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Thermal neutron field characteristics in the neutron exposure accelerator system for biological effect experiments (NASBEE) facility
Nuclear Inst. and Methods in Physics Research, A 949 (2020) 162724
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Thermal neutron field characteristics in the neutron exposure accelerator system for biological effect experiments (NASBEE) facility Ryo Ogawara a ,∗, Satoshi Kodaira b , Mitsuru Suda a , Takuya Hagihara a , Tsuyoshi Hamano a a
National Institute of Radiological Sciences, National Institutes for Quantum and Radiological Science and Technology, 4-9-1 Anagawa, Inage-ku, Chiba, Japan Center for Advanced Radiation Emergency Medicine, National Institutes for Quantum and Radiological Science and Technology, 4-9-1 Anagawa, Inage-ku, 263-8555 Chiba, Japan b
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Keywords: Accelerator-based neutron source Thermal neutrons fields Boron neutron capture therapy Fast neutron dosimetry
ABSTRACT The neutron exposure accelerator system for biological effect experiments (NASBEE) facility was constructed to investigate the biological effects of fast neutrons produced by the 9 Be(p, n)9 B and 9 B(D, n)10 B reactions. In this work, we describe the modification of the NASBEE facility for the development of thermal neutron detectors and radiology applications. We characterized thermal neutron fields created with a polyethylene (PE) moderator. The maximum thermal neutron fluxes via proton (600 μA) and deuteron (350 μA) beams were estimated to be 5.7 × 107 and 1.2 × 108 cm−2 s−1 , respectively, by using the 30-mm-thick PE moderator set in front of the irradiation sample The ratios of the gamma ray absorbed dose to thermal neutron fluence were 3.6 × 10−12 Gy cm2 for the 9 Be(p, n)9 B reaction and 6.1 × 10−11 Gy cm2 for the 9 B(D, n)10 B reaction. The thermal neutron field in the NASBEE facility can be used for various applications of thermal neutrons and is now open to external users.
1. Introduction Accelerator-based neutron sources have several advantages compared with conventional nuclear reactor neutron sources, including flexible intensity control and no requirement for nuclear fuel. Boron neutron capture therapy (BNCT) using an accelerator-based thermal neutron source is a promising candidate for cancer radiation therapy [1–3]. However, the thermal neutron field generated by accelerators has high dose contamination by fast neutrons and gamma rays [4,5]. The amount of contamination depends on the design of the irradiation components, such as the moderator and shielding material [1,3]. Therefore, the thermal neutron flux and the contamination of the thermal neutron dose must be assessed quantitatively for various applications. In thermal neutron fields, absorbed doses of gamma rays have often been measured with thermoluminescence dosimeters (TLD) [6] or optically stimulated luminescence (OSL) [7] dosimeters, because these detectors have a low sensitivity for fast neutrons. Total absorbed doses due to fast neutron interactions have been estimated by CR39 plastic nuclear track detectors [8,9], or neutron spectrometry with the ICRP 74 flux-to-dose conversion factor [10]. Equivalent doses of 60 Co gamma rays for fast neutrons were evaluated with an ionization chamber for biological effect experiments [11]. However, the ionization chamber is sensitive to both fast neutrons and gamma rays. Subtraction analysis between the ionization chamber and gamma ray dosimeters, such as TLD and OSL dosimeters, is required for fast neutron dosimetry.
The thermal neutron flux is evaluated by the gold foil activation method [12], and other radiation detectors, such as scintillators with optical fiber detectors [13,14] and glass dosimeters [15]. The neutron exposure accelerator system for biological effect experiments (NASBEE) facility [9,11] was constructed for studies of the biological effects of fast neutrons (∼2 MeV) at the National Institute of Radiological Sciences (NIRS), National Institutes for Quantum and Radiological Science and Technology (QST), Chiba, Japan. Recently, the demand for thermal neutron fields has increased at the NASBEE facility to develop thermal neutron detectors and radiology applications related to BNCT. The thermal neutrons are generally produced via a fast neutron moderator in front of the irradiation samples. The thermal neutron parameters, such as flux and absorbed dose contamination due to fast neutrons and gamma rays, depend on the configuration setup (e.g., moderator thickness, distance between source and moderator/sample, and surrounding shielding material) [1,3]. In this work, we describe the design for thermal neutron irradiation at the NASBEE facility and the characteristics of the thermal neutron field. 2. Materials and methods 2.1. NASBEE facility The NASBEE facility was originally constructed to investigate the biological effects of fast neutrons after the criticality accident at a
∗ Corresponding author. E-mail address: [email protected] (R. Ogawara).
https://doi.org/10.1016/j.nima.2019.162724 Received 18 October 2018; Received in revised form 5 July 2019; Accepted 6 September 2019 Available online 28 September 2019 0168-9002/© 2019 Elsevier B.V. All rights reserved.
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Nuclear Inst. and Methods in Physics Research, A 949 (2020) 162724
Fig. 1. Schematic of the experimental setup in the conventional irradiation room of the NASBEE facility.
Table 1 Maximum thermal neutron flux and dose contamination of fast neutrons and gamma rays in the neutron fields of the 9 Be(p, n)9 B and 9 Be(D, n)10 B reactions. The errors were estimated only as statistical uncertainty. 9
Neutron production PE moderator thickness [mm] Thermal neutron flux [cm−2 s−1 μA−1 ] (Cd ratio) Absorbed dose [Gy C−1 ] Dose contamination [Gy cm2 ]
Be(p, n)9 B
9
Be(D, n)10 B
30 (0.95 ± 0.01) × 105 (4.63 ± 0.10) 0.34 ± 0.01 0.34 ± 0.01 (3.57 ± 0.09) × 10−12 (3.63 ± 0.07) × 10−12
Fast neutrons Gamma rays Fast neutrons Gamma rays
uranium fuel conversion test facility belonging to JCO, in Tokaimura, Japan [9,11]. In the NASBEE facility, neutron beams are produced by 9 Be(p, n)9 B and 9 Be(D, n)10 B reactions [16] by using a multi-cusp negative ion source that produces a proton or a deuteron beam and also includes a 2 MV tandem accelerator and a solid 9 Be target (𝜙120 mm, 200 μm thick). Neutron beams are irradiated from the collimator in a polyethylene (PE) shield covering the 9 Be target (Fig. 1). The maximum beam currents of the 4 MeV protons and deuterons are 600 and 350 μA, respectively.
