Design, testing and optimization of a neutron radiography system based on a Deuterium–Deuterium (D–D) neutron generator K. Bergaoui, N. Reguigui, C. K. Gary, J. T. Cremer, J. H. Vainionpaa & M. A. Piestrup Journal of Radioanalytical and Nuclear Chemistry An International Journal Dealing with All Aspects and Applications of Nuclear Chemistry ISSN 0236-5731 J Radioanal Nucl Chem DOI 10.1007/s10967-013-2729-y
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Author's personal copy J Radioanal Nucl Chem DOI 10.1007/s10967-013-2729-y
Design, testing and optimization of a neutron radiography system based on a Deuterium–Deuterium (D–D) neutron generator K. Bergaoui • N. Reguigui • C. K. Gary • J. T. Cremer • J. H. Vainionpaa • M. A. Piestrup
Received: 6 March 2013 Ó Akade´miai Kiado´, Budapest, Hungary 2013
Abstract Simulations show that significant improvement in imaging performance can be achieved through collimator design for thermal and fast neutron radiography with a laboratory neutron generator. The radiography facility used in the measurements and simulations employs a fully high-voltage-shielded, axial D–D neutron generator with a radio frequency driven ion source. The maximum yield of such generators is about 1010 fast neutrons per seconds (E = 2.45 MeV). Both fast and thermal neutron images were acquired with the generator and a Charge Coupled Devices camera. To shorten the imaging time and decrease the noise from gamma radiation, various collimator designs were proposed and simulated using Monte Carlo N-Particle Transport Code (MCNPX 2.7.0). Design considerations included the choice of material, thickness, position and aperture for the collimator. The simulation results and optimal configurations are presented. Keywords Thermal neutron radiography Fast neutron radiography D–D Neutron generator Monte Carlo simulation (MCNPX) Collimator CCD camera
K. Bergaoui (&) N. Reguigui Unite´ de Recherche ‘‘Maıˆtrise et De´veloppement des Techniques Nucle´aires a` Caracte`re Pacifique’’, National Center of Nuclear Sciences and Technologies, Technopole Sidi Thabet, BP 72, 2020 Tunis, Tunisia e-mail:
[email protected];
[email protected] C. K. Gary J. T. Cremer J. H. Vainionpaa M. A. Piestrup Adelphi Technology Inc., 2003 East Bayshore Road, Redwood City, CA 94063, USA
Introduction Neutron radiography (NR) is an important tool in nondestructive examination, which has been adopted in industrial, medical, metallurgical, nuclear and explosive inspections [1]. From the 1990s to the present day, neutron radiography imaging systems and the methods used to analyse the images have continued to advance [2]. It offers some very explicit advantages over c-ray (or X-ray) imaging. Neutron cross-sections, being almost independent of the atomic number (Z) of the material, result in neutron imaging being capable of discerning materials of similar Z and/or low Z materials even when they are present inside high Z surroundings. Also, hydrogen, which is a very important element in determining the properties of materials, can be imaged even if present in minute quantities due to its significant neutron scattering and absorption cross sections. Neutrons also offer the advantage of being able to differentiate between isotopes of an element. Furthermore, radioactive materials which cannot be imaged using photons due to fogging of the detector can be imaged with neutrons using the transfer technique [3]. The neutron radiography facility discussed consists of three major components: a neutron source, neutron collimator and image processing system (Fig. 1). The neutron collimator is a critical component in neutron radiography because it affects directly the characteristics of the neutron beam and the quality of the radiographic image [4, 5]. Thermal and fast neutron images were acquired using a D–D neutron generator and image intensified Charge Coupled Devices (CCD) camera focused on a neutron sensitive scintillator. While these images demonstrate the ability of laboratory based neutron radiography, they could benefit from decreased imaging time and reduced background. The
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Author's personal copy J Radioanal Nucl Chem Fig. 1 The principle of a neutron radiographic facility (D: diameter of the aperture next the neutron generator and D0: diameter of the aperture next to the image plane)
neutron flux, spatial collimation and neutron to gamma ration (neutron/gamma) can be improved through collimation. A range of collimator designs were considered and modelled using Monte Carlo code Version MCNP 2.7.0 [6].
