INTENSE PULSED NEUTRON GENERATION TO DETECT ILLICIT MATERIALS.

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1 INTENSE PULSED NEUTRON GENERATION TO DETECT ILLICIT MATERIALS. C B Franklyn Radiation Science Department, Necsa, PO Box 582, Pretoria 0001, South Africa. Fax: chris.franklyn@necsa.co.za 1.Introduction The project to develop an intense pulsed neutron generator has evolved over the three year research period from a small scale technologically unproven system to a large scale technologically proven facility. The challenges experienced during this time were numerous and in most instances overcome or by-passed. Prospects for continued success of the project are currently still balanced on a knife-edge, but the knowledge and experience gained will be preserved for the benefit of the scientific community within South Africa. This report describes most aspects of the project development both from a technical and knowledge base perspective. 2. Intense pulsed neutron source development. 2.1 Plasma immersion ion implantation method. The original project description was for the development of an intense pulsed neutron generator based on the principle of the plasma immersion ion implantation (PIII) technique. This was based on original calculations by Uhm 1 indicating that, with a sufficiently powerful pulsed power supply, D-D fusion neutrons with a flux in excess of neutrons per second could be generated. Feasibility studies had been performed with a lower power (25 kv 2.5 Joule) PIII system, however it was realized that considerable untested technology in magnetic pulse compression was required to operate a system at 100 kv, 10 3 Joule. Although it was still considered feasible, limited human resources became a major constraint in continuing this avenue of research and development. It was therefore decided to shelve this approach in favour of an alternative technology opportunity that became available to Necsa. 1

2 It is also interesting to note that a similar plasma technique, inertial electrostatic confinement, as promoted especially by J Sved of NSD-fusion 2, has great potential although still very much in the development stage in terms of demonstrating reliably intense neutron flux yields. 2.2 Accelerator based neutron source. The use of accelerators to generate fast neutrons has been available for many decades with impressive intensities generated, especially by spallation neutron sources. Of interest to the project has been the development of more compact accelerator based intense neutron sources, especially systems that could be easily relocated as required. In the mid 1990 s the South African mining industry embarked on a technology research and development project, in which Necsa participated, to facilitate more efficient detection and extraction of precious minerals from rock using nuclear techniques. A technique identified was based on fast neutron resonance radiography, which required the development of a robust, intense fast neutron source where the neutron energy could be selected, as well as novel fast neutron detection techniques. Such a system was developed, based on the principle of radio frequency quadruple (RFQ) linear accelerators. Due to changing market patterns the technology was, in 2007, disbanded by the mining industry, which provided an opportunity for Necsa to take over the facility and apply the technology towards this IAEA project and more generally undertake basic and applied research for the benefit of the community. RFQ accelerators have proven to be useful tools for generating intense beams of ions of specific energy. Two RFQ accelerator systems, specifically designed to accelerate deuterium ions, are now being operated at PLABS and are referred to here as the ADM and D-100. Although the systems are similar, differences in their operation characteristics are illustrated in Table 1. Both systems function under the same basic principle, consisting of two independent RFQ acceleration cavities, one cavity capable of adding 4 MeV of kinetic energy to a deuteron, the other cavity capable of supplying a further 1 MeV of kinetic energy. 2

3 Features ADM D-100 operating frequency (MHz) output energy (MeV) maximum beam pulse width (ms) repetition rate (Hz) maximum RF duty factor 1.2 % 20 % pulsed RF power requirement (kw) 280/ /200 linac length (m) Average extracted beam current (ma) Neutron flux from D 2 target (n.s -1 ) TABLE I. Operating specifications of the two accelerator systems The novelty of coupling the two cavities is encompassed in the selection of the relative phase of the RF power in the second cavity with respect to the first cavity. The effect is to retard, act neutrally, or accelerate the beam as it traverses the second cavity, resulting in a range of mono-energetic deuterons (D + ) or protons (H + ). Figure 1 illustrates, for the ADM system, the measured emitted deuteron energy as a function of the relative RF phase which correlated well with predictions. Although beam transmission efficiency is degraded when the RF is out of phase for any ion energy between 3.5 and 5.0 MeV, a beam transmission of >96% is still achieved. Figure 1. Measured deuteron energy as a function of the relative phase of the RF power in the ADM system. 3

