TY - JOUR
AU1 - Gómez-Ros,, J.M.
AU2 - Bedogni,, R.
AU3 - Bortot,, D.
AU4 - Domingo,, C.
AU5 - Esposito,, A.
AU6 - Introini,, M.V.
AU7 - Lorenzoli,, M.
AU8 - Mazzitelli,, G.
AU9 - Moraleda,, M.
AU1 - Pola,, A.
AU1 - Sacco,, D.
AB - Abstract This communication describes two new instruments, based on multiple active thermal neutron detectors arranged within a single moderator, that permit to unfold the neutron spectrum (from thermal to hundreds of MeV) and to determine the corresponding integral quantities with only one exposure. This makes them especially advantageous for neutron field characterisation and workplace monitoring in neutron-producing facilities. One of the devices has spherical geometry and nearly isotropic response, the other one has cylindrical symmetry and it is only sensitive to neutrons incident along the cylinder axis. In both cases, active detectors have been specifically developed looking for the criteria of miniaturisation, high sensitivity, linear response and good photon rejection. The calculated response matrix has been validated by experimental irradiations in neutron reference fields with a global uncertainty of 3%. The measurements performed in realistic neutron fields permitted to determine the neutron spectra and the integral quantities, in particular H*(10). INTRODUCTION The measurement of neutron spectra is an essential issue for the accurate determination of radiation protection quantities whose dependence on fluence varies nearly two orders of magnitude depending on neutron energy, according to the variation in the quality factor Q values with neutron energy. The accurate determination of radiation protection quantities for neutrons depends critically of the fluence spectrum(1). One of the most widely used techniques for measuring these neutron spectra is the Bonner Sphere Spectrometer (BSS) that has demonstrated its capability as a well-established technique(2). Nevertheless, the use of BSS requires multiple exposures with long time irradiation sessions so it is not suitable to operate as a real-time monitor or in pulse fields. During the last years, two new instruments called respectively Spherical Spectrometer (SP2) and Cylindrical Spectrometer (CYSP) have been developed within the framework of a collaboration between Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT, Spain), Instituto Nazionale di Fisica Nucleare (INFN, Italy), Politecnico di Milano (Italy) and Universidad Autónoma de Barcelona (UAB, Spain) supported by Spanish and Italian projects. Both instruments are based on multiple active thermal neutron detectors located in different positions within a single moderating structure(3–6). Thus, the response of the detector located at position α, Mα, is as follows: Mα=∫dEΦERα(E)(1) where ΦE is the energy distribution of fluence and Rα(E) is the response function for the detector located at position α. This equation is formally equal to the integral equation describing the response of a BSS(2) so the same unfolding methods can be applied to obtain the neutron spectrum from the measured values once re response matrix is known. New low cost active thermal neutron detectors have been developed by depositing a 30 μm layer of 6LiF on commercially available windowless diodes(7–9). The process allows to produce simultaneously many detectors with no other costs than the compound to be deposited and the diodes themselves. As it is described in the following sections, the SP2 is a spherical active neutron spectrometer consisting of multiple thermal neutron detectors arranged along three perpendicular axis within a single moderating sphere A nearly isotropic response is obtained by averaging the response of the detectors placed at the same radial distance, thus resulting in a device than requires only one exposure to determine the whole spectrum(4, 5). For the CYSP, the thermal neutron detectors are located along the axis of a cylindrical moderating structure made of polyethylene, borated plastic and lead that provides a strong directional response within the energy interval from thermal to hundreds of MeV, being nearly insensitive to neutrons coming from directions other than the cylinder axis. Therefore it will be especially suitable for applications where the