- Email: [email protected]
Photoneutron production by a 25 MeV electron linac for BNCT application
Annals of Nuclear Energy 54 (2013) 192–196
Contents lists available at SciVerse ScienceDirect
Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene
Photoneutron production by a 25 MeV electron linac for BNCT application Fatemeh Torabi a, S. Farhad Masoudi a,⇑, Faezeh Rahmani b a b
Department of Physics, K.N. Toosi University of Technology, P.O. Box 15875-4416, Tehran, Iran Department of Radiation Application, Shahid Beheshti University, P.O. Box 1983963113, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 8 August 2012 Received in revised form 26 October 2012 Accepted 1 November 2012 Available online 20 December 2012 Keywords: Photoneutron source Electron linear accelerator Beam shaping assembly MCNPX BNCT Neutron yield
a b s t r a c t Recently, extensive research has been performed to produce neutron beams in hospitals using electron linear accelerator for neutron medical applications such as Boron Neutron Capture Therapy. In this article, we present the results of Monte Carlo calculations, using MCNPX code, for the possibility of setting up a 25 MeV electron linac based photoneutron source for BNCT applications. Photon and photoneutron converters with different materials in various geometries were studied to achieve maximum neutron yield. In addition, the possibility of using an optimized neutron beam in BNCT was investigated by proposing the optimized beam shaping assembly. It was shown that our designed neutron beam, which passes through the optimized BSA, meets the IAEA criteria for in-air parameters. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Neutron sources are important tools for a variety of applications in different fields of science and engineering. Neutrons have many properties that make them useful for certain types of researches. Neutron can travel large distances through some materials without being scattered or absorbed. It can be used to locate hydrogen and light atoms precisely. So it is ideal for many applications that require detection of light materials such as explosive mines in land, corrosion in fighter jet wings and light oxygen in YBCO (Yttrium– Barium–Copper Oxide). In addition to industrial applications, the neutron is a useful particle in medical physics (Yamamoto et al., 2002; Auterinen et al., 2004; Huang et al., 2005; Koivunoro et al., 2004). For example, Boron Neutron Capture Therapy (BNCT) is an effective therapeutic modality to treat malignant tumors such as brain tumors and head and neck cancers (Yu et al., 2011; Rao et al., 2004). There are a wide variety of different neutron sources like radioactive sources (Ghassoun et al., 2009), reactors (Auterinen et al., 2004; Kiger et al., 1999, 2004), and accelerator-based neutron sources (Elshahat et al., 2007; Bleuel et al., 1998; Tahara et al., 2006; Rahmani et al., 2011). However, some neutron sources such as reactors are not appropriate in medical field because of some disadvantages such as radioactive waste materials, large dimensions and the problems in maintenance. Unlike the reactor based neutron sources, accelerator-based neutron sources have advantages such as: ⇑ Corresponding author. E-mail address: [email protected] (S.F. Masoudi). 0306-4549/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.anucene.2012.11.001
– No radioactive waste materials are generated. – Shielding is needed only during accelerator ON condition. – Having small dimensions in comparison to reactor based neutron sources. – Possibility to be pulsed. – Modulating the flux intensity can be done by modifying the parameters of accelerated particle for different applications. Recently, the electron linac-based neutron sources have been studied extensively (Jallu et al., 1999; Rahmani and Shahriari, 2010; Huang et al., 2006; Rahmani et al., 2011). Although these neutron sources have the above mentioned advantages, their main disadvantage is low neutron production (Auditore et al., 2005). Nevertheless, acceptable source strength, at least for some applications like BNCT, can be obtained using appropriate materials in an optimized geometry. In the present study, the optimization process for photoneutron source was investigated using the MCNPX (Monte Carlo Code), considering a 25 MeV electron linear. Also the application of the proposed photoneutron source for BNCT treatment was studied. 2. Materials and methods 2.1. The photon target As the probable energy of the electron source in linac is 25 MeV emitted with the highest probability from one point, in our simulation we assumed that the electron source in SDEF card is a small disk with the monoenergetic 25 MeV electron beam. Since, the produced electrons pass through the high atomic Z number target, a
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cascade shower of bremsstrahlung photons is produced. The produced photons participate in (c, n) reaction within photoneutron target. Photoneutrons are produced when the energy of photons are higher than the threshold energy of the (c, n) reaction. Depending on the photoneutron target, the threshold energy varies from 7 MeV to 18 MeV (except Be, D). A photon converter design is the first step in investigating the photoneutron source in which the photon beam is produced through (e, c) reaction. For optimizing the photon production, some appropriate targets such as tungsten (W), tantalum (Ta), rhenium (Re) and lead (Pb) were considered in two different geometries: cylindrical (with different thicknesses and radiuses) and hemispherical (with different radiuses). 2.2. The photoneutron target Design of the photoneutron converter was performed to calculate the highest neutron yield obtained from the optimized photon converter. The neutron yields produced by different photoneutron converters were studied, considering the (c, n) energy threshold and related cross-section. To achieve the maximum photoneutron yield, the best material in the optimized geometry should be selected. Uranium as the best photoneutron converter with the high (c, n) cross section was studied in different geometries to determine an efficient geometry which provides the highest neutron yield. Monte Carlo calculations for the proposed geometries were carried out again in the presence of lead (Pb) as a reflector to obtain the highest forward neutron yield. Forward neutron yield was calculated by assuming that Pb completely surrounds the photoneutron converter. Also, several materials were examined as the photoneutron converter such as U, Th, W, Ta, Au, Ag, BeD2, Be. The optimized geometry was considered in all calculations.
