- Email: [email protected]
First test of SP2: A novel active neutron spectrometer condensing the functionality of Bonner spheres in a single moderator
Nuclear Instruments and Methods in Physics Research A 767 (2014) 159–162
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
First test of SP2: A novel active neutron spectrometer condensing the functionality of Bonner spheres in a single moderator R. Bedogni a,n, D. Bortot b,c, B. Buonomo a, A. Esposito a, J.M. Gómez-Ros a,d, M.V. Introini b,c, M. Lorenzoli b,c, A. Pola b,c, D. Sacco a,e a
INFN-LNF Laboratori Nazionali di Frascati, Via E. Fermi 40, 00044 Frascati, Italy Politecnico di Milano—Dipartimento di Energia, Via Ponzio 34/3, 20133 Milano, Italy c INFN—sezione di Milano, Via Celoria 16, 20133 Milano, Italy d CIEMAT, Av. Complutense 40, 28040 Madrid, Spain e INAIL—DPIA Via di Fontana Candida n.1, 00040 Monteporzio C., Italy b
art ic l e i nf o
a b s t r a c t
Article history: Received 6 June 2014 Received in revised form 30 July 2014 Accepted 5 August 2014 Available online 12 August 2014
The NESCOFI@BTF (2011–2013) international collaboration was established to develop realtime neutron spectrometers to simultaneously cover all energy components of neutron fields, from thermal up to hundreds MeV. This communication concerns a new spherical spectrometer, called SP\widehat2, which condenses the functionality of an Extended Range Bonner Sphere Spectrometer (ERBSS) into a single moderator embedding multiple active thermal neutron detectors. The possibility of achieving the complete spectrometric information in a single exposure constitutes a great advantage compared to the ERBSS. The first experimental test of the instrument, performed with a reference 241Am–Be source in different irradiation geometries, is described. The agreement between observed and simulated response is satisfactory for all tested geometries. & 2014 Elsevier B.V. All rights reserved.
Keywords: Neutron spectrometry Moderator NESCOFI@BTF Bonner spheres
1. Introduction The NESCOFI@BTF (2011–2013) international collaboration was established with the aim of developing real-time neutron spectrometers to simultaneously cover all energy components of neutron fields, from thermal up to hundreds MeV. The final users of these instruments will be neutron-producing facilities, ranging from medical to industrial and research fields. The basic idea is to condense the functionality of an Extended Range Bonner Sphere Spectrometer (ERBSS) [1] into a single moderator embedding multiple active thermal neutron detectors. The expected performance, in terms of accuracy and spectrometric capability, are those of the ERBSS. However, the possibility of achieving the complete spectrometric information in a single exposure constitutes a great advantage compared to ERBSS. The project proposed to develop two different spectrometers: – CYSP (CYlindrical SPectrometer), with unidirectional response [2]; – SP2 (SPherical SPectrometer), with isotropic response. This paper is focused on the SP2. The spectrometer was designed through an extensive Monte Carlo simulation campaign with the MCNPX 2.6 [3] code that allowed optimizing the radius of the n
Corresponding author.
http://dx.doi.org/10.1016/j.nima.2014.08.004 0168-9002/& 2014 Elsevier B.V. All rights reserved.
spherical polyethylene moderator, the internal distribution of the thermal neutron detectors and the thickness and location of an internal lead shell for enhancing the high-energy response. The final design consists of a 25 cm diameter polyethylene sphere embedding 31 thermal neutron detectors arranged in symmetrical positions along the three axes. The internal one cm thick lead shell, acting as (n, xn) degrader, allows extending the energy interval of the response up to hundreds of MeV [4]. The SP2 response matrix is fully known by Monte Carlo simulation. The average reading of the six detectors located at the same radial position proved to be nearly independent from the irradiation geometry, thus constituting an isotropic data, which is only function of the radial position and the neutron energy. This is the basis for using SP2 as a spectrometer [4]. According to this theoretical design, a passive prototype equipped with Dysprosium activation foils was built, in order to experimentally validate the simulated response matrix. The device was irradiated with quasi-monoenergetic reference neutron fields from 147 keV to 14.8 MeV available at PTB Braunschweig. This campaign allowed estimating in 73% the overall uncertainty of the simulated SP2 response matrix, for the investigated energy range [5]. A further stage was the development of customized active thermal neutron detectors fulfilling the following characteristics: (1) miniaturization (the target dimension for a single detector is in the order of 1 cm);
160
R. Bedogni et al. / Nuclear Instruments and Methods in Physics Research A 767 (2014) 159–162
(2) sensitivity and linearity (the spectrometers should work with dose rates ranging from μSv/h up to Sv/h); (3) excellent photon rejection; (4) low-cost (a single SP2 includes thirty-one detectors, thus excluding for budget reasons practically all commercially available active detectors). Among the two solid-state devices developed by the project [6,7], the TNPD type (thermal neutron pulse detector) was used in this work. The TNPD produces a pulse-height distribution from which the thermal neutron fluence is derived. Typical thermal neutron sensitivity is 0.03 cm2 (counts per unit fluence) [7]. This work presents a first test of the SP2, equipped with TNPD active detectors, in different irradiation geometries, using a reference source of 241Am–Be (strength 2.09E þ6 s 1) available at INFN-LNF calibration laboratory.
