PRAMANA c Indian Academy of Sciences Vol. 85, No. 3 journal of September 2015 physics pp. 497 504 Nuclear fission as a tool to contrast the contraband of special nuclear material VIESTI GIUSEPPE 1, CESTER DAVIDE 1, NEBBIA GIANCARLO 2,, STEVANATO LUCA 1, NERI FRANCESCO 3, PETRUCCI STEFANO 3, SELMI SIMONE 3 and TINTORI CARLO 3 1 Dipartimento di Fisica ed Astronomia, Università di Padova, Via Marzolo 8, Padova I-35131, Italy 2 INFN Sezione di Padova, Via Marzolo 8, Padova I-35131, Italy 3 CAEN S.p.A., Via Vetraia 11, I-55049, Viareggio (LU), Italy Corresponding author. E-mail: giancarlo.nebbia@pd.infn.it DOI: 10.1007/s12043-015-1061-1; epublication: 25 August 2015 Abstract. An integrated mobile system for port security is presented. The system was designed to perform passive measurements of neutrons and γ -rays to search and identify radioactive and special nuclear materials as well as for the active investigations by using the tagged neutron inspection technique of suspect dangerous materials. The discrimination between difficult-to-detect uranium samples and high Z materials as lead was specifically studied. The system has been employed in laboratory detection tests and in a seaport field test. Keywords. Associated particle technique; fast neutron inspection; detection of special nuclear material. PACS Nos 28.20. V; 29.30.Kv; 89.20.Bb; 89.40. Cc 1. Introduction The SLIMPORT project, financed by the Italian Ministry for the Economic Development (MISE), was dedicated to the development of an integrated package of tools forming a complete security system to monitor transport of persons and merchandise in seaports. In this framework, a mobile inspection station (called SMANDRA, the Italian acronym stands for Sistema Mobile per Analisi Non Distruttive e RAdiometriche i.e., Mobile System for Non-Destructive Analysis and Radiometric Measurements) has been developed. The aim of SMANDRA is to search and identify sources of ionizing radiation by passive measurements of γ -rays and neutrons or to identify dangerous and/or illegal materials inside volumes tagged as suspect by previous conventional surveys such as Pramana J. Phys., Vol. 85, No. 3, September 2015 497
Viesti Giuseppe et al X-ray scans. Thus the SMANDRA system is a second line active inspection tool. The SMANDRA system consists of two units: (1) A passive unit consisting of two γ -ray detectors (5 5 NaI(Tl) and 2 2 LaBr(Ce) scintillation detectors) and two neutron counters (5 2 liquid scintillator and 3 He proportional counter for fast/slow neutron measurements). This unit hosts batteries, power supplies, front-end electronics and CPU. (2) An active unit consisting of a portable sealed neutron generator type TPA-17 from EADS-SODERN. The first unit can be used in standalone mode as a high-efficiency spectroscopic radiometer for the detection of ionizing radiation, to search and identify radioactive material as well as special nuclear material (SNM). It can also be used as detector package connected to the active unit for the interrogation of voxels inside a load by means of the tagged neutron inspection system (TNIS) technique [1]. The SMANDRA system has been fully described in [2] and only a short description is provided here. Laboratory results obtained so far are also summarized [3,4] discussing in detail the active detection of SNM. Finally, results from the recent field demonstration are presented. 2. The SMANDRA system The complete SMANDRA system during laboratory tests with SNM is shown in figure 1. The dual use of SMANDRA system (active and passive interrogations) sets stringent requirements: (1) Low background, high-efficiency detectors for γ -rays and neutrons, with the need to discriminate the two components of radiation in the passive mode use. (2) High count rate capability detectors to be operated in coincidence with the associated particle counter hosted in the neutron generator. For γ -ray detection, photon spectroscopy is performed by using both the highresolution LaBr(Ce) detector and the high-efficiency NaI(Tl) scintillator. The LaBr(Ce) detector offers the ultimate energy resolution for scintillators but the one used in SMANDRA has a limited volume compared to other scintillation materials. Moreover, LaBr(Ce) suffers from the internal activity that sets limits in the capability of detecting weak sources [3]. Consequently, the NaI(Tl) scintillator was selected