Fig. 1. The photon yield produced by bombarding W with 25 MeV electron beam.
2.3. BSA design In recent years, different studies have focused on using linac for BNCT; however it seems that most of the reported neutron fluxes lack enough epithermal neutron flux to be employed in BNCT. In comparison with other studies, this neutron beam seems to be strong enough for BNCT especially for treatment of deep-seated brain tumors. To obtain an adequate epithermal neutron flux, the neutron beam must be shaped through the appropriate beam shaping assembly (BSA). Thus, an optimum BSA including the moderators, the reflector, the collimator and the gamma shield was proposed. During optimization process, different compositions of materials were investigated to meet the in-air parameters at the exit port.
Fig. 2. The photon yield in the different targets with optimized thickness.
3. Results and discussion 3.1. Photon converter Fig. 1 shows the photon yield for the 25 MeV electron beam bombarding tungsten target in cylindrical geometry. As seen, by increasing the radius more than 1 cm, no considerable increasing can be achieved. Therefore optimized dimensions were chosen as 0.3 cm in thickness and 1 cm in radius. Similar calculations were performed for Ta, Pb and Re. The optimized thickness and radius were determined to achieve the maximum photon yield. The results of comparing photon yields in different materials are shown in Fig. 2. Considering optimum thickness and radius, the photon yields produced by cylindrical W, Ta and Re have almost the same results. The photon yields of the selected targets in hemispherical geometry with different radiuses were also calculated to obtain
Fig. 3. The photon yield by bombarding different targets in the hemispherical geometry with 25 MeV electron beam.
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Fig. 4. The cylindrical photoneutron converter (labeled by 2) surrounds the optimized photon converter (labeled by 1).
Fig. 7. Neutron yield of uranium photoneutron source in three different geometries of Fig. 6.
Table 1 Forward neutron yield in four different geometries. Configuration Neutron yield (n/mA)
Geometry 1 3.55 1013
Geometry 2 3.86 1013
Geometry 3 4.75 1013
Cylindrical shape 1.88 1013
Fig. 5. Photoneutron yield in the cylindrical photoneutron converter with different radiuses and thicknesses.
optimized radius. As shown in Fig. 3, the photon production in the hemispherical targets with optimized radiuses is better than in the cylindrical geometry. The maximum photon yields in W and Re are approximately the same; however W was selected because it is more accessible. In conclusion, we chose tungsten in hemispherical geometry with 0.4 cm radius as the best photon converter.
Fig. 8. Photoneutron yield of the different photoneutron converters in the optimized geometry.
Fig. 6. Three different geometries for the photoneutron converter containing the hemispherical photon converter.
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F. Torabi et al. / Annals of Nuclear Energy 54 (2013) 192–196 Table 2 The probable (c, n) reactions for the different materials used as photoneutron sources. Probable (c, n) reactions
Threshold energies (MeV) 238
(c, n) (c, 2n) (c, 3n)
U
6.15 11.28 17.82
232
Th
6.44 11.56 18.35
180
W
8.41 15.35 23.57
182
183
184
186
8.07 14.75 23.16
6.19 14.26 20.94
7.41 13.60 21.67
7.19 12.95 20.36
W
W
W
181
W
Ta
7.58 14.22 22.13
197
Au
8.07 14.71 23.08
107
Ag
9.54 17.47 27.51
9
Be
1.67 20.56 31.24
Table 3 The in-air parameters of optimized BSA configuration in comparison with the IAEA criteria.
uepi (n/cm2 s)
D_ fn =uepi
D_ c =uepi
(Gy cm2)
(Gy cm2)
uepi/ uthermal
IAEA criteria >5 108 <2 10–13 <2 10–13 >20 MgF2 15 cm, TiF3 30 cm 7.2 108 /mA 2.96 10–14 8.65 10–14 55.5 MgF2 15 cm, TiF3 35 cm 4.9 108 /mA 2.24 10–14 8.66 10–14 49.0
Fig. 9. The neutron spectrum of photoneutron source; (1) our proposed source with a 25 MeV electron linac, (2) our proposed source with a 20 MeV electron linac, and (3) the result of reference (Rahmani and Shahriari, 2010).