Materials and methods Neutron generator D–D The deuterium–deuterium 2.5 MeV neutron generator developed by Adelphi Technology Inc. [7], uses a deuteron beam produced with radio frequency (RF) plasma and acceleration voltage in the range from 100 to 140 keV (Fig. 2). The deuterium ion beam was directed at the titanium-covered cathode, which adsorbs deuterons and works as a deuterium target [8]. The neutron source operates in continuous mode, producing up to 1010 neutrons per second, though an intensity of 109 n/s was used in the measurements presented. Due to the physics of D–D reaction, the generated neutrons have a well-defined energy maximum close to 2.5 MeV, slightly dependent on the emission angle. The energy and this anisotropy were included in the MCNP simulations for the moderator design. The effective fast neutron source size is 2–3 cm [9].
The dual MCP light amplifier and a coupled CCD chip are thermoelectrically cooled to -20 °C. A tapered fiber optic directs the amplified light to the CCD sensor. The dual MCP amplifier produces high photon gains up to 106 at a very low dark count levels of 1 or 2 electron counts per frame at 15 frames per second. In our experiments the dual MCP amplifier was set to an overall photon amplification gain of 5 9 105. The Stanford Photonics CCD camera uses a Sony XX285 scientific grade image sensor, which has a full frame pixel count of 1,380 by 1,024 K, where the pixels are 6.47 microns square. The 1.6–1 reducing fiber optic taper creates an effective square pixel with 10.35-lm sides at the input image plane. The dual MCP image intensifier has a 50 line-pairs/mm resolution, which corresponds to the effective pixel size. An image is created by summing multiple frames. The short 15-s frame times reduce the dark count accumulation to 1 or 2 dark counts per frame, and the data acquisition software further reduces
The CCD detector A camera box allows the Stanford Photonics CCD camera and its electronics to be placed outside the direct neutron beam exposure. A scintillator is mounted on the inner surface of the 3 mm wall of the box. A mirror reflects light from the scintillator out of the neutron/gamma path to a 5 cm diameter lens with f-stop equal to 1.0. The lens focuses the light onto the dual microchannel plate (MCP).
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Fig. 2 The D–D-110 neutron generator
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noise by filtering out cosmic ray noise and spot noise from each of the incoming frames. The radiography system The design of the collimator will directly affect fundamental properties of the resulting neutron beam. The design choices that are made must recognize the relationship between all of the various components and features. The key issues in the design of any facility for neutron imaging include the neutron flux and L/D ratio [10]. Collimator parameters The parameters most important to the quality of the image data are the ratio of the collimator length to diameter of the aperture (Collimation Ratio (L/D)) [11], Beam Divergence [11], Gamma content [12] and the thermal neutron content (TNC) [10]. Collimator design Geometric Shape of the Collimator: The divergent beam collimator is commonly used after the conclusion of Barton in 1967 that divergent beam collimators produce highest resolution [13]. Among them the most commonly used physical form is a truncated cone or pyramid [11]. In this study, we used a divergent beam collimator. Materials of the Walls and their lining: The most important item of each collimator is its lining. Unlike charged particles, neutrons cannot be focused [11]. Hence the neutron beam must be collimated by suitable lining in the collimator. To prevent stray neutrons from reaching the radiographed object and to reduce the scattering of neutrons within the collimator, the lining must be made with neutron absorbing material. Some materials suitable for this purpose are: boron, cadmium, dysprosium, europium, gadolinium and indium [11]. The effectiveness of these materials varies with the neutron energy spectrum. The use of boron is recommended because it gets less activated, which facilitates maintenance of the collimator [11]. Generally boron in the form of Boral (B4C) is used in the collimator [13]. Borated Polyethylene (PE-B) (2.5 cm) and Lead (1 cm) are chosen as our collimator wall material. Gamma and Neutron Filters: To filter out the gamma rays from the beam, Lead (Pb) and Bismuth (Bi) filters are used [11]. Neutron filters are also required to filter out the fast neutrons from the beam. Single crystal sapphire (Al2O3), silicon, quartz and Bismuth act as fast neutron filters [14, 15]. Sapphire (Al2O3) is an effective fastneutron filter because its transmission for neutrons of
wavelengths less than 0.04-nm (500 MeV) is less than 3 % for a filter thickness of 100-mm [14]. For gamma filter we used Lead for its effectiveness in attenuating the gamma and its durability. Bismuth may be used as the gamma filter; it is usually more expensive than Lead.