2. Measurement set-up To study the spectrometer response as a function of the irradiation geometry, the SP2 was equipped with a single “radius” of five detectors occupying radial positions 0.0 cm (centre), 5.5 cm, 7.5 cm, 9.5 cm and 11 cm. The shallowest position at R¼ 12.35 cm, corresponding to an un-moderated detector to measure thermal neutrons present in the incident field, was not used, because no thermal neutrons are present in the Am–Be spectrum. The orientation of the spectrometer with respect to the Am–Be source was then changed in order to achieve the different irradiation geometries (1 0 0), ( 1 0 0), (0 1 0), (0 1 0), (0 0 1) and (0 0 1) (see Fig. 1). The results from the last four symmetrical geometries were averaged and referred as a single “lateral” geometry (LAT). The difference in sensitivity among different TNPDs was accounted by introducing individual correction factors, previously determined by exposing every TNPD in the same thermal neutron field as explained in Ref. [7]. This correction allows reducing to about 73% the detector-to-detector response variability. A dedicated multi-detector analog compact board was developed to acquire the signals from the multiple detectors of the SP2. The board includes eight analog channels, each formed by a bias regulator, a charge preamplifier and a shaper amplifier. The eight amplified analog signals are sent to a commercial digitizer programmed within the LabView environment. This allows to simultaneously acquire on a PC the pulse height spectra from all TNPDs. For every irradiation geometry the Am–Be source was placed at 1 m from the SP2 center (point of test). The shadow-cone technique was adopted to account for the room- and air- scattered
Fig. 1. Irradiation geometries chosen for the experiment. Cut-view of the SP2 from top. (0 1 0), (0 1 0), (0 0 1) and (0 0 1) symmetrical geometries are grouped into a single “lateral” geometry, called LAT.
Fig. 2. Irradiation set-up for the “cone configuration”. The source-to-cone distance was 3 cm, whilst the source-to-detector distance was 100 cm.
component of the neutron field, so that twelve exposures were performed in total. Fig. 2 shows the experimental set-up in the case of the (1 0 0) irradiation with the shadow-cone (“cone configuration”). The source-to-cone distance and the source-to-detector distance were equal to 3 cm and 100 cm, respectively.
3. Results Fig. 3 shows the pulse height spectra from the five TNPDs for an exposure time of 54,000 s, in (1 0 0) geometry and in total-field condition (no shadow-cone). The initial portion of the spectra is influenced by the secondary electrons due to the moderation photons plus those present in the free-in-air-field. The broad peaked structure above 1 V is due to the charged particles produced by thermal neutrons in the converter covering the solid-state sensor. Previous optimization tests allowed fixing a photon-to-neutron discrimination threshold to 0.6 V, which corresponds to a local minimum in the spectra. For a given geometry, the number of pulses in each detector integrated from 0.6 V to 3 V was normalized to the exposure time and corrected for the scattered radiation by subtracting the corresponding quantity obtained with the shadow-cone. This difference was corrected for the individual sensitivity of the TNPDs and normalized to the free-in-air neutron fluence rate at the point of test (sphere center). The resulting array represents the “observed” SP2 count per unit fluence, symbolized with Cg(R), where pedix “g” refers to the specific irradiation geometry and R is the radius corresponding to the detector position: R¼ 0.0 cm (centre), 5.5 cm, 7.5 cm, 9.5 cm and 11 cm. In parallel, the SP2 response was calculated with MCNPX. The model includes accurate description of the SP2 in terms of materials and densities. The simulations determined the number of thermal neutron capture reactions in the converter of the TNPD in different radial positions, per unit fluence from a 241Am–Be source placed at 1 m from the sphere centre, in an ideal scatter-free scenario. The simulated number of capture reactions per unit fluence is termed Mg(R). The ratio Fg(R)¼ Cg(R)/Mg(R) represents the spectrometer calibration factor. If the Monte Carlo model well represents the real geometry, then the ratio Fg(R) is constant as the irradiation geometry and the detector position change. Thus the accuracy of the simulation model can be quantified by the constancy of F. Table 1 shows Fg(R) for different radial positions and geometries. Because the (0 1 0), (0 1 0), (0 0 1) and (0 0 1) geometries are symmetrical, they have averaged and reported as a single geometry (LAT, meaning “lateral”). Uncertainties of experimental values, in the order of 3% to 5%, are due to the residual detector-to-detector response variability and to the counting statistics. The values of Fg(R) are less than 1 because not all charged particles generated in the converter produce a detectable signal.