3.2. Photoneutron converter At first step, the photoneutron converter was assumed as cylindrical uranium which is shown in Fig. 4. Neutron yield was determined by changing height and radius of this cylinder. It is shown in Fig. 5, a cylinder with 4 cm in thickness and approximately 5–6 cm in radius can be considered as the optimized cylindrical photoneutron converter. Three different shapes of uranium as the photoneutron target are shown in Fig. 6. Fig. 7 shows the photoneutron yields of these geometries in different radiuses (spherical: geometry 2 or hemispherical: geometries 1 and 3). Fig. 7 shows that the neutron yields of the mentioned geometries in radius of 6 cm are approximately the same. Table 1 shows the results of the photoneutron converter with Pb reflector in the four studied geometries.
The results of neutron yield in different photoneutron targets such as U, Th, W, Ta, Au, Ag, BeD2 and Be are shown in Fig. 8. The probable (complete) (c, n) reactions for these targets are shown in Table 2. According to the results of Fig. 8, uranium seems to be the best photoneutron converter. In summary, the best materials and their geometries to obtain maximum photoneutron yield are as follows: hemispherical tungsten with 0.4 cm in thickness as the photon converter which was located in the middle of hemispherical uranium with 6 cm in radius. In Fig. 9, the neutron spectrum of our proposed photoneutron source for a 25 MeV and a 20 MeV electron linac and the result of (Rahmani and Shahriari, 2010) have been illustrated. Our proposed photoneutron source has a reasonable high energy neutron yield near the epithermal energy even using a 20 MeV linac. As our proposed neutron beam has a suitable neutron flux near the epithermal energy, it can be used to produce neutrons with high epithermal neutron flux using suitable beam shaping assembly. Enough epithermal neutron flux is critical in some applications such as the treatment of deep-seated brain tumors by BNCT. In the next section, the properties of the obtained neutron beam were studied after beam shaping assembly at the beam exit, and the in-air parameters were determined. 3.3. BSA design for BNCT application Based on the better transmission of epithermal neutron flux and by removing beam contamination, the BSA was designed cylindrical which includes (See Fig. 10):
Fig. 10. The photoneutron target and final BSA configuration (1) photoneutron source, (2) Pb as the reflector, (3) MgF2 as the first moderator, (4) TiF3 as the second moderator, (5) Bi as the gamma shield, (6) Ni as the collimator.
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– 15 cm MgF2 and 35 cm TiF3 as the first and the second moderators. – 20 cm bismuth as the best gamma shield to reduce the emission of high energy photons at the beam port. – 1 cm Nickel as the best collimator and neutron guide considering its low absorption cross section in the epithermal region. – Lead as the best reflector surrounding the entire BSA configuration. Table 3 shows the in-air parameters of the optimized BSA configuration for the designed photoneutron source. The results of the present study show that the proposed neutron beam meets the IAEA criteria [IAEA-TECHDOC-1223, 2001] and has a suitable epithermal neutron flux. 4. Conclusion The photoneutron source based on high energy electron linac was optimized to produce the maximum neutron yield. Calculations were performed using MCNPX Mont Carlo Code for 25 MeV electron beam bombarding different photon targets in different geometries. High photon production achieved through the use of tungsten in the hemispherical shape. Also, the study of photoneutron production performed for several materials in different geometries. The hemispherical design of uranium was selected as the best photoneutron converter which produces the highest neutron yield towards the beam port. According to the results of the present study the produced neutron beam had enough intensity to meet the demand of many applications like BNCT. Therefore, the optimal beam shaping assembly was designed based on the best moderators (MgF2 and TiF3), the reflector (Pb), the gamma shield (Bi) and the collimator (Ni). The investigation of the beam quality at the beam port indicates that the beam meets the IAEA criteria values and it is proper to treat deep-seated brain tumors. References Auditore, L., Barna, R.C., De Pasquale, D., Italiano, A., Trifiro, A., Trimarchi, M., 2005. Study of a 5 MeV electron linac based neutron source. Nucl. Instrum. Method. Phys. Res. B 229, 137–143.