Experimental neutron radiography configuration In this experimental step, a D–D neutron generator was used to illuminate a set of objects, and a scintillator coupled to a CCD camera was used to record thermal and fast neutron images. The generator was housed in a polyethylene block 61 cm high and 61 cm deep. A 15-cm 9 10cm square opening extended from the wall of the generator to the edge of the polyethylene block with a length of 20 cm. This opening served as the collimation for the experiment. The set of objects that were imaged is shown in Fig. 3. The objects were chosen to provide varying contrast for fast and thermal neutrons as well as gammas and X-rays. A 4-step lead phantom was used to provide contrast for X-rays and gammas. Polyethylene blocks (2.5 and 1.3 cm thick) provide contrast for fast and thermal neutrons. Beryllium, in the form of a biconcave lens, has a cross section that is relatively larger for fast neutrons. Electrical tape, due to the presence of hydrogen as well as higher Z materials has good contrast for both thermal neutrons and gammas. The smaller 6.25 and 12.5-mm steps of the lead were eclipsed the bottom half of roll of black electrical tape The 2.5 cm and 5 cm thick portion of the 4-step lead phantom covers a 1.25 cm thick polyethylene plate. On top of the lead phantom and eclipsing the 1.25 cm thick polyethylene is a 2.5 cm thickness of the 4-step silicon stacked-chip phantom. Silicon has a low cross section for all the radiation measured, but is significantly more sensitive to photons than neutrons. This variety of materials, and the use of know thicknesses provides qualitative information similar to that from more expensive calibrated beam purity and sensitivity indicators. In the fast neutron radiography we placed the object at a distance of 1.52 meter from the D–D generator. A 4-cm thick BC-408 plastic scintillator was placed immediately behind the objects to convert the fast neutron signal to visible light imaged by the CCD camera. For the thermal neutron radiography we placed 2.5 cm of polyethylene before the object to moderate the fast neutrons and the 10 cm 9 10 cm by 5 mm thick plastic scintillator on the inner wall of the camera box was replaced by a 10 cm 9 10 cm by 100 micron thick gadolinium-based, thermal neutron phosphor, Gd2O2S:Tb, which produced light upon thermal neutron capture by the gadolinium.
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second was a divergent collimator (L = 50 cm, D = 1 cm and D0 = 12 cm). The walls were made of (PE-B) with depth 2.5 cm covered by 1 cm of Lead. In the third stage (3), we studied the influence of gamma and neutron filters on the parameters associated with neutron radiography. In our study, we tested sapphire (Al2O3) for fast neutron filtering and lead for gamma filtering (Figs. 6, 7). In the fourth stage (4), we fixed the best configuration from the last stages and we calculated fth, TNC and (n/c) parameter with a variable collimator length (L), diameter of the aperture (D) and diameter of the aperture next to the image plan (D0). Fast neutron radiography
Fig. 3 Set of objects that are imaged by thermal and fast neutron radiography. Assorted objects were 4-step lead and silicon-chip Rose phantoms, beryllium biconcave neutron lens, black electrical tape, 1.25 and 2.5 cm polyethylene blocks
Simulation model Thermal neutron radiography Simulations of the thermal neutron radiography facility were implemented in four stages: In the first stage (1), we tested the best position of the divergent collimator (the length of the collimator L = 50 cm, diameter of the aperture D = 1 cm and the aperture next to the image plane D0 = 12 cm), from the neutron generator with a variable thickness of polyethylene, between the neutron generator and the aperture of the collimator (Fig. 4). In the second stage (2), different configurations were considered to obtain the maximum thermal neutron flux (fth) at the image plane (Fig. 5), the first configuration was composed of a divergent collimator (L = 50 cm, D = 1 cm and D0 = 24 cm) and walls made of 2.5-cm deep (PE-B) covered by 1 cm of lead. The walls started 15 cm from the neutron generator. The second configuration was similar as in the first with walls starting 40.35 cm from the neutron generator. The third configuration was composed of a divergent collimator (L = 50 cm, D = 1 cm and D0 = 12 cm), 2 cm of Polyethylene between the aperture of collimator and the neutron generator and the walls made of (PE-B) with depth 2.5 cm covered by 1 cm of Lead. The fourth configuration included two collimators. The first was a conic collimator with a length of 13 and radii 4 and 0.5 cm [16]. The larger radii was close to the source, with 2 cm of Polyethylene between the aperture of collimator and the neutron generator. The
