Radiation Portal Monitor with 10 B+ZnS(Ag) Neutron Detector Performance for the Detection of Special Nuclear Materials

Transcription

1 Radiation Portal Monitor with 10 B+ZnS(Ag) Neutron Detector Performance for the Detection of Special Nuclear Materials Karen A. Guzmán-García 1,*, Hector Rene Vega-Carrillo 2, Eduardo Gallego 1/2 Juan Antonio González 3, Roberto Méndez 4, Alfredo Lorente 1/3 & Sviatoslv Ibañez-Fernandez 1/4 1 Departamento de Ingeniería Energética, ETSI Industriales Universidad Politécnica de Madrid, C. José Gutiérrez Abascal 2, Madrid, Spain 2 Unidad Académica de Estudios Nucleares, Universidad Autónoma de Zacatecas C. Ciprés No , Zacatecas, Zac. Mexico 3 Laboratorio de Ingeniería Nuclear, ETSI Caminos Canales y puertos Universidad Politécnica de Madrid, C. Prof. Aranguren 3, 28040, Madrid Spain 4 Laboratorio de Patrones neutrónicos. CIEMAT, Av. Complutense 40, 28040, Madrid Spain * ingkarenguzman@gmail.com Abstract In homeland security, neutron detection is used to prevent the smuggling of Special Nuclear Materials. Thermal neutrons are normally detected with 3 He proportional counters, in the Radiation Portal Monitors, RPMs, however due to the 3 He shortage new procedures are being studied. In this work Monte Carlo methods, using the MCNP6 code, have been used to study the neutron detection features of a 10 B+ZnS(Ag) under real conditions inside of a RPM. The performance for neutron detection was carried out for 252 Cf, 238 U and 239 Pu under different conditions. In order to mimic an actual situation occurring at border areas, a sample of SNM sited inside a vehicle was simulated and the RPM with 10 B+ZnS(Ag) response was calculated. At 200 cm the 10 B+ZnS(Ag) on RPM response is close to 2.5 cps-ng 252 Cf, when the 252 Cf neutron source is shielded with 0.5 cm-thick lead and 2.5 cm-thick polyethylene fulfilling the ANSI recommendations. Three different geometries of neutron detectors of 10 B+ZnS(Ag) in a neutron detection system in RPM were modeled. Therefore, the 10 B+ZnS(Ag) detectors are an innovative and viable replacement for the 3 He detectors in the RPM. Keywords: Special nuclear materials, Neutron, Radiation portal monitors, MCNP6 72

2 1.- INTRODUCTION Since terrorist attacks in the United States in September of 2001, heightened concern with regard to critical infrastructure security and methods necessary for guaranteeing the safety of the general public [Spence 2011]. The Radiation Portal Monitors (RPMs) deployed at ports, railways, airports and vehicle checkpoints, used to screen in vessel, vehicles, cargo and individuals in order to thwart the illicit trafficking of SNM [Weltz et al., 2015]. The RPMs have played important role in preventing the illicit trafficking and transport of nuclear and radioactive materials [Kwak et al., 2010]. With an increase in the capabilities and sophistication of terrorist networks worldwide comes a corresponding increase in the portability of radiological or nuclear devices being detonated in any place of the world. One method to decrease the risk associated with this threat is to interdict the material during transport of goods. Current RPMs have limitations in their ability to detect shielded nuclear materials [Spence 2011]. But also still play an important role in national security. Current RPM deployed at borders are generally equipped with two types of radiation sensors; high efficiency gamma detectors commonly based on plastic scintillators of Poly- Vinyl-Toluene (PVT), and neutron detectors that are exclusively based on helium-3, 3 He proportional counters, consisted of 3 He tubes surrounded in polyethylene for neutron detection, and plastic scintillators for gamma ray detection [Peerani et al., 2012]. Neutron detection is important for homeland security efforts, including monitors national point of entry the presence of special nuclear material (SNM), which is defined as plutonium and uranium enriched in 233 U or 235 U and is a fissile component of nuclear weapons [Weltz et al., 2015; Kouzes et al., 2009]. Most current neutron detection systems used for nuclear security are based on 3 He (technology) proportional counter, these are robust, gamma insensitive, and remain an efficient, proven technology for detecting thermal neutrons. However, a global shortage of 73

3 3 He has caused the price to surge and limits the future supply of these neutron detectors, stimulating the development of alternative neutron detection technology [Weltz et al., 2015; Peerani et al., 2012]. The aim of this work is to study the neutron detection features of the 10 B+ZnS(Ag) detectors, using Monte-Carlo neutron-transport, with the MCNP6 code under real conditions inside on RPMs, in border areas, with a vehicle-based simulation in similarly to the RPMs benchmark simulation, in order to study the performance for each detector for detect a neutron source of 252 Cf, and SNM, (HUE 70% 238 U) and 239 Pu under two different conditions; bare and shielding, for each detection system. 74