Auterinen, I., Seren, T., Anttila, K., Kosunen, A., Savolainen, S., 2004. Measurement of free beam neutron spectra at eight BNCT facilities worldwide. Appl. Radiat. Isot. 61, 1021–1026. Bleuel, D.L., Donahue, R.L., Ludewigt, B.A., Vujic, J., 1998. Designing acceleratorbased epithermal neutron beams for boron neutron capture therapy. Med. Phys. 25, 1725–1734. IAEA-TECDOC-1223, 2001. Current Status of Neutron Capture Therapy. International Atomic Energy Agency. Elshahat, B.A., Naqvi, A.A., Maalej, N., Abdalla, Kh., 2007. Design calculation of an accelerator based BSA for BNCT of brain cancer. Radioanal. Nucl. Chem. 274, 539–544. Ghassoun, J., Chkillou, B., Jehouani, A., 2009. Spatial and spectral characteristics of a compact system neutron beam designed for BNCT facility. Appl. Radiat. Isot. 67, 560–564. Huang, W.L., Li, Q.F., Lin, Y.Z., 2005. Calculation of photoneutrons produced in the targets of electron linear accelerators for radiography and radiotherapy applications. Nucl. Instrum. Method. Phys. Res. B 229, 339–347. Huang, W.L., Li, Q.F., Lin, Y.Z., Su, Q., Luo, Y.S., 2006. Measurements of photoneutrons produced by a 15 MeV electron linac for radiography applications. Nucl. Instrum. Method.Phys. Res. B 251, 361–366. Jallu, F., Lyoussi, A., Payan, E., Recroix, H., Mariani, A., Nurdin, G., Buisson, A., Allano, J., 1999. Photoneutron production in tungsten, praseodymium, copper and beryllium by using high energy electron linear accelerator. Nucl. Instrum. Method.Phys. Res. B 155, 373–381. Kiger, W.S., Sakamoto, S., Harling, O.K., 1999. Neutronic design of a fission converter-based epithermal neutron beam for neutron capture therapy. Nucl. Sci. Eng. 131, 1–22. Kiger, W.S., Lu, X.Q., Harling, O.K., Riley, K.J., Binns, P.J., Kaplan, J., Patel, H., Zamenhof, R.G., Shibata, Y., Kaplan, I.D., Busse, P.M., Palmer, M.R., 2004. Preliminary treatment planning and dosimetry for a clinical trial of neutron capture therapy using a fission converter epithermal neutron beam. Appl. Radiat. Isot. 61, 1075–1081. Koivunoro, H., Bleuel, D.L., Nastasi, U., Nastasi, U., lou, T.P., Reijonen, J., Leung, K.N., 2004. BNCT dose distribution in liver with epithermal D–D and D–T fusionbased neutron beams. Appl. Radiat. Isot. 61, 853–859. Rahmani, F., Shahriari, M., 2010. Hybrid photoneutron source optimization for electron accelerator-based BNCT. Nucl. Instrum. Method. Phys. Res. A 618, 48– 53. Rahmani, F., Shahriari, M., Minoochehr, A., Nedaie, H., 2011. Feasibility study on the use of uranium in photoneutron target and BSA optimization for linac based BNCT. Nucl. Instrum. Method. Phys. Res. A 641, 136–140. Rao, M., Trivillin, V.A., Heber, E.M., Cantarelli, M.A., Itoiz, M.E., Nigg, D.W., Rebagliati, R.J., Batistoni, D., Schwint, A.E., 2004. BNCT of 3 cases of spontaneous head and neck cancer in feline patients. Appl. Radiat. Isot. 61, 947–952. Tahara, Y., ODA, Y., Shiraki, T., Tsutsui, T., Yokobori, H., Yonai, S., Baba, M., Nakamura, T., 2006. Engineering design of a spallation reaction-based neutron generator for Boron Capture Therapy. Nucl. Sci. Technol. 43, 9–19. Yamamoto, T., Matsumura, A., Yamamoto, K., Kumada, H., Shibata, Y., Nose, T., 2002. In Phantom two-dimensional thermal neutron distribution for intraoperative boron neutron capture therapy of brain tumors. Phys. Med. Biol. 47, 2387–2396. Yu, H.T., Liu, Y.W., Lin, T.Y., Wang, L.W., 2011. BNCT treatment planning of recurrent head-and-neck cancer using THOR plan. Appl. Radiat. Isot. 69, 1907–1910.
