ISSN 10628738, Bulletin of the Russian Academy of Sciences. Physics, 2011, Vol. 75, No. 4, pp. 449–453. © Allerton Press, Inc., 2011. Original Russian Text © E.S. Konobeevsky, L.N. Latysheva, N.M. Sobolevsky, R.D. Ili c' , 2011, published in Izvestiya Rossiiskoi Akademii Nauk. Seriya Fizicheskaya, 2011, Vol. 75, No. 4, pp. 485–489.
Optimizing the Collimator/Shielding Configuration of the NG430 Neutron Generator E. S. Konobeevskya, L. N. Latyshevaa, N. M. Sobolevskya, and R. D. Ilic' b a
Institute for Nuclear Research, Russian Academy of Sciences, Moscow, 117312 Russia b VINCA Institute of Nuclear Sciences, Belgrade, 11001 Serbia email:
[email protected]
Abstract—Computer optimization of the collimator and shielding of the NG430 neutron generator at the Institute for Nuclear Research, Russian Academy of Sciences, is performed. The purpose of the optimization is to reach an acceptable value of the ratio Q between the neutron fluence in the investigated target (the signal) and the neutron fluence in the area of the detecting equipment (the background). The fluences of fast neu trons in the target and detectors are calculated by the Monte Carlo method. The influence of the walls of the experimental hall is taken into account. The optimal configuration of the assembly that provides the required Q value is found. DOI: 10.3103/S1062873811040277
INTRODUCTION The NG430 neutron generator at the Institute for Nuclear Research, Russian Academy of Sciences, is used for investigating the interactions of neutrons with fewnucleon systems at an energy of 14 MeV. Neu trons are produced in the 3H(dn)4He reaction. Deu terons accelerated to an energy of 430 keV interact with a solid tritium target. The deuteron current can reach a value of 20 mA. The thickness of the deposited tritium layer on the target surface is 1.5 mg cm–2. The activity of the target is 400–600 Ci. The intensity of the neutron source at the specified parameters reaches 1013 neutrons s–1. The NG430 neutron generator is currently being upgraded in connection with its moving to a new underground experimental hall. The purpose of this work is the numerical optimization of the collimator and shielding of the neutron generator to achieve an acceptable ratio between the useful signal and the background; below, this ratio is denoted by Q. The Q value is defined as the ratio between the neu tron fluence in the target (the signal) and the neutron fluence in the area of detectors (the background) (Fig. 1).The preliminary estimation of this value for the future experiments is Q ≥ 300. The parameters for optimizing the Q value are the collimator thickness along the neutron beam axis and the shielding thick ness around the detectors, collimator, and DT neutron source. The passage of neutrons through the collimator and shielding, taking into account the effect of the walls, floor, and roof of the experimental hall, was modeled by the Monte Carlo method using the MCNP5 [1] and SHIELD [2] transport codes. The
neutron fluences in the experimental hall’s roof were estimated to ensure radiation safety on the territory of the institute. LAYOUT OF THE EXPERIMENTAL HALL The horizontal and vertical sections of the under ground experimental hall in which the NG430 neu tron generator will be placed are shown in Figs. 1a, 1b, 1c. The hall has the following dimensions: the length along the neutron beam (the Z axis) is 14 m, the height (along the Х axis) is 6.5 m, and the width (along the Y axis) is 8 m. The hall is surrounded by concrete walls on all sides. The thickness of these walls is 1 m. The concrete wall along the positive direction of the Z axis is located at a considerable distance and is ignored in the calculations. Figure 1b (the vertical cross section of the hall) shows the roof of the experimental hall. It consists of layers of concrete and soil, both one meter thick. The surface of the soil layer is even with that of the ground of the institute’s campus. Figure 1c shows the shield ing made of borated polyethylene around the colli mator and detector area (cross hatching) and the area around the DT neutron source (grey area). COLLIMATOR CONFIGURATION The neutron collimator, with its thickness of L = 180 cm along the direction of the neutron beam, is shown in Fig. 2. The collimator is an assembly of rect angular boxes (parallelepipeds). The configuration is symmetric along the X and Y axes. In the center of the collimator is a cylindrical channel with a radius of 3 cm that forms the neutron beam. The boxes form
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beam were modeled. The thicknesses of the boxes in the collimator varied in proportion to the total thick ness of the collimator.
