- Email: [email protected]
Designing a new graphite illuminator for imaging facility of INUS to improve neutron beam uniformity and intensity
Applied Radiation and Isotopes 148 (2019) 204–212
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Designing a new graphite illuminator for imaging facility of INUS to improve neutron beam uniformity and intensity
T
E. Nazemia,∗, M. Dincab, A. Movafeghia, B. Rokroka, M.H. Choopan Dastjerdia a b
Nuclear Science and Technology Research Institute, Tehran, Iran Institute for Nuclear Research (INR), Campului 1, Mioveni, 115400, Romania
H I GH L IG H T S
graphite illuminator is proposed to improve the neutron beam characteristics in INUS imaging facility. • AThenewshape of proposed illuminator is a cylinder whose one side is oriented. • To obtain optimum shape, 3 effective parameters were investigated by MCNPX code. • The results indicatedilluminator that the optimum illuminator should have a thickness of 10 cm, angle of 54.5° and position of 0. •
A R T I C LE I N FO
A B S T R A C T
Keywords: Neutron imaging Graphite illuminator Beam profile MCNP
A cylindrical graphite illuminator with a thickness of 6.5 cm and diameter of 18 cm was installed inside the collimator of INUS (Instalatie de Neutronografie UScata) neutron imaging facility in the past. The graphite illuminator is usually utilized inside the collimator of neutron imaging facility to provide an intense and approximately uniform beam of neutrons at the outlet of collimator. With the mentioned existing illuminator in INUS imaging facility, the thermal neutron flux at the exit of collimator was measured 7.2 × 104 n/cm2/s. Also the obtained neutron beam profile in this facility shows that it is not completely uniform at the imaging screen and the intensity of neutrons at the top and bottom of beam profile are not the same. Hence, in this paper a new graphite illuminator is proposed to improve the neutron beam characteristics in INUS imaging facility. Monte Carlo N-Particle (MCNP) code was implemented in this study for evaluating the proposed illuminator. The shape of proposed illuminator is a cylinder whose one side is inclined. Three quality factors of thermal neutron intensity, thermal neutron beam uniformity and gamma radiation dose rate were used to evaluate performance of the new illuminator. In order to obtain optimum illuminator shape, three effective parameters of thickness, angle of inclined side and position of the illuminator inside the collimator were investigated in this research. The investigation was carried out on thicknesses in the range of 5 to 25 cm with a step of 5 cm, angles in the range of 10 to 60° with a step of 10° and positions of −5, 0 and 5 cm with respect to center of reactor core. After investigating and interpolating the results, it was found that the proposed illuminator with a thickness of 10 cm, angle of 54.5° and position of 0 can produce a uniform beam profile, increase the thermal neutron intensity up to 7.1% and also decrease the neutron to gamma ratio up to 5% in comparison with the existing one.
1. Introduction
20 cm lead rings as a primary aperture to limit neutrons entering the collimator, a combination of 1.3 cm boral plate and 20 cm lead ring as a secondary aperture to prevent direct reaching of the neutron from illuminator to the beam tube's wall, and a 3 cm bismuth box to stop the gamma rays. All the components of collimator were inserted into a stainless steel tube. The designed collimator has geometrical parameters of: pin-hole aperture diameter D = 45 mm, distance illuminator surface - pin-hole aperture = 1555 mm, L/D = 92.8, beam
The tangential beam tube of the TRIGA Annular Core Pulsing Reactor (ACPR) in Institute for Nuclear Research (INR) is used for both neutron and gamma radiography. The collimator of INUS (Instalatie de Neutronografie UScata) imaging facility encompasses a graphite cylinder with thickness of 6.5 cm as an illuminator to provide a uniformly intense source of neutrons, a combination of 50 cm polyethylene and
∗
Corresponding author. E-mail address: [email protected] (E. Nazemi).
https://doi.org/10.1016/j.apradiso.2019.04.012 Received 24 December 2018; Received in revised form 10 March 2019; Accepted 8 April 2019 Available online 09 April 2019 0969-8043/ © 2019 Elsevier Ltd. All rights reserved.
Applied Radiation and Isotopes 148 (2019) 204–212
E. Nazemi, et al.
Fig. 1. Schematic view of simulated geometries: (a) INUS facility including reactor core, collimator and illuminator (b) New proposed illuminator (c) Reference illuminator.
et al., 2016; Burgio and Rosa, 2004; Stamatelatos et al., 2007; Basturk et al., 2005; Jamro et al., 2017; Tsai et al., 2009; Tharwat et al., 2014; Amini et al., 2015; Milczarek et al., 2005; Kontogeorgakos and Stamatelatos, 2010; Mishra et al., 2006) and also X-ray radiography instruments (Nazemi et al., 2018, 2019a, 2019b) which demonstrates the capability of this code in this research field. A geometry same as that used in previous work (Dinca et al., 2017) was simulated for INUS neutron imaging facility in this research. The only difference with the previous work is that the detailed components of collimator such as lead and polycarbonate rings which are used to make a divergent beam, were not simulated in this work in order to speed up the calculations. This was applied because various parameters were supposed to be investigated in this study which increases the number of simulations. To consider the effect of mentioned components, an ideal diaphragm (ring) with outer and inner diameter of 12 cm and 4.5 cm was positioned at distance of 160 cm from the center. The importance of neutron in diaphragm was considered zero. In this way, the neutron can just pass through the center of diaphragm (aperture). Between reactor core and primary part of collimator (about 1 m in length) was filled with demineralized water. After this length, around of collimator was filled with concrete. A volume cylindrical source was used for modelling the reactor core. To define the energy spectrum of the neutron, the obtained data in the previous research was used (Dinca et al., 2006). In the mentioned previous work, the transport program WIMS D4 was involved to model the reactor core and calculate the neutron spectrum. The applied model consists cylindrical rings that cover the central hole, ACPR fuel, water etc. The calculated neutron flux for 69 broad groups in a thin volume at the edge of the core, was collapsed in 3 groups. The normalized weights of calculated neutron flux were used in the input file of the MCNP code in this study. In the case of gamma, a spectrum that was generally calculated by M. Roos (1959) for thermal reactors, was used.