Contents lists available at SciVerse ScienceDirect
Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene
Photoneutron production by a 25 MeV electron linac for BNCT application Fatemeh Torabi a, S. Farhad Masoudi a,⇑, Faezeh Rahmani b a b
Department of Physics, K.N. Toosi University of Technology, P.O. Box 15875-4416, Tehran, Iran Department of Radiation Application, Shahid Beheshti University, P.O. Box 1983963113, Tehran, Iran
a r t i c l e
i n f o
Article history: Received 8 August 2012 Received in revised form 26 October 2012 Accepted 1 November 2012 Available online 20 December 2012 Keywords: Photoneutron source Electron linear accelerator Beam shaping assembly MCNPX BNCT Neutron yield
a b s t r a c t Recently, extensive research has been performed to produce neutron beams in hospitals using electron linear accelerator for neutron medical applications such as Boron Neutron Capture Therapy. In this article, we present the results of Monte Carlo calculations, using MCNPX code, for the possibility of setting up a 25 MeV electron linac based photoneutron source for BNCT applications. Photon and photoneutron converters with different materials in various geometries were studied to achieve maximum neutron yield. In addition, the possibility of using an optimized neutron beam in BNCT was investigated by proposing the optimized beam shaping assembly. It was shown that our designed neutron beam, which passes through the optimized BSA, meets the IAEA criteria for in-air parameters. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Neutron sources are important tools for a variety of applications in different fields of science and engineering. Neutrons have many properties that make them useful for certain types of researches. Neutron can travel large distances through some materials without being scattered or absorbed. It can be used to locate hydrogen and light atoms precisely. So it is ideal for many applications that require detection of light materials such as explosive mines in land, corrosion in fighter jet wings and light oxygen in YBCO (Yttrium– Barium–Copper Oxide). In addition to industrial applications, the neutron is a useful particle in medical physics (Yamamoto et al., 2002; Auterinen et al., 2004; Huang et al., 2005; Koivunoro et al., 2004). For example, Boron Neutron Capture Therapy (BNCT) is an effective therapeutic modality to treat malignant tumors such as brain tumors and head and neck cancers (Yu et al., 2011; Rao et al., 2004). There are a wide variety of different neutron sources like radioactive sources (Ghassoun et al., 2009), reactors (Auterinen et al., 2004; Kiger et al., 1999, 2004), and accelerator-based neutron sources (Elshahat et al., 2007; Bleuel et al., 1998; Tahara et al., 2006; Rahmani et al., 2011). However, some neutron sources such as reactors are not appropriate in medical field because of some disadvantages such as radioactive waste materials, large dimensions and the problems in maintenance. Unlike the reactor based neutron sources, accelerator-based neutron sources have advantages such as: ⇑ Corresponding author. E-mail address: [email protected] (S.F. Masoudi). 0306-4549/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.anucene.2012.11.001
– No radioactive waste materials are generated. – Shielding is needed only during accelerator ON condition. – Having small dimensions in comparison to reactor based neutron sources. – Possibility to be pulsed. – Modulating the flux intensity can be done by modifying the parameters of accelerated particle for different applications. Recently, the electron linac-based neutron sources have been studied extensively (Jallu et al., 1999; Rahmani and Shahriari, 2010; Huang et al., 2006; Rahmani et al., 2011). Although these neutron sources have the above mentioned advantages, their main disadvantage is low neutron production (Auditore et al., 2005). Nevertheless, acceptable source strength, at least for some applications like BNCT, can be obtained using appropriate materials in an optimized geometry. In the present study, the optimization process for photoneutron source was investigated using the MCNPX (Monte Carlo Code), considering a 25 MeV electron linear. Also the application of the proposed photoneutron source for BNCT treatment was studied. 2. Materials and methods 2.1. The photon target As the probable energy of the electron source in linac is 25 MeV emitted with the highest probability from one point, in our simulation we assumed that the electron source in SDEF card is a small disk with the monoenergetic 25 MeV electron beam. Since, the produced electrons pass through the high atomic Z number target, a
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193
cascade shower of bremsstrahlung photons is produced. The produced photons participate in (c, n) reaction within photoneutron target. Photoneutrons are produced when the energy of photons are higher than the threshold energy of the (c, n) reaction. Depending on the photoneutron target, the threshold energy varies from 7 MeV to 18 MeV (except Be, D). A photon converter design is the first step in investigating the photoneutron source in which the photon beam is produced through (e, c) reaction. For optimizing the photon production, some appropriate targets such as tungsten (W), tantalum (Ta), rhenium (Re) and lead (Pb) were considered in two different geometries: cylindrical (with different thicknesses and radiuses) and hemispherical (with different radiuses). 2.2. The photoneutron target Design of the photoneutron converter was performed to calculate the highest neutron yield obtained from the optimized photon converter. The neutron yields produced by different photoneutron converters were studied, considering the (c, n) energy threshold and related cross-section. To achieve the maximum photoneutron yield, the best material in the optimized geometry should be selected. Uranium as the best photoneutron converter with the high (c, n) cross section was studied in different geometries to determine an efficient geometry which provides the highest neutron yield. Monte Carlo calculations for the proposed geometries were carried out again in the presence of lead (Pb) as a reflector to obtain the highest forward neutron yield. Forward neutron yield was calculated by assuming that Pb completely surrounds the photoneutron converter. Also, several materials were examined as the photoneutron converter such as U, Th, W, Ta, Au, Ag, BeD2, Be. The optimized geometry was considered in all calculations.