(3.34 ± 0.05) × 105 (4.44 ± 0.09) 3.79 ± 0.05 2.03 ± 0.04 (11.3 ± 0.33) × 10−12 (6.07 ± 0.16) × 10−12
scintillator (Canberra). Neutron flux 𝛷 estimated from the activated gold foil was obtained with Eq. (1). 𝛷=
1 𝐶 𝜆 ( ) ( ) 𝑁𝐴𝑢 𝜎 𝜀𝐼𝑔 𝐹𝑠 1 − 𝑒−𝜆𝑡𝑖 𝑒−𝜆𝑡𝑐 1 − 𝑒−𝜆𝑡𝑚
(1)
Here, 𝜆 is the decay constant of 198 Au (= 0.693/𝑇1∕2 , where 𝑇1∕2 is the half-life), 𝑁Au is the number of 197 Au nuclei, and 𝜎 is the crosssection of 197 Au(n, 𝛾)198 Au reactions (98.5 b). For 412 keV gamma rays, the detection efficiency, 𝜀, is 48%, the intensity abundance, 𝐼g is 95.6%, and self-shielding factor 𝐹S of the thermal neutrons is calculated to be 0.993 [17]. 𝑡i , 𝑡c , and 𝑡m are the times of neutron exposure, cooling, and spectrum measurement, respectively. Cd ratio 𝑅Cd is defined as the measured flux ratio of bare gold foil, 𝛷bare , to Cd-covered foil, 𝛷Cd . 𝛷bare contains thermal neutron fluxes 𝛷th and epithermal neutron fluxes 𝛷epi . 𝛷Cd contains epithermal neutrons because 133 Cd has a large neutron absorption cross-section. Because 𝑅Cd = (𝛷th + 𝛷epi )/𝛷epi , thermal neutron fluxes (< 0.55 eV of Cd cut-off energy [18]) were obtained by 𝛷th = 𝛷bare (1 – 1/𝑅Cd ). The statistical uncertainty of
2.2. Radiation detectors and dosimeters Thermal neutron fluxes were obtained by neutron activation analysis using bare gold foils (0.5 × 0.5 × 0.01 mm, Nilaco Corp.) and Cd covers (8 × 8 × 0.5 mm) [12]. After thermal neutron irradiation, the event count, C, of the total absorption peak of 412 keV gamma rays emitted by 198 Au was measured using a well-type NaI(Tl) inorganic 2
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Nuclear Inst. and Methods in Physics Research, A 949 (2020) 162724
(SSD), which was defined as the distance from the 9 Be target to the PE moderator surface in front of the detector, could be controlled in the range 600–1500 mm by the lifting stage. Ten cuboid PE moderators 300 × 300 mm in size and 10, 20, 30, 40, or 50 mm thick (two moderators of each thickness) were used. The thickness of the PE moderator placed on the back of the detector was fixed as 70 mm. The distance was 20 mm between the PE moderators on the front and back detectors. 2.4. Experiments The thermal neutron flux, Cd ratio, absorbed dose, and dose contamination due to fast neutrons and gamma rays depending on PE moderator thickness (0–240 mm) were measured. The PE moderator thicknesses that obtained the maximum thermal neutron fluxes were determined for the 9 Be(p, n)9 B and 9 Be(D, n)10 B reactions. The profiles and SSD dependences of the thermal neutron field characteristics were measured for this PE moderator thickness. The profile measurements were performed from −150 to 150 mm at 30 mm intervals at SSDs of 600 and 1500 mm. Profile shapes were evaluated by a fitting analysis with the polynomial function 𝑓1 (x) = a(x − 𝑥o )4 + b(x − 𝑥o )2 + c, where a, b, c, and 𝑥o are fitting parameters. Areas with intensities of more than 95% in the profiles were defined as flat areas. The SSD dependences (SSDs of 600, 900, 1200, and 1500 mm) of the thermal neutron fields characteristics were evaluated in both reactions. The curves of SSD dependences were evaluated by fitting analysis with the function 𝑓2 (x) = a/(x − 𝑥o )2 , where a and 𝑥o are fitting parameters. Thermal neutron fluxes (units: cm−2 s−1 μA−1 ) were normalized by using the primary beam current. The absorbed doses of fast neutrons and gamma rays were normalized by using the total irradiation charges for the 9 Be target as Gy C−1 . The dose contaminations (units: Gy cm2 ) of fast neutrons and gamma rays were defined as the ratio of the absorbed dose to the thermal neutron fluence. 2.5. Geant4 Monte Carlo simulations The neutron energy spectra for the 9 Be(p, n)9 B and 9 Be(D, n)10 B reactions in NASBEE thermal neutron fields were simulated with Geant 4.10.3 [19–21]. The QGSP_BIC_AllHP model and TENDL-2015 (TALYSbased evaluated nuclear data library) were used in the simulation [22, 23]. The quark–gluon string precompound (QGSP) and binary cascade (BIC) were used for particle energy ranges of 12 GeV–100 TeV and 200 MeV–9.9 GeV. In previous work, the neutron production yields of the 9 Be(p, n)9 B reactions obtained with the BIC model were inconsistent with ENDF/B-VII nuclear data library in the proton energy range of < 50 MeV [24]. Therefore, the TENDL libraries were used in conjunction with the ENDF/B-VII data library in an energy range of 0–200 MeV.
