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Neutronic design and simulated performance of Peking University Neutron Imaging Facility (PKUNIFTY)
Nuclear Instruments and Methods in Physics Research A 651 (2011) 67–72
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Neutronic design and simulated performance of Peking University Neutron Imaging Facility (PKUNIFTY)$ Weiwei Wen, Hang Li, Yubin Zou n, Guoyo Tang, Dawei Mo, Yuanrong Lu, Zuiyu Guo State Key Laboratory of Nuclear Physics and Technology & School of Physics, Peking University, Beijing 100871, China
a r t i c l e in f o
abstract
Available online 30 December 2010
The Peking University Neutron Imaging Facility (PKUNIFTY) is a Radio Frequency Quadruple (RFQ) accelerator based system. The fast neutrons are produced by 2 MeV deuterons bombarding beryllium target. The moderator, reflector, shielding and collimator have been optimized with Monte–Carlo simulation to improve the neutron beam quality. The neutrons are thermalized in water cylinder of F26 26 cm2 with a polyethylene disk in front of Be target. The size of deuteron beam spot is optimized considering both the thermal neutron distribution and the demand of target cooling. The shielding is a combination of 8 cm thick lead and 42 cm thick boron doped polyethylene. The thermal neutrons are extracted through a rectangular inner collimator and a divergent outer collimator. The thermal neutron beam axis is perpendicular to the D + beam line in order to reduce the fast neutron and the g ray components in the imaging beam. When the neutron yield is 3 1012 n/s and the L/D is 50, the thermal neutron flux is 5 105 n/cm2/s at the imaging plane, the Cd ratio is 1.63 and the n/g ratio is 1.6 1010 n/ cm2/Sv. & 2011 Elsevier B.V. All rights reserved.
Keywords: D–Be neutron source Neutron radiography Neutronic design Monte–Carlo simulation
1. Introduction Neutron radiography as a non-destructive testing tool has already been applied in many fields [1]. The Peking University Neutron Imaging Facility (PKUNIFTY) [2], which is being installed and will be completed early 2011, is based on a Compact Accelerator-driven Neutron Source (CANS) and will be used for education and training, neutron radiography technology developments, and investigation of neutron radiography applications. That accelerator is a RFQ (Radio Frequency Quadrupole) accelerator, which is suitable to accelerate low-energy high-intensity ion beams. In CANS the fast neutrons can be generated by accelerated ions bombarding certain target. However in many cases the cold, thermal or epithermal neutrons are required, then the moderator should be used to slow down the fast neutrons. The idea to carry out thermal neutron radiography with cyclotron and Van de Graaff accelerator can be traced back to 1980s [3,4]. The reactions Be(p, n) and Be(d, n) were used and the design of moderator was investigated. In 1990s RFQ based thermal neutron radiography facility was proposed by ANL and MIT [5,6], and the design of its moderator was discussed [7,8]. Recent example is LENS (low energy neutron source) project at Indiana $ This work is supported by National Natural Science Foundation of China under the Key Project 10735020, and National Basic Research Program of China under contract No. 2010CB833106. n Corresponding author. Tel./fax: + 86 10 6276 7895. E-mail address: [email protected] (Y. Zou).
0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.12.194
University, which is a cold neutron source using a combination of RFQ and DTL (Drift Tube Linac) accelerators. Its neutronic design has been intensively studied [9]. PKUNIFTY tried to meet the basic requirements of neutron radiography with the size as small as possible and the cost as low as possible. The reaction Be(d, n) is selected for neutron generation. The deuteron beam energy is designed as 2 MeV and the rated average beam current is 4 mA, which gives a fast neutron yield of 3 1012 n/cm2/s [9]. Using Be(d, n) reaction a higher neutron yield can be obtained at lower beam energy, but that reaction also gives higher g yield. In order to get higher quality of imaging neutron beam, the neutronic design of PKUNITFY moderator assembly including moderator, reflector, shielding and collimator has been studied and simulated with Monte–Carlo (MC) code based on Geant4 [10].
2. Study on the effects of main parameters In order to obtain an optimized neutronic design, the materials and dimensions of the moderator and reflector, the size of beam spot on the target, and the angle between the collimator axis and D + beam tube axis were investigated with simplified model. The effects of those parameters’ variation were simulated, and the results provide important information to the neutronic design of the moderator assembly. The simulated results below are all normalized by the source neutron yield, and their relative errors are all below 5%.