diameter = 288.3 mm at object's plane, beam divergence at object's plane = 3.97°, collimator divergence = 3.34° (Dinca et al., 2017). It is worth to mention that designing and constructing of every neutron radiography collimator is done based on a unique set of existing constraints such as budget, beam tube dimensions, weight restrictions, resolution requirements, beam size and etc (MacGillivray). The illuminator is usually a graphite block which is positioned close to the flux centerline for a tangential beam or close to the reactor core for a radial beam (MacGillivray). As mentioned recently, the illuminator is responsible for providing an intense and approximately uniform beam of neutrons at the outlet of collimator. Since the illuminator scatters gamma radiations in addition to neutrons, its thickness and shape affects the intensity and energy spectrum of both mentioned radiations. Thus, the dimensions and shape of illuminator should be selected such that to increase the thermal neutron flux and decrease the intensity of gamma radiations. Besides, the uniformity of thermal neutron beam profile at the imaging plane is affected by the shape of illuminator and should be considered during the designing process. The purpose of this paper is designing of a new graphite illuminator for INUS imaging facility using a Monte Carlo method. The effective parameters such as shape, dimensions and position of illuminator inside the collimator is investigated in this study in order to improve the intensity and uniformity of thermal neutrons at the imaging plane. 2. Materials and methods Monte Carlo N-Particle (MCNP) code was implemented to investigate the reference and new proposed illuminator. MCNP code is a powerful tool to simulate transporting of different kinds of radiations such as neutron, gamma, electrons and etc. (Pelowitz, 2005). In recent years, the MCNP code has been widely utilized in modelling of research reactors and neutron radiography facilities (Dinca et al., 2006; Dastjerdi 205
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At first, a graphite cylinder with a thickness and diameter of 6.5 cm and 18 cm was simulated as the reference illuminator. Indeed, this graphite illuminator is the existing one in the neutron imaging facility of INUS and was considered as a reference in this study. In other words, all the results of new proposed illuminators are compared with the results of reference one. It should be noticed that the TMCCS continuous energy cross section library (source B-V.0) was utilized for graphite in the input file of MCNP code. In addition, an MT card with an S (α,β) thermal neutron scattering data set was used for graphite to take the scattering properties of graphite into account for low energy neutrons. The shape of new proposed illuminator in this work is a little different from the reference illuminator. As it is schematically shown in Fig. 1, it is a cylinder whose one side is inclined. It is worth to mention that the idea of inclined side has been originated from X-ray tube's inclined target. Three quality factors of thermal neutron intensity, thermal neutron beam uniformity and gamma dose rate were used to evaluate performance of the new illuminator. In order to obtain optimum illuminator shape, three effective parameters of thickness, angle of inclined side and position of the illuminator inside the collimator were investigated in this research. The investigation was carried out on thicknesses in the range of 5 to 25 cm with a step of 5 cm, angles in the range of 10 to 60° with a step of 10° and positions of −5, 0 and 5 cm with respect to center of reactor core. The two quality factors of thermal neutron intensity and gamma dose rate were calculated inside the aperture of collimator using tally F4. The main reason for selecting the aperture position instead of the outlet of collimator for calculating the mentioned parameters, was decreasing the required time for running of MCNP code. Since the obtained results of new illuminator would be interpreted relative to the reference illuminator in this study, there is no difference to calculate flux at the aperture or outlet of collimator, because both of them would be decreased with a same rate and their relation is kept almost constant. The flux of thermal neutrons was directly calculated using tally F4. Regarding the gamma dose rate, at first the average flux of gamma rays was calculated using the tally F4, and then converted into a dose rate using the DEn and DFn cards in the input file of the MCNPX code. It is worth noting that the photon flux to dose rate conversion factors, utilized in the DEn and DFn cards, were obtained from ICRP-21 (International Commission On Radiological Protection (ICRP), 1971). For assessing the neutron beam uniformity, the mesh tally type 1 with a 22 × 22 cm rectangular grid and pixel size of 5 × 5 mm positioned at a distance of 20 cm from the outlet of collimator, was utilized. It should be noted that the mesh tally is a technique in MCNPX code to display the particle dose, flux, or other quantities on a cylindrical, spherical, or rectangular grid (Pelowitz, 2005).
reach at the bottom of imaging screen in comparison with top of that. As a result, the neutron beam profile is not completely uniform at the imaging screen. The reason of non-uniformity is that more number of neutrons, which come from the reactor, impact to the closer side of illuminator (top of the illuminator) and consequently more neutrons are scattered from this side related to the farther side (bottom of the illuminator). It should be noted that the close and far sides are determined with respect to the distance between illuminator and reactor core. Since there is a diaphragm between the illuminator and imaging screen, more neutrons that are scattered from the top of illuminator will reach to the bottom of imaging screen and vice versa the less scattered neutrons from the bottom of illuminator will reach to the top of imaging screen. Using the simulation, the thermal neutron intensity and gamma radiation dose rate were calculated 5.8 × 10−8 1/cm2 and 1.2 × 10−11 μSv/h inside the collimator's diaphragm, respectively. The neutron beam profile which is shown in Fig. 3, was calculated at a distance of 20 cm from the exit of collimator. The calculated neutron intensity and gamma dose rate from simulations are not absolute values same as experimental results, but they are proportional to the experimental ones. As shown in Figs. 2 and 3, the relative difference of neutron intensities between the top and bottom of experimental and simulated beam profile is about 24.7% and 27.6%, respectively. Although the pixel size of simulated and experimental images are not same, the results indicate that the simulated beam profile is in good agreement with the experimental one. 3.2. New proposed illuminator The relative difference of neutron intensities between the top and bottom of beam profile for the new proposed and reference illuminator is shown in Fig. 4. The relative difference between obtained results from the new proposed and reference (existing) illuminator for thermal neutron intensity and gamma dose rate quantities are also shown in Fig. 5 and Fig. 6, respectively. From Fig. 4 it is apparent that in all three positions the relative difference of thermal neutron flux between top and bottom of the beam profile almost increases by increasing the thickness of proposed illuminator. The obtained relative difference for reference illuminator is also 27.6%. In positions of −5 cm and 0, for all thicknesses the relative difference is regularly decreased by increasing the angle of inclined side. On the contrary, no regular manner is observed for thickness of 5 cm. As it can be obviously seen from Fig. 5, in all positions the relative difference of thermal neutron flux between obtained results from the new proposed and reference illuminator increases when thickness of proposed illuminator is raised. In position of −5 cm, by increasing the angle of inclined side, the thermal neutron intensity decreases, while in positions of 0 and 5 cm, unless the thickness of 5 cm, the thermal neutron intensity increases first and then decreases. Indeed, in positions of 0 and 5 cm the thermal neutron intensity has a maximum value at the angles of 30° and 40°, respectively. As indicated in Fig. 6, similar to thermal neutrons, gamma radiation dose rate in all positions increases when the thickness of proposed illuminator is raised. In position of −5 cm, when the angle of inclined side is increased, gamma radiation dose rate for thickness of 5 cm is regularly decreased but for thickness of 10 cm and other thicknesses it has a maximum value at the angles of 20° and 30°, respectively. In position of 0, gamma dose rate for thickness of 5 cm is regularly decreased, for thickness of 10 and 15 cm has a maximum value at the angle of 50° and for thicknesses of 20 and 25 cm it is regularly increased by increasing the angle. In position of 5 cm, for all thicknesses the gamma dose rate is regularly raised by increasing the angle.