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In the fast neutron radiography facility the quality of the NR image is determined by the collimator ratio (L/D) in a similar way as in the case of thermal neutron flux. The imaging quality of a fast neutron radiography facility would be further characterized by the beam quality profile, described by the number of uncollided fast neutrons that reach the detector position within the neutron beam [16]. The fast neutron radiography facility was examined in two stages: In the first stage (1), different configurations are considered to obtain the maximum fast neutron flux at the image plane (Fig. 7). The configurations of the collimator are shown in the Fig. 8. The first configuration was composed by divergent collimator (L = 50 cm, D = 1 cm and D0 = 12 cm). The second configuration was composed by divergent collimator (L = 50 cm, D = 1 cm and D0 = 12 cm) and the walls were made of (PE-B) with depth of 3 cm covered by 1 cm of lead. Configuration # 3 we used divergent collimator and the walls are made of iron with depth 3 cm covered by 1 cm of Lead. In the second stage (2), we calculated fast neutron flux, and uncollided fast neutrons parameters with a variable collimator length, diameter of the aperture D and diameter with D0 = 24 cm.
Results and discussion A thermal neutron image of the assorted objects was acquired with a 17 min exposure using an intensity of 109 n/s, and is shown in Fig. 9a. The fast neutron image shown in Fig. 9b was acquired in a longer 67 min exposure. The relatively long image times can be shortened by using the full output of the generator (which requires greater shielding for personnel safety), but improvements to the geometry can reduce exposure times without increasing radiation dose. Importantly, significant degradation of the image quality was observed due to gamma radiation
Author's personal copy J Radioanal Nucl Chem
Fig. 4 MCNP model of the D–D generator with moderator, shielding and collimator for various polyethylene thicknesses between the collimator and the generator
background [17]. The thermal neutron image shows absorption of some of the signal by the lead, and the contrast for the tape, which contains higher Z elements in addition to hydrogen is equal to or greater than that for the polyethylene, while one would expect the polyethylene to have greater contrast for a pure thermal beam. Significant neutron radiation is present as can be seen from the contrast from the polyethylene and beryllium, both of which have very small gamma absorption, especially in comparison to the lead. Similarly, in the fast neutron image, gammas are indicated by lead contrast, but the presence of fast neutrons can be seen from the beryllium and especially silicon contrast. Some of the contrast from the polyethylene and tape could be due to thermal neutrons. A better collimator system would reduce the background noise and improve the Single/Noise radio (SNR) and
thereby improve resolution and contrast. Thus, the modeling effort concentrated on increasing the neutron flux and improving the n/c ratio to reduce quantum noise and gamma background. The goals of the computer modeling of the fast and thermal neutron radiography system were to optimized useful neutron flux for a given source intensity, provide a large L/D for good resolution, and minimize the stray neutron and gamma background, while providing sufficient shielding for radiation protection. Polyethylene and paraffin were used for neutron shielding and lead for gamma shielding. For the setup shown in Fig. 4 the neutron flux at the image plane in the case (L = 50 cm, D = 1 cm and D0 = 22 cm) as a function of collimator position is given in Fig. 10. The maximum fth occurs for a (0 cm) of polyethylene between the aperture and the neutron generator.
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Fig. 5 MCNP model of the thermal neutron radiography facility with different types of collimators
Fig. 6 MCNP model of the thermal neutron radiography facility with the sapphire fast neutron filter
Fig. 7 MCNP model of the thermal neutron radiography facility with the Lead gamma filter
This position (0 cm) was used in all successive designs presented. The neutron flux at the image plane for different configurations of the collimator, as shown in Fig. 5, was calculated. These results are given in Fig. 11. The best configuration is number # 2 corresponding to maximum fth at the image plane. Neutron energy spectrum calculated at aperture and image plane for the configuration # 2 of neutron generator based thermal neutron radiography facility is shown in (Fig. 12).