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2.1. Moderator and reflector In our simplified model the moderator, reflector and shielding layer are supposed in a coaxial right cylinder structure, which horizontal profile is shown in Fig. 1. The Be target locates at the center of the cylinder, and can be treated as an 8 cm-diamter and 0.4 cm-thick disk, which is perpendicular to the F8 cm D + beam tube. The X and Y coordinates are defined as shown in Fig. 1 with the origin at the center of moderator. The spectrum and angular distribution of emitted neutrons adopt the data given in Ref. [11]. The average energy of emitted neutrons is about 2 MeV, which can be effectively slowed down by elastic scattering with hydrogen-rich moderator [6]. The moderator material is always surrounded by reflector material to reflect the diffused away neutrons back to the moderator in order to enhance the neutron flux at the entrance of the collimator. The good reflector material should have high scattering cross-section and low absorption cross-section with thermal neutrons. Based on the above analysis and the cost, we choose water (H2O) and polyethylene (PE) as the candidate moderator material, as well as H2O, PE and graphite (C) as the candidate reflector material. Several combinations of moderator and reflector with different materials and dimension have been
simulated to get the highest thermal neutron flux in the moderator with the dimension as small as possible. The thermal neutron flux distribution at X–Y plane in the moderator area of 10 cm oxo10 cm and 0oyo10 cm was investigated. Firstly we assumed the reflector size is infinite and optimized the moderator radius. Then the outer radius of the reflector was optimized with the optimal moderator size. For each step we calculated the curve of thermal neutron peak flux vs. the moderator or reflector radius. The optimal radius is corresponding to the point, where the curve tends to saturation. The optimized results of different combinations of moderator and reflector are shown in Table 1 and from Table 1we can find out that the combinations A, B and C almost have the same peak flux of thermal neutrons, but combination A has the smallest dimension and the largest area in which the thermal neutron flux is higher than 90% of the peak flux. The larger high flux area in the moderator will benefit the uniformity of the extracted neutron flux. The combinations D and E have the lower peak flux of thermal neutrons, therefore as a moderator material water is not so good as PE in our case. 2.2. Size of beam spot The 2 MeV/4 mA deuteron beam will deposit 8 kW thermal power on the target. The target is water cooled to keep the moderator temperature below 60 1C. A large beam spot is desired to reduce the powder density on the target so that the target is easier to be cooled. However the fast neutron density on the target is reduced either. To study the effect of spot size on thermal neutron distribution in the moderator, a PE cylinder with a height of 40 cm and a radius of 20 cm is set as the moderator and reflector, which is larger than the Case A in Table 1 so that the dimension influence can be neglected. Distributions of thermal neutron flux with different size of beam spot are shown in Table 2. Table 2 shows that the area of high thermal neutron flux decreases quickly with the increase in spot size , while the peak flux of the thermal neutron decreases smoothly and the position of the peak neutron flux does not change very much with the spot size increasing. Those results indicate that the beam spot size should be as small as possible to obtain larger area of high thermal neutron flux. So the diameter of beam spot on the target should be carefully selected to get a balance between the neutronic and target cooling. 2.3. Shielding
Fig. 1. Horizontal profile of the moderator, the reflector and the shield.
Table 1 Optimized dimension of different combinations and their peak thermal neutron flux. Combination Moderator material
Reflecter material
Optimized dimensiona (cm)
Peak flux of thermal neutronsb (n/cm2/s)
High flux areac (cm2)
A B C D E
PE C H2O H2O C
14 8+ 19 10+ 10 16 10+ 10
0.0145 0.0150 0.0149 0.0105 0.0113
7.92 7.60 4.16 6.48 3.76
PE PE PE H2O H2O
a Radius of moderator plus thickness of reflector, total radius for using same material. b Normalized to the source neutron yield. c Area in which the thermal neutron flux is higher than 90% peak flux.
To reduce the total cost the accelerator and the target of PKUNIFTY will be installed together in an existing neutron experiment hall, of which the wall is made of 1.5 m thick concrete. The neutron yield of PKUNIFTY exceeds the shield ability of the Table 2 Distributions of thermal neutron flux with different size of beam spot. Radius of D + beam (cm)
Peak flux of thermal neutrona (n/cm2/s)
Coordinate of peak flux (x, y) (cm)
Area at X–Y plane in which the thermal neutron flux 40.013 (cm2)
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
0.0160 0.0160 0.0157 0.0153 0.0147 0.0147 0.0141 0.0132
(0, (0, (0, (0, (0, (0, (0, (0,
11.04 10.24 10.64 9.76 7.28 5.76 2.24 0.40
a
Normalized to the source neutron yield.
3.5) 3.5) 3.5) 3.6) 3.6) 3.6) 3.6) 3.7)
W. Wen et al. / Nuclear Instruments and Methods in Physics Research A 651 (2011) 67–72
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Fig. 2. Layout of the X–Y plane profile of calculation model with deflected collimator.