3. Results 3.1. Existing (reference) illuminator Since in this paper all the evaluations of new proposed illuminator would be done in comparison with the existing illuminator and all the results will be interpreted relatively, it is better to first explain the neutron beam characteristics of INUS imaging facility obtained from experiment and simulation. Some measured beam characteristics of INUS imaging facility is reported in (Dinca et al., 2017). The thermal neutron beam intensity and gamma dose rate were reported 7.2 × 104 n/cm2/s and 940 μSv/h, respectively. The experimental image of beam profile (white beam), when there is no object between exit of collimator and imaging screen, is shown in Fig. 2. The detection system in the experiment included a Starlight XPRESS SXV-H9 CCD camera and a 6 LiF-ZnS scintillator screen. An exposure time of 40 s was selected for imaging. The distance between collimator's diaphragm to scintillator screen and L/D ratio for this experiment were 4177 mm and 92.8, respectively. As it can be seen from this figure, more thermal neutrons
4. Discussions As indicated in results section, all the parameters of position, angle 206
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Fig. 2. The experimental image of beam profile obtained in imaging facility of INUS.
Fig. 3. The simulated image of beam profile for the existing illuminator in imaging facility of INUS.
and 10 cm obtained from previous step, thickness of 10 cm which is related to positions of −5 cm and 0, is selected. In Fig. 4a and b, if a line is plotted parallel to x-axis and in value of zero difference, it would cross the curve of 10 cm thickness at the angle of about 54.5°. In other words, in both positions of −5 cm and 0, for the thickness of 10 cm and the angle of 54.5° we can have an almost zero difference intensity. As shown in Fig. 5a and b, the neutron intensity relative difference between the reference illuminator and the proposed illuminator at the angle of 54.5° for positions of −5 cm and 0 is −3.7% and 7.1%, respectively. Now it is time of assessing the third priority. In this case, the less gamma radiation dose rate, the better. As indicated in Fig. 6, the relative gamma radiation dose rate at the angle of 54.5° for positions of
of inclined side and thickness of proposed illuminator have significant influences on the beam quality. Therefore, optimum parameters should be chosen. To do this purpose, at first priorities were listed in this way: 1- beam uniformity, 2- thermal neutron intensity, 3- gamma radiation dose rate. In this case of beam uniformity, a zero difference between intensities of top and bottom part of beam profile is ideal. Looking at Fig. 4, it can be concluded that for positions of −5 cm and 0, illuminator with a thickness of 5 and 10 cm could make a zero difference and for position of 5 cm a thickness of 5 cm can just do it. In the case of thermal neutron intensity, the more thermal neutron intensity, the better. Observing Fig. 5, it is clear that by increasing the thickness of illuminator the intensity is raised. Thus, among the thicknesses of 5 cm 207
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Fig. 4. The relative difference of thermal neutron flux between top and bottom of the beam profile when the proposed illuminator is placed at a position of: (a) −5 cm (b) 0 (c) 5 cm.
−5 cm and 0 is −11.9% and 12.8%, respectively. In neutron radiography, the parameter of neutron to gamma ratio is usually used for evaluating the effect of gamma radiation dose rate on image. The ratio is recommended to be more than 1 × 106 n/cmˆ2.mrem (Berger, 1960; Domanus and Markgraf, 1987; Domanus et al., 1992). As reported in
reference (Dinca et al., 2017), this ratio for imaging facility of INUS with the presence of reference illuminator was calculated 4.67 × 106 n/ cmˆ2.mrem. The relative difference of neutron to gamma ratio between the reference illuminator and the proposed illuminator with the angle of 54.5° in positions of −5 cm and 0 was calculated 9% and −5% from 208
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Fig. 5. Relative difference between thermal neutron flux obtained from the new proposed and reference illuminator when the proposed illuminator is placed at a position of: (a) −5 cm (b) 0 (c) 5 cm.
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Fig. 6. Relative difference between obtained gamma radiation dose rates from the new proposed and reference illuminator when the proposed illuminator is placed at a position of: (a) −5 cm (b) 0 (c) 5 cm. 210
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Fig. 7. The neutron beam profile obtained from the optimum proposed illuminator which is positioned in 0 and has a thickness of 10 cm and an angle of 54.5°.
neutron imaging facilities in the world that use graphite illuminator inside the collimator.
simulations in this study, respectively. Since the neutron to gamma ratio for the reference illuminator that was measured experimentally is 4.67 times (467%) larger than the threshold of recommended value, the effect of new proposed illuminator with the angle of 54.5° in positions of −5 cm and 0 would be negligible. Finally, it can be concluded that the new proposed illuminator with the angle of 54.5° in position of 0 is more appropriate than position −5 cm, because thermal neutron intensity in this position is 7.1% more than the reference illuminator while, in position of −5 cm it is 3.7% less than the reference one. The neutron beam profile obtained from the optimum proposed illuminator at the exit of collimator, is shown in Fig. 7. As it can be seen from this figure, the beam profile is more uniform in comparison with the beam profile obtained from the reference one (Fig. 3).