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Y Fig. 1. General plan of the experimental hall where the NG430 neutron generator will be placed: (a) horizontal section; (b) vertical section, (c) additional shielding made of borated polyethylene in the horizontal section. All dimensions are given in centimeters. S is the source of DT neutrons; C is the collimator of L thickness; T is the target; D is the detectors; W is the hall’s walls; G is the ground, P1 is the shielding of the collimator and detectors; P2 is the shielding of DT neutron source.
ing the collimator are made from different materi als: borated polyethylene, iron, concrete, and lead. In optimizing the collimator configuration, several variants with different thicknesses L along the neutron
LAYOUT OF DETECTORS The detectors in which the neutron fluence was scored consist of concentric cylindrical layers mounted with a pitch of 20 cm and a thickness along the beam axis of 10 cm; the distance between layers is 15 cm (see Fig. 1). Altogether, there are eight volumes arranged along the radius (N = 1–8) and five layers along the beam axis. The area in which the detectors are mounted extends from the collimator’s surface to 120 cm along the direction of the neutron beam and from 20 to 180 cm in the radial direction. The target is a cylinder with a radius of 3 cm and a length of 6 cm. In modeling the passage of neutrons, both the target and the detectors were filled with air. The neutron fluences in the target and in the detectors were calculated by track length estimation. Only neutrons with energies of Еn ≥ 1 MeV were scored, as was dictated by the detecting conditions in the future experiments. RESULTS FROM SIGNAL/BACKGROUND ESTIMATES The signal/background ratio Q was optimized in several steps because we had to introduce additional shielding around the detecting area and the DT neu
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tron source in order to ensure an acceptable Q value and permissible radiation conditions at the roof of the experimental hall. In the first stage of modeling, only the collimator’s thickness L along the neutron beam axis was varied: L = 74, 108, 144, 180, 216 cm. The dependences of the Q value on the detector number N in the radial direction for three detector positions along the beam axis (labeled as Z1, Z2, and Z3) are shown in Fig. 3. These positions correspond to distances from the col limator’s surface along the beam axis of 8, 33, and 83 cm, respectively. The Q values for various collima tor thicknesses L are marked by different symbols. From the presented curves, we can see that the Q value varies from 100 to 1000. The Q < 300 values were not acceptable for further experiments, so geometry optimization aimed at reaching Q > 300 values was required for all of the considered cases. From Fig. 3, we also can see that for a collimator with a thickness of L = 216 cm, the Q values are lower than for other variants. In the case of L = 72 cm, the Q values are comparable to the values for the other variants at large Z, but are appreciably lower than for L = 108 cm at low Z and N. This is why variants with collimator thicknesses of L = 108, 144, and 180 cm were chosen for further optimization of the neutron generator. In the second stage of modeling, a shielding layer of borated polyethylene with a thickness of 20 cm was introduced around the collimator and the detecting area (see Fig. 1c) in order to increase the Q values. The results are shown in Fig. 4. We can see that after plac ing shielding around the collimator and detectors, the Q values lie in the interval of 250–2700 and are accept able for most detector positions in further experi ments. In the final stage of modeling, we studied the pos sibility of further increasing the Q values and investi gated the dependence of Q on the thickness of the shielding layer around the collimator and detectors. We considered a case where the collimator’s thickness was L = 108 cm. The thickness of the shielding layer was varied in the range of 5–40 cm. In addition, the DT neutron source was surrounded by borated poly ethylene layers with thicknesses of 20 and 40 cm (see Fig. 1c) in order to protect the detector from albedo neutrons from the walls of the hall. The Q values for the above geometries are shown in Fig. 5. The black markers indicate the Q values for the model in which the shielding is only around the colli mator and detectors. The model in which there is full shielding around the collimator, detectors, and the DT neutron source is shown by the white markers. From Fig. 5, we can see that adding a layer of shielding around the DT neutron source increases the Q values by a factor of 2–5 and ensures high values of Q for all detector positions. For instance, when there is shielding with a thickness of 20 cm around the colli mator, detectors, and DT neutron source, the Q value
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Q (a) 1000 800 600 400 200 0 600 (b) 500 400 300 200 100 0 300 (c) 250 200 Thickness of collimator 72 cm 100 108 cm 144 cm 50 180 cm 216 cm 0 1 2 3 150
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Fig. 3. Dependence of Q on the detector number in the radial direction N in the (a) Z1, (b) Z2, and (c) Z3 positions along the beam axis when there is no shielding around the collimator, detecting area, or neutron source.