R. Bedogni et al. / Nuclear Instruments and Methods in Physics Research A 767 (2014) 159–162
161
Fig. 3. Pulse height spectra, in counts, from the five TNPDs, positioned (from top-left to bottom-right) at R ¼0.0 cm, 5.5 cm, 7.5 cm, 9.5 cm and 11 cm, after 54,000 s exposure time in (1 0 0) geometry and in total-field condition (no shadow-cone). The X-axis indicates the amplitude of the signals, in V. The Y-axis indicates the number of pulses per channel (one channel corresponds to approx. 6 mV). Table 1 SP2 calibration factor, Fg(R), derived for different irradiation geometries and different detector radial positions. The symmetrical geometries (0 1 0), (0 1 0), (0 0 1) and (0 0 1) were averaged and reported as a single “lateral” geometry (LAT). Detector radial position (cm)
0.0 5.5 7.5 9.5 11.0
Irradiation geometry 100
1 0 0
LAT
0.1697 0.005 0.1877 0.006 0.1697 0.005 0.1747 0.005 0.1777 0.005
0.1737 0.006 0.1897 0.008 0.1757 0.009 0.1977 0.012 0.1787 0.016
0.1747 0.005 0.1887 0.006 0.1747 0.006 0.1757 0.006 0.1757 0.006
The best estimation for the SP2 calibration factor F, obtained by a weighted average of the Fg (R) values in Table 1, using the inverse square of uncertainty as weighting factor, is F¼ (0.176 70.003). Uncertainties due to device positioning ( 71%) and Am–Be source strength (71%) are also accounted. Multiplying F by the simulated response Mg(R) and the free-inair neutron fluence rate at the point of test, obtains the “expected” count rates shown in Figs. 4–6 (one per geometry). These are compared with the “observed” count rates. Data from Table 1 and Figs. 4–6 confirm the satisfactory degree of accuracy of the simulation model, for the energy range of the Am–Be source.
Fig. 4. Experimental and expected counts from the TNPDs positioned at R ¼0.0 cm, 5.5 cm, 7.5 cm, 9.5 cm and 11 cm in the (1 0 0) geometry. Line is only eye guide.
4. Conclusions The first test of an active mono-sphere multiple-detector neutron spectrometer, called SP2, has been performed using the reference field from an Am–Be source (energy range from about 0.1 to 10 MeV). To study the spectrometer response as a function of the irradiation geometry, the SP2 was equipped with a single “radius” of five detectors occupying radial positions 0.0 cm (centre), 5.5 cm, 7.5 cm, 9.5 cm and 11 cm. The orientation of the spectrometer with respect to the Am–Be source was changed in order to achieve the different irradiation geometries (1 0 0), ( 1 0 0), (0 1 0), (0 1 0), (0 0 1) and (0 0 1). The last four symmetrical geometries were combined into a single “lateral” geometry (LAT). The count rates in the five detectors, after correcting for detector individual sensitivity and for the room-
Fig. 5. Experimental and expected counts from the TNPDs positioned at R ¼0.0 cm, 5.5 cm, 7.5 cm, 9.5 cm and 11 cm in the ( 1 0 0) geometry. Line is only eye guide.