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apradiso.2019.04.012. References Amini, M., Fadaei, A.H., Gharib, M., 2015. Analyzing the effect of geometric factors on designing neutron radiography system. Appl. Radiat. Isot. 105, 249–256. Basturk, M., Kardjilov, N., Lehmann, E., Zawisky, M., 2005. Monte Carlo simulation of neutron transmission of boron-alloyed steel. IEEE Trans. Nucl. Sci. 52 (1), 394–399. Berger, H., 1960. Neutron Radiography. ANL-6346. pp. 12–37 i960. Burgio, N., Rosa, R., 2004. Monte Carlo design for a new neutron collimator at the ENEA Casaccia TRIGA reactor. Appl. Radiat. Isot. 61 (4), 663–666. Dastjerdi, M.C., Khalafi, H., Kasesaz, Y., Mirvakili, S.M., Emami, J., Ghods, H., Ezzati, A., 2016. Design, construction and characterization of a new neutron beam for neutron radiography at the Tehran Research Reactor. Nucl. Instrum. Methods Phys. Res. Sect. A Accel. Spectrom. Detect. Assoc. Equip. 818, 1–8. Dinca, M., Pavelescu, M., Iorgulis, C., 2006. Collimated neutron beam for neutron radiography. Rom. J. Phys. 51 (3/4), 435. Dinca, M., Iorgulis, C., Barbos, D., Mitrea, L.D., 2017. Status of the imaging facility INUS at INR. Phys. Procedia 88, 167–174. Domanus, J.C., Markgraf, J.C., 1987. Collimators for Thermal eutron Radiography: an Overview. Domanus, J.C., Bayon, G., Harms, A.A., Greim, L., Leeflang, H.P., Markgraf, J.F.W., Matfield, R., Taylor, D.J., 1992. Practical Neutron Radiography. EUR, Luxembourg). International Commission On Radiological Protection (ICRP), April 1971. Data for Protection against Ionizing Radiation from External Sources: Supplement to ICRP Publication 15. Pergamon Press ICRP-21. Jamro, R., Kardjilov, N., HairieRabir, M., Zain, M.R.M., Mohamed, A.A., Ali, N.M., Idris, F., Ahmad, M.H.A.R.M., Yazid, K., Yazid, H., Azman, A., 2017. Monte Carlo simulation for designing collimator of the neutron radiography facility in Malaysia. Phys. Procedia 88, 361–368. Kontogeorgakos, D., Stamatelatos, I.E., 2010. Validation of the Monte Carlo model of the Greek research reactor core. Nucl. Technol. 170 (3), 460–464. MacGillivray, G.M., Neutron Radiography Collimator Design. Nray Services Inc, RR# 1 Petawawa, Ontario, Canada. Milczarek, J.J., Trzciński, A., El Abd, A.E.G., Czachor, A., 2005. Monte Carlo code for neutron radiography. Nucl. Instrum. Methods Phys. Res. Sect. A Accel. Spectrom. Detect. Assoc. Equip. 542 (1–3), 237–240. Mishra, K.K., Hawari, A.I., Gillette, V.H., 2006. Design and performance of a thermal neutron imaging facility at the North Carolina State University PULSTAR reactor. IEEE Trans. Nucl. Sci. 53 (6), 3904–3911. Nazemi, E., Rokrok, B., Movafeghi, A., Dastjerdi, M.C., 2018. Simulation of a complete X-
5. Conclusions In this paper, an attempt has been made to propose a new graphite illuminator for neutron imaging facility of INUS with the advantage of having a more uniform and intense beam at the imaging screen. The MCNP code was utilized for simulating the neutron imaging facility of INUS which was including reactor core, radiation shield, neutron collimator and graphite illuminator. The shape of proposed illuminator is a cylinder that its one side is inclined. To evaluate performance of the new illuminator, three quality factors of thermal neutron beam uniformity, thermal neutron intensity and gamma radiation dose rate were considered. Three effective parameters of thickness, angle of inclined side and position of the illuminator inside the collimator were surveyed to achieve optimum illuminator shape. The investigation was done on thicknesses in the range of 5 to 25 cm with a step of 5 cm, angles in the range of 10 to 60° with a step of 10° and positions of −5, 0 and 5 cm with respect to center of reactor core. Finally, by comparing the obtained results for various parameters, it was realized that the proposed illuminator with a thickness of 10 cm, angle of 54.5° and position of 0 can generate a uniform beam profile at the imaging screen. In addition, it can improve the thermal neutron intensity up to 7.1% in comparison with the existing one without having a significant effect of gamma dose rate on the image. Although the proposed procedure in this paper was just done for special case of INUS neutron imaging facility, it can be applied for other 211
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Laboratory, Los Alamos. Roos, M., 1959. Sources of gamma radiation in a reactor core (No. AE–19). AB Atomenergi. Stamatelatos, I.E., Varvayanni, M., Tzika, F., Ale, A.B.F., Catsaros, N., 2007. Monte Carlo simulation of the Greek Research Reactor neutron irradiation facilities. Nucl. Instrum. Methods Phys. Res. Sect. B Beam Interact. Mater. Atoms 263 (1), 136–139. Tharwat, M., Mohamed, N., Mongy, T., 2014. Image enhancement using MCNP5 code and MATLAB in neutron radiography. Appl. Radiat. Isot. 89, 30–36. Tsai, P.E., Liu, Y.H., Huang, C.K., Liu, H.M., Jiang, S.H., 2009. Neutron flux mapping inside a cubic and a head PMMA phantom using indirect neutron radiography. Appl. Radiat. Isot. 67 (7–8), S190–S194.