Fig. 1. The photon yield produced by bombarding W with 25 MeV electron beam.
2.3. BSA design In recent years, different studies have focused on using linac for BNCT; however it seems that most of the reported neutron fluxes lack enough epithermal neutron flux to be employed in BNCT. In comparison with other studies, this neutron beam seems to be strong enough for BNCT especially for treatment of deep-seated brain tumors. To obtain an adequate epithermal neutron flux, the neutron beam must be shaped through the appropriate beam shaping assembly (BSA). Thus, an optimum BSA including the moderators, the reflector, the collimator and the gamma shield was proposed. During optimization process, different compositions of materials were investigated to meet the in-air parameters at the exit port.
Fig. 2. The photon yield in the different targets with optimized thickness.
3. Results and discussion 3.1. Photon converter Fig. 1 shows the photon yield for the 25 MeV electron beam bombarding tungsten target in cylindrical geometry. As seen, by increasing the radius more than 1 cm, no considerable increasing can be achieved. Therefore optimized dimensions were chosen as 0.3 cm in thickness and 1 cm in radius. Similar calculations were performed for Ta, Pb and Re. The optimized thickness and radius were determined to achieve the maximum photon yield. The results of comparing photon yields in different materials are shown in Fig. 2. Considering optimum thickness and radius, the photon yields produced by cylindrical W, Ta and Re have almost the same results. The photon yields of the selected targets in hemispherical geometry with different radiuses were also calculated to obtain
Fig. 3. The photon yield by bombarding different targets in the hemispherical geometry with 25 MeV electron beam.
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Fig. 4. The cylindrical photoneutron converter (labeled by 2) surrounds the optimized photon converter (labeled by 1).
Fig. 7. Neutron yield of uranium photoneutron source in three different geometries of Fig. 6.
Table 1 Forward neutron yield in four different geometries. Configuration Neutron yield (n/mA)
Geometry 1 3.55 1013
Geometry 2 3.86 1013
Geometry 3 4.75 1013
Cylindrical shape 1.88 1013
Fig. 5. Photoneutron yield in the cylindrical photoneutron converter with different radiuses and thicknesses.
optimized radius. As shown in Fig. 3, the photon production in the hemispherical targets with optimized radiuses is better than in the cylindrical geometry. The maximum photon yields in W and Re are approximately the same; however W was selected because it is more accessible. In conclusion, we chose tungsten in hemispherical geometry with 0.4 cm radius as the best photon converter.
Fig. 8. Photoneutron yield of the different photoneutron converters in the optimized geometry.
Fig. 6. Three different geometries for the photoneutron converter containing the hemispherical photon converter.
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F. Torabi et al. / Annals of Nuclear Energy 54 (2013) 192–196 Table 2 The probable (c, n) reactions for the different materials used as photoneutron sources. Probable (c, n) reactions
Threshold energies (MeV) 238
(c, n) (c, 2n) (c, 3n)
U
6.15 11.28 17.82
232
Th
6.44 11.56 18.35
180
W
8.41 15.35 23.57
182
183
184
186
8.07 14.75 23.16
6.19 14.26 20.94
7.41 13.60 21.67
7.19 12.95 20.36
W
W
W
181
W
Ta
7.58 14.22 22.13
197
Au
8.07 14.71 23.08
107
Ag
9.54 17.47 27.51
9
Be
1.67 20.56 31.24
Table 3 The in-air parameters of optimized BSA configuration in comparison with the IAEA criteria.
uepi (n/cm2 s)
D_ fn =uepi
D_ c =uepi
(Gy cm2)
(Gy cm2)
uepi/ uthermal
IAEA criteria >5 108 <2 10–13 <2 10–13 >20 MgF2 15 cm, TiF3 30 cm 7.2 108 /mA 2.96 10–14 8.65 10–14 55.5 MgF2 15 cm, TiF3 35 cm 4.9 108 /mA 2.24 10–14 8.66 10–14 49.0
Fig. 9. The neutron spectrum of photoneutron source; (1) our proposed source with a 25 MeV electron linac, (2) our proposed source with a 20 MeV electron linac, and (3) the result of reference (Rahmani and Shahriari, 2010).