experiment hall. A shield around the target is necessary. And a lot of electronic devices of the accelerator are installed beside the beam line. The shield should protect these devices from radiation damages. The reference shows some ordinary electronic devices can work properly when the device’s cumulative fast neutron (E410 keV) fluence below 1011–1012 n/cm2 and cumulative g dose below 103 Gy [12]. We designed the shield assuming our electronic devices can work in the similar situation. Here we only consider a simplified model, in which the moderator and reflector are surrounded by a layer of lead, and then a layer of boron doped PE. The ion beam tube and collimator are omitted in this model. The lead can block the g rays produced by 9Be(d, n) reaction and emitted during neutron moderation and capture. The boron doped PE can further moderate the fast neutrons and absorb the thermal neutrons. The thickness of lead and boron doped PE as well as the boron carbide (B4C) mass fraction in boron doped PE have been optimized by MC simulation. The B4C mass fraction in boron doped PE is optimized to 2%, which can absorb thermal neutrons completely. More B4C will reduce the neutron moderation capability of the material. The simulation results indicate that a lead layer of 8 cm and a boron doped PE layer of 42 cm are adequate. In that case the neutron flux will be attenuated with a coefficient of 10 9, and the g ray is less than 1 mSv/h. The electronic devices 1 m away from the moderator center can work normally for several tens of thousands hours. And with the shield wall of the experiment hall, the equivalent dose rate is below 0.1 mSv/h outside the hall. With 250 working days the dose is much lower than 1 mSv, which is the dose limit for public exposure in China. For the real setup, the neutron and g ray will leak from the beam tube and collimator. Some local shield should be installed.
2.4. Angle between the collimator axis and D + beam tube axis The fast neutrons and g rays, which are background components of collimated thermal neutron beam, must be reduced in order to improve the imaging quality. The reduction of fast neutron and g
ray components has been investigated by changing the angle between the collimator axis and D + beam tube axis [3]. The calculation model contains the cylindrical moderator and shielding as shown in Fig. 2. The Be target makes an angle of 451 with the X-axis to reduce the beam power density on target, and the axis of the divergent collimator is at an angle of y with the D + beam tube axis. After optimization the target center is at a distance of 4 cm from the moderator center and the collimator entrance, 1 cm away from the moderator center. Fig. 3 shows the simulated y-dependence of the neutron flux of different components at the imaging plane with L/D¼ 50, and Fig. 4 shows the y-dependence of the n/g ratio at the same condition. As Fig. 3 shows, the fast neutron flux drastically reduces with the increase of y value, while the thermal neutron and epithermal neutron (0.5 eV oEo10 keV) flux does not change much with y value. That is because the non-scattered fast neutrons emit from the target and leak directly through the collimator; however, the thermal and epithermal neutrons have scattered many times and almost uniformly disperse in the moderator. Fig. 4 shows the n/g ratio increases rapidly with the y value increasing when y is larger than 601. Consequently, an angle of 901 between the collimator axis and the D + beam tube axis is a reasonable value to decrease the fast neutrons and g rays at imaging plane. (The fast neutron component is not in the same scale as thermal and epithermal neutrons)
3. Moderator assembly design and simulated results 3.1. Overview of moderator assembly The moderator assembly was designed based on the above investigation, the mechanical and the target cooling considerations. Although the Case A in Table 1 gives a better result, it is difficult to put the beam tube, the target with cooling structure and the collimator in PE tightly due to their too complicated shapes. In order to realize easier installation, a modified Case C is adopted. In this scheme a PE plate is
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(2) the thermal neutron flux at imaging plane should be as high as possible; (3) the L/D can be changed to meet different test requirements.
The outer collimator is a divergent quadrangular with a divergence angle of 2.31 to fit the imaging area as mentioned above. Its inner surface is covered by a layer of 0.3 cm thick B4C
Fig. 3. Neutron flux of different components vs. deflecting angle y. (The fast neutron component is not in the same scale as thermal and epithermal neutrons).
Fig. 5. Moderator assembly configuration in central horizontal profile. (The shielding layer is not in scale).
Table 3 Calculated values of thermal neutron flux, Cd ratio and n/g as function of L/D. L (cm)
D (cm)
L/D
Thermal neutron fluxa (n/cm2/s)
Cd ratio
n/g ratio (n/cm2/Sv)
100 100 100 100
1 2 3 4
100 50 33 25
4.96E-08 1.79E-07 3.37E-07 5.20E-07
1.33 1.63 2.12 1.75
6.14E+ 09 1.57E+ 10 2.10E+ 10 4.64E+ 10
Fig. 4. The n/g ratio at the imaging plane vs. deflecting angle y. a
Normalized to the source neutron yield.
inserted between the target housing and the extraction surface of collimator as main moderator, and the surrounding water is the main reflector. But actually there is no clear boundary between the moderator and reflector. The closed water container is made of aluminum with a height of 26 cm and a radius of 13 cm for inner dimension. The shielding includes a lead layer of 8 cm and a boron doped PE layer of 42 cm. The angle between the collimator axis and the D + beam tube axis is 901, and the collimator consists of an inner collimator and an outer collimator with a B4C aperture in between. The designed moderator assembly configuration is shown in Fig. 5. The calculation shows that the thermal neutron flux at the collimator entrance of this scheme decreases only 3% compared with Case A. 3.2. Collimator structure and entrance position The design criteria of the collimator structure and entrance position are: (1) Field of view can achieve a 20 20 cm2 at 2 m away from the aperture, and 10 10 cm2 at 1 m away;
Fig. 6. Neutron spectrum at the imaging plane when D ¼ 2 cm, L¼ 100 cm.