scatter, were normalized to the free-in-air neutron fluence rate at the point of test. The “observed” counts per unit fluence were divided by the Monte Carlo response for the corresponding
162
R. Bedogni et al. / Nuclear Instruments and Methods in Physics Research A 767 (2014) 159–162
ambient-dose-equivalent of about 400 nSv/burst (Am–Be equivalent), and 50% for 42000 nSv/burst. These Figures, to be checked on the field, are compatible with those of the best-performing instruments of a recent EURADOS inter-comparison [8]. To conclude: after additional experimental tests at different neutron energies and pulsed fields will be completed, a new wellestablished instrument, able to determine the whole neutron spectrum in a single exposure, will be available to the scientific community.
Acknowledgements
Fig. 6. Experimental and expected counts from the TNPDs positioned at R ¼0.0 cm, 5.5 cm, 7.5 cm, 9.5 cm and 11 cm in the LAT geometry. Line is only eye guide.
This work has been supported by the NESCOFI@BTF and NEURAPID Projects (INFN—Commissione Scientifica Nazionale 5, Italy), the FIS2012-39104-C02-02 Project (MINECO, Spain) and the EU FP7 CRISP (WP14) project. References
irradiation geometries and detector positions, obtaining different estimations for the spectrometer calibration factor. The best estimation of the SP2 calibration factor F, obtained by a weighted average of the values in Table 1, is F¼(0.176 70.003). The accuracy of this calibration factor is confirmed by the expected vs. observed count rates comparisons shown in Figs. 4–6. For the energy range of the Am–Be source, and for the different irradiation geometries investigated, the response of the SP2 is known with a satisfactory degree of accuracy (overall uncertainty few percent). Because SP2 is likely to be used in pulsed fields, a special consideration is needed about the expected performance of the device in these fields. In “single pulse” scenario, the population of diffusing neutrons will reach the thermal neutron sensors according to e t/T time distribution, with time constant in the order of 102 μs [8]. The shaping time of the SP2 analog amplifiers may be fixed to 25 ns, thus the single-count pulse duration will be in the order of 100 ns. By conservatively assuming this number as dead-time, and convoluting the dead-time correction with the mentioned exponential time distribution, the count loss will be: 10% for an
[1] S. Agosteo, R. Bedogni, M. Caresana, N. Charitonidis, M. Chiti, A. Esposito, M. Ferrarini, C. Severino, M. Silari, Nucl. Instrum. Methods Phys. Res., Sect. A 694 (2012) 55. [2] R. Bedogni, J.M. Gómez-Ros, D. Bortot, A. Pola, M.V. Introini, A. Esposito, A. Gentile, G. Mazzitelli, B. Buonomo, Radiat. Prot. Dosim. (2013), http://dx. doi.org/10.1093/rpd/nct286. [3] D.B. Pelowitz (Ed.), MCNPX User’s Manual Version 2.6, Report LA-CP-07-1473, 2008. [4] J.M. Gomez-Ros, R. Bedogni, M. Moraleda, A. Esposito, A. Pola, M.V. Introini, G. Mazzitelli, L. Quintieri, B. Buonomo, Nucl. Instrum. Methods Phys. Res., Sect. A 677 (2012) 4. [5] R. Bedogni, J.M. Gómez-Ros, A. Pola, M.V. Introini, D. Bortot, A. Gentile, A. Esposito, G. Mazzitelli, B. Buonomo, L. Quintieri, L. Foggetta, Nucl. Instrum. Methods Phys. Res., Sect. A 714 (2013) 110. [6] A. Pola, D. Bortot, M.V. Introini, R. Bedogni, A. Gentile, A. Esposito, J.M. GómezRos, E. Passoth, A. Prokofiev, Radiat. Prot. Dosim. (2013), http://dx.doi.org/ 10.1093/rpd/nct298. [7] R. Bedogni, D. Bortot, A. Pola, M.V. Introini, A. Gentile, A. Esposito, J.M. GómezRos, M. Palomba, A. Grossi, Radiat. Prot. Dosim. (2013), http://dx.doi.org/ 10.1093/rpd/nct319. [8] M. Caresana, A. Denker, A. Esposito, M. Ferrarini, N. Golnik, E. Hohmann, A. Leuschner, M. Luszik-Bhadra, G. Manessi, S. Mayer, K. Ott, J. Röhrich, M. Silari, F. Trompier, M Volnhals, M Wielunski, Nucl. Instrum. Methods Phys. Res., Sect. A 737 (2014).
