Special Nuclear Material detection studies with the SMANDRA mobile system
G. Viesti 1*, D. Cester 1, G. Nebbia 2, L. Stevanato 1,2, F. Neri 3, S. Petrucci 3, S. Selmi 3, C. Tintori 3, P. Peerani 4, A. Tomanin 4
1 Dipartimento di Fisica, Università di Padova-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
4 European Commission EC–JRC–ITU, Ispra, Italy
In most harbors worldwide inspection systems dedicated to the security of passengers and freight are based on fixed x-ray portals, radiation portal monitors and handheld radiometers. The SLIMPORT project, financed by the Italian Ministry of Industry, is dedicated to the development of an integrated package of tools forming a complete security system to monitor movement of persons and merchandise in harbors.
In this framework we have developed a mobile inspection station (called SMANDRA, Stazione Mobile per Analisi Non-Distruttive e RAdiometriche). The system is conceived as an instrument to perform non-destructive analysis, usable in conjunction with present monitoring devices such as radiation portal monitors, x-ray scanners and others. The aim of the system is to search and identify sources of ionizing radiation or identify dangerous and/or illegal materials inside volumes tagged as “suspect” by conventional surveys. The system is made of two pieces each weighting less than 50 kg and with volume less than 0.1 m3 as follows :
a- A passive unit including two gamma-ray detectors (5”x5” NaI(Tl) and 2”x2” LaBr) and two neutron counters (liquid scintillator and 3He proportional counter for fast/slow neutrons). The unit hosts batteries, power supplies, front-end electronics and CPU.
b- An active unit including a portable sealed neutron generator based on the Associated Particle Imaging (API) technique.
The first unit can be used in standalone mode as a high efficiency spectroscopic radiometer for the detection of ionizing radiation such as gamma-rays, fast and thermal neutrons to search and identify radioactive material as well as Special Nuclear Material (SNM). It can be used as well as detector package connected to the second unit for active interrogation of voxels inside a load by tagged neutron inelastic scattering imaging. SMANDRA will be a mobile system transported by a non-specialized light vehicle, easily operated by non-specialized personnel.
The front end electronics consists of a single VME card hosting FADC units (CAEN-V1720 8 Channel 12bit 250 MS/s Digitizer) used to perform digital pulse processing by FPGA. This allows one to obtain directly the energy of the detected events as well as to build coincidences and time measurements. Neutron/gamma discrimination for the liquid scintillator detector is also performed by the same module.
The passive unit a) passed laboratory test to verify the compliance with IEC standards.
The capability of the SMANDRA system in detecting special nuclear material has been explored with different Plutonium and Uranium samples available at the PERLA Laboratory of JRC-Ispra. After calibration with common gamma-ray and Californium sources, the SMANDRA mobile system was exposed to e set of different Plutonium sources that included both small (6 g) samples with enrichment between 61 and 93% in 239Pu and larger samples (up to 170 g). Most of the time the Pu samples were shielded with lead to suppress the very strong emission of 59 keV gamma ray from the 241Am isotope. Moreover, two uranium samples ( 50g with 90% and 400g with 10% enrichment on 235U, respectively) were also studied.
Gamma ray spectra taken with the 2” x 2” LaBr and the 5” x5” NaI(Tl) detectors were analyzed by using our software that automatically searches for peaks, subtracts the continuous part of the gamma ray spectrum and extracts the relevant parameters of the gamma lines.
Results demonstrated that even with the lead shield or for large samples with extremely high self-absorption, characteristic medium energy transitions (373, 414 keV in 239Pu and 662, 772 keV in 241Am) are clearly seen in the spectra. Moreover, the SMANDRA neutron detectors registered a strong signal decreasing, as expected, with the enrichment in 239Pu.
As for the U samples, the neutron signal is very low and requires long measuring times to reach a statistical significance. However, also in this case, characteristic gamma ray transitions are detected: the 186 keV from 235U for the high enrichment sample and 767 e 1001 keV from the 238U in addition to the 186 keV for the low enrichment sample. Active investigation tests using 14 MeV neutrons have been also completed. Results will be presented.