varies in the range of 600–5000. For a similar case but with a thickness of 40 cm, the Q value varies from 600 to 24000. NEUTRON FLUENCE AT THE ROOF OF THE EXPERIMENTAL HALL The roof of the experimental hall where the neu tron generator will be placed is at the same level as the
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Fig. 4. Dependence of Q on the detector number in the radial direction N in the (a) Z1, (b) Z2, (c) and Z3 positions along the beam axis when there is shielding 20 cm thick around the collimator and detecting area.
Fig. 5. Dependence of Q on the detector number in the radial direction N in the (a) Z1, (b) Z2, and (c) Z3 positions along the beam axis when there is shielding of various thicknesses around the collimator, detecting area, and neutron source. The collimator’s thickness is L = 108 cm.
surface of the ground in the institute campus. It is nec essary to ensure the permissible radiation conditions at the institute when operating the neutron generator. In this work, the neutron fluences at the hall’s roof were estimated during modeling. Estimates were made for neutrons of all energies from thermal to 14 MeV.
Figure 6 shows a schematic of the detecting layers on the surface of the hall’s roof. Altogether, there are 10 layers 135–150 cm in size along the beam axis and 10 cm thick laterally across the hall. The asterisk rep resents the projection of the neutron source onto the roof’s plane.
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Neuron fluences (neutrons cm–2 s–1) of all energies (En > 0) through the roof zone at a neutron source intensity of 1013 neutrons s–1 Shielding/zone no. 0 cm 10 cm 20 cm 40 cm 20 cm, total 40 cm, total
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The neutron fluences in the detecting layers at an intensity of the DT source of 1013 neutrons s–1 are listed in the table. The neutron fluences for when there is shielding made of borated polyethylene around the detectors and collimator are shown in the first four rows of the table. It is clear that in this version of the geometry, the neutron fluences in some layers on the roof’s surface reach values of 20 neutrons cm–2 s–1, which is higher than the permissible limits. In the last two rows of the table, the neutron flu ences are shown for when there is shielding around the detecting area, collimator, and DT neutron source (see Fig. 1a). In these versions of the geometry, the neutron fluences at the roof decrease to a maximum value of 6 neutrons cm–2 s–1 when the thickness of the shielding is 20 cm, and to 0.5 neutrons cm–2 s–1 at a shielding thickness of 40 cm. These parameters corre
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spond to permissible radiation conditions on the insti tute campus. CONCLUSIONS We have optimized the configuration of the NG 430 neutron generator at the Institute for Nuclear Research, Russian Academy of Sciences. Our aim was to reach acceptable values of the signal/background ratio Q in the detecting area of further physical exper iments. The neutron fluences at the roof of the under ground experimental hall in which the neutron gener ator will be placed were calculated in order to estimate the radiation conditions on the institute’s campus. The modeling was performed by the Monte Carlo method using the MCNP5 and SHIELD transport codes. When the thickness of the collimator along the neutron beam is ~110 cm, the Q value is around 1000, which is quite sufficient for further experiments. Shielding the detector and collimator with a borated polyethylene layer 10–40 cm thick increases the Q value to 2700. Additionally shielding the DT neutron source with a borated polyethylene layer 20–40 cm thick increases the Q value to 104 and more, while ensuring permissi ble radiation conditions on the institute’s campus.
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Fig. 6. Arrangement of detectors on the roof of the experi mental hall where the NG430 neutron generator will be placed. The dimensions are given in centimeters. The pro jection of the neutron source on the plane of the roof is indicated with an asterisk.
REFERENCES 1. MCNP—A General Monte Carlo NParticle Transport Code, Ver. 5, vol. II: User’s Guide, LACP–03–0245, April 24, 2003. 2. http://www.inr.ru/shield
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