4 2.6 Proton energy (MeV) Relative RF phase to booster (Deg.) Figure 2. Extracted proton energy from the D-100 system as a function of relative RF phase for 35 kw (red square) and 10kW (blue diamond) booster power. Figure 2 illustrates the level of extra energy added to a hydrogen beam in the D-100 system where not only the phase, but the RF power itself plays a role in the acceleration of the ion, due to the structure of the booster cavity. Figure 3 is a photograph of the two RFQ accelerators side by side in the experimental hall, PLABS, at Necsa. Figure 3. Photograph of the D-100 (left) and ADM (right) RFQ accelerators. 4

5 For the generation of neutrons, through the d(d,n) 3 He reaction, a combined spinning disc and differential pumping system is used to isolate a deuterium gas target from the main vacuum of the accelerator 3. Such a system removes the effect of energy loss and dispersion for a beam having to traverse a gas containment window. Dependent on the phase between the two RF systems of the accelerators and the radial location of the neutron detection system with respect to the incident beam direction, neutron beams of energy from 2 to 8 MeV can be generated with a total flux of n.s -1 for the ADM and n.s -1 for the D-100. Figure 4 illustrates the neutron flux emanating from the ADM when operated at 100µA at 4 and 5 MeV incident D + beam energy, interacting with a 3 cm thick deuterium gas target at 3 bar pressure. Neutron flux (10 4 n.mm -2 ) Angle from beam axis (Deg.) Figure 4. Neutron flux at 4 (red) and 5 MeV (blue) incident D + beam on a 3 cm 3 bar deuterium gas target The ADM and D-100 systems were originally designed for the generation of and neutrons per second respectively at either 7.2 MeV or 8.2 MeV, primarily in a 30 forward cone. The neutron energy spread in the region of interest is less than 600 kev. By varying the relative RF phase, neutrons between 6.6 and 8.2 MeV can be produced in the incident ion beam direction. Neutron beams as low as 2 MeV can be utilized by moving away from the incident beam direction, albeit at lower intensity. One can therefore adapt accelerator operation to scan objects on and off neutron resonance energies for elements such as C, N and O. 5

6 Investigations using solid targets such as Be and Li will also be considered, wherein white spectrum radiographic techniques are used. Furthermore the scope of using a proton beam from the accelerator will also be investigated, e.g. Li(p,n) and Be(p,n) reactions. Compounded with the accelerator, gas target and neutron detector development, is the need to simulate the complete system, especially from a radiation shielding perspective. Considerable effort has gone into modelling the neutron shielding required for the dual accelerator systems using the MCNP code, including the effects on the detection system if working with a target typically consisting of a cargo container. Furthermore, it was known that the original shielding of the PLABS building was inadequate when operating the D-100 at full power. The results of MNCP simulations have already been implemented for the extra shielding required due to the inclusion of the ADM facility in the experimental hall. Analyses are still in progress for the further shielding requirements when operating the D-100 at full power. 3. Detection systems Since the application of the system is for neutron radiography, several methods of image capture have been developed. In the first system a mirror reflects light from a neutron scintillator onto a CCD camera, thus protecting the CCD camera from the full intensity of the primary neutron beam. This system has operated reliably for many years and will continue to be applied with the ADM. With improvements in semiconductor based photon collection devices, an alternative imaging system was developed. Earlier studies with a 20 x 25 cm amorphous silicon flat panel detector indicated potential viability 4. Drawbacks were encountered due to the use of low efficiency scintillator material, as well as degradation of the electronics associated with the detector readout which was in line of sight with the primary neutron flux. A larger amorphous silicon array was acquired from Perkin Elmer where the readout electronics are situated at the edges of the panel. This amorphous silicon panel has a sensitive area of 410 mm by 410 mm and pixel size of 400 microns. A specially made combination of Bicron scintillating fibre blocks is mounted directly onto the panel. Each fibre block (10 mm x 10 mm) consists of 289 BCF-28 fibres of 6

7 diameter 0.25 mm single cladding. The depth of the block is 70 mm. Each block of fibres was specially produced such that when combined to the full array, the fibres extended a focal length of 980 mm. Tests so far have indicated that the light collection efficiency is extremely low, requiring neutron fluxes of at least per second. This therefore rules out the use of this technology on the present ADM. A similar fibre scintillation block array is also used on the D-100 system which relies on the use of an array of CCD cameras to collect the scintillation light from the 40 x 40 cm 2 detector area, see Figure 5. The light from the array is effectively split into four segments by using a multiple mirror in the shape of a pyramid. The light from each mirror is amplified by an image intensifier and split by a semi-mirror to be detected by two CCD cameras. One camera is triggered for high energy