neutron spectrum as a function of the emission angle needs to be measured. MATERIALS AND METHODS Active thermal neutron detectors Because of the number of thermal neutron detectors that should be embedded in the devices, new thermal neutron active detectors needed to be developed focusing on four main characteristics:(9) low cost, reduced size, good photon rejection and linear H*(10) response within the required range (from fractions of μSv/h to hundreds of mSv/h). As it has been described in previously published papers(7, 8), the new detectors have been produced by deposition of a 30 μm layer of 6LiF on commercially available windowless p-i-n diodes, with active area either of 1 × 1 cm2 (overall dimension around 1.5 × 1.2 × 0.4 cm3) or 2.8 × 2.8 cm2 (overall dimension around 3.5 × 3.5 × 0.6 cm3). The linearity of the response to thermal neutrons has been verified in the ENEA-Casaccia TRIGA reactor(9) for thermal fluence rates in the range 1.7 × 103–1.7 × 105 cm−2s−1. Moreover, their individual sensitivity has been measured by irradiation in a cylindrical cavity, 10 cm high and 4 cm diameter, along the axis of a moderating polyethylene cylinder, 25 cm height and 25 cm diameter, with a standard X3 Am–Be source(8). The response variability is reduced to less than ±3%. Simulation and design Both the optimised design of the spectrometers and the calculation of the corresponding response matrix have been performed using the Monte Carlo code MCNPX 2.7(10), with the ENDF/B-VII cross-section library for neutrons with energies below 20 MeV and the thermal neutron S(α,β) cross-section data for polyethylene(11). Neutron transport above 20 MeV has been modelled using Bertini intra-nuclear cascade model and Dresner evaporation model(12). A cut-off in the number of histories has been applied to obtain statistical uncertainties lower than 3% in all cases. Modelling of the geometries has been done using the MCAM interface program(13, 14). The response of the detectors has been calculated in two ways: (1) assuming its proportionality to the number of neutron induced reactions, N(n,α), within the 6LiF converter layer volume normalised per unit incident fluence which was obtained using the modified track-length scoring option for the fluence (F4 tally), i.e.: N(n,x)=∫dEΦEVρatσ(n,x)(2) where σ(n,α) is the microscopic cross-section for (n,α) reactions, V is the volume, ρat is the atomic density and ΦE is the energy distribution of neutron fluence; and (2) calculating the deposited energy in Si by secondary charged particles (F6 tally multiplied by mass). THE NEW NEUTRON SPECTROMETERS Two new neutron spectrometers have been developed based on the arrangement of the thermal neutron detectors described in Section 2.1 at selected positions within a moderating volume of polyethylene. The pulses from the detectors are processed through an analog chain made of charge preamplifier and shaper amplifier, followed by a commercial digitizer NI USB 6366 operating in streaming mode. The thermal neutron contribution is separated from the continuous distribution of secondary electrons in the measured pulse height distribution with a threshold placed at 0.6 V. The spherical spectrometer SP2 The design of the instruments is based on the results of detailed simulation studies(3, 4) demonstrating that a single moderating sphere embedding thermal neutron detectors symmetrically arranged permits to obtain a nearly isotropic fluence response and spectrometric capability in the energy range from thermal up to hundreds of MeV. The SP2 spectrometer, as it is shown in Figure 1, consists of thirty thermal neutron detectors along three perpendicular axes at five radial distances (6, 9, 10.5, 12 and 13.5 cm) plus another one at the centre of a polyethylene sphere of diameter 29.5 cm. An internal 1 cm thick lead shell between 3.5 and 4.5 works as an energy converter via (n,xn) reactions both for the central detector and for those located at nearest distances, thus enhancing the response above 20 MeV. As it has been discussed in previous works(3, 4), a nearly isotropic response is obtained by averaging the individual response of every six detectors located at the same radial distance, i.e. Equation (1) becomes: M̅r=∫dEΦER̅r(E)(3) where ΦE is