ray digital radiographic system for industrial applications. Appl. Radiat. Isot. 139, 294–303. Nazemi, E., Movafeghi, A., Rokrok, B., Dastjerdi, M.C., 2019a. A novel method for predicting pixel value distribution non-uniformity due to heel effect of X-ray tube in industrial digital radiography using artificial neural network. J. Nondestruct. Eval. 38 (1). Nazemi, E., Rokrok, B., Movafeghi, A., Dastjerdi, M.C., Dinca, M., 2019b. Obtaining optimum exposure conditions for digital X-ray radiography of fresh nuclear fuel rods. Nucl. Instrum. Methods Phys. Res. Sect. A Accel. Spectrom. Detect. Assoc. Equip. 923, 88–96. Pelowitz, D.B., 2005. MCNP-X TM User's Manual, Version 2.5.0. LA-CP-05e0369. National
212
Contents lists available at ScienceDirect
Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso
Designing a new graphite illuminator for imaging facility of INUS to improve neutron beam uniformity and intensity
T
E. Nazemia,∗, M. Dincab, A. Movafeghia, B. Rokroka, M.H. Choopan Dastjerdia a b
Nuclear Science and Technology Research Institute, Tehran, Iran Institute for Nuclear Research (INR), Campului 1, Mioveni, 115400, Romania
H I GH L IG H T S
graphite illuminator is proposed to improve the neutron beam characteristics in INUS imaging facility. • AThenewshape of proposed illuminator is a cylinder whose one side is oriented. • To obtain optimum shape, 3 effective parameters were investigated by MCNPX code. • The results indicatedilluminator that the optimum illuminator should have a thickness of 10 cm, angle of 54.5° and position of 0. •
A R T I C LE I N FO
A B S T R A C T
Keywords: Neutron imaging Graphite illuminator Beam profile MCNP
A cylindrical graphite illuminator with a thickness of 6.5 cm and diameter of 18 cm was installed inside the collimator of INUS (Instalatie de Neutronografie UScata) neutron imaging facility in the past. The graphite illuminator is usually utilized inside the collimator of neutron imaging facility to provide an intense and approximately uniform beam of neutrons at the outlet of collimator. With the mentioned existing illuminator in INUS imaging facility, the thermal neutron flux at the exit of collimator was measured 7.2 × 104 n/cm2/s. Also the obtained neutron beam profile in this facility shows that it is not completely uniform at the imaging screen and the intensity of neutrons at the top and bottom of beam profile are not the same. Hence, in this paper a new graphite illuminator is proposed to improve the neutron beam characteristics in INUS imaging facility. Monte Carlo N-Particle (MCNP) code was implemented in this study for evaluating the proposed illuminator. The shape of proposed illuminator is a cylinder whose one side is inclined. Three quality factors of thermal neutron intensity, thermal neutron beam uniformity and gamma radiation dose rate were used to evaluate performance of the new illuminator. In order to obtain optimum illuminator shape, three effective parameters of thickness, angle of inclined side and position of the illuminator inside the collimator were investigated in this research. The investigation was carried out on thicknesses in the range of 5 to 25 cm with a step of 5 cm, angles in the range of 10 to 60° with a step of 10° and positions of −5, 0 and 5 cm with respect to center of reactor core. After investigating and interpolating the results, it was found that the proposed illuminator with a thickness of 10 cm, angle of 54.5° and position of 0 can produce a uniform beam profile, increase the thermal neutron intensity up to 7.1% and also decrease the neutron to gamma ratio up to 5% in comparison with the existing one.
1. Introduction
20 cm lead rings as a primary aperture to limit neutrons entering the collimator, a combination of 1.3 cm boral plate and 20 cm lead ring as a secondary aperture to prevent direct reaching of the neutron from illuminator to the beam tube's wall, and a 3 cm bismuth box to stop the gamma rays. All the components of collimator were inserted into a stainless steel tube. The designed collimator has geometrical parameters of: pin-hole aperture diameter D = 45 mm, distance illuminator surface - pin-hole aperture = 1555 mm, L/D = 92.8, beam
The tangential beam tube of the TRIGA Annular Core Pulsing Reactor (ACPR) in Institute for Nuclear Research (INR) is used for both neutron and gamma radiography. The collimator of INUS (Instalatie de Neutronografie UScata) imaging facility encompasses a graphite cylinder with thickness of 6.5 cm as an illuminator to provide a uniformly intense source of neutrons, a combination of 50 cm polyethylene and
∗
Corresponding author. E-mail address: [email protected] (E. Nazemi).
https://doi.org/10.1016/j.apradiso.2019.04.012 Received 24 December 2018; Received in revised form 10 March 2019; Accepted 8 April 2019 Available online 09 April 2019 0969-8043/ © 2019 Elsevier Ltd. All rights reserved.
Applied Radiation and Isotopes 148 (2019) 204–212
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Fig. 1. Schematic view of simulated geometries: (a) INUS facility including reactor core, collimator and illuminator (b) New proposed illuminator (c) Reference illuminator.