to be used as a highefficiency device for the detection and identification of weak γ sources with a simple decay scheme, when the energy resolution is not required to discriminate γ transitions with similar energies. As for the neutron detectors, the 3 He proportional counter with a polyethylene moderator is a typical choice for such systems operated in passive mode. This counter provides the information about the total neutron yield without the possibility of discriminating fast from thermal neutrons. However, the direct detection of fast neutrons both in passive and active modes is an important task that justifies the use of liquid organic scintillator (a 5 2 cell). It is also worth mentioning that the time resolution of liquid scintillators is very important in performing active interrogations. 498 Pramana J. Phys., Vol. 85, No. 3, September 2015
Nuclear fission as a tool to contrast the contraband of special nuclear material Figure 1. The SMANDRA system during active interrogation of a highly enriched uranium sample placed in front of the detector unit. The box containing the EADS- SODERN TPA-17 neutron generator is shown on the left. In the SMANDRA system, both types of measurements (passive and active) are managed by a simple CAEN VME electronic front-end based on fast digitizers. The front end makes use of a battery-operated VME minicrate (four slots) with a Bridge USB V1718. The minicrate hosts a HV system (V6533 Programmable HV Power Supply (six Ch., 4 kv, 3 ma, 9 W) and a V1720 eight channel 12 bit 250 MS/s Digitizer. Inside the V1720, digital pulse processing (DPP) algorithms are implemented by using field programmable gate array (FPGA), providing online, the time stamp for each event, the complete integration of the signal, a partial integration of the signal used for pulse shape discrimination in the liquid scintillator and the possibility of storing a selected part of the digitized signal. The latter feature is required to reconstruct off-line coincidences and for the time measurements in active mode. Intense laboratory work has been carried out to characterize the detector performances with the VME front-end by comparing with the data obtained from conventional NIM electronics read-out. In particular, the NaI(Tl) and LaBr(Ce) detectors have been characterized in terms of energy resolution and pulse amplitude stability as a function of counting rate. Also LaBr(Ce) with digital signal processing has been operated up to a rate of 340 khz with excellent results [3]. The neutron-gamma discrimination is also performed online by the FPGA which provides both the total integration of the liquid scintillator signal (total light) and the integration of the prompt part of the distribution (prompt light). The ratio between the delayed light (obtained by the difference between the total and the prompt ones) and the total light is used to perform online pulse shape discrimination as a function of the total light [2]. Pramana J. Phys., Vol. 85, No. 3, September 2015 499
Viesti Giuseppe et al In active interrogations the associated particle detector signal is also processed in the V1720 card. The α-particles emitted in the 3 H( 2 H, 4 He)n neutron source reaction are indeed detected in a fast YAP(Ce) scintillation detector embedded inside the neutron generator, coupled to an external HAMAMATSU R1450 PMT. According to the results of laboratory-active interrogations performed so far, the associated particle detector covers a fraction of solid angle of about 10 3 such that a rate of 10 khz characterizes the operation of the neutron generator at a total flux of 10 7 neutrons/s. Under this condition, the spot of the tagged neutron beam at the object position (located 30 cm from the passive unit front face) has been measured to be about 15 cm (FWHM), depending on the acceptance of the YAP(Ce) detector. In the active mode operation we directly stored all the event singles processed by the V1720 card running at a typical total rate of about 50 khz recording the interesting part of the digitized signals. Offline software analyses the event files reconstructing the coincidence events and the time correlation between detectors. The time resolution depends on the way of handling the data. The best results have been obtained using a digital constant fraction discriminator [2] that allows one to obtain time resolutions of the order of about δt = 1ns(FWHM)for the LaBr(Ce) and δt = 5 ns (FWHM) for the NaI(Tl) detectors with the thresholds set at 500 kev. It is worth mentioning that a 5 ns time resolution reflects a depth resolution of about 25 cm for the inspected voxel for 14 MeV tagged neutrons. 