/Be . J. Aerospace Technol. Manag. 1 ( 2 ), 234 – 246 ( 2009 ). Google Scholar CrossRef Search ADS 10 International Organization for Standardization (ISO 8529) . Reference Neutrons Radiations Part 1: characteristics and methods of production. ISO ( 2001 ). 11 International Organization for Standardization (ISO 8529) . Reference Neutrons Radiations Part 2: calibration fundamentals of radiation protection devices related to the basic quantities characterizing the radiation field. ISO ( 2001 ). 12 Bardell , A. G. , Burke , M. , Hunt , J. B. , Tagziria , H. and Thomas , D. J. Anisotropy of emission from radionuclide neutron sources. NPL Report CIRM 24 ( Teddington, UK : NPL ) ( 1998 ). © The Author(s) 2018. Published by Oxford University Press. All rights reserved. For Permissions, please email: firstname.lastname@example.org This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/about_us/legal/notices)
Radiation Protection Dosimetry – Oxford University Press
Published: May 17, 2018
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