et al., 2016; Burgio and Rosa, 2004; Stamatelatos et al., 2007; Basturk et al., 2005; Jamro et al., 2017; Tsai et al., 2009; Tharwat et al., 2014; Amini et al., 2015; Milczarek et al., 2005; Kontogeorgakos and Stamatelatos, 2010; Mishra et al., 2006) and also X-ray radiography instruments (Nazemi et al., 2018, 2019a, 2019b) which demonstrates the capability of this code in this research field. A geometry same as that used in previous work (Dinca et al., 2017) was simulated for INUS neutron imaging facility in this research. The only difference with the previous work is that the detailed components of collimator such as lead and polycarbonate rings which are used to make a divergent beam, were not simulated in this work in order to speed up the calculations. This was applied because various parameters were supposed to be investigated in this study which increases the number of simulations. To consider the effect of mentioned components, an ideal diaphragm (ring) with outer and inner diameter of 12 cm and 4.5 cm was positioned at distance of 160 cm from the center. The importance of neutron in diaphragm was considered zero. In this way, the neutron can just pass through the center of diaphragm (aperture). Between reactor core and primary part of collimator (about 1 m in length) was filled with demineralized water. After this length, around of collimator was filled with concrete. A volume cylindrical source was used for modelling the reactor core. To define the energy spectrum of the neutron, the obtained data in the previous research was used (Dinca et al., 2006). In the mentioned previous work, the transport program WIMS D4 was involved to model the reactor core and calculate the neutron spectrum. The applied model consists cylindrical rings that cover the central hole, ACPR fuel, water etc. The calculated neutron flux for 69 broad groups in a thin volume at the edge of the core, was collapsed in 3 groups. The normalized weights of calculated neutron flux were used in the input file of the MCNP code in this study. In the case of gamma, a spectrum that was generally calculated by M. Roos (1959) for thermal reactors, was used.
diameter = 288.3 mm at object's plane, beam divergence at object's plane = 3.97°, collimator divergence = 3.34° (Dinca et al., 2017). It is worth to mention that designing and constructing of every neutron radiography collimator is done based on a unique set of existing constraints such as budget, beam tube dimensions, weight restrictions, resolution requirements, beam size and etc (MacGillivray). The illuminator is usually a graphite block which is positioned close to the flux centerline for a tangential beam or close to the reactor core for a radial beam (MacGillivray). As mentioned recently, the illuminator is responsible for providing an intense and approximately uniform beam of neutrons at the outlet of collimator. Since the illuminator scatters gamma radiations in addition to neutrons, its thickness and shape affects the intensity and energy spectrum of both mentioned radiations. Thus, the dimensions and shape of illuminator should be selected such that to increase the thermal neutron flux and decrease the intensity of gamma radiations. Besides, the uniformity of thermal neutron beam profile at the imaging plane is affected by the shape of illuminator and should be considered during the designing process. The purpose of this paper is designing of a new graphite illuminator for INUS imaging facility using a Monte Carlo method. The effective parameters such as shape, dimensions and position of illuminator inside the collimator is investigated in this study in order to improve the intensity and uniformity of thermal neutrons at the imaging plane. 2. Materials and methods Monte Carlo N-Particle (MCNP) code was implemented to investigate the reference and new proposed illuminator. MCNP code is a powerful tool to simulate transporting of different kinds of radiations such as neutron, gamma, electrons and etc. (Pelowitz, 2005). In recent years, the MCNP code has been widely utilized in modelling of research reactors and neutron radiography facilities (Dinca et al., 2006; Dastjerdi 205
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At first, a graphite cylinder with a thickness and diameter of 6.5 cm and 18 cm was simulated as the reference illuminator. Indeed, this graphite illuminator is the existing one in the neutron imaging facility of INUS and was considered as a reference in this study. In other words, all the results of new proposed illuminators are compared with the results of reference one. It should be noticed that the TMCCS continuous energy cross section library (source B-V.0) was utilized for graphite in the input file of MCNP code. In addition, an MT card with an S (α,β) thermal neutron scattering data set was used for graphite to take the scattering properties of graphite into account for low energy neutrons. The shape of new proposed illuminator in this work is a little different from the reference illuminator. As it is schematically shown in Fig. 1, it is a cylinder whose one side is inclined. It is worth to mention that the idea of inclined side has been originated from X-ray tube's inclined target. Three quality factors of thermal neutron intensity, thermal neutron beam uniformity and gamma dose rate were used to evaluate performance of the new illuminator. In order to obtain optimum illuminator shape, three effective parameters of thickness, angle of inclined side and position of the illuminator inside the collimator were investigated in this research. The investigation was carried out on thicknesses in the range of 5 to 25 cm with a step of 5 cm, angles in the range of 10 to 60° with a step of 10° and positions of −5, 0 and 5 cm with respect to center of reactor core. The two quality factors of thermal neutron intensity and gamma dose rate were calculated inside the aperture of collimator using tally F4. The main reason for selecting the aperture position instead of the outlet of collimator for calculating the mentioned parameters, was decreasing the required time for running of MCNP code. Since the obtained results of new illuminator would be interpreted relative to the reference illuminator in this study, there is no difference to calculate flux at the aperture or outlet of collimator, because both of them would be decreased with a same rate and their relation is kept almost constant. The flux of thermal neutrons was directly calculated using tally F4. Regarding the gamma dose rate, at first the average flux of gamma rays was calculated using the tally F4, and then converted into a dose rate using the DEn and DFn cards in the input file of the MCNPX code. It is worth noting that the photon flux to dose rate conversion factors, utilized in the DEn and DFn cards, were obtained from ICRP-21 (International Commission On Radiological Protection (ICRP), 1971). For assessing the neutron beam uniformity, the mesh tally type 1 with a 22 × 22 cm rectangular grid and pixel size of 5 × 5 mm positioned at a distance of 20 cm from the outlet of collimator, was utilized. It should be noted that the mesh tally is a technique in MCNPX code to display the particle dose, flux, or other quantities on a cylindrical, spherical, or rectangular grid (Pelowitz, 2005).