W. Wen et al. / Nuclear Instruments and Methods in Physics Research A 651 (2011) 67–72
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Fig. 7. Distribution of the thermal neutron flux along the Y-axis at L¼100 cm.
mixed with epoxy resin to eliminate neutrons to be scattered from and through the collimator walls. The beam defining aperture is a 1 cm thick B4C disk loaded in an aluminum cage with a hole in the center. There are four apertures with the diameter (D) of 1, 2, 3 and 4 cm, respectively. The distance between aperture and imaging plane (L) can be varied from 1 to 2 m, so the L/D ratio can be adjusted from 25 to 200. The cross-section of the inner collimator is a square, and its side length is 4 cm. The entrance surface of the inner collimator is parallel to the target so that the neutron flux is uniform. As the simulation results, the target center is 4 cm away from the center of water container, and the center of entrance surface of inner collimator is 1.5 cm away. The aperture is 9 cm away from the water container center so that the thermal neutron flux at the imaging plane is high and uniform.
3.3. Simulated results of the imaging neutron beam MC simulations were performed to assess the parameters of the imaging neutron beam. The calculated values of the thermal neutron flux, Cd ratio and n/g ratio at different L/D combinations are summarized in Table 3. From this table, the n/g is all higher than 1 1010 n/cm2/Sv except the L/D ¼100, and the thermal neutron flux can be 5 105 n/cm2/s at L/D ¼50 when the fast neutron yield is 3 1012 n/s. Whereas, the Cd ratio of about 2 is small because there are still many epithermal neutrons contained in the imaging neutrons as mentioned in Section 2.4. Fig. 6 shows the neutron spectrum at the imaging plane when D ¼2 cm, L¼100 cm. The thermal neutron is the main component with the peak value at 0.025 eV. The energy selection imaging system with time-of-flight method is being studied to increase the Cd ratio. Fig. 7 shows that the distributions of the thermal neutron flux along the Y-axis with different D values at L¼100 cm. The distribution along the Z-axis is almost the same as the Y-axis. The field of view at that distance is 10 10 cm2. The neutron flux at the edge of the field of view is 7% lower than the center part.
4. Summary The moderator assembly of PKUNIFY including the moderator, the reflector, the shield and the collimator has been designed and simulated using MC method. With the simulation results, the moderator was designed as a compound structure with PE as main moderator surrounded by H2O as main reflector for high neutron flux, small dimension and simple machining. The deuteron beam spot size was optimized to meet the requirements of both the larger area in the moderator with high neutron flux and the target cooling. The shield with lead and boron doped PE in adequate thickness was designed. The angle between the collimator axis and the D + beam tube axis was optimized to be 901 to reduce the fast neutrons and g rays proportion at the imaging plane. As the optimization results, the thermal neutron flux at the imaging plane can reach 5 105 n/cm2/s with L/D ¼50 when the fast neutron yield is 3 1012 n/s. The n/g ratio is around 1 1010 n/ cm2/Sv with different aperture size. The L/D ratio is 25–200 adjustable. The field of view can achieve 20 20 cm2 at the position of 2 m away from the aperture. And the thermal neutron flux uniformity within the field of view is better than 7%. Simulated results also show that the Cd ratio is still low. An energy selection imaging system is being studied to increase the Cd ratio. References ¨ [1] E. Lehmann, G. Kuhne, P. Vontobel, G. Frei, IEEE Trans. Nucl. Sci. NS-52 (2005) 317. [2] Zhiyu Guo, Yubin Zou, Yuanrong Lu, et al, Neutron radiography with compact accelerator at Peking University: problems and solutions, in: Proceedings of UCANS-I, Beijing, August 15–17, 2010. (in preparation). [3] S. Tazawa, T. Nakanii, Present status of the cyclotron-based neutron radiography. In: S. Fujine, K. Kanda, G. Matsumoto, J. Barton (Eds.), Neutron Radiography (3): Proceedings of the Third World Conference (1989) 213–220. [4] Mitsutaka Kakeno, Yoshiaki Kido, Jun-ichi Kawamoto. Neutron radiography facilities using a 3 MV van de Graaff accelerator. In: S. Fujine, K. Kanda, G. Matsumoto, J. Barton (Eds.), Neutron Radiography (3), in: Proceedings of the Third World Conference, 1989, 245–252. [5] G.H. Gillespie, G.E. McMichael, G.R. Imel, SPIE Proc. 2867 (1997) 343. [6] R.C. Lanza, Y. Fink., E.B. Iverson, E.W. McFarland, S.H. Shi, SPIE Proc. 2867 (1997) 347.
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[7] G.R. Imel, R.C. Singleterry, G.H. Gillespie, G.E. McMichael, Nucl. Instr. and Meth. A 377 (1996) 20. [8] D.B. Puffer, Moderator design for accelerator based neutron radiography and tomography systems. MS thesis of MIT, 1994. [9] C.M. Lavelle, D.V. Baxter, A. Bogdanov, et al., Nucl. Instr. and Meth. A 587 (2008) 324.