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
First test of SP2: A novel active neutron spectrometer condensing the functionality of Bonner spheres in a single moderator R. Bedogni a,n, D. Bortot b,c, B. Buonomo a, A. Esposito a, J.M. Gómez-Ros a,d, M.V. Introini b,c, M. Lorenzoli b,c, A. Pola b,c, D. Sacco a,e a
INFN-LNF Laboratori Nazionali di Frascati, Via E. Fermi 40, 00044 Frascati, Italy Politecnico di Milano—Dipartimento di Energia, Via Ponzio 34/3, 20133 Milano, Italy c INFN—sezione di Milano, Via Celoria 16, 20133 Milano, Italy d CIEMAT, Av. Complutense 40, 28040 Madrid, Spain e INAIL—DPIA Via di Fontana Candida n.1, 00040 Monteporzio C., Italy b
art ic l e i nf o
a b s t r a c t
Article history: Received 6 June 2014 Received in revised form 30 July 2014 Accepted 5 August 2014 Available online 12 August 2014
The NESCOFI@BTF (2011–2013) international collaboration was established to develop realtime neutron spectrometers to simultaneously cover all energy components of neutron fields, from thermal up to hundreds MeV. This communication concerns a new spherical spectrometer, called SP\widehat2, which condenses the functionality of an Extended Range Bonner Sphere Spectrometer (ERBSS) into a single moderator embedding multiple active thermal neutron detectors. The possibility of achieving the complete spectrometric information in a single exposure constitutes a great advantage compared to the ERBSS. The first experimental test of the instrument, performed with a reference 241Am–Be source in different irradiation geometries, is described. The agreement between observed and simulated response is satisfactory for all tested geometries. & 2014 Elsevier B.V. All rights reserved.
Keywords: Neutron spectrometry Moderator NESCOFI@BTF Bonner spheres
1. Introduction The NESCOFI@BTF (2011–2013) international collaboration was established with the aim of developing real-time neutron spectrometers to simultaneously cover all energy components of neutron fields, from thermal up to hundreds MeV. The final users of these instruments will be neutron-producing facilities, ranging from medical to industrial and research fields. The basic idea is to condense the functionality of an Extended Range Bonner Sphere Spectrometer (ERBSS) [1] into a single moderator embedding multiple active thermal neutron detectors. The expected performance, in terms of accuracy and spectrometric capability, are those of the ERBSS. However, the possibility of achieving the complete spectrometric information in a single exposure constitutes a great advantage compared to ERBSS. The project proposed to develop two different spectrometers: – CYSP (CYlindrical SPectrometer), with unidirectional response [2]; – SP2 (SPherical SPectrometer), with isotropic response. This paper is focused on the SP2. The spectrometer was designed through an extensive Monte Carlo simulation campaign with the MCNPX 2.6 [3] code that allowed optimizing the radius of the n
Corresponding author.
http://dx.doi.org/10.1016/j.nima.2014.08.004 0168-9002/& 2014 Elsevier B.V. All rights reserved.
spherical polyethylene moderator, the internal distribution of the thermal neutron detectors and the thickness and location of an internal lead shell for enhancing the high-energy response. The final design consists of a 25 cm diameter polyethylene sphere embedding 31 thermal neutron detectors arranged in symmetrical positions along the three axes. The internal one cm thick lead shell, acting as (n, xn) degrader, allows extending the energy interval of the response up to hundreds of MeV [4]. The SP2 response matrix is fully known by Monte Carlo simulation. The average reading of the six detectors located at the same radial position proved to be nearly independent from the irradiation geometry, thus constituting an isotropic data, which is only function of the radial position and the neutron energy. This is the basis for using SP2 as a spectrometer [4]. According to this theoretical design, a passive prototype equipped with Dysprosium activation foils was built, in order to experimentally validate the simulated response matrix. The device was irradiated with quasi-monoenergetic reference neutron fields from 147 keV to 14.8 MeV available at PTB Braunschweig. This campaign allowed estimating in 73% the overall uncertainty of the simulated SP2 response matrix, for the investigated energy range [5]. A further stage was the development of customized active thermal neutron detectors fulfilling the following characteristics: (1) miniaturization (the target dimension for a single detector is in the order of 1 cm);
160
R. Bedogni et al. / Nuclear Instruments and Methods in Physics Research A 767 (2014) 159–162
(2) sensitivity and linearity (the spectrometers should work with dose rates ranging from μSv/h up to Sv/h); (3) excellent photon rejection; (4) low-cost (a single SP2 includes thirty-one detectors, thus excluding for budget reasons practically all commercially available active detectors). Among the two solid-state devices developed by the project [6,7], the TNPD type (thermal neutron pulse detector) was used in this work. The TNPD produces a pulse-height distribution from which the thermal neutron fluence is derived. Typical thermal neutron sensitivity is 0.03 cm2 (counts per unit fluence) [7]. This work presents a first test of the SP2, equipped with TNPD active detectors, in different irradiation geometries, using a reference source of 241Am–Be (strength 2.09E þ6 s 1) available at INFN-LNF calibration laboratory.