reach at the bottom of imaging screen in comparison with top of that. As a result, the neutron beam profile is not completely uniform at the imaging screen. The reason of non-uniformity is that more number of neutrons, which come from the reactor, impact to the closer side of illuminator (top of the illuminator) and consequently more neutrons are scattered from this side related to the farther side (bottom of the illuminator). It should be noted that the close and far sides are determined with respect to the distance between illuminator and reactor core. Since there is a diaphragm between the illuminator and imaging screen, more neutrons that are scattered from the top of illuminator will reach to the bottom of imaging screen and vice versa the less scattered neutrons from the bottom of illuminator will reach to the top of imaging screen. Using the simulation, the thermal neutron intensity and gamma radiation dose rate were calculated 5.8 × 10−8 1/cm2 and 1.2 × 10−11 μSv/h inside the collimator's diaphragm, respectively. The neutron beam profile which is shown in Fig. 3, was calculated at a distance of 20 cm from the exit of collimator. The calculated neutron intensity and gamma dose rate from simulations are not absolute values same as experimental results, but they are proportional to the experimental ones. As shown in Figs. 2 and 3, the relative difference of neutron intensities between the top and bottom of experimental and simulated beam profile is about 24.7% and 27.6%, respectively. Although the pixel size of simulated and experimental images are not same, the results indicate that the simulated beam profile is in good agreement with the experimental one. 3.2. New proposed illuminator The relative difference of neutron intensities between the top and bottom of beam profile for the new proposed and reference illuminator is shown in Fig. 4. The relative difference between obtained results from the new proposed and reference (existing) illuminator for thermal neutron intensity and gamma dose rate quantities are also shown in Fig. 5 and Fig. 6, respectively. From Fig. 4 it is apparent that in all three positions the relative difference of thermal neutron flux between top and bottom of the beam profile almost increases by increasing the thickness of proposed illuminator. The obtained relative difference for reference illuminator is also 27.6%. In positions of −5 cm and 0, for all thicknesses the relative difference is regularly decreased by increasing the angle of inclined side. On the contrary, no regular manner is observed for thickness of 5 cm. As it can be obviously seen from Fig. 5, in all positions the relative difference of thermal neutron flux between obtained results from the new proposed and reference illuminator increases when thickness of proposed illuminator is raised. In position of −5 cm, by increasing the angle of inclined side, the thermal neutron intensity decreases, while in positions of 0 and 5 cm, unless the thickness of 5 cm, the thermal neutron intensity increases first and then decreases. Indeed, in positions of 0 and 5 cm the thermal neutron intensity has a maximum value at the angles of 30° and 40°, respectively. As indicated in Fig. 6, similar to thermal neutrons, gamma radiation dose rate in all positions increases when the thickness of proposed illuminator is raised. In position of −5 cm, when the angle of inclined side is increased, gamma radiation dose rate for thickness of 5 cm is regularly decreased but for thickness of 10 cm and other thicknesses it has a maximum value at the angles of 20° and 30°, respectively. In position of 0, gamma dose rate for thickness of 5 cm is regularly decreased, for thickness of 10 and 15 cm has a maximum value at the angle of 50° and for thicknesses of 20 and 25 cm it is regularly increased by increasing the angle. In position of 5 cm, for all thicknesses the gamma dose rate is regularly raised by increasing the angle.
3. Results 3.1. Existing (reference) illuminator Since in this paper all the evaluations of new proposed illuminator would be done in comparison with the existing illuminator and all the results will be interpreted relatively, it is better to first explain the neutron beam characteristics of INUS imaging facility obtained from experiment and simulation. Some measured beam characteristics of INUS imaging facility is reported in (Dinca et al., 2017). The thermal neutron beam intensity and gamma dose rate were reported 7.2 × 104 n/cm2/s and 940 μSv/h, respectively. The experimental image of beam profile (white beam), when there is no object between exit of collimator and imaging screen, is shown in Fig. 2. The detection system in the experiment included a Starlight XPRESS SXV-H9 CCD camera and a 6 LiF-ZnS scintillator screen. An exposure time of 40 s was selected for imaging. The distance between collimator's diaphragm to scintillator screen and L/D ratio for this experiment were 4177 mm and 92.8, respectively. As it can be seen from this figure, more thermal neutrons
4. Discussions As indicated in results section, all the parameters of position, angle 206
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Fig. 2. The experimental image of beam profile obtained in imaging facility of INUS.
Fig. 3. The simulated image of beam profile for the existing illuminator in imaging facility of INUS.
and 10 cm obtained from previous step, thickness of 10 cm which is related to positions of −5 cm and 0, is selected. In Fig. 4a and b, if a line is plotted parallel to x-axis and in value of zero difference, it would cross the curve of 10 cm thickness at the angle of about 54.5°. In other words, in both positions of −5 cm and 0, for the thickness of 10 cm and the angle of 54.5° we can have an almost zero difference intensity. As shown in Fig. 5a and b, the neutron intensity relative difference between the reference illuminator and the proposed illuminator at the angle of 54.5° for positions of −5 cm and 0 is −3.7% and 7.1%, respectively. Now it is time of assessing the third priority. In this case, the less gamma radiation dose rate, the better. As indicated in Fig. 6, the relative gamma radiation dose rate at the angle of 54.5° for positions of
of inclined side and thickness of proposed illuminator have significant influences on the beam quality. Therefore, optimum parameters should be chosen. To do this purpose, at first priorities were listed in this way: 1- beam uniformity, 2- thermal neutron intensity, 3- gamma radiation dose rate. In this case of beam uniformity, a zero difference between intensities of top and bottom part of beam profile is ideal. Looking at Fig. 4, it can be concluded that for positions of −5 cm and 0, illuminator with a thickness of 5 and 10 cm could make a zero difference and for position of 5 cm a thickness of 5 cm can just do it. In the case of thermal neutron intensity, the more thermal neutron intensity, the better. Observing Fig. 5, it is clear that by increasing the thickness of illuminator the intensity is raised. Thus, among the thicknesses of 5 cm 207
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Fig. 4. The relative difference of thermal neutron flux between top and bottom of the beam profile when the proposed illuminator is placed at a position of: (a) −5 cm (b) 0 (c) 5 cm.
−5 cm and 0 is −11.9% and 12.8%, respectively. In neutron radiography, the parameter of neutron to gamma ratio is usually used for evaluating the effect of gamma radiation dose rate on image. The ratio is recommended to be more than 1 × 106 n/cmˆ2.mrem (Berger, 1960; Domanus and Markgraf, 1987; Domanus et al., 1992). As reported in
reference (Dinca et al., 2017), this ratio for imaging facility of INUS with the presence of reference illuminator was calculated 4.67 × 106 n/ cmˆ2.mrem. The relative difference of neutron to gamma ratio between the reference illuminator and the proposed illuminator with the angle of 54.5° in positions of −5 cm and 0 was calculated 9% and −5% from 208
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Fig. 5. Relative difference between thermal neutron flux obtained from the new proposed and reference illuminator when the proposed illuminator is placed at a position of: (a) −5 cm (b) 0 (c) 5 cm.