4 2.- MATERIALS AND METHODS 2.1.-Description of the N-15, N-48 and N-15 plus of 10 B+ZnS(Ag) First of all three different detectors are going to be explained, they are detectors of 10 B+ZnS(Ag), N-15 ndetbrick, N-48 and the N-15plus. The N-15 and N-48 were manufactured by BridgePort Instruments, LLC. Both use a mixture of 10 B high enrichment, with a scintillator detector ZnS(Ag), 10 B+ZnS(Ag), as neutron detection. The screens are on polymethyl methacrylate, PMMA, with twofold, as a neutron moderator and as light guide, guide the pulses to a photomultiplier tube embedded high voltage supply, and a multichannel analyzer emorpho digital electronics. The sensitive area of each detector is formed by 5 transparent layers of 10 B+ZnS(Ag) deposited on 4 plates of PMMA of 23 x 36 x cm ( N-15), 120 x 15.2 x cm (N- 48). The PMMA acts as light guide and as moderator. All is surrounded by ~8µm thick aluminum mylar as light reflector. The internal configuration is shown in Figure 1. Figure 1.- Internal configuration N-15 and N-48 75

5 These detectors has similar geometry, both are rectangular and with the same internal array. The external size of the N-15 detector is 23 x 36 x 4 cm, and the external dimensions of the N- 48 detector are x 16.7 x 6.35 cm. Each detector has an outer moderator t of highdensity polyethylene, (0.94gr/cm 3 ), HDPE. For the N-15 detector the moderator thickness is 24 mm in the front, lateral faces, and top, bottom 36 mm while 48 mm-thick in the back ( mm). The N- 48 is 25 mm-thick in front, top, bottom, and lateral faces, while 50 mm-thick in the back (25+50 mm) as shown in Figure 2. Figure 2.- Configuration N-15 and N-48 detectors, bare and moderated. A new model of neutron detector was studied; prototype, [Guzmán-García et al., 2016] based on previous studies of the N-15 neutron detector, and was called N-15 plus, which is an improvement geometry respect to the manufactured detector, N-15 ndetbrick. The difference respect to the actual N-15 is the PMMA thickness, from to cm and the sensitive area has 30% of 10 B higher than the actual detector, The internal configuration can be seen in Figure 3. 76

6 Figure 3.- N-15 plus model MCNP configuration. In summary, the N-15 and N-48 are detectors manufactured and with similar internal geometry varying in the detection area and the N-15plus detector is an MCNP6 model based on previous studies from the N-15 detector [Guzmán-García et al., 2016]. These neutron detectors are studied as an alternative to the 3 He detectors actual installed in the RPM, for this reason, models in RPM were made Monte Carlo Calculations Radiation Portal Monitors, RPMs, models description Using MCNP code version 6.1 [Pelowitz et al., 2014], models of three neutron detections system for RPMs were built including all the each detectors details such as: the sensitive layers, the PMMA, PTM and their moderators, where the PTM was modeled as ta empty cylinder of glass. Each neutron detections systems (N-15, N-48 and N-15plus) are described below. A model was made with the detector N-15 ndetbrick, three N-15 detectors were positioning in the parallel inside on an iron structure of 200 x 40 x 15 cm dimensions, each detector has the moderator of mm HDPE, the first detector (N-15C) was positioned to 75cm from the midpoint from the detector respect to the ground, Figure 4. 77

7 Figure 4.- N-15, Neutron detector System. Other model was made the detector N-48, was modeled together with a PVT gamma detector of 190 x 41 x 5 cm dimenstions, surrounded by aluminum and shielding with lead and polyethylene, both, N-48 neutron detector and PVT gamma detector are inside on an iron structure of 235 x 80 x 18 cm dimensions, this configuration can see in Figure 5. Figure 5.- Radiation Portal Monitor, RPM, PVT gamma and N

