POSITION SENSITIVE DETECTION OF CONCEALED SUBSTANCES EMPLOYING PULSED SLOW NEUTRONS

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1 POSITION SENSITIVE DETECTION OF CONCEALED SUBSTANCES EMPLOYING PULSED SLOW NEUTRONS R. E. Mayer A. Tartaglione, J. J. Blostein, M. Schneebeli, P. D Avanzo, L. Capararo Centro Atómico Bariloche and Instituto Balseiro (GAEN) Comisión Nacional de Energía Atómica and Universidad Nacional de Cuyo mayer@cab.cnea.gov.ar also at CONICET Summary Identification of pure substances and later simple mixtures, were carried out at conveniently short counting times. Position definition of an isolated sample object was achieved through slow neutron time of flight. Although several substances were employed, emphasis was placed on chlorine and nitrogen. Scientific scope of project under the CRP Develop nuclear techniques in support of existing screening capacity for bulk cargo shipments. It is aimed at identifying elemental composition associated to explosives and discerning licit from illicit materials traffic, through neutron interactions yielding information on the spatial location by neutron time-of-flight. Seek this objective by means of testing the applicability of moderated neutrons, paying special attention to their influence in position definition uncertainty and employing existing laboratory facility during development stage. Plans for next 8 months Proceed to test the already installed capacity to identify sample position and composition, currently limited to a few chemical elements and low space resolution. This next step must record experimental spectra of complex arrays of matrix elements that enclose the sample substance. Validation of a Montecarlo scheme useful to represent the neutron time-of-flight spread of the moderated neutron pulse responsible for the position definition uncertainty of a detected volume of substance.

2 Given a sample substance included in a concealing matrix, carry out experimental tests of the usefulness of a complementary irradiation with fast neutrons which undergo moderation inside this matrix and later induce neutron-gamma processes. The time-of-flight position identification by means of externally moderated neutrons is thus lost when switching to internal moderation in the matrix, in exchange for the possible addition of a capacity to detect the sample substance hidden in a highly shielding moderating environment. Results To simulate a lower intensity neutron source, the experimental set-up was positioned at least five meters away from the target of a 25 MeV linear electron accelerator (LINAC) of the traveling wave type. This set-up poses the draw-back of not allowing to irradiate a high volume sample, for the neutron beam is 50 millimeter diameter in all cases. The low intensity falling on the sample is in the order of 200 thermal n/cm 2 sec and 90 near epithermal n/cm 2 sec (above cadmium cut-off energy) with a 40 mm thick polyethylene slab neutron moderator. Some initial success has been attained in adapting the existing facility to serve as the neutron source for the combined neutron time-of-flight and gamma-prompt experiments (early activation decay photons of short lived isotopes, included). Gamma detection at the initial step was carried out by only one 2 x2 NaI(Tl) commercial gamma detector. Those first results [] prompted to construct a more appropriate array consisting of four commercial 2 x2 NaI(Tl) scintillators including their original PMTs with ad hoc voltage dividers and only one appropriate preamplifier. This adaptation was necessary to reduce the initial dead time after each accelerator burst. Under appropriate shielding, it was possible to sort-out the gamma response to neutron interactions in the presence of a combined pulsed high energy X- ray and neutron field. A number of spectra were recorded for several substances with the initial experimental geometry described through a gamma detector highly coupled to the sample. This laboratory style situation was far from a realistic approach to container scanning, but yielded a number of initial results useful throughout the following more realistic stages. The recording times were purposely restricted to 5 minutes and the displayed data are in all cases background subtracted (with neutron monitor normalization). Two approaches were tested to seek a more trustworthy substance identification with low resolution gamma detectors, through the recording of gamma spectra induced by neutrons of different known energy ranges, as presented in []. First, the cadmium difference method was 2

3 used to show the gamma response to moderated epithermal neutrons and the response to thermal neutrons (black symbols), shown in Figs. through 4. thermal neutron absorption epithermal neutron absorption thermal neutron absorption epithermal neutron absorption ee + aniq.; 5 kev 32 S; 840,993 kev 35 Cl; 57,073 kev ee + aniq; 5 kev 35 Cl; 788,8 kev a. u. 35 Cl; 64,86 kev ( 35 Cl; 60,072 kev ) 35 Cl; 95,4 kev E E [kev] E [kev] Fig.. Sulfur 40 g Fig.2. Chlorine in NaCl 25 cm 3 ee + anih.; 5 kev thermal neutron absorption epithermal neutron absorption 56 Fe; kev(de) 56 Fe; kev( se) 56 Fe; kev Ag - 7,45 kev 09 Ag - 98,72 kev Ag Th - Bg Th Ag Epi - Bg Epi E-6 56 Fe; k ev E Ag - 235,62 kev Fe; kev Ag - 549,56 kev 56 Fe; 352,35 kev E [kev] E γ [kev] Fig.3. Fe sample 6 mm thick Fig.4. Silver 25 mm thick sheet Irradiation with (near) epithermal neutrons (energy above cadmium cut-off) induce in samples a gamma response markedly different from that due to thermal neutrons (energy below cadmium cut-off). Fig.4 shows that effect in silver-09 which leads to expect the possibility of increased certitude in identification of a particular elemental component in the spectrum of a complex substance, through the recording of both kinds of spectra. 3