Fig. 2. PE moderator thickness dependence of thermal neutron fluxes (black) and Cd ratio (white) in neutron fields from (a) 9 Be(p, n)9 B and (b) 9 Be(D, n)10 B reactions. Statistical errors are smaller than the symbol size.
the thermal neutron flux was derived from the error propagation of the event count. The dosimetry systems for fast neutrons and gamma rays were calibrated by the air kerma of 60 Co gamma rays at NIRS, QST. Thus, all absorbed doses described below are mean equivalent doses of 60 Co gamma rays. The absorbed dose of gamma rays in the NASBEE neutron fields was measured with an Al2 O3 :C OSL dosimeter (InLight, Nagase Landauer, Ltd.) and a reader system (microStar reader, Nagase Landauer, Ltd.). Because the ionization chamber (1 cm3 , IC-17A, Far West Technology, Inc.) was sensitive to fast neutrons and gamma rays [9], the absorbed dose of fast neutrons was evaluated by the difference among the OSL dosimeters and the ionization chamber. The uncertainties of the OSL dosimeters and the ionization chamber were estimated by the standard deviations of four samples and five exposures, respectively.
3. Results and discussions 3.1. Dependence of PE moderator thickness The maximum thermal neutron fluxes were 0.95 ± 0.01× 105 cm−2 s−1 μA−1 (Cd ratio of 4.63 ± 0.10) and 3.34 ± 0.05× 105 cm−2 s−1 μA−1 (Cd ratio of 4.44 ± 0.09) in the neutron fields from the 9 Be(p, n)9 B and 9 Be(D, n)10 B reactions, respectively, using the 30-mm-thick PE moderator (Fig. 2(a, b) and Table 1).The error bars in the figures are much smaller than the data symbols. The profiles (Section 3.2) and SSD dependences (Section 3.3) were measured with the 30-mmthick PE moderator. The fluxes were low compared with the therapeutic intensity required for BNCT (>1 × 109 cm2 s−1 ) [2]. In the near future, the thermal neutron flux intensity will be increased toward >1 × 109 cm2 s−1 for the 9 Be(p, n)9 B reactions by optimizing the moderator designs. The Cd ratio in the 9 Be(p, n)9 B reaction using the >100-mm-thick PE moderator was 1.6 times larger than that of the 9 Be(D, n)10 B reaction (Fig. 2(a, b)). 9 Be(D, n)10 B reactions produce higher energy
2.3. Setup configuration An aluminum stage (356 × 356 × 10 mm) with a square hole (290 × 290 × 10 mm) held PE moderators of arbitrary thickness in front of the detectors (Fig. 1). The aluminum stage was mounted on a lifting stage made of carbon fiber-reinforced plastic (CFRP) in a conventional irradiation room at the NASBEE facility. The source-to-surface distance 3
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Fig. 3. Neutron energy spectra of the 9 Be(p, n)9 B (solid line) and reactions (dotted line) for a 30-mm-thick PE moderator.
Nuclear Inst. and Methods in Physics Research, A 949 (2020) 162724
9
Be(D, n)10 B
neutrons than 9 Be(p, n)9 B reactions for the same primary beam energy [16]. The energy spectrum obtained by Geant4 simulations with a 30-mm-thick PE moderator was different from the measured spectrum (Fig. 3). The fast neutron energies for the 9 Be(p, n)9 B and 9 Be(D, n)10 B reactions were distributed below about 2 and 10 MeV, respectively. The difference in Cd ratio was due to the neutron energy spectrum in both reactions. The absorbed doses of fast neutrons and gamma rays were both 0.34 ± 0.01 Gy C−1 in the 9 Be(p, n)9 B reaction, and were 3.79 ± 0.05 Gy C−1 and 2.03 ± 0.04 Gy C−1 in the 9 Be(D, n)10 B reaction, respectively (Fig. 4(a, b) and Table 1). For the 9 Be target irradiated with the same charge, the absorbed doses of fast neutrons and gamma rays in the 9 Be(D, n)10 B reaction were 11 and 6 times larger than those of the 9 Be(p, n)9 B reaction, respectively. The slopes of the absorbed gamma ray doses for both reactions were lower than those of the absorbed fast neutron doses (Fig. 4(a, b)). For >30 and >70 mm thick PE moderators in the 9 Be(p, n)9 B and 9 Be(D, n)10 B reactions, the absorbed doses of fast neutrons were smaller than those of gamma rays. Fig. 5 and Table 1 show the dose contaminations of fast neutrons and gamma rays in both reactions. The fast neutron dose contamination decreased with increasing PE moderator thickness. In contrast, the minimum gamma ray dose contamination was achieved with 30- and 40-mm-thick PE moderators for the 9 Be(p, n)9 B and 9 Be(D, n)10 B reactions, respectively. These contaminations were one order of magnitude larger than the doses recommended by IAEA TECDOC-1223 in the therapeutic facility [1,25]. Beta particles with endpoint energies of 2.863 MeV emitted by 28 Al (produced by neutron capture by 27 Al) may affect the response in the OSL dosimeter. We expected that the absorbed dose measured with the OSL dosimeter would increase with the thermal neutron flux depending on moderator thickness. However, we did not observe this trend (Fig. 4(a, b)). We think that this effect was significantly lower than the background gamma rays because of the high contamination in the field compared with the therapeutic facilities (Fig. 5) [25].
Fig. 4. PE moderator thickness dependences of fast neutron (white), gamma ray (gray), and total (black) absorbed doses in neutron fields produced by the (a) 9 Be(p, n)9 B and (b) 9 Be(D, n)10 B reactions.
3.2. Profile measurements The uniform field of thermal neutrons, Cd ratio, and the absorbed doses were estimated by fitting analysis with 𝑓1 (x) (Figs. 6(a–d) and 7(a–d) and Table 2). The error of the flat areas was estimated by error propagation of the fitting parameters of 𝑓1 (x) and statistical uncertainty. The uniform field of the thermal neutron flux profile for an SSD of 1500 mm was spread compared with that for an SSD of 600 mm in the neutron fields of the 9 Be(p, n)9 B and 9 Be(D, n)10 B reactions. The
Fig. 5. PE moderator thickness dependences of dose contaminations for fast neutrons (triangles) and gamma rays (circles) in NASBEE neutron fields for the 9 Be(p, n)9 B (gray) and 9 Be(D, n)10 B (white) reactions.
4
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Fig. 6. Profiles of measured thermal neutron fluxes (black) and Cd ratio (white) in the neutron fields of the (a, b) 9 Be(p, n)9 B and (c, d) 9 Be(D, n)10 B reactions at SSDs of (a, c) 600 and (b, d) 1500 mm. Solid and dotted lines are fitting function 𝑓1 (𝑥) of the measured thermal neutron fluxes and Cd ratio, respectively.