[10] S. Agostinelli, J. Allison, K. Amako, et al., Nucl. Instr. and Meth. A 506 (2003) 250. [11] Tetsuo Inada, Kiyomitsu Kawachi, J. Nucl. Sci. Technol. 5 (1) (1968) 22. [12] Panxun Chen, Radiation Effects on Semiconductor Devices and Integrated Circuits, National Defence Industry Press, Beijing, 2005 384–391. (in Chinese.
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
Neutronic design and simulated performance of Peking University Neutron Imaging Facility (PKUNIFTY)$ Weiwei Wen, Hang Li, Yubin Zou n, Guoyo Tang, Dawei Mo, Yuanrong Lu, Zuiyu Guo State Key Laboratory of Nuclear Physics and Technology & School of Physics, Peking University, Beijing 100871, China
a r t i c l e in f o
abstract
Available online 30 December 2010
The Peking University Neutron Imaging Facility (PKUNIFTY) is a Radio Frequency Quadruple (RFQ) accelerator based system. The fast neutrons are produced by 2 MeV deuterons bombarding beryllium target. The moderator, reflector, shielding and collimator have been optimized with Monte–Carlo simulation to improve the neutron beam quality. The neutrons are thermalized in water cylinder of F26 26 cm2 with a polyethylene disk in front of Be target. The size of deuteron beam spot is optimized considering both the thermal neutron distribution and the demand of target cooling. The shielding is a combination of 8 cm thick lead and 42 cm thick boron doped polyethylene. The thermal neutrons are extracted through a rectangular inner collimator and a divergent outer collimator. The thermal neutron beam axis is perpendicular to the D + beam line in order to reduce the fast neutron and the g ray components in the imaging beam. When the neutron yield is 3 1012 n/s and the L/D is 50, the thermal neutron flux is 5 105 n/cm2/s at the imaging plane, the Cd ratio is 1.63 and the n/g ratio is 1.6 1010 n/ cm2/Sv. & 2011 Elsevier B.V. All rights reserved.
Keywords: D–Be neutron source Neutron radiography Neutronic design Monte–Carlo simulation
1. Introduction Neutron radiography as a non-destructive testing tool has already been applied in many fields [1]. The Peking University Neutron Imaging Facility (PKUNIFTY) [2], which is being installed and will be completed early 2011, is based on a Compact Accelerator-driven Neutron Source (CANS) and will be used for education and training, neutron radiography technology developments, and investigation of neutron radiography applications. That accelerator is a RFQ (Radio Frequency Quadrupole) accelerator, which is suitable to accelerate low-energy high-intensity ion beams. In CANS the fast neutrons can be generated by accelerated ions bombarding certain target. However in many cases the cold, thermal or epithermal neutrons are required, then the moderator should be used to slow down the fast neutrons. The idea to carry out thermal neutron radiography with cyclotron and Van de Graaff accelerator can be traced back to 1980s [3,4]. The reactions Be(p, n) and Be(d, n) were used and the design of moderator was investigated. In 1990s RFQ based thermal neutron radiography facility was proposed by ANL and MIT [5,6], and the design of its moderator was discussed [7,8]. Recent example is LENS (low energy neutron source) project at Indiana $ This work is supported by National Natural Science Foundation of China under the Key Project 10735020, and National Basic Research Program of China under contract No. 2010CB833106. n Corresponding author. Tel./fax: + 86 10 6276 7895. E-mail address: [email protected] (Y. Zou).
0168-9002/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nima.2010.12.194
University, which is a cold neutron source using a combination of RFQ and DTL (Drift Tube Linac) accelerators. Its neutronic design has been intensively studied [9]. PKUNIFTY tried to meet the basic requirements of neutron radiography with the size as small as possible and the cost as low as possible. The reaction Be(d, n) is selected for neutron generation. The deuteron beam energy is designed as 2 MeV and the rated average beam current is 4 mA, which gives a fast neutron yield of 3 1012 n/cm2/s [9]. Using Be(d, n) reaction a higher neutron yield can be obtained at lower beam energy, but that reaction also gives higher g yield. In order to get higher quality of imaging neutron beam, the neutronic design of PKUNITFY moderator assembly including moderator, reflector, shielding and collimator has been studied and simulated with Monte–Carlo (MC) code based on Geant4 [10].
2. Study on the effects of main parameters In order to obtain an optimized neutronic design, the materials and dimensions of the moderator and reflector, the size of beam spot on the target, and the angle between the collimator axis and D + beam tube axis were investigated with simplified model. The effects of those parameters’ variation were simulated, and the results provide important information to the neutronic design of the moderator assembly. The simulated results below are all normalized by the source neutron yield, and their relative errors are all below 5%.