4 The next step was carried out with the improved four 2 x2 NaI(Tl) scintillators, setting the emphasis on the time of flight (TOF) capacity to identify the sample position in more realistic conditions as concerning distances and surrounding materials which tend to hide the sample substance in some cases. This array was also employed to test the Hydrogen (water) Iron Nitrogen and iron Iron only Water and iron Nitrogen selected ROI capacity to detect nitrogen, and to measure its proportion respect to hydrogen content and respect to iron, through the N/H and Nitrogen N/Fe count ratios. An example of a liquid N 2 (LN 2 ) sample set among surrounding Iron selected ROI objects of water and iron, is displayed in The N/Fe ratios obtained for several successive determinations with and without N 2 are displayed in Fig.6 in the order they were measured at close range. Recording times were 5 minutes. The sensitivity to N 2 presence is clear E [MeV] Fig.5. PHA gamma spectra: water, Fe and N2. Hydrogen E (MeV) Fig Nitrogen ROI/Iron ROI Iron and nitrogen Iron without nitrogen Iron and water "time" Fig.6. Ratios of areas (ROI) of Fig.6 One other situation tested comprised the LN 2 sample hidden among cooking flour bags. The corresponding pulse height spectra (PHA) are shown in Fig.7. The Flour only Flour and nitrogen Background presence of the N 2 sample is not completely obscured by the flour E E [MeV] Fig7. PHA gamma spectra. N2 and flour surrounding, where even its own N is seen by the experiment when the LN 2 is removed from the neutron beam. In these determinations the volume of LN 2 irradiated by the direct beam (neutrons) was 25 cm 3. 4

5 Concerning sample position determination through moderated neutron TOF, Figs. 8 and 9 show the sensitivity evinced experimentally, using the 4 NaI(Tl) array and kg NaCl sample. 6 kg NaCl LARGE MOVEMENT 740 µs kg NaCl SMALL MOVEMENT 890 µs m flight path 7 m flight path 8 m flight path 680 µs 0 cm 0 cm -0 cm 5 cm -5 cm Time-of-flight [µs] Fig.8. TOF spectra. 2m range variation Time-of-flight [µs] Fig.9. TOF spectra. 0.3m range variation Recording times were 0 minutes at all distances. Further investigation into the subject led to test once more the cadmium difference method. The last figure shows these differences for the kg NaCl sample at two positions 50 cm apart. The epithermal tail arising from the 4 cm thick neutron moderator, to the left, at short TOF values, vanishes, thus leaving a somewhat clearer picture as concerning sample position. Figs.9 and 0 were recorded at 6 m moderatorsample distance GAMMAS FROM THERMAL NEUTRONS 4 cm polyethylene slab NaCl 30 cm from detector NaCl 80 cm from detector Time-of-flight [µs] Fig.0. TOF spectra. Cd difference method Final Remarks The above described flux allowed the experiments on the samples with only one gamma detector, to be performed during 5 min counting times at close range and later experiments 5

6 with a modest gamma detector array, to be carried out at realistic distances over 5 and 0 minutes recording times. The feasibility of this thermal/epithermal substance identification resource, greatly depends on the trade-off between number and disposition of gamma detectors and source intensity not restricted to a collimated beam. The position determination capability through slow neutron TOF is not sufficient for imaging, but it is convenient to give an idea of the place in a container where to look for a suspect object, while if the substance is dispersed the TOF spectrum will be broad, indicating that situation. The recording of complex spectra from real mixtures of substances is work still to be done, in order to assess the real capability to contribute with a new capacity to act in collaboration with other scanning tools, for the fast screening of unknown objects. Aknowledgements Financial support has been received from project 06/C98 from Universidad Nacional de Cuyo. Reference [] IAEA Proceedings Series, STI/PUB/300, 2007, ISBN