Table 2 Flat profile areas of thermal neutron fluxes and absorbed doses of fast neutrons and gamma rays using a 30-mm-thick PE moderator. SSDs were 600 and 1500 mm for both interactions. The errors were estimated by the error propagation of the fitting parameters and statistical uncertainties. Neutron production
SSDa [mm]
Flat area [mm] Thermal neutron flux
9
Be(p,n)9 B
9 Be(D,n)9 B
a
600 1500 600 1500
78 ± 1.3 100 ± 1.0 76 ± 1.2 97 ± 1.1
Cd ratio
192 164 200 168
± ± ± ±
2.3 2.8 1.9 2.5
Absorbed dose Fast neutrons & gamma rays (IC)
Gamma rays (OSL)
Fast neutrons (IC - OSL)
96 ± 0.7 121 ± 1.6 124 ± 1.2 161 ± 1.2
97 ± 2.9 114 ± 3.1 102 ± 1.2 155 ± 2.1
91 ± 3.2 118 ± 4.4 133 ± 2.1 157 ± 4.2
A source-to-surface distance (SSD) was defined as a distance from 9 Be target to PE filter placed on detectors front.
neutron beam injected into the PE moderator at an SSD of 1500 mm was spread out more that at an SSD of 600 mm. The Cd ratios showed the opposite trend to the flux, and the flat areas were larger than the thermal neutron fluxes. The flat areas for an SSD of 1500 mm were also more spread out than those for an SSD of 600 mm (Fig. 6 a–d) for the absorbed doses of fast neutrons and gamma rays. The flat areas for the 9 Be(p, n)9 B reaction at SSDs of 600 and 1500 mm were smaller than those of the 9 Be(D, n)10 B reaction for both absorbed doses. The profiles of the absorbed doses were spread out compared with the profiles of the thermal neutron fluxes.
3.3. SSD dependences In the neutron fields produced by the 9 Be(p, n)9 B and 9 Be(D, n)10 B reactions, the SSD dependences of the neutron fluxes were inversely proportional to the square of the SSD (Figs. 7a, b and 8a, b). The intensity ratio mainly caused by the solid-angle effect; it was calculated as roughly (600/1500)2 = 0.16. The relative intensities of the thermal neutron flux at SSDs of 600 to 1500 mm were 0.13 and 0.14 in the 9 Be(p, n)9 B and 9 Be(D, n)10 B reactions, respectively. Since fast neutrons are scattered with target shield, angle distributions of fast neutrons are spread at collimator outlet of the target shield. The 5
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Nuclear Inst. and Methods in Physics Research, A 949 (2020) 162724
Fig. 7. Profiles of fast neutron (white), gamma ray (gray), and total (black) absorbed doses for the (a, b) 9 Be(p, n)9 B and (c, d) 9 Be(D, n)10 B reactions at SSDs of (a, c) 600 and (b, d) 1500 mm. Dotted, dashed, and solid lines are fitting function 𝑓1 (x) of the fast neutron, gamma ray, and total absorbed doses, respectively.
angle distributions are influence the fast neutron flux injected to PE moderator. Therefore, it is considered that the intensity ratio between SSDs of 600 mm and 1500 mm are lower than the solid-angle effect. The Cd ratios of both reactions were close to the values for SSDs of 600–1500 mm. The average Cd ratios for SSDs of 600–1500 mm were 4.75 ± 0.10 and 4.64 ± 0.15 for the 9 Be(p, n)9 B and 9 Be(D, n)10 B reactions, respectively.
3.4. Applications Thermal neutron fields with verified performance, including their contamination by fast neutrons and gamma rays, can be used to develop and calibrate thermal neutron detectors, to optimize filters for BNCT fields [26], and to develop materials for effective shielding in accelerator neutron fields [27]. Additionally, radiobiology experiments can be performed to characterize biological responses to accelerator BNCT fields.
The systematic uncertainty of the gold foil activation method was estimated from the reproducibility measurement. The flux of thermal neutrons was measured three times at the isocenter for an SSD of 600 mm with a 30-mm-thick PE moderator (Figs. 2(a, b), 6(a, c), and 8(a, b)). The systematic uncertainty was estimated to be 2.8% for the gold foil activation method.
4. Conclusions The thermal neutron fields from the 9 Be(p, n)9 B and 9 Be(D, n)10 B reactions were characterized in the accelerator neutron exposure facility, NASBEE. The maximum thermal neutron fluxes from the proton (600 μA) and deuteron (350 μA) beams were 5.7 × 107 and 1.2 × 108 cm−2 s−1 , respectively, using a 30-mm-thick PE moderator. The ratios of gamma ray absorbed dose to thermal neutron fluence were 3.6 × 10−12 Gy cm2 for the 9 Be(p, n)9 B reaction and 6.1 × 10−11 Gy cm2 for the 9 B(D, n)10 B reaction. The developed thermal neutron field in the NASBEE facility is useful for developing thermal neutron detectors and radiology applications related to BNCT, and is now open to external users.
The absorbed doses of fast neutrons and gamma rays in both reactions were also inversely proportional to the square of the SSD (Fig. 9 a, b). The relative absorbed doses of fast neutrons and gamma rays for SSDs of 600 and 1500 mm were 0.14 and 0.12 for the 9 Be(p, n)9 B reaction, and 0.15 and 0.12 for the 9 Be(D, n)10 B reaction, respectively. This suggests that gamma rays from the target shield were influence for the absorbed doses of gamma rays in SSD of 600 mm compared with 1500 mm. 6
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Fig. 8. SSD dependences of measured thermal neutron flux (black) and Cd ratio (white) with a 30-mm-thick PE moderator in the neutron fields of the (a) 9 Be(p, n)9 B and (b) 9 Be(D, n)10 B reactions. Solid lines are fitting function 𝑓2 (x) for the measured thermal neutron fluxes.
Fig. 9. SSD dependences of fast neutron (white), gamma ray (gray), and total (black) absorbed doses using a 30-mm-thick PE moderator in the neutron fields of the (a) 9 Be(p, n)9 B and (b) 9 Be(D, n)10 B reactions. Dotted, dashed, and solid lines are the fitting function 𝑓2 (x) for the fast neutron, gamma ray, and total absorbed doses.