the energy distribution of fluence, M̅r is the averaged response of the detectors located at the same radial distance r and R̅r(E) is the corresponding response function averaged for these detectors(3). Figure 1. Open in new tabDownload slide (a) Schematic 3D view of the SP2, showing the position of the detectors (cavities for cables have been removed to simplify the figure) and (b) assembled SP2 with the associated electronic. Figure 1. Open in new tabDownload slide (a) Schematic 3D view of the SP2, showing the position of the detectors (cavities for cables have been removed to simplify the figure) and (b) assembled SP2 with the associated electronic. Figure 2 shows the response matrix R̅r(E) for three irradiation geometries: one isotropic and two parallel along incident directions (1 0 0) and (1 1 1), both referred to the three perpendicular axes of detectors. As it can be seen, the anisotropy is practically negligible for all the radial distances but for the detectors located at the shallowest positions (r = 13.5 cm). Figure 2. Open in new tabDownload slide Response matrix for the SP2 calculated assuming three irradiation geometries: isotropic (continuous line); along one of the axis of detectors, labelled (1 0 0) (dashed line); and along a direction with equal angle to the three axes of detectors, labelled (1 1 1) (dotted line). Figure 2. Open in new tabDownload slide Response matrix for the SP2 calculated assuming three irradiation geometries: isotropic (continuous line); along one of the axis of detectors, labelled (1 0 0) (dashed line); and along a direction with equal angle to the three axes of detectors, labelled (1 1 1) (dotted line). The influence of this variations in the response matrix depending on the neutron incidence geometry was studied by simulating the exposure to different neutron sources under the three irradiation geometries, isotropic, (1 0 0) and (1 1 1), as well as considering an isotropic source in a concrete wall room. Then, the unfolded spectrum was obtained using the code FRUIT(15) with the response matrix calculated for isotropic irradiation(3, 4). The discrepancy between the unfolded values and the reference ones for the total fluence and the ambient dose equivalent are less than 9 and 6% in all the cases. Only in the very unusual case of a directional thermal neutron field, the anisotropy effect due to the shallowest detectors could be significant but the problem will be solved just with the addition of extra detectors at the corresponding radial distance(3). A previous version of the SP2 equipped with Dysprosium activation foils was used to measure the photoneutron spectrum in the treatment room of a 15 MV Varian CLINAC accelerator, comparing the results with those obtained with a BSS set(16). This comparison also provided a very satisfactory result either for the unfolded spectrum as well as for the determination of the integral quantities fluence and ambient dose equivalent (Table 1). Table 1. Comparison of the neutron fluence and ambient dose equivalent per photon absorbed dose, measured with a BSS and the SP2 in the treatment room of a 15 MV Varian CLINAC accelerator. Quantity . BSS . SP2 . Φ (×108 cm−2) 4.77 ± 0.15 4.35 ± 0.30 H*(10)/D (mSv/Gy) 0.38 ± 0.02 0.36 ± 0.04 Quantity . BSS . SP2 . Φ (×108 cm−2) 4.77 ± 0.15 4.35 ± 0.30 H*(10)/D (mSv/Gy) 0.38 ± 0.02 0.36 ± 0.04 Table 1. Comparison of the neutron fluence and ambient dose equivalent per photon absorbed dose, measured with a BSS and the SP2 in the treatment room of a 15 MV Varian CLINAC accelerator. Quantity . BSS . SP2 . Φ (×108 cm−2) 4.77 ± 0.15 4.35 ± 0.30 H*(10)/D (mSv/Gy) 0.38 ± 0.02 0.36 ± 0.04 Quantity . BSS . SP2 . Φ (×108 cm−2) 4.77 ± 0.15 4.35 ± 0.30 H*(10)/D (mSv/Gy) 0.38 ± 0.02 0.36 ± 0.04 A partial validation of the SP2 response matrix has been done using the reference INFN-LNF (Laboratori Nazionale di Frascati) Am–Be source to compare the measured and the simulated response of the detectors located along a radius of the sphere for three irradiation geometries(17). The comparison between measured and calculated count rates (after correction for individual sensitivity and applying the calibration factor to the simulated values) resulted to be accurately enough to determine the response of