2. Measurement set-up To study the spectrometer response as a function of the irradiation geometry, the SP2 was equipped with a single “radius” of five detectors occupying radial positions 0.0 cm (centre), 5.5 cm, 7.5 cm, 9.5 cm and 11 cm. The shallowest position at R¼ 12.35 cm, corresponding to an un-moderated detector to measure thermal neutrons present in the incident field, was not used, because no thermal neutrons are present in the Am–Be spectrum. The orientation of the spectrometer with respect to the Am–Be source was then changed in order to achieve the different irradiation geometries (1 0 0), ( 1 0 0), (0 1 0), (0 1 0), (0 0 1) and (0 0 1) (see Fig. 1). The results from the last four symmetrical geometries were averaged and referred as a single “lateral” geometry (LAT). The difference in sensitivity among different TNPDs was accounted by introducing individual correction factors, previously determined by exposing every TNPD in the same thermal neutron field as explained in Ref. [7]. This correction allows reducing to about 73% the detector-to-detector response variability. A dedicated multi-detector analog compact board was developed to acquire the signals from the multiple detectors of the SP2. The board includes eight analog channels, each formed by a bias regulator, a charge preamplifier and a shaper amplifier. The eight amplified analog signals are sent to a commercial digitizer programmed within the LabView environment. This allows to simultaneously acquire on a PC the pulse height spectra from all TNPDs. For every irradiation geometry the Am–Be source was placed at 1 m from the SP2 center (point of test). The shadow-cone technique was adopted to account for the room- and air- scattered
Fig. 1. Irradiation geometries chosen for the experiment. Cut-view of the SP2 from top. (0 1 0), (0 1 0), (0 0 1) and (0 0 1) symmetrical geometries are grouped into a single “lateral” geometry, called LAT.
Fig. 2. Irradiation set-up for the “cone configuration”. The source-to-cone distance was 3 cm, whilst the source-to-detector distance was 100 cm.
component of the neutron field, so that twelve exposures were performed in total. Fig. 2 shows the experimental set-up in the case of the (1 0 0) irradiation with the shadow-cone (“cone configuration”). The source-to-cone distance and the source-to-detector distance were equal to 3 cm and 100 cm, respectively.
3. Results Fig. 3 shows the pulse height spectra from the five TNPDs for an exposure time of 54,000 s, in (1 0 0) geometry and in total-field condition (no shadow-cone). The initial portion of the spectra is influenced by the secondary electrons due to the moderation photons plus those present in the free-in-air-field. The broad peaked structure above 1 V is due to the charged particles produced by thermal neutrons in the converter covering the solid-state sensor. Previous optimization tests allowed fixing a photon-to-neutron discrimination threshold to 0.6 V, which corresponds to a local minimum in the spectra. For a given geometry, the number of pulses in each detector integrated from 0.6 V to 3 V was normalized to the exposure time and corrected for the scattered radiation by subtracting the corresponding quantity obtained with the shadow-cone. This difference was corrected for the individual sensitivity of the TNPDs and normalized to the free-in-air neutron fluence rate at the point of test (sphere center). The resulting array represents the “observed” SP2 count per unit fluence, symbolized with Cg(R), where pedix “g” refers to the specific irradiation geometry and R is the radius corresponding to the detector position: R¼ 0.0 cm (centre), 5.5 cm, 7.5 cm, 9.5 cm and 11 cm. In parallel, the SP2 response was calculated with MCNPX. The model includes accurate description of the SP2 in terms of materials and densities. The simulations determined the number of thermal neutron capture reactions in the converter of the TNPD in different radial positions, per unit fluence from a 241Am–Be source placed at 1 m from the sphere centre, in an ideal scatter-free scenario. The simulated number of capture reactions per unit fluence is termed Mg(R). The ratio Fg(R)¼ Cg(R)/Mg(R) represents the spectrometer calibration factor. If the Monte Carlo model well represents the real geometry, then the ratio Fg(R) is constant as the irradiation geometry and the detector position change. Thus the accuracy of the simulation model can be quantified by the constancy of F. Table 1 shows Fg(R) for different radial positions and geometries. Because the (0 1 0), (0 1 0), (0 0 1) and (0 0 1) geometries are symmetrical, they have averaged and reported as a single geometry (LAT, meaning “lateral”). Uncertainties of experimental values, in the order of 3% to 5%, are due to the residual detector-to-detector response variability and to the counting statistics. The values of Fg(R) are less than 1 because not all charged particles generated in the converter produce a detectable signal.