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Nuclear Inst. and Methods in Physics Research, A journal homepage: www.elsevier.com/locate/nima
Thermal neutron field characteristics in the neutron exposure accelerator system for biological effect experiments (NASBEE) facility Ryo Ogawara a ,∗, Satoshi Kodaira b , Mitsuru Suda a , Takuya Hagihara a , Tsuyoshi Hamano a a
National Institute of Radiological Sciences, National Institutes for Quantum and Radiological Science and Technology, 4-9-1 Anagawa, Inage-ku, Chiba, Japan Center for Advanced Radiation Emergency Medicine, National Institutes for Quantum and Radiological Science and Technology, 4-9-1 Anagawa, Inage-ku, 263-8555 Chiba, Japan b
ARTICLE
INFO
Keywords: Accelerator-based neutron source Thermal neutrons fields Boron neutron capture therapy Fast neutron dosimetry
ABSTRACT The neutron exposure accelerator system for biological effect experiments (NASBEE) facility was constructed to investigate the biological effects of fast neutrons produced by the 9 Be(p, n)9 B and 9 B(D, n)10 B reactions. In this work, we describe the modification of the NASBEE facility for the development of thermal neutron detectors and radiology applications. We characterized thermal neutron fields created with a polyethylene (PE) moderator. The maximum thermal neutron fluxes via proton (600 μA) and deuteron (350 μA) beams were estimated to be 5.7 × 107 and 1.2 × 108 cm−2 s−1 , respectively, by using the 30-mm-thick PE moderator set in front of the irradiation sample The ratios of the gamma ray absorbed dose to thermal neutron fluence were 3.6 × 10−12 Gy cm2 for the 9 Be(p, n)9 B reaction and 6.1 × 10−11 Gy cm2 for the 9 B(D, n)10 B reaction. The thermal neutron field in the NASBEE facility can be used for various applications of thermal neutrons and is now open to external users.
1. Introduction Accelerator-based neutron sources have several advantages compared with conventional nuclear reactor neutron sources, including flexible intensity control and no requirement for nuclear fuel. Boron neutron capture therapy (BNCT) using an accelerator-based thermal neutron source is a promising candidate for cancer radiation therapy [1–3]. However, the thermal neutron field generated by accelerators has high dose contamination by fast neutrons and gamma rays [4,5]. The amount of contamination depends on the design of the irradiation components, such as the moderator and shielding material [1,3]. Therefore, the thermal neutron flux and the contamination of the thermal neutron dose must be assessed quantitatively for various applications. In thermal neutron fields, absorbed doses of gamma rays have often been measured with thermoluminescence dosimeters (TLD) [6] or optically stimulated luminescence (OSL) [7] dosimeters, because these detectors have a low sensitivity for fast neutrons. Total absorbed doses due to fast neutron interactions have been estimated by CR39 plastic nuclear track detectors [8,9], or neutron spectrometry with the ICRP 74 flux-to-dose conversion factor [10]. Equivalent doses of 60 Co gamma rays for fast neutrons were evaluated with an ionization chamber for biological effect experiments [11]. However, the ionization chamber is sensitive to both fast neutrons and gamma rays. Subtraction analysis between the ionization chamber and gamma ray dosimeters, such as TLD and OSL dosimeters, is required for fast neutron dosimetry.
The thermal neutron flux is evaluated by the gold foil activation method [12], and other radiation detectors, such as scintillators with optical fiber detectors [13,14] and glass dosimeters [15]. The neutron exposure accelerator system for biological effect experiments (NASBEE) facility [9,11] was constructed for studies of the biological effects of fast neutrons (∼2 MeV) at the National Institute of Radiological Sciences (NIRS), National Institutes for Quantum and Radiological Science and Technology (QST), Chiba, Japan. Recently, the demand for thermal neutron fields has increased at the NASBEE facility to develop thermal neutron detectors and radiology applications related to BNCT. The thermal neutrons are generally produced via a fast neutron moderator in front of the irradiation samples. The thermal neutron parameters, such as flux and absorbed dose contamination due to fast neutrons and gamma rays, depend on the configuration setup (e.g., moderator thickness, distance between source and moderator/sample, and surrounding shielding material) [1,3]. In this work, we describe the design for thermal neutron irradiation at the NASBEE facility and the characteristics of the thermal neutron field. 2. Materials and methods 2.1. NASBEE facility The NASBEE facility was originally constructed to investigate the biological effects of fast neutrons after the criticality accident at a
∗ Corresponding author. E-mail address: [email protected] (R. Ogawara).
https://doi.org/10.1016/j.nima.2019.162724 Received 18 October 2018; Received in revised form 5 July 2019; Accepted 6 September 2019 Available online 28 September 2019 0168-9002/© 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic of the experimental setup in the conventional irradiation room of the NASBEE facility.
Table 1 Maximum thermal neutron flux and dose contamination of fast neutrons and gamma rays in the neutron fields of the 9 Be(p, n)9 B and 9 Be(D, n)10 B reactions. The errors were estimated only as statistical uncertainty. 9
Neutron production PE moderator thickness [mm] Thermal neutron flux [cm−2 s−1 μA−1 ] (Cd ratio) Absorbed dose [Gy C−1 ] Dose contamination [Gy cm2 ]
Be(p, n)9 B
9
Be(D, n)10 B
30 (0.95 ± 0.01) × 105 (4.63 ± 0.10) 0.34 ± 0.01 0.34 ± 0.01 (3.57 ± 0.09) × 10−12 (3.63 ± 0.07) × 10−12
Fast neutrons Gamma rays Fast neutrons Gamma rays
uranium fuel conversion test facility belonging to JCO, in Tokaimura, Japan [9,11]. In the NASBEE facility, neutron beams are produced by 9 Be(p, n)9 B and 9 Be(D, n)10 B reactions [16] by using a multi-cusp negative ion source that produces a proton or a deuteron beam and also includes a 2 MV tandem accelerator and a solid 9 Be target (𝜙120 mm, 200 μm thick). Neutron beams are irradiated from the collimator in a polyethylene (PE) shield covering the 9 Be target (Fig. 1). The maximum beam currents of the 4 MeV protons and deuterons are 600 and 350 μA, respectively.