For the configuration # 2 we fixed (L = 50 cm and D = 1 cm) and we varied the value of D0 (see Table 1). We choose the value of D0 = 26 cm as the optimal condition since it yields the maximum TNC and (n/c) parameter while maintaining a large neutron flux. We also calculated the fth, (n/c) parameter and TNC at the image plane with neutron fast filer sapphire (Al2O3) (see Table 2). The presence of the sapphire filter decreases the thermal neutron flux fth, and the (n/c) parameter, but increases the TNC. The same result was reported by Fantidis [18].
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3 cm B-PE + 1cm of lead
(a) Configuration 1
6 cm of iron + 1cm of lead
3 cm of iron+ 1cm of lead
(b) Configuration 2
8 cm of iron
+ 1cm of lead
(c) Configuration 3
8 cm of iron + 3 cm B-PE+ 1cm of lead
(d) Configuration 4
(e) Configuration 5
(f) Configuration 6
8 cm of Tungsten + 3 cm B-PE + 1cm of lead
6 cm of iron + 3 cm B-PE 1cm of lead
6 cm of iron + 3 cm B-PE + 1cm of lead
(g) Configuration 7
(h) Configuration 8
(i) Configuration 9
Fig. 8 MCNP model of the fast neutron radiography facility with different types of collimators
For the presence of lead as a gamma filter at the collimator, the thermal neutron flux (fth), TNC and the (n/c) parameter decrease (Table 3), except in the case where the thickness of lead was equal to 5 cm, which resulted in a slight increase in (n/c) parameter. In the optimization process of the collimator, the collimator length (L) and the inlet aperture (D) were varied in
order to attain the values for which the maximum thermal flux is obtained in the image plane. These results are given in Table 4. The object to detector distance was considered to be 0.5 cm. An energy boundary of 0.01–0.3 eV was used to define the thermal neutron flux. The fth would vary from 2.38 9 102 up to 3.29 9 104 n cm-2 s-1 for a maximum neutron yield by the generator
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Author's personal copy J Radioanal Nucl Chem Fig. 9 Radiographs of the objects taken using D–D neutron generator based thermal neutron (a) and fast neutron (b) radiography facility
10 5
0 cm of Polyethylene 2 cm of Polyethylene 4 cm of Polyethylene 6cm of Polyethylene
104
Neutron Flux (ncm-2s-1)
Neutron Flux (ncm-2s-1)
105
103
102
10 4
Configuration 1 Configuration 2 Configuration 3 Configuration 4
10 3
10 2
1E-8 1E-7 1E-6 1E-5 1E-4 1E-3
0,01
0,1
0,5 1
2
5
Neutron Energy (MeV)
1E-8 1E-7 1E-6 1E-5 1E-4 1E-3
0,01
0,1
0,5 1
2
5
Neutron Energy (MeV)
Fig. 10 The Neutron flux at the image plane in the case of L/D = 50 cm for various polyethylene thicknesses
Fig. 11 The Neutron flux at the image plate in the case L/D = 50 cm for different configurations of the collimator
of 1010 ns-1. The TNC varies from 8 to 33 %. The fth is comparable with fluxes from low power research reactors [19, 20]. These values are higher in comparison to those calculated assuming a yield of 1011 ns-1 by Fantidis [16]. In this work we used a different collimator configuration. For all the values of L and D considered, the ratio (n/c) was higher than 106 (n/cm2 mR). The values for neutron flux; the ratio (n/c), and TNC are higher than another study by Fantidis et al. [2] that used a 50 mg 252Cf source for neutron radiography system and achieved fh equal to 1.57 9 104 ncm-2 s-1, the ratio (n/c) = 9.71 9 103 ncm-2 mSv-1 and the TNC = 0.51 % in the case of (L/D = 50 cm). The present results are also higher than those from a SbBe neutron source (fth = 6.13 9 102 ncm-2 s-1, TNC = 21.2 % and (n/c) = 51.0 cm-2 Sv-1 in the case
of (L/D = 50 cm) reported by Fantidis et al. [21], and those reported for an 241Am–Be neutron source (fth equal to 7 .83 9 103 cm-2 s-1 with (n/c) ratio of 5.72 9 1012 cm-2 mSv-1) reported by Jafari and Feghhi [22]. In the case of the fast neutron radiography facility, we present in Table 5 the value of fast neutron flux for each of the collimator configurations. The best configuration is number 6 corresponding to the maximum fast neutron flux, equal to 1.08 9 105 ncm-2 s-1. The fast neutron flux at the image plane varies from 4.76 9 102 to 1.83 9 105 ncm-2 s-1 (Table 6). These values are comparable with low power research reactors [23], and are some were higher in comparison to those found for a neutron generator with a yield of 1011 ns-1 according to Fantidis et al. [16].