the SP2 for the energy range of the Am–Be source (within an overall uncertainty <3%). The cylindrical spectrometer CYSP The CYSP consists of seven active thermal neutron detectors located along the axis of a polyethylene cylinder located inside a more complex structure (Figure 3a and b). The geometry has been optimised after evaluating an exhaustive set of possible configurations looking for a collimated directional response in the energy range from thermal up to some hundreds of MeV(6). The resulting instrument is a cylinder 65 cm long and 50 cm diameter consisting of two main parts: a collimator 30 cm long with a 15 cm diameter mouth covered by a 5 mm layer of borated plastic and a main body with seven detectors located along the axis at distances 4, 6, 8, 10, 12, 14 and 21 cm from the end of the collimator. A lead disk 1 cm thick is located at 17 cm to increase the response to high energy neutrons of detectors located at deeper positions (12, 14 and mainly 21 cm) through the (n,xn) reactions that allows to extend the sensitivity up to the high energy range. A ring of air holes around the main axis cylindrical axis enhance neutron streaming towards the detectors located at deepest positions(6). The cylindrical volume allocating the detectors, the air channels and the lead disk are also surrounded by a 5 mm layer of borated plastic to increase thermal neutron capture. Figure 3. Open in new tabDownload slide (a) Schematic 3D view of the CYSP, showing the position of the detectors along the axis of the cylinder as well as the lead and borated plastic layers; (b) cross-sectional diagram of the CYSP (arrow indicates the direction of the incident radiation); and (c) built prototype, showing the borated plastic layers, air channels and holes for detectors and cables. Figure 3. Open in new tabDownload slide (a) Schematic 3D view of the CYSP, showing the position of the detectors along the axis of the cylinder as well as the lead and borated plastic layers; (b) cross-sectional diagram of the CYSP (arrow indicates the direction of the incident radiation); and (c) built prototype, showing the borated plastic layers, air channels and holes for detectors and cables. Figure 4. Open in new tabDownload slide Response matrix for the CYSP. Figure 4. Open in new tabDownload slide Response matrix for the CYSP. The instrument has been fabricated using the thermal neutron detectors described in Section 2.1 (Figure 3c) and the response matrix has been calculated according to the procedure described in Section 2.2 for energies from 10–9 to 103 MeV assuming irradiation with a plane parallel mononergetic beam. As it is shown in Figure 5, the response to neutrons with energies from thermal up to around 10 MeV decreases as the detector depth increases. For energies above 10 MeV, the lead shell converter increases the response of detectors located at deeper. Figure 5. Open in new tabDownload slide Comparison of CYSP experimentally measured counts per unit fluence rate compared with the simulated values for exposure to 252Cf and monochromatic beams 144 keV, 565 keV, 2.0 MeV. 3.5 MeV, 5.0 MeV and 16.5 MeV. Figure 5. Open in new tabDownload slide Comparison of CYSP experimentally measured counts per unit fluence rate compared with the simulated values for exposure to 252Cf and monochromatic beams 144 keV, 565 keV, 2.0 MeV. 3.5 MeV, 5.0 MeV and 16.5 MeV. The calculated response matrix (shown in Figure 4) has been partially validated with the monochromatic reference neutron fields of 0.144, 0.565, 2.0, 3.5, 5.0 and 16.5 MeV and the 252Cf source available at National Physical Laboratory (NPL, UK)(18). The theoretically calculated response matrix agrees with the experimentally measured response within an overall 3% uncertainty, thus confirming the design of the instrument is able to eliminate the response to scattered neutrons at least up to about 10 MeV. An additional validation of the instrument has been recently performed in the 12 × 12 × 6 m3 irradiation room of Politecnico di Milano. The neutron spectrum at 1 m from an Eckert and Ziegler Cesio s.r.o. CZ/1003/S—96 241Am–Be source has been measured using a BSS with the shadow cone technique to discriminate uncollide component.and the CYSP(19). Both the source and the