8 The N-15plus model were complemented defining three N-15plus detectors with two PVT gamma detectors of 40 x 60 x 4 cm, the PVTs surrounded by aluminum and a lead/polyethylene shield, all inside on an iron structure of 200 x 90 x 15 cm, the N-15 plus detectors were positioning in parallel, the first detector were positioned to 75 cm from the midpoint from the detector respect to the ground, Figure 6. Figure 6.- Radiation Portal Monitor, RPM, two PVT gamma and three N-15plus detectors Model of vehicle with SNM in customs. A vehicle-based simulation was performed similarly to the RPMs benchmark simulations, each RPM described above were defined as an emplaced stationary detector objects pass by the monitor, RPM, and are screened for the presence of radioactive materials. In this case we suppose cargo scanning scenario, and we defined a vehicle passing on the RPMs. The vehicle was defined as the Annex A, informative for the test vehicle [CEI-IEC 2004]. The dimensions of the box are 530 x 270 x 200 cm, and the cab has 120 x 250 x170 cm. In the box different materials were defined as a scrap, with ~730 kg, these scrap materials are steel, cooper, bronze, lead, zinc, aluminum, brass, and iron, simulating a neutron source of 252 Cf or SNM, HEU or 239 Pu. Scrap materials can see in Table 1. 79

9 Table 1.-Scrap, different material in the box inside the vehicle Density Weight Material [g/cm 3 ] [kg] Steel Steel Cooper Bronze Lead Zinc ~ Aluminum ~ Brass Iron ~ TOTAL ~ In the vehicle center in the box in the middle of the scrap were defined a sphere of ~1.87cm, ~500 g of U, 70% enriched HEU (2,144n/s) [Spence 2011], first bare, and then shielded in concentric spheres of polyethylene and lead. Other case was a sphere of ~1.87cm, ~515 g of 239 Pu (40,000 n/s ) [Perani et al., 2012], first bare, and then shielded in concentric spheres of polyethylene and lead. Other case was a neutron source of 252 Cf moderated with 0.7 cm of HDPE. The different materials in the box and these materials were in randomly defined position and Figure 7. Figure 7.- Scrap and the source 80

10 The sources are characterized by a Maxwell energy distribution. In the use of MCNP, the Watts fission spectrum is used an alternative to the Maxwell fission spectrum for induced fission of the 252 Cf, 239 Pu and 238 U. The Watts fission spectrum is modeled by the following equation: ( ) ( ) ( ( ) (1) In order to determine the H*(10) received by a driver when transport these type of materials in the interior of the cab. The driver was defined as the BOMAB Phantoms [Nuclear Technology Services, Inc 2016] that is an acronym for Bottle Manikin Absorber, theses BOMAB are designed to imitate, as nearly possible, human tissue for the purpose of calibrating whole body counters, these phantom consists on the cylinders of various shapes (circular and elliptical) dimensions which together comprise the average person. High density polyethylene, approximately 4.76 mm tick, is used in all parts of the phantom the dimensions are shown in the Table 2. Figure 8. Figure 8.- BOMAB phantom 81

11 Table 2.-BOMAB Phantom dimensions Quantity Shape Cross section (cm 2 ) Vertical height (cm) Head 1 Ellipse 19x14 20 Neck1 Circle 13Ø 10 Thorax 1 Ellipse 36x20 20 Lumbar 1 Ellipse 36x20 20 Thighs 2 Circle 16 Ø 40 Legs 2 Circle 13 Ø 40 Arms 2 Circle 10.5 Ø 60 For each case in three different positions, first in front of the N-15 detection systems, then in front of the RPM-N-48, and finally in the RPM-N-15plus, all these model can see in Figure 9. In all the Monte Carlo calculations the number of histories was large enough to obtain uncertainties less than 5%. For the calculations, the cross sections were taken from the ENDF/B-VI library, where the S(α, β) treatment was included to take into account the thermalized neutron interactions [Vega-Carrillo et al., 2014]. 82

12 Figure 9.- Vehicle-based model with all RPMs in 3 different positions. 83

13 3.- RESULTS Table 3 shows the calculated detectors response for each source in the three positions described above, the response are the amount of 10 B(n,α) 7 Li reactions per neutron form the source. Table 3.- Results of each position and with the different materials System Neutron detector system three N-15 ndetbrick detectors Source Position 1 Position 2 Position 3 conditions 252 Cf* 2.31 ± ± ± 0.09 HEU** 2.59 ± ± ±0.10 HEU Shield 1.54 ± ± ± Pu*** ± ± ± Pu Shield ± ± ± Cf* 1.36 ± ,77 ± ± 0.02 LEFT Radiation Portal Monitor with N- 48neutron detector and PVT gamma detector RIGHT Radiation Portal Monitor with N- 48 neutron detector and PVT gamma detector HEU** 1.37 ± ± ± 0.02 HEU Shield 0.82 ± ± ± Pu*** ± ± ± Pu Shield ± ± ± Cf* 1.35 ± ,79 ± ± 0.02 HEU** 1.35 ± ± ± 0.02 HEU Shield 0.81 ± ± ± Pu*** ± ± ± Pu Shield ± ± ± Cf* 0.53 ± ± ± 0.04 Radiation Portal Monitor with N- 15plus and two PVT gamma detector HEU** 0.54 ± ± ± 0.04 HEU Shield 0.31 ± ± ± Pu*** ± ± ± Pu Shield 6.78 ± ± ± 0.05 Detector in front the source N-15 ndetbrick N-48 N-15 plus *1 ng 252Cf produces 2,340 n/s [Vega-Carrillo 1988] **500g HEU 70% produces 2,144 n/s [Spencer 2011] ***500g 239Pu produces 40,000 n/s [Perani 2012] 84