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Author's personal copy J Radioanal Nucl Chem Table 1 The thermal neutron radiography parameters with variable aperture next the image plan D0 for L/D = 50 cm (Abreviations: D0: diameter of the aperture next the image plan; L: length of collimator; D0 (cm)
D (cm)
L (cm)
D: diameter of the aperture next the neutron generator; fth: thermal neutron flux; TNC: Thermal neutron content and n/c: the neutron to gamma ratio) fth (ncm-2 s-1)
D/L (cm)
n=c (n/cm2 mR)
TNC (%)
12
1
50
50
7.02E?03
19
1.07E?06
18
1
50
50
1.46E?04
27
1.86E?06
22
1
50
50
1.98E?04
31
2.13E?06
26 30
1 1
50 50
50 50
2.21E?04 2.29E?04
33 32
2.83E?06 2.37 E?06
40
1
50
50
2.62E?04
31
2.63E?06
Table 2 The thermal neutron radiography parameters with variable sapphire filter length for L/D = 50 cm Sapphire filter (cm)
fth (ncm-2 s-1)
TNC (%)
Table 4 The thermal neutron radiography parameters for different L/D ratio
n=c (n/cm2 mR)
L
D
L/D
fth (ncm-2 s-1)
TNC (%)
n=c (n/cm2 mR)
25
5.91E?05
0
2.21E?04
33
2.83E?06
Experimental collimation
2
2.21E?04
32
2.39E?06
150
6
2.10E?04
35
2.31E?06
Optimized collimation
10
1.87E?04
42
2.08E?06
150
15
10
3.25E?04
27
1.65E?06
14
1.42E?04
43
1.62E?06
50
1
50
2.21E?04
33
2.83E?06
18
1.02E?04
45
1.23E?06
50
2
25
2.36E?04
28
2.48E?06
22
7.69E?03
49
1.02E?06
50
4
12.5
3.29E?04
24
3.16E?06
75
1
75
4.00E?03
22
1.60E?06
75
2
37.5
5.96E?03
23
2.41E?06
75 100
4 1
18.5 100
7.78E?03 1.22E?03
17 16
2.80E?06 1.10E?06
100
2
50
1.65E?03
13
100
4
25
1.97E?03
100
6
125
1
125
2
125
4
125 125 150
1
150 150 150
Table 3 The thermal neutron radiography parameters with variable Lead filter length for L/D = 50 cm Lead filter (cm)
fth (ncm-2 s-1)
TNC (%)
n=c (n/cm2 mR)
0
2.21E?04
33
2.83E?06
5
9.20E?03
27
4.19E?06
10
4.95E?03
23
3.71E?06
15
3.89E?03
29
3.15E?06
20
1.91E?03
10
7
21
1.52E?06
At image plane At aperature plane
Neutron Flux (ncm-2s-1)
106
105
104
103 1E-8 1E-7 1E-6 1E-5 1E-4 1E-3
0,01
0,1
0,5 1
2
5
Neutron Energy (MeV)
Fig. 12 The Neutron flux calculated at aperture and image plane for the configuration 2 of neutron generator based thermal neutron radiography facility
15
10
9.25
1.85E?06 1.89E?06
3.80E?03
11
2.72E?06
3.81E?02
9
1.34E?06
62.5
8.34E?02
13
4.14E?06
31.4
8.94E?02
8
1.79E?06
6
20.83
1.73E?03
10
3.26E?06
8
15.62
3.13E?03
11
4.38E?06
150
2.38E?02
13
1.12E?06
2
75
2.97E?02
9
1.49E?06
4
37.5
6.78E?02
11
2.51E?06
6
25
9.73E?02
8
2.55E?06
18.75
1.33E?03
8
3.38E?06
15
2.02E?03
8
4.13E?06
150
8
150
10
16.66
3.06E?03
125
Tables 4 and 6 also include a comparison of the arrangement used in the experimental images with a similar setup (same L/D) using an optimized collimator. As can be seen from the first two rows Table 4, the collimator slightly increases the thermal neutron flux and TNC, while greatly reducing the gamma background. From Table 6, it can be seen that the collimator increases the uncollided neutron flux. Greater gains in image quality can be anticipated from changing to other L/D ratios as evident from the results in the tables.