(3.34 ± 0.05) × 105 (4.44 ± 0.09) 3.79 ± 0.05 2.03 ± 0.04 (11.3 ± 0.33) × 10−12 (6.07 ± 0.16) × 10−12
scintillator (Canberra). Neutron flux 𝛷 estimated from the activated gold foil was obtained with Eq. (1). 𝛷=
1 𝐶 𝜆 ( ) ( ) 𝑁𝐴𝑢 𝜎 𝜀𝐼𝑔 𝐹𝑠 1 − 𝑒−𝜆𝑡𝑖 𝑒−𝜆𝑡𝑐 1 − 𝑒−𝜆𝑡𝑚
(1)
Here, 𝜆 is the decay constant of 198 Au (= 0.693/𝑇1∕2 , where 𝑇1∕2 is the half-life), 𝑁Au is the number of 197 Au nuclei, and 𝜎 is the crosssection of 197 Au(n, 𝛾)198 Au reactions (98.5 b). For 412 keV gamma rays, the detection efficiency, 𝜀, is 48%, the intensity abundance, 𝐼g is 95.6%, and self-shielding factor 𝐹S of the thermal neutrons is calculated to be 0.993 [17]. 𝑡i , 𝑡c , and 𝑡m are the times of neutron exposure, cooling, and spectrum measurement, respectively. Cd ratio 𝑅Cd is defined as the measured flux ratio of bare gold foil, 𝛷bare , to Cd-covered foil, 𝛷Cd . 𝛷bare contains thermal neutron fluxes 𝛷th and epithermal neutron fluxes 𝛷epi . 𝛷Cd contains epithermal neutrons because 133 Cd has a large neutron absorption cross-section. Because 𝑅Cd = (𝛷th + 𝛷epi )/𝛷epi , thermal neutron fluxes (< 0.55 eV of Cd cut-off energy [18]) were obtained by 𝛷th = 𝛷bare (1 – 1/𝑅Cd ). The statistical uncertainty of
2.2. Radiation detectors and dosimeters Thermal neutron fluxes were obtained by neutron activation analysis using bare gold foils (0.5 × 0.5 × 0.01 mm, Nilaco Corp.) and Cd covers (8 × 8 × 0.5 mm) [12]. After thermal neutron irradiation, the event count, C, of the total absorption peak of 412 keV gamma rays emitted by 198 Au was measured using a well-type NaI(Tl) inorganic 2
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(SSD), which was defined as the distance from the 9 Be target to the PE moderator surface in front of the detector, could be controlled in the range 600–1500 mm by the lifting stage. Ten cuboid PE moderators 300 × 300 mm in size and 10, 20, 30, 40, or 50 mm thick (two moderators of each thickness) were used. The thickness of the PE moderator placed on the back of the detector was fixed as 70 mm. The distance was 20 mm between the PE moderators on the front and back detectors. 2.4. Experiments The thermal neutron flux, Cd ratio, absorbed dose, and dose contamination due to fast neutrons and gamma rays depending on PE moderator thickness (0–240 mm) were measured. The PE moderator thicknesses that obtained the maximum thermal neutron fluxes were determined for the 9 Be(p, n)9 B and 9 Be(D, n)10 B reactions. The profiles and SSD dependences of the thermal neutron field characteristics were measured for this PE moderator thickness. The profile measurements were performed from −150 to 150 mm at 30 mm intervals at SSDs of 600 and 1500 mm. Profile shapes were evaluated by a fitting analysis with the polynomial function 𝑓1 (x) = a(x − 𝑥o )4 + b(x − 𝑥o )2 + c, where a, b, c, and 𝑥o are fitting parameters. Areas with intensities of more than 95% in the profiles were defined as flat areas. The SSD dependences (SSDs of 600, 900, 1200, and 1500 mm) of the thermal neutron fields characteristics were evaluated in both reactions. The curves of SSD dependences were evaluated by fitting analysis with the function 𝑓2 (x) = a/(x − 𝑥o )2 , where a and 𝑥o are fitting parameters. Thermal neutron fluxes (units: cm−2 s−1 μA−1 ) were normalized by using the primary beam current. The absorbed doses of fast neutrons and gamma rays were normalized by using the total irradiation charges for the 9 Be target as Gy C−1 . The dose contaminations (units: Gy cm2 ) of fast neutrons and gamma rays were defined as the ratio of the absorbed dose to the thermal neutron fluence. 2.5. Geant4 Monte Carlo simulations The neutron energy spectra for the 9 Be(p, n)9 B and 9 Be(D, n)10 B reactions in NASBEE thermal neutron fields were simulated with Geant 4.10.3 [19–21]. The QGSP_BIC_AllHP model and TENDL-2015 (TALYSbased evaluated nuclear data library) were used in the simulation [22, 23]. The quark–gluon string precompound (QGSP) and binary cascade (BIC) were used for particle energy ranges of 12 GeV–100 TeV and 200 MeV–9.9 GeV. In previous work, the neutron production yields of the 9 Be(p, n)9 B reactions obtained with the BIC model were inconsistent with ENDF/B-VII nuclear data library in the proton energy range of < 50 MeV [24]. Therefore, the TENDL libraries were used in conjunction with the ENDF/B-VII data library in an energy range of 0–200 MeV.
Fig. 2. PE moderator thickness dependence of thermal neutron fluxes (black) and Cd ratio (white) in neutron fields from (a) 9 Be(p, n)9 B and (b) 9 Be(D, n)10 B reactions. Statistical errors are smaller than the symbol size.
the thermal neutron flux was derived from the error propagation of the event count. The dosimetry systems for fast neutrons and gamma rays were calibrated by the air kerma of 60 Co gamma rays at NIRS, QST. Thus, all absorbed doses described below are mean equivalent doses of 60 Co gamma rays. The absorbed dose of gamma rays in the NASBEE neutron fields was measured with an Al2 O3 :C OSL dosimeter (InLight, Nagase Landauer, Ltd.) and a reader system (microStar reader, Nagase Landauer, Ltd.). Because the ionization chamber (1 cm3 , IC-17A, Far West Technology, Inc.) was sensitive to fast neutrons and gamma rays [9], the absorbed dose of fast neutrons was evaluated by the difference among the OSL dosimeters and the ionization chamber. The uncertainties of the OSL dosimeters and the ionization chamber were estimated by the standard deviations of four samples and five exposures, respectively.