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2.1. Moderator and reflector In our simplified model the moderator, reflector and shielding layer are supposed in a coaxial right cylinder structure, which horizontal profile is shown in Fig. 1. The Be target locates at the center of the cylinder, and can be treated as an 8 cm-diamter and 0.4 cm-thick disk, which is perpendicular to the F8 cm D + beam tube. The X and Y coordinates are defined as shown in Fig. 1 with the origin at the center of moderator. The spectrum and angular distribution of emitted neutrons adopt the data given in Ref. [11]. The average energy of emitted neutrons is about 2 MeV, which can be effectively slowed down by elastic scattering with hydrogen-rich moderator [6]. The moderator material is always surrounded by reflector material to reflect the diffused away neutrons back to the moderator in order to enhance the neutron flux at the entrance of the collimator. The good reflector material should have high scattering cross-section and low absorption cross-section with thermal neutrons. Based on the above analysis and the cost, we choose water (H2O) and polyethylene (PE) as the candidate moderator material, as well as H2O, PE and graphite (C) as the candidate reflector material. Several combinations of moderator and reflector with different materials and dimension have been
simulated to get the highest thermal neutron flux in the moderator with the dimension as small as possible. The thermal neutron flux distribution at X–Y plane in the moderator area of 10 cm oxo10 cm and 0oyo10 cm was investigated. Firstly we assumed the reflector size is infinite and optimized the moderator radius. Then the outer radius of the reflector was optimized with the optimal moderator size. For each step we calculated the curve of thermal neutron peak flux vs. the moderator or reflector radius. The optimal radius is corresponding to the point, where the curve tends to saturation. The optimized results of different combinations of moderator and reflector are shown in Table 1 and from Table 1we can find out that the combinations A, B and C almost have the same peak flux of thermal neutrons, but combination A has the smallest dimension and the largest area in which the thermal neutron flux is higher than 90% of the peak flux. The larger high flux area in the moderator will benefit the uniformity of the extracted neutron flux. The combinations D and E have the lower peak flux of thermal neutrons, therefore as a moderator material water is not so good as PE in our case. 2.2. Size of beam spot The 2 MeV/4 mA deuteron beam will deposit 8 kW thermal power on the target. The target is water cooled to keep the moderator temperature below 60 1C. A large beam spot is desired to reduce the powder density on the target so that the target is easier to be cooled. However the fast neutron density on the target is reduced either. To study the effect of spot size on thermal neutron distribution in the moderator, a PE cylinder with a height of 40 cm and a radius of 20 cm is set as the moderator and reflector, which is larger than the Case A in Table 1 so that the dimension influence can be neglected. Distributions of thermal neutron flux with different size of beam spot are shown in Table 2. Table 2 shows that the area of high thermal neutron flux decreases quickly with the increase in spot size , while the peak flux of the thermal neutron decreases smoothly and the position of the peak neutron flux does not change very much with the spot size increasing. Those results indicate that the beam spot size should be as small as possible to obtain larger area of high thermal neutron flux. So the diameter of beam spot on the target should be carefully selected to get a balance between the neutronic and target cooling. 2.3. Shielding
Fig. 1. Horizontal profile of the moderator, the reflector and the shield.
Table 1 Optimized dimension of different combinations and their peak thermal neutron flux. Combination Moderator material
Reflecter material
Optimized dimensiona (cm)
Peak flux of thermal neutronsb (n/cm2/s)
High flux areac (cm2)
A B C D E
PE C H2O H2O C
14 8+ 19 10+ 10 16 10+ 10
0.0145 0.0150 0.0149 0.0105 0.0113
7.92 7.60 4.16 6.48 3.76
PE PE PE H2O H2O
a Radius of moderator plus thickness of reflector, total radius for using same material. b Normalized to the source neutron yield. c Area in which the thermal neutron flux is higher than 90% peak flux.
To reduce the total cost the accelerator and the target of PKUNIFTY will be installed together in an existing neutron experiment hall, of which the wall is made of 1.5 m thick concrete. The neutron yield of PKUNIFTY exceeds the shield ability of the Table 2 Distributions of thermal neutron flux with different size of beam spot. Radius of D + beam (cm)
Peak flux of thermal neutrona (n/cm2/s)
Coordinate of peak flux (x, y) (cm)
Area at X–Y plane in which the thermal neutron flux 40.013 (cm2)
0 0.5 1.0 1.5 2.0 2.5 3.0 3.5
0.0160 0.0160 0.0157 0.0153 0.0147 0.0147 0.0141 0.0132
(0, (0, (0, (0, (0, (0, (0, (0,
11.04 10.24 10.64 9.76 7.28 5.76 2.24 0.40
a
Normalized to the source neutron yield.
3.5) 3.5) 3.5) 3.6) 3.6) 3.6) 3.6) 3.7)
W. Wen et al. / Nuclear Instruments and Methods in Physics Research A 651 (2011) 67–72
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Fig. 2. Layout of the X–Y plane profile of calculation model with deflected collimator.