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Author's personal copy J Radioanal Nucl Chem Table 5 Fast neutron flux at the image plan with different collimators configurations
Number of configurations
Fast neutron flux Ffast (ncm-2 s-1)
Configuration 1
4.29E?04
Configuration 2
4.44E?04
Configuration 3
6.11E?04
Configuration 4
8.20E?04
Configuration 5
8.95E?04
Configuration 6
1.08E?05
Configuration 7 Configuration 8
5.52E?04 4.12E?04
Configuration 9
7.01E?04
Table 6 The fast neutron radiography parameters for different L/D ratio L
D
L/D
Ffast (ncm-2 s-1)
Uncollided Ffast (ncm-2 s-1)
lG (cm)
1.88E?04
0.05
experimental arrangement. The optimized design for thermal neutron radiography provides a thermal neutron flux of 3.29 9 104 ncm-2 s-1 at the image plane using an L/D ratio of 12.5. The resulting neutron to gamma ratio is 3.16 9 106 n/cm2 mR. For the fast neutron radiography facility the maximum neutron flux corresponding to the optimal configuration and L/D = 12.5 cm provided a fast neutron flux equal to 1.83 9 105 ncm-2 s-1 and an uncollided fast neutron flux 7.55 9 104 ncm-2 s1. The next stage of this project will be to construct the proposed collimators and measure the flux, gamma rejection and resolution that can be achieved. Acknowledgments This paper was developed under (IAEA TUN2003 project) ‘‘Installation of neutron activation analysis laboratory based on a neutron generator’’. The authors would like to thank Dr. Fantidis G. and Dr. Nicolaou GE., from the University of Thrace, Xanthi, Greece for their help and also would like to thank the Radiation Safety Information Computational Center (RSICC) for providing the MCNP code.
Experimental collimation 150
15
10
3.69E?04
Optimized collimation 150
15
10
6.26E?04
2.19E?04
0.05
50
1
50
1.08E?05
3.06E?04
0.010
50
2
25
1.34E?05
4.85E?04
0.020
50
4
12.5
1.83E?05
7.55E?04
0.040
100
1
100
9.67E?03
3.46E?03
0.005
100 100
2 4
50 25
1.35E?04 2.75E?04
7.51E?03 1.68E?04
0.010 0.020
100
6
16.66
3.71E?04
1.98E?04
0.030
150
1
150
2.66E?03
1.18E?03
0.003
150
2
75
5.16E?03
2.87E?03
0.006
150
4
37.5
1.06E?04
6.27E?03
0.013
150
6
25
1.3E?04
8.75E?03
0.020
150
8
18.75
2.06E?04
9.01E?03
0.026
150
10
15
2.07E?04
9.02E?03
0.033
200
1
200
5.63E?02
4.76E?02
0.002
200
2
100
2.19E?03
1.39E?03
0.005
200
4
50
5.68E?03
3.89E?03
0.010
200
8
25
9.78E?03
5.05E?03
0.020
200
10
20
1.08E?04
5.15E?03
0.025
Conclusions A fast and thermal neutron radiography facility has been constructed using a laboratory neutron generator. To improve image quality, a collimator structure has been designed using the MCNPX to reduce background noise and increase the thermal and fast neutron flux. The collimator provides significant improvement in neutron flux and a decrease in background radiation compared to the
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