3. Results and discussions 3.1. Dependence of PE moderator thickness The maximum thermal neutron fluxes were 0.95 ± 0.01× 105 cm−2 s−1 μA−1 (Cd ratio of 4.63 ± 0.10) and 3.34 ± 0.05× 105 cm−2 s−1 μA−1 (Cd ratio of 4.44 ± 0.09) in the neutron fields from the 9 Be(p, n)9 B and 9 Be(D, n)10 B reactions, respectively, using the 30-mm-thick PE moderator (Fig. 2(a, b) and Table 1).The error bars in the figures are much smaller than the data symbols. The profiles (Section 3.2) and SSD dependences (Section 3.3) were measured with the 30-mmthick PE moderator. The fluxes were low compared with the therapeutic intensity required for BNCT (>1 × 109 cm2 s−1 ) [2]. In the near future, the thermal neutron flux intensity will be increased toward >1 × 109 cm2 s−1 for the 9 Be(p, n)9 B reactions by optimizing the moderator designs. The Cd ratio in the 9 Be(p, n)9 B reaction using the >100-mm-thick PE moderator was 1.6 times larger than that of the 9 Be(D, n)10 B reaction (Fig. 2(a, b)). 9 Be(D, n)10 B reactions produce higher energy
2.3. Setup configuration An aluminum stage (356 × 356 × 10 mm) with a square hole (290 × 290 × 10 mm) held PE moderators of arbitrary thickness in front of the detectors (Fig. 1). The aluminum stage was mounted on a lifting stage made of carbon fiber-reinforced plastic (CFRP) in a conventional irradiation room at the NASBEE facility. The source-to-surface distance 3
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Fig. 3. Neutron energy spectra of the 9 Be(p, n)9 B (solid line) and reactions (dotted line) for a 30-mm-thick PE moderator.
Nuclear Inst. and Methods in Physics Research, A 949 (2020) 162724
9
Be(D, n)10 B
neutrons than 9 Be(p, n)9 B reactions for the same primary beam energy [16]. The energy spectrum obtained by Geant4 simulations with a 30-mm-thick PE moderator was different from the measured spectrum (Fig. 3). The fast neutron energies for the 9 Be(p, n)9 B and 9 Be(D, n)10 B reactions were distributed below about 2 and 10 MeV, respectively. The difference in Cd ratio was due to the neutron energy spectrum in both reactions. The absorbed doses of fast neutrons and gamma rays were both 0.34 ± 0.01 Gy C−1 in the 9 Be(p, n)9 B reaction, and were 3.79 ± 0.05 Gy C−1 and 2.03 ± 0.04 Gy C−1 in the 9 Be(D, n)10 B reaction, respectively (Fig. 4(a, b) and Table 1). For the 9 Be target irradiated with the same charge, the absorbed doses of fast neutrons and gamma rays in the 9 Be(D, n)10 B reaction were 11 and 6 times larger than those of the 9 Be(p, n)9 B reaction, respectively. The slopes of the absorbed gamma ray doses for both reactions were lower than those of the absorbed fast neutron doses (Fig. 4(a, b)). For >30 and >70 mm thick PE moderators in the 9 Be(p, n)9 B and 9 Be(D, n)10 B reactions, the absorbed doses of fast neutrons were smaller than those of gamma rays. Fig. 5 and Table 1 show the dose contaminations of fast neutrons and gamma rays in both reactions. The fast neutron dose contamination decreased with increasing PE moderator thickness. In contrast, the minimum gamma ray dose contamination was achieved with 30- and 40-mm-thick PE moderators for the 9 Be(p, n)9 B and 9 Be(D, n)10 B reactions, respectively. These contaminations were one order of magnitude larger than the doses recommended by IAEA TECDOC-1223 in the therapeutic facility [1,25]. Beta particles with endpoint energies of 2.863 MeV emitted by 28 Al (produced by neutron capture by 27 Al) may affect the response in the OSL dosimeter. We expected that the absorbed dose measured with the OSL dosimeter would increase with the thermal neutron flux depending on moderator thickness. However, we did not observe this trend (Fig. 4(a, b)). We think that this effect was significantly lower than the background gamma rays because of the high contamination in the field compared with the therapeutic facilities (Fig. 5) [25].
Fig. 4. PE moderator thickness dependences of fast neutron (white), gamma ray (gray), and total (black) absorbed doses in neutron fields produced by the (a) 9 Be(p, n)9 B and (b) 9 Be(D, n)10 B reactions.
3.2. Profile measurements The uniform field of thermal neutrons, Cd ratio, and the absorbed doses were estimated by fitting analysis with 𝑓1 (x) (Figs. 6(a–d) and 7(a–d) and Table 2). The error of the flat areas was estimated by error propagation of the fitting parameters of 𝑓1 (x) and statistical uncertainty. The uniform field of the thermal neutron flux profile for an SSD of 1500 mm was spread compared with that for an SSD of 600 mm in the neutron fields of the 9 Be(p, n)9 B and 9 Be(D, n)10 B reactions. The
Fig. 5. PE moderator thickness dependences of dose contaminations for fast neutrons (triangles) and gamma rays (circles) in NASBEE neutron fields for the 9 Be(p, n)9 B (gray) and 9 Be(D, n)10 B (white) reactions.
4
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Fig. 6. Profiles of measured thermal neutron fluxes (black) and Cd ratio (white) in the neutron fields of the (a, b) 9 Be(p, n)9 B and (c, d) 9 Be(D, n)10 B reactions at SSDs of (a, c) 600 and (b, d) 1500 mm. Solid and dotted lines are fitting function 𝑓1 (𝑥) of the measured thermal neutron fluxes and Cd ratio, respectively.