experiment hall. A shield around the target is necessary. And a lot of electronic devices of the accelerator are installed beside the beam line. The shield should protect these devices from radiation damages. The reference shows some ordinary electronic devices can work properly when the device’s cumulative fast neutron (E410 keV) fluence below 1011–1012 n/cm2 and cumulative g dose below 103 Gy [12]. We designed the shield assuming our electronic devices can work in the similar situation. Here we only consider a simplified model, in which the moderator and reflector are surrounded by a layer of lead, and then a layer of boron doped PE. The ion beam tube and collimator are omitted in this model. The lead can block the g rays produced by 9Be(d, n) reaction and emitted during neutron moderation and capture. The boron doped PE can further moderate the fast neutrons and absorb the thermal neutrons. The thickness of lead and boron doped PE as well as the boron carbide (B4C) mass fraction in boron doped PE have been optimized by MC simulation. The B4C mass fraction in boron doped PE is optimized to 2%, which can absorb thermal neutrons completely. More B4C will reduce the neutron moderation capability of the material. The simulation results indicate that a lead layer of 8 cm and a boron doped PE layer of 42 cm are adequate. In that case the neutron flux will be attenuated with a coefficient of 10 9, and the g ray is less than 1 mSv/h. The electronic devices 1 m away from the moderator center can work normally for several tens of thousands hours. And with the shield wall of the experiment hall, the equivalent dose rate is below 0.1 mSv/h outside the hall. With 250 working days the dose is much lower than 1 mSv, which is the dose limit for public exposure in China. For the real setup, the neutron and g ray will leak from the beam tube and collimator. Some local shield should be installed.
2.4. Angle between the collimator axis and D + beam tube axis The fast neutrons and g rays, which are background components of collimated thermal neutron beam, must be reduced in order to improve the imaging quality. The reduction of fast neutron and g
ray components has been investigated by changing the angle between the collimator axis and D + beam tube axis [3]. The calculation model contains the cylindrical moderator and shielding as shown in Fig. 2. The Be target makes an angle of 451 with the X-axis to reduce the beam power density on target, and the axis of the divergent collimator is at an angle of y with the D + beam tube axis. After optimization the target center is at a distance of 4 cm from the moderator center and the collimator entrance, 1 cm away from the moderator center. Fig. 3 shows the simulated y-dependence of the neutron flux of different components at the imaging plane with L/D¼ 50, and Fig. 4 shows the y-dependence of the n/g ratio at the same condition. As Fig. 3 shows, the fast neutron flux drastically reduces with the increase of y value, while the thermal neutron and epithermal neutron (0.5 eV oEo10 keV) flux does not change much with y value. That is because the non-scattered fast neutrons emit from the target and leak directly through the collimator; however, the thermal and epithermal neutrons have scattered many times and almost uniformly disperse in the moderator. Fig. 4 shows the n/g ratio increases rapidly with the y value increasing when y is larger than 601. Consequently, an angle of 901 between the collimator axis and the D + beam tube axis is a reasonable value to decrease the fast neutrons and g rays at imaging plane. (The fast neutron component is not in the same scale as thermal and epithermal neutrons)
3. Moderator assembly design and simulated results 3.1. Overview of moderator assembly The moderator assembly was designed based on the above investigation, the mechanical and the target cooling considerations. Although the Case A in Table 1 gives a better result, it is difficult to put the beam tube, the target with cooling structure and the collimator in PE tightly due to their too complicated shapes. In order to realize easier installation, a modified Case C is adopted. In this scheme a PE plate is
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(2) the thermal neutron flux at imaging plane should be as high as possible; (3) the L/D can be changed to meet different test requirements.
The outer collimator is a divergent quadrangular with a divergence angle of 2.31 to fit the imaging area as mentioned above. Its inner surface is covered by a layer of 0.3 cm thick B4C
Fig. 3. Neutron flux of different components vs. deflecting angle y. (The fast neutron component is not in the same scale as thermal and epithermal neutrons).
Fig. 5. Moderator assembly configuration in central horizontal profile. (The shielding layer is not in scale).
Table 3 Calculated values of thermal neutron flux, Cd ratio and n/g as function of L/D. L (cm)
D (cm)
L/D
Thermal neutron fluxa (n/cm2/s)
Cd ratio
n/g ratio (n/cm2/Sv)
100 100 100 100
1 2 3 4
100 50 33 25
4.96E-08 1.79E-07 3.37E-07 5.20E-07
1.33 1.63 2.12 1.75
6.14E+ 09 1.57E+ 10 2.10E+ 10 4.64E+ 10
Fig. 4. The n/g ratio at the imaging plane vs. deflecting angle y. a
Normalized to the source neutron yield.
inserted between the target housing and the extraction surface of collimator as main moderator, and the surrounding water is the main reflector. But actually there is no clear boundary between the moderator and reflector. The closed water container is made of aluminum with a height of 26 cm and a radius of 13 cm for inner dimension. The shielding includes a lead layer of 8 cm and a boron doped PE layer of 42 cm. The angle between the collimator axis and the D + beam tube axis is 901, and the collimator consists of an inner collimator and an outer collimator with a B4C aperture in between. The designed moderator assembly configuration is shown in Fig. 5. The calculation shows that the thermal neutron flux at the collimator entrance of this scheme decreases only 3% compared with Case A. 3.2. Collimator structure and entrance position The design criteria of the collimator structure and entrance position are: (1) Field of view can achieve a 20 20 cm2 at 2 m away from the aperture, and 10 10 cm2 at 1 m away;
Fig. 6. Neutron spectrum at the imaging plane when D ¼ 2 cm, L¼ 100 cm.