R. Bedogni et al. / Nuclear Instruments and Methods in Physics Research A 767 (2014) 159–162
161
Fig. 3. Pulse height spectra, in counts, from the five TNPDs, positioned (from top-left to bottom-right) at R ¼0.0 cm, 5.5 cm, 7.5 cm, 9.5 cm and 11 cm, after 54,000 s exposure time in (1 0 0) geometry and in total-field condition (no shadow-cone). The X-axis indicates the amplitude of the signals, in V. The Y-axis indicates the number of pulses per channel (one channel corresponds to approx. 6 mV). Table 1 SP2 calibration factor, Fg(R), derived for different irradiation geometries and different detector radial positions. The symmetrical geometries (0 1 0), (0 1 0), (0 0 1) and (0 0 1) were averaged and reported as a single “lateral” geometry (LAT). Detector radial position (cm)
0.0 5.5 7.5 9.5 11.0
Irradiation geometry 100
1 0 0
LAT
0.1697 0.005 0.1877 0.006 0.1697 0.005 0.1747 0.005 0.1777 0.005
0.1737 0.006 0.1897 0.008 0.1757 0.009 0.1977 0.012 0.1787 0.016
0.1747 0.005 0.1887 0.006 0.1747 0.006 0.1757 0.006 0.1757 0.006
The best estimation for the SP2 calibration factor F, obtained by a weighted average of the Fg (R) values in Table 1, using the inverse square of uncertainty as weighting factor, is F¼ (0.176 70.003). Uncertainties due to device positioning ( 71%) and Am–Be source strength (71%) are also accounted. Multiplying F by the simulated response Mg(R) and the free-inair neutron fluence rate at the point of test, obtains the “expected” count rates shown in Figs. 4–6 (one per geometry). These are compared with the “observed” count rates. Data from Table 1 and Figs. 4–6 confirm the satisfactory degree of accuracy of the simulation model, for the energy range of the Am–Be source.
Fig. 4. Experimental and expected counts from the TNPDs positioned at R ¼0.0 cm, 5.5 cm, 7.5 cm, 9.5 cm and 11 cm in the (1 0 0) geometry. Line is only eye guide.
4. Conclusions The first test of an active mono-sphere multiple-detector neutron spectrometer, called SP2, has been performed using the reference field from an Am–Be source (energy range from about 0.1 to 10 MeV). To study the spectrometer response as a function of the irradiation geometry, the SP2 was equipped with a single “radius” of five detectors occupying radial positions 0.0 cm (centre), 5.5 cm, 7.5 cm, 9.5 cm and 11 cm. The orientation of the spectrometer with respect to the Am–Be source was changed in order to achieve the different irradiation geometries (1 0 0), ( 1 0 0), (0 1 0), (0 1 0), (0 0 1) and (0 0 1). The last four symmetrical geometries were combined into a single “lateral” geometry (LAT). The count rates in the five detectors, after correcting for detector individual sensitivity and for the room-
Fig. 5. Experimental and expected counts from the TNPDs positioned at R ¼0.0 cm, 5.5 cm, 7.5 cm, 9.5 cm and 11 cm in the ( 1 0 0) geometry. Line is only eye guide.
scatter, were normalized to the free-in-air neutron fluence rate at the point of test. The “observed” counts per unit fluence were divided by the Monte Carlo response for the corresponding
162
R. Bedogni et al. / Nuclear Instruments and Methods in Physics Research A 767 (2014) 159–162
ambient-dose-equivalent of about 400 nSv/burst (Am–Be equivalent), and 50% for 42000 nSv/burst. These Figures, to be checked on the field, are compatible with those of the best-performing instruments of a recent EURADOS inter-comparison [8]. To conclude: after additional experimental tests at different neutron energies and pulsed fields will be completed, a new wellestablished instrument, able to determine the whole neutron spectrum in a single exposure, will be available to the scientific community.