3.2. Photoneutron converter At first step, the photoneutron converter was assumed as cylindrical uranium which is shown in Fig. 4. Neutron yield was determined by changing height and radius of this cylinder. It is shown in Fig. 5, a cylinder with 4 cm in thickness and approximately 5–6 cm in radius can be considered as the optimized cylindrical photoneutron converter. Three different shapes of uranium as the photoneutron target are shown in Fig. 6. Fig. 7 shows the photoneutron yields of these geometries in different radiuses (spherical: geometry 2 or hemispherical: geometries 1 and 3). Fig. 7 shows that the neutron yields of the mentioned geometries in radius of 6 cm are approximately the same. Table 1 shows the results of the photoneutron converter with Pb reflector in the four studied geometries.
The results of neutron yield in different photoneutron targets such as U, Th, W, Ta, Au, Ag, BeD2 and Be are shown in Fig. 8. The probable (complete) (c, n) reactions for these targets are shown in Table 2. According to the results of Fig. 8, uranium seems to be the best photoneutron converter. In summary, the best materials and their geometries to obtain maximum photoneutron yield are as follows: hemispherical tungsten with 0.4 cm in thickness as the photon converter which was located in the middle of hemispherical uranium with 6 cm in radius. In Fig. 9, the neutron spectrum of our proposed photoneutron source for a 25 MeV and a 20 MeV electron linac and the result of (Rahmani and Shahriari, 2010) have been illustrated. Our proposed photoneutron source has a reasonable high energy neutron yield near the epithermal energy even using a 20 MeV linac. As our proposed neutron beam has a suitable neutron flux near the epithermal energy, it can be used to produce neutrons with high epithermal neutron flux using suitable beam shaping assembly. Enough epithermal neutron flux is critical in some applications such as the treatment of deep-seated brain tumors by BNCT. In the next section, the properties of the obtained neutron beam were studied after beam shaping assembly at the beam exit, and the in-air parameters were determined. 3.3. BSA design for BNCT application Based on the better transmission of epithermal neutron flux and by removing beam contamination, the BSA was designed cylindrical which includes (See Fig. 10):
Fig. 10. The photoneutron target and final BSA configuration (1) photoneutron source, (2) Pb as the reflector, (3) MgF2 as the first moderator, (4) TiF3 as the second moderator, (5) Bi as the gamma shield, (6) Ni as the collimator.
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– 15 cm MgF2 and 35 cm TiF3 as the first and the second moderators. – 20 cm bismuth as the best gamma shield to reduce the emission of high energy photons at the beam port. – 1 cm Nickel as the best collimator and neutron guide considering its low absorption cross section in the epithermal region. – Lead as the best reflector surrounding the entire BSA configuration. Table 3 shows the in-air parameters of the optimized BSA configuration for the designed photoneutron source. The results of the present study show that the proposed neutron beam meets the IAEA criteria [IAEA-TECHDOC-1223, 2001] and has a suitable epithermal neutron flux. 4. Conclusion The photoneutron source based on high energy electron linac was optimized to produce the maximum neutron yield. Calculations were performed using MCNPX Mont Carlo Code for 25 MeV electron beam bombarding different photon targets in different geometries. High photon production achieved through the use of tungsten in the hemispherical shape. Also, the study of photoneutron production performed for several materials in different geometries. The hemispherical design of uranium was selected as the best photoneutron converter which produces the highest neutron yield towards the beam port. According to the results of the present study the produced neutron beam had enough intensity to meet the demand of many applications like BNCT. Therefore, the optimal beam shaping assembly was designed based on the best moderators (MgF2 and TiF3), the reflector (Pb), the gamma shield (Bi) and the collimator (Ni). The investigation of the beam quality at the beam port indicates that the beam meets the IAEA criteria values and it is proper to treat deep-seated brain tumors. References Auditore, L., Barna, R.C., De Pasquale, D., Italiano, A., Trifiro, A., Trimarchi, M., 2005. Study of a 5 MeV electron linac based neutron source. Nucl. Instrum. Method. Phys. Res. B 229, 137–143.