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Fig. 7. Distribution of the thermal neutron flux along the Y-axis at L¼100 cm.
mixed with epoxy resin to eliminate neutrons to be scattered from and through the collimator walls. The beam defining aperture is a 1 cm thick B4C disk loaded in an aluminum cage with a hole in the center. There are four apertures with the diameter (D) of 1, 2, 3 and 4 cm, respectively. The distance between aperture and imaging plane (L) can be varied from 1 to 2 m, so the L/D ratio can be adjusted from 25 to 200. The cross-section of the inner collimator is a square, and its side length is 4 cm. The entrance surface of the inner collimator is parallel to the target so that the neutron flux is uniform. As the simulation results, the target center is 4 cm away from the center of water container, and the center of entrance surface of inner collimator is 1.5 cm away. The aperture is 9 cm away from the water container center so that the thermal neutron flux at the imaging plane is high and uniform.
3.3. Simulated results of the imaging neutron beam MC simulations were performed to assess the parameters of the imaging neutron beam. The calculated values of the thermal neutron flux, Cd ratio and n/g ratio at different L/D combinations are summarized in Table 3. From this table, the n/g is all higher than 1 1010 n/cm2/Sv except the L/D ¼100, and the thermal neutron flux can be 5 105 n/cm2/s at L/D ¼50 when the fast neutron yield is 3 1012 n/s. Whereas, the Cd ratio of about 2 is small because there are still many epithermal neutrons contained in the imaging neutrons as mentioned in Section 2.4. Fig. 6 shows the neutron spectrum at the imaging plane when D ¼2 cm, L¼100 cm. The thermal neutron is the main component with the peak value at 0.025 eV. The energy selection imaging system with time-of-flight method is being studied to increase the Cd ratio. Fig. 7 shows that the distributions of the thermal neutron flux along the Y-axis with different D values at L¼100 cm. The distribution along the Z-axis is almost the same as the Y-axis. The field of view at that distance is 10 10 cm2. The neutron flux at the edge of the field of view is 7% lower than the center part.
4. Summary The moderator assembly of PKUNIFY including the moderator, the reflector, the shield and the collimator has been designed and simulated using MC method. With the simulation results, the moderator was designed as a compound structure with PE as main moderator surrounded by H2O as main reflector for high neutron flux, small dimension and simple machining. The deuteron beam spot size was optimized to meet the requirements of both the larger area in the moderator with high neutron flux and the target cooling. The shield with lead and boron doped PE in adequate thickness was designed. The angle between the collimator axis and the D + beam tube axis was optimized to be 901 to reduce the fast neutrons and g rays proportion at the imaging plane. As the optimization results, the thermal neutron flux at the imaging plane can reach 5 105 n/cm2/s with L/D ¼50 when the fast neutron yield is 3 1012 n/s. The n/g ratio is around 1 1010 n/ cm2/Sv with different aperture size. The L/D ratio is 25–200 adjustable. The field of view can achieve 20 20 cm2 at the position of 2 m away from the aperture. And the thermal neutron flux uniformity within the field of view is better than 7%. Simulated results also show that the Cd ratio is still low. An energy selection imaging system is being studied to increase the Cd ratio. References ¨ [1] E. Lehmann, G. Kuhne, P. Vontobel, G. Frei, IEEE Trans. Nucl. Sci. NS-52 (2005) 317. [2] Zhiyu Guo, Yubin Zou, Yuanrong Lu, et al, Neutron radiography with compact accelerator at Peking University: problems and solutions, in: Proceedings of UCANS-I, Beijing, August 15–17, 2010. (in preparation). [3] S. Tazawa, T. Nakanii, Present status of the cyclotron-based neutron radiography. In: S. Fujine, K. Kanda, G. Matsumoto, J. Barton (Eds.), Neutron Radiography (3): Proceedings of the Third World Conference (1989) 213–220. [4] Mitsutaka Kakeno, Yoshiaki Kido, Jun-ichi Kawamoto. Neutron radiography facilities using a 3 MV van de Graaff accelerator. In: S. Fujine, K. Kanda, G. Matsumoto, J. Barton (Eds.), Neutron Radiography (3), in: Proceedings of the Third World Conference, 1989, 245–252. [5] G.H. Gillespie, G.E. McMichael, G.R. Imel, SPIE Proc. 2867 (1997) 343. [6] R.C. Lanza, Y. Fink., E.B. Iverson, E.W. McFarland, S.H. Shi, SPIE Proc. 2867 (1997) 347.
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[10] S. Agostinelli, J. Allison, K. Amako, et al., Nucl. Instr. and Meth. A 506 (2003) 250. [11] Tetsuo Inada, Kiyomitsu Kawachi, J. Nucl. Sci. Technol. 5 (1) (1968) 22. [12] Panxun Chen, Radiation Effects on Semiconductor Devices and Integrated Circuits, National Defence Industry Press, Beijing, 2005 384–391. (in Chinese.