Acknowledgements
Fig. 6. Experimental and expected counts from the TNPDs positioned at R ¼0.0 cm, 5.5 cm, 7.5 cm, 9.5 cm and 11 cm in the LAT geometry. Line is only eye guide.
This work has been supported by the NESCOFI@BTF and NEURAPID Projects (INFN—Commissione Scientifica Nazionale 5, Italy), the FIS2012-39104-C02-02 Project (MINECO, Spain) and the EU FP7 CRISP (WP14) project. References
irradiation geometries and detector positions, obtaining different estimations for the spectrometer calibration factor. The best estimation of the SP2 calibration factor F, obtained by a weighted average of the values in Table 1, is F¼(0.176 70.003). The accuracy of this calibration factor is confirmed by the expected vs. observed count rates comparisons shown in Figs. 4–6. For the energy range of the Am–Be source, and for the different irradiation geometries investigated, the response of the SP2 is known with a satisfactory degree of accuracy (overall uncertainty few percent). Because SP2 is likely to be used in pulsed fields, a special consideration is needed about the expected performance of the device in these fields. In “single pulse” scenario, the population of diffusing neutrons will reach the thermal neutron sensors according to e t/T time distribution, with time constant in the order of 102 μs [8]. The shaping time of the SP2 analog amplifiers may be fixed to 25 ns, thus the single-count pulse duration will be in the order of 100 ns. By conservatively assuming this number as dead-time, and convoluting the dead-time correction with the mentioned exponential time distribution, the count loss will be: 10% for an
[1] S. Agosteo, R. Bedogni, M. Caresana, N. Charitonidis, M. Chiti, A. Esposito, M. Ferrarini, C. Severino, M. Silari, Nucl. Instrum. Methods Phys. Res., Sect. A 694 (2012) 55. [2] R. Bedogni, J.M. Gómez-Ros, D. Bortot, A. Pola, M.V. Introini, A. Esposito, A. Gentile, G. Mazzitelli, B. Buonomo, Radiat. Prot. Dosim. (2013), http://dx. doi.org/10.1093/rpd/nct286. [3] D.B. Pelowitz (Ed.), MCNPX User’s Manual Version 2.6, Report LA-CP-07-1473, 2008. [4] J.M. Gomez-Ros, R. Bedogni, M. Moraleda, A. Esposito, A. Pola, M.V. Introini, G. Mazzitelli, L. Quintieri, B. Buonomo, Nucl. Instrum. Methods Phys. Res., Sect. A 677 (2012) 4. [5] R. Bedogni, J.M. Gómez-Ros, A. Pola, M.V. Introini, D. Bortot, A. Gentile, A. Esposito, G. Mazzitelli, B. Buonomo, L. Quintieri, L. Foggetta, Nucl. Instrum. Methods Phys. Res., Sect. A 714 (2013) 110. [6] A. Pola, D. Bortot, M.V. Introini, R. Bedogni, A. Gentile, A. Esposito, J.M. GómezRos, E. Passoth, A. Prokofiev, Radiat. Prot. Dosim. (2013), http://dx.doi.org/ 10.1093/rpd/nct298. [7] R. Bedogni, D. Bortot, A. Pola, M.V. Introini, A. Gentile, A. Esposito, J.M. GómezRos, M. Palomba, A. Grossi, Radiat. Prot. Dosim. (2013), http://dx.doi.org/ 10.1093/rpd/nct319. [8] M. Caresana, A. Denker, A. Esposito, M. Ferrarini, N. Golnik, E. Hohmann, A. Leuschner, M. Luszik-Bhadra, G. Manessi, S. Mayer, K. Ott, J. Röhrich, M. Silari, F. Trompier, M Volnhals, M Wielunski, Nucl. Instrum. Methods Phys. Res., Sect. A 737 (2014).