3. Detection and identification of radioactive sources Laboratory tests have been carried out to verify the possibility of detecting the presence of radioactive material (γ -ray or neutron sources) and identifying the type of sources. As a guidance, the IEC 62327 standard for hand-held instruments for the detection and identification of radionuclides has been considered. A 3 s time lapse has been selected to verify the presence of alarms in NaI(Tl) for γ -rays, whereas a 10 s cycle is used for neutrons. SMANDRA detects a 0.4 MBq 60 Co source at 270 cm from the front face of the detector (with an equivalent dose of 20 nsv/h) and 0.4 MBq 241 Am at 80 cm from the front face of the detector (with an equivalent dose of 2.5 nsv/h) with PD = 90% at CF = 95%. This result needs to be compared with the IEC62372 requirement of detection for a source that produces 500 nsv/h at the front face of the detector. After the alarm, the identification of γ -source requires a measurement of 1 min. For neutrons, SMANDRA, after a proper energy windowing, detects in 10 s, the weak 252 Cf source placed at 140 cm from the detector surface with PD > 90% at CF = 95%, demonstrating a sensitivity about 60 times larger than required by the IEC62372. 4. Detection and identification of special nuclear material A campaign dedicated to the detection of SNM has been carried out at the PERLA Laboratory of the Joint Research Center of the European Commission in Ispra using several Pu and U samples having different enrichments and weights [4]. SMANDRA was used in passive as well in active modes. To summarize, the Pu samples were easily identified by their neutron emission and characteristic γ -ray signatures fully exploited in the LaBr(Ce) detector. 500 Pramana J. Phys., Vol. 85, No. 3, September 2015
Nuclear fission as a tool to contrast the contraband of special nuclear material The detection of U samples appeared difficult for which the neutron yield is quite low and the characteristic γ -ray signature is also low in energy (and then easily masked or attenuated by shielding). This is demonstrated in figure 2 where the effect of attenuation due to lead shielding is detailed for the characteristic γ -rays emitted froma1gsourceof weapon-grade plutonium (93% 239 Pu) and uranium (93% 235 Pu). In case of WGPu, the 239 Pu and 241 Am transitions (E γ = 414 and 662 kev) still have a yield of about 100 Hz after 2.5 cm of shielding making the detection possible at close contact. Consequently, it appeared interesting to study the detection of U samples in active interrogations. Several chemicals and uranium samples have been bombarded with typical measuring time of 10 min at a neutron total flux of 10 7 neutron/s. First, it is verified that inorganic or iron-based materials are easily identified by their well-known coincident γ -ray spectrum whereas the U samples cannot be discriminated from other heavy metals such as Pb that exhibit featureless coincident γ -ray spectra. It has been verified that the discrimination between U samples and other materials can be achieved by analysing the correlation between SMANDRA detectors. In particular, it was observed that when the liquid scintillator is used to separate γ -rays from neutrons, a good U discrimination is achieved on plotting the ratio between the triple (liquid scintillator NaI(Tl) YAP(Ce)) and the double (liquid scintillator YAP(Ce)) coincidences in γ -ray and neutron events [4]. Typical results are shown in figure 3. In other words, as the NaI(Tl) mainly detects γ -rays because of the difference in its intrinsic efficiency for photons and neutrons, we are plotting the probability of multiple γ events vs. the γ neutron coincidences, and this plot clearly separates U from other samples, including Figure 2. Gamma-ray yield from 1 g weapon-grade plutonium and uranium samples as a function of the thickness of lead shield: 241 Am, E γ = 662 kev (diamond); 239 Pu, E γ = 414 kev (square); 235 U, E γ = 186 kev (yellow triangle); 238 U, E γ = 1001 kev (green triangle). Pramana J. Phys., Vol. 85, No. 3, September 2015 501