Table 2 Flat profile areas of thermal neutron fluxes and absorbed doses of fast neutrons and gamma rays using a 30-mm-thick PE moderator. SSDs were 600 and 1500 mm for both interactions. The errors were estimated by the error propagation of the fitting parameters and statistical uncertainties. Neutron production
SSDa [mm]
Flat area [mm] Thermal neutron flux
9
Be(p,n)9 B
9 Be(D,n)9 B
a
600 1500 600 1500
78 ± 1.3 100 ± 1.0 76 ± 1.2 97 ± 1.1
Cd ratio
192 164 200 168
± ± ± ±
2.3 2.8 1.9 2.5
Absorbed dose Fast neutrons & gamma rays (IC)
Gamma rays (OSL)
Fast neutrons (IC - OSL)
96 ± 0.7 121 ± 1.6 124 ± 1.2 161 ± 1.2
97 ± 2.9 114 ± 3.1 102 ± 1.2 155 ± 2.1
91 ± 3.2 118 ± 4.4 133 ± 2.1 157 ± 4.2
A source-to-surface distance (SSD) was defined as a distance from 9 Be target to PE filter placed on detectors front.
neutron beam injected into the PE moderator at an SSD of 1500 mm was spread out more that at an SSD of 600 mm. The Cd ratios showed the opposite trend to the flux, and the flat areas were larger than the thermal neutron fluxes. The flat areas for an SSD of 1500 mm were also more spread out than those for an SSD of 600 mm (Fig. 6 a–d) for the absorbed doses of fast neutrons and gamma rays. The flat areas for the 9 Be(p, n)9 B reaction at SSDs of 600 and 1500 mm were smaller than those of the 9 Be(D, n)10 B reaction for both absorbed doses. The profiles of the absorbed doses were spread out compared with the profiles of the thermal neutron fluxes.
3.3. SSD dependences In the neutron fields produced by the 9 Be(p, n)9 B and 9 Be(D, n)10 B reactions, the SSD dependences of the neutron fluxes were inversely proportional to the square of the SSD (Figs. 7a, b and 8a, b). The intensity ratio mainly caused by the solid-angle effect; it was calculated as roughly (600/1500)2 = 0.16. The relative intensities of the thermal neutron flux at SSDs of 600 to 1500 mm were 0.13 and 0.14 in the 9 Be(p, n)9 B and 9 Be(D, n)10 B reactions, respectively. Since fast neutrons are scattered with target shield, angle distributions of fast neutrons are spread at collimator outlet of the target shield. The 5
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Fig. 7. Profiles of fast neutron (white), gamma ray (gray), and total (black) absorbed doses for the (a, b) 9 Be(p, n)9 B and (c, d) 9 Be(D, n)10 B reactions at SSDs of (a, c) 600 and (b, d) 1500 mm. Dotted, dashed, and solid lines are fitting function 𝑓1 (x) of the fast neutron, gamma ray, and total absorbed doses, respectively.
angle distributions are influence the fast neutron flux injected to PE moderator. Therefore, it is considered that the intensity ratio between SSDs of 600 mm and 1500 mm are lower than the solid-angle effect. The Cd ratios of both reactions were close to the values for SSDs of 600–1500 mm. The average Cd ratios for SSDs of 600–1500 mm were 4.75 ± 0.10 and 4.64 ± 0.15 for the 9 Be(p, n)9 B and 9 Be(D, n)10 B reactions, respectively.
3.4. Applications Thermal neutron fields with verified performance, including their contamination by fast neutrons and gamma rays, can be used to develop and calibrate thermal neutron detectors, to optimize filters for BNCT fields [26], and to develop materials for effective shielding in accelerator neutron fields [27]. Additionally, radiobiology experiments can be performed to characterize biological responses to accelerator BNCT fields.
The systematic uncertainty of the gold foil activation method was estimated from the reproducibility measurement. The flux of thermal neutrons was measured three times at the isocenter for an SSD of 600 mm with a 30-mm-thick PE moderator (Figs. 2(a, b), 6(a, c), and 8(a, b)). The systematic uncertainty was estimated to be 2.8% for the gold foil activation method.
4. Conclusions The thermal neutron fields from the 9 Be(p, n)9 B and 9 Be(D, n)10 B reactions were characterized in the accelerator neutron exposure facility, NASBEE. The maximum thermal neutron fluxes from the proton (600 μA) and deuteron (350 μA) beams were 5.7 × 107 and 1.2 × 108 cm−2 s−1 , respectively, using a 30-mm-thick PE moderator. The ratios of gamma ray absorbed dose to thermal neutron fluence were 3.6 × 10−12 Gy cm2 for the 9 Be(p, n)9 B reaction and 6.1 × 10−11 Gy cm2 for the 9 B(D, n)10 B reaction. The developed thermal neutron field in the NASBEE facility is useful for developing thermal neutron detectors and radiology applications related to BNCT, and is now open to external users.
The absorbed doses of fast neutrons and gamma rays in both reactions were also inversely proportional to the square of the SSD (Fig. 9 a, b). The relative absorbed doses of fast neutrons and gamma rays for SSDs of 600 and 1500 mm were 0.14 and 0.12 for the 9 Be(p, n)9 B reaction, and 0.15 and 0.12 for the 9 Be(D, n)10 B reaction, respectively. This suggests that gamma rays from the target shield were influence for the absorbed doses of gamma rays in SSD of 600 mm compared with 1500 mm. 6
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Fig. 8. SSD dependences of measured thermal neutron flux (black) and Cd ratio (white) with a 30-mm-thick PE moderator in the neutron fields of the (a) 9 Be(p, n)9 B and (b) 9 Be(D, n)10 B reactions. Solid lines are fitting function 𝑓2 (x) for the measured thermal neutron fluxes.
Fig. 9. SSD dependences of fast neutron (white), gamma ray (gray), and total (black) absorbed doses using a 30-mm-thick PE moderator in the neutron fields of the (a) 9 Be(p, n)9 B and (b) 9 Be(D, n)10 B reactions. Dotted, dashed, and solid lines are the fitting function 𝑓2 (x) for the fast neutron, gamma ray, and total absorbed doses.
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