Auterinen, I., Seren, T., Anttila, K., Kosunen, A., Savolainen, S., 2004. Measurement of free beam neutron spectra at eight BNCT facilities worldwide. Appl. Radiat. Isot. 61, 1021–1026. Bleuel, D.L., Donahue, R.L., Ludewigt, B.A., Vujic, J., 1998. Designing acceleratorbased epithermal neutron beams for boron neutron capture therapy. Med. Phys. 25, 1725–1734. IAEA-TECDOC-1223, 2001. Current Status of Neutron Capture Therapy. International Atomic Energy Agency. Elshahat, B.A., Naqvi, A.A., Maalej, N., Abdalla, Kh., 2007. Design calculation of an accelerator based BSA for BNCT of brain cancer. Radioanal. Nucl. Chem. 274, 539–544. Ghassoun, J., Chkillou, B., Jehouani, A., 2009. Spatial and spectral characteristics of a compact system neutron beam designed for BNCT facility. Appl. Radiat. Isot. 67, 560–564. Huang, W.L., Li, Q.F., Lin, Y.Z., 2005. Calculation of photoneutrons produced in the targets of electron linear accelerators for radiography and radiotherapy applications. Nucl. Instrum. Method. Phys. Res. B 229, 339–347. Huang, W.L., Li, Q.F., Lin, Y.Z., Su, Q., Luo, Y.S., 2006. Measurements of photoneutrons produced by a 15 MeV electron linac for radiography applications. Nucl. Instrum. Method.Phys. Res. B 251, 361–366. Jallu, F., Lyoussi, A., Payan, E., Recroix, H., Mariani, A., Nurdin, G., Buisson, A., Allano, J., 1999. Photoneutron production in tungsten, praseodymium, copper and beryllium by using high energy electron linear accelerator. Nucl. Instrum. Method.Phys. Res. B 155, 373–381. Kiger, W.S., Sakamoto, S., Harling, O.K., 1999. Neutronic design of a fission converter-based epithermal neutron beam for neutron capture therapy. Nucl. Sci. Eng. 131, 1–22. Kiger, W.S., Lu, X.Q., Harling, O.K., Riley, K.J., Binns, P.J., Kaplan, J., Patel, H., Zamenhof, R.G., Shibata, Y., Kaplan, I.D., Busse, P.M., Palmer, M.R., 2004. Preliminary treatment planning and dosimetry for a clinical trial of neutron capture therapy using a fission converter epithermal neutron beam. Appl. Radiat. Isot. 61, 1075–1081. Koivunoro, H., Bleuel, D.L., Nastasi, U., Nastasi, U., lou, T.P., Reijonen, J., Leung, K.N., 2004. BNCT dose distribution in liver with epithermal D–D and D–T fusionbased neutron beams. Appl. Radiat. Isot. 61, 853–859. Rahmani, F., Shahriari, M., 2010. Hybrid photoneutron source optimization for electron accelerator-based BNCT. Nucl. Instrum. Method. Phys. Res. A 618, 48– 53. Rahmani, F., Shahriari, M., Minoochehr, A., Nedaie, H., 2011. Feasibility study on the use of uranium in photoneutron target and BSA optimization for linac based BNCT. Nucl. Instrum. Method. Phys. Res. A 641, 136–140. Rao, M., Trivillin, V.A., Heber, E.M., Cantarelli, M.A., Itoiz, M.E., Nigg, D.W., Rebagliati, R.J., Batistoni, D., Schwint, A.E., 2004. BNCT of 3 cases of spontaneous head and neck cancer in feline patients. Appl. Radiat. Isot. 61, 947–952. Tahara, Y., ODA, Y., Shiraki, T., Tsutsui, T., Yokobori, H., Yonai, S., Baba, M., Nakamura, T., 2006. Engineering design of a spallation reaction-based neutron generator for Boron Capture Therapy. Nucl. Sci. Technol. 43, 9–19. Yamamoto, T., Matsumura, A., Yamamoto, K., Kumada, H., Shibata, Y., Nose, T., 2002. In Phantom two-dimensional thermal neutron distribution for intraoperative boron neutron capture therapy of brain tumors. Phys. Med. Biol. 47, 2387–2396. Yu, H.T., Liu, Y.W., Lin, T.Y., Wang, L.W., 2011. BNCT treatment planning of recurrent head-and-neck cancer using THOR plan. Appl. Radiat. Isot. 69, 1907–1910.