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Fig. 6. Relative difference between obtained gamma radiation dose rates from the new proposed and reference illuminator when the proposed illuminator is placed at a position of: (a) −5 cm (b) 0 (c) 5 cm. 210
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Fig. 7. The neutron beam profile obtained from the optimum proposed illuminator which is positioned in 0 and has a thickness of 10 cm and an angle of 54.5°.
neutron imaging facilities in the world that use graphite illuminator inside the collimator.
simulations in this study, respectively. Since the neutron to gamma ratio for the reference illuminator that was measured experimentally is 4.67 times (467%) larger than the threshold of recommended value, the effect of new proposed illuminator with the angle of 54.5° in positions of −5 cm and 0 would be negligible. Finally, it can be concluded that the new proposed illuminator with the angle of 54.5° in position of 0 is more appropriate than position −5 cm, because thermal neutron intensity in this position is 7.1% more than the reference illuminator while, in position of −5 cm it is 3.7% less than the reference one. The neutron beam profile obtained from the optimum proposed illuminator at the exit of collimator, is shown in Fig. 7. As it can be seen from this figure, the beam profile is more uniform in comparison with the beam profile obtained from the reference one (Fig. 3).
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.apradiso.2019.04.012. References Amini, M., Fadaei, A.H., Gharib, M., 2015. Analyzing the effect of geometric factors on designing neutron radiography system. Appl. Radiat. Isot. 105, 249–256. Basturk, M., Kardjilov, N., Lehmann, E., Zawisky, M., 2005. Monte Carlo simulation of neutron transmission of boron-alloyed steel. IEEE Trans. Nucl. Sci. 52 (1), 394–399. Berger, H., 1960. Neutron Radiography. ANL-6346. pp. 12–37 i960. Burgio, N., Rosa, R., 2004. Monte Carlo design for a new neutron collimator at the ENEA Casaccia TRIGA reactor. Appl. Radiat. Isot. 61 (4), 663–666. Dastjerdi, M.C., Khalafi, H., Kasesaz, Y., Mirvakili, S.M., Emami, J., Ghods, H., Ezzati, A., 2016. Design, construction and characterization of a new neutron beam for neutron radiography at the Tehran Research Reactor. Nucl. Instrum. Methods Phys. Res. Sect. A Accel. Spectrom. Detect. Assoc. Equip. 818, 1–8. Dinca, M., Pavelescu, M., Iorgulis, C., 2006. Collimated neutron beam for neutron radiography. Rom. J. Phys. 51 (3/4), 435. Dinca, M., Iorgulis, C., Barbos, D., Mitrea, L.D., 2017. Status of the imaging facility INUS at INR. Phys. Procedia 88, 167–174. Domanus, J.C., Markgraf, J.C., 1987. Collimators for Thermal eutron Radiography: an Overview. Domanus, J.C., Bayon, G., Harms, A.A., Greim, L., Leeflang, H.P., Markgraf, J.F.W., Matfield, R., Taylor, D.J., 1992. Practical Neutron Radiography. EUR, Luxembourg). International Commission On Radiological Protection (ICRP), April 1971. Data for Protection against Ionizing Radiation from External Sources: Supplement to ICRP Publication 15. Pergamon Press ICRP-21. Jamro, R., Kardjilov, N., HairieRabir, M., Zain, M.R.M., Mohamed, A.A., Ali, N.M., Idris, F., Ahmad, M.H.A.R.M., Yazid, K., Yazid, H., Azman, A., 2017. Monte Carlo simulation for designing collimator of the neutron radiography facility in Malaysia. Phys. Procedia 88, 361–368. Kontogeorgakos, D., Stamatelatos, I.E., 2010. Validation of the Monte Carlo model of the Greek research reactor core. Nucl. Technol. 170 (3), 460–464. MacGillivray, G.M., Neutron Radiography Collimator Design. Nray Services Inc, RR# 1 Petawawa, Ontario, Canada. Milczarek, J.J., Trzciński, A., El Abd, A.E.G., Czachor, A., 2005. Monte Carlo code for neutron radiography. Nucl. Instrum. Methods Phys. Res. Sect. A Accel. Spectrom. Detect. Assoc. Equip. 542 (1–3), 237–240. Mishra, K.K., Hawari, A.I., Gillette, V.H., 2006. Design and performance of a thermal neutron imaging facility at the North Carolina State University PULSTAR reactor. IEEE Trans. Nucl. Sci. 53 (6), 3904–3911. Nazemi, E., Rokrok, B., Movafeghi, A., Dastjerdi, M.C., 2018. Simulation of a complete X-
5. Conclusions In this paper, an attempt has been made to propose a new graphite illuminator for neutron imaging facility of INUS with the advantage of having a more uniform and intense beam at the imaging screen. The MCNP code was utilized for simulating the neutron imaging facility of INUS which was including reactor core, radiation shield, neutron collimator and graphite illuminator. The shape of proposed illuminator is a cylinder that its one side is inclined. To evaluate performance of the new illuminator, three quality factors of thermal neutron beam uniformity, thermal neutron intensity and gamma radiation dose rate were considered. Three effective parameters of thickness, angle of inclined side and position of the illuminator inside the collimator were surveyed to achieve optimum illuminator shape. The investigation was done on thicknesses in the range of 5 to 25 cm with a step of 5 cm, angles in the range of 10 to 60° with a step of 10° and positions of −5, 0 and 5 cm with respect to center of reactor core. Finally, by comparing the obtained results for various parameters, it was realized that the proposed illuminator with a thickness of 10 cm, angle of 54.5° and position of 0 can generate a uniform beam profile at the imaging screen. In addition, it can improve the thermal neutron intensity up to 7.1% in comparison with the existing one without having a significant effect of gamma dose rate on the image. Although the proposed procedure in this paper was just done for special case of INUS neutron imaging facility, it can be applied for other 211
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