14 The MCNP results are by emitted neutron, so in each case the results were multiplied by the corresponding intensity source, in 252 Cf for nano-gram of californium, HEU 70% for 500 g and 239 Pu for 515 g. Table 4 shows the comparative of each neutron detection system respect to each source, when the vehicle pass in front of them. Table 4.- Comparative (RPMs) N-15, N-48 and N-15plus Description Source conditions N-15 ndetbrick (three neutron detectors) N-48 (right) (one neutron detector) N-15 plus (three neutron detectors) 252 Cf* 2.31 ± ,79 ± ± 0.04 Comparative with three different systems when the source is in front of each HEU** 2.59 ± ± ± 0.04 HEU Shield 1.54 ± ± ± Pu*** ± ± ± Pu Shield ± ± ± 0.05 Table 5 shows the H*(10) for neutrons, for the driver for each case of positions and materials described in the model. The MCNP results are by emitted neutron, and are gives in psv/s, the conversion was made for give the results in µsv/h, in each case the results were multiplied by the corresponding intensity source. 85

15 Table 5.- H*(10)neutrons calculated for each material Section 252 Cf µsv/h n (10-4 ) HEU µsv/h n (10-4 ) HEU Shield µsv/h n (10-4 ) 239 Pu µsv/h n (10-4 ) 239 Pu Shield µsv/h n (10-4 ) Head (1) 4.94 ± ± ± ± ± 1.2 Neck (1) 4.61 ± ± ± ± ± 2.0 Thorax (1) 3.17 ± ± ± ± ± 0.45 Lumbar (1) 3.11 ± ± ± ± ± 0.46 Legs (2-2) ± ± ± ± ± 46 Arms (2) 7.83± ± ± ± ± 3.3 TOTAL ± ± ± ± ±62 86

16 4.- DISCUSSION Table 3 shows the calculated response of three neutron detection systems inside a RPM, the N-15, N-48 and N-15plus, in counts per neutron multiplied by the intensity of each source, the neutrons source was located at 200 cm from the detector, in the position 1 the N-15 detector was in the center and we can see than in this case is the more efficient, and the same behavior occur for each RPM when the source is just in front of the neutron detectors in each system, in this table we can see the response of each RPM en all positions. Table 4, is in summary all response only when the neutron source is just in 200 cm in front and we can see that the N-15plus, RPM, shows the best response, in efficiency, and it can detect a 2.5cps/ng 252 Cf as the ANSI required [ANSI 2006], the model propuse of improvement of N-15 can be detect 2.5 cps/ng 252 Cf. Table 5, shows the H*(10) ambient dose equivalent due to neutrons calculated, when the driver transport, a neutron source of 252 Cf+HDPE, a Sphere of HEU and 239 Pu, bare and shielded by borated polyethylene and lead. In the case that the driver unaware that the box contain some nuclear material mixed in the scrap. The Marjory received dose is when the vehicle transport 239 Pu, and in all cases in the legs received the more H*(10). 87

17 5.- CONCLUSIONS The increased probability of nuclear attack and the scarcity of the elemental material of 3 He for the neutron detectors, forced to seek interesting alternatives. In this paper three neutron detectors have been studied in similar situation of operation, these models are based in previous studies [Guzman-Garcia et al., 2016a; Guzman-Garcia et al., 2016b]. The reactions in 10 B was calculated using Monte Carlo methods with MCNP6 code, each total response were adjusted with their own efficiency factor determined previously, to determine the counts rates in each case. The 10 B+ZnS(Ag) detectors are an interesting alternative to replace 3 He detectors in RPMs. The N- 15 detector is considered suitable for portable backpack systems. The N-48 detector is close to be considered a replacement for 3 He detectors in RPM. But the N-15 plus as an improvement in the geometry of the detector raising the amount of 10 B and the PMMAthick, increases the detector efficiency aiming to reach 2.5 cps/ng 252 Cf, defined in the ANSI standards a goal to use this type of detectors as an alternative in RPMs [ANSI 2006]. Acknowledgments The first autor, K.A. Guzmán-García, thanks the scholarship granted by the CONACyT and the COZCyT, Mexico. 88

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