spectrometers reference positions were at 2.3 m above the floor. Because of the CYSP response matrix was calculated assuming broad parallel irradiation condition(6) and the divergence of the field cannot be ignored at 1 m distance, the CYSP measurements have been modified using calculated geometric coefficients to correct the effect of such divergence. Table 2 shows, the values of the integral quantities total fluence rate, Φ̇ ⁠, and ambient dose equivalent rate, Ḣ⁎(10) ⁠, due to the uncollide component, obtained from the unfolded spectra measured with the BSS using the shadow cones and directly with the CYSP. The values are the same within the reported uncertainties, thus confirming the new spectrometer is practically insensitive to scattered neutron radiation impinging laterally on the cylinder, as it was expected from the design simulations(6). Table 2. Comparison between measurements of the uncollide neutron field from the 241Am–Be source in the calibration room of Politecnico di Milano made: with a BSS using the shadow cone technique to separate the uncollide component; and directly with the cylindrical spectrometer CYSP. Quantity . BSS(*) . CYSP . Φ̇ (cm−2 s−1) 17.3 ± 0.5 18.2 ± 0.7 Ḣ⁎(10) (mSv h−1) 24.4 ± 1.0 25.7 ± 1.1 h*(10) (pSv cm2) 391 ± 3 394 ± 6 Quantity . BSS(*) . CYSP . Φ̇ (cm−2 s−1) 17.3 ± 0.5 18.2 ± 0.7 Ḣ⁎(10) (mSv h−1) 24.4 ± 1.0 25.7 ± 1.1 h*(10) (pSv cm2) 391 ± 3 394 ± 6 (*)Uncollide component. Table 2. Comparison between measurements of the uncollide neutron field from the 241Am–Be source in the calibration room of Politecnico di Milano made: with a BSS using the shadow cone technique to separate the uncollide component; and directly with the cylindrical spectrometer CYSP. Quantity . BSS(*) . CYSP . Φ̇ (cm−2 s−1) 17.3 ± 0.5 18.2 ± 0.7 Ḣ⁎(10) (mSv h−1) 24.4 ± 1.0 25.7 ± 1.1 h*(10) (pSv cm2) 391 ± 3 394 ± 6 Quantity . BSS(*) . CYSP . Φ̇ (cm−2 s−1) 17.3 ± 0.5 18.2 ± 0.7 Ḣ⁎(10) (mSv h−1) 24.4 ± 1.0 25.7 ± 1.1 h*(10) (pSv cm2) 391 ± 3 394 ± 6 (*)Uncollide component. CONCLUSIONS Two single-moderator neutron spectrometers, respectively called SP2 and CYSP, have been developed. Their energy response matrix is similar to the BSS with the advantage that they can determine the neutron spectrum with a single exposure. The SP2 is a nearly isotropic spherical spectrometer, especially suitable for workplace monitoring application. On the contrary, the CYSP is a directional spectrometer which is practically insensitive to scattered contributions. Therefore, the CYSP is appropriated for characterising direct neutron spectra when such scattered neutrons (albedo) can contribute significantly to the radiation field. ACKNOWLEDGEMENTS This work is supported by projects FIS2012-39104-C02 (MINECO, Spain), FIS2015-64793-C2 (MINECO, Spain) and NEURAPID (INFN—Commissione Scientifica Nazionale 5, Italy). The authors wish to thank FDS Team, China, for providing the MCAM software. REFERENCES 1 Thomas , D. J. Neutron spectrometry . Radiat. Meas. 45 , 1178 – 1185 ( 2010 ). Google Scholar Crossref Search ADS WorldCat 2 Thomas , D. J. and Alevra , A. V. Bonner sphere spectrometers—a critical review . Nucl. Instrum. Methods A 476 , 12 – 20 ( 2002 ). Google Scholar Crossref Search ADS WorldCat 3 Gómez-Ros , J. M. , Bedogni , B., Moraleda , M., Delgado , A., Romero , A. and Esposito , A. 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Methods A 782 , 35 – 39 ( 2015 ). Google Scholar Crossref Search ADS WorldCat 19 Pola , A. , Bedogni , R., Domingo , C., Gómez-Ros , J. M., Introini , M. V., Martínez-Rovira , I. and Romero-Expósito , M. Neutron spectrometry of a 241Americium-Beryllium neutron source using the broad-energy range neutron spectrometers CYSP and Bonner Spheres . Appl. Radiat. Isotop. (submitted for publication). OpenURL Placeholder Text WorldCat © The Author 2017. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oup.com
TI - TWO NEW SINGLE-EXPOSURE, MULTI-DETECTOR NEUTRON SPECTROMETERS FOR RADIATION PROTECTION APPLICATIONS IN WORKPLACE MONITORING
JF - Radiation Protection Dosimetry
DO - 10.1093/rpd/ncw349
DA - 2017-04-01
UR - https://www.deepdyve.com/lp/oxford-university-press/two-new-single-exposure-multi-detector-neutron-spectrometers-for-eozpOM9hgP
SP - 104
VL - 173
IS - 1-3
DP - DeepDyve
ER -