Viesti Giuseppe et al Figure 3. Correlation between the triple (α-particle liquid scintillator NaI(Tl) detectors) and double (α-particle liquid scintillator detectors) coincidences for neutrons and γ -rays identified in the liquid scintillator. The square refers to iron, the diamond to lead, the cross to organic and the triangles to uranium (full triangle denotes 2.5 kg sample with 4.4% enrichment on 235 U, empty triangle denotes 2.5 kg sample with enrichment on 235 U). lead, as demonstrated in [4]. This was due to the presence of neutron-induced fission in U samples. More recently, we have also studied the effect of shielding on this type of discrimination by simply using a 252 Cf source to produce fission events. Results are reported in figure 4 relative to the unshielded source. It seems that using 1 or 2 cm lead shielding immediately inhibits the detection of γ -rays in the NaI(Tl) detectors thus lowering the triple/double ratio. This means that a shielded fission source will be easily confused with other materials. Figure 4. Dependence of the triples/doubles ratio for γ -rays and neutrons as a function of lead shield thickness when NaI(Tl) detector is used to build the triple events (squares). The triangles refer to a system in which NaI(Tl) is replaced by a liquid scintillator where only neutrons are selected. The data without shield are normalized to (1, 1) point whereas the other data points refer to 1 and 2 cm lead shields. 502 Pramana J. Phys., Vol. 85, No. 3, September 2015
Nuclear fission as a tool to contrast the contraband of special nuclear material A second test was performed by replacing the NaI(Tl) detector with a second liquid scintillator in which only neutrons are selected by the software. In this case, as the neutrons are scarcely attenuated by lead, the triple/double ratio of a fissile source remains close to that of an unshielded source. This evidence will guide us to design a new version of SMANDRA passive unit that is able to distinguish fission sources in the presence of different shielding. 5. Field demonstration The passive unit of the SMANDRA system has been employed recently in a field demonstration at La Spezia seaport (Italy) together with other participants to the task SlimChek of the SLIMPORT project, as documented in figure 5. The demonstration was directed to the National Firefighter Corp and was structured in the following way: (1) The SMANDRA system was used to determine the position of a weak radioactive source (about 20 kbq) located inside a shipping container and to identify the radioactive material. (2) After that a remote-controlled forklift entered the container to remove some pellets of materials around the source position. (3) Finally, a remote-controlled robotic arm entered the container for catching and transporting the source on a safety dump located outside the container. This demonstration was very successful. Figure 5. Pictures of the seaport demonstration. The SMANDRA system showing the robotic arm (a) and searching for radioactive source (b). Pramana J. Phys., Vol. 85, No. 3, September 2015 503
Viesti Giuseppe et al 6. Summary and conclusions The mobile SMANDRA inspection system has been tested in laboratory conditions for two distinct tasks: as a high sensitivity passive spectroscopic system and as a complete inspection system using tagged neutrons. Uranium samples are discriminated from non-fissile heavy elements by taking advantage of the large fission cross-section that significantly increases the possibility of detection for neutron γ -ray or neutron neutron coincidences. Results obtained so far demonstrate the good capability of the present prototype and will guide us in preparing a more advanced version of the system. References [1] S Pesente, G Nebbia, M Lunardon, G Viesti, S Blagus, K Nad, D Sudac, V Valkovic, I Lefesvre and M J Lopez-Jimenez, Nucl. Instrum. Methods B 241, 743 (2005) [2] D Cester, D Fabris, M Lunardon, S Moretto, G Nebbia, S Pesente, L Stevanato, G Viesti, F Neri, S Petrucci, S Selmi and C Tintori, IEEE Proc. ANIMMA, ISBN number 978-1-4577-0926-5, 6172933 (2011) [3] L Stevanato, D Cester, G Nebbia, G Viesti, F Neri, S Petrucci, S Selmi and C Tintori, Nucl. Instrum. Methods A 678, 83 (2012) [4] D Cester, G Nebbia, L Stevanato, G Viesti, F Neri, S Petrucci, S Selmi, C Tintori, P Peerani and A Tomanin, Nucl. Instrum. Methods A 663, 55 (2012) 504 Pramana J. Phys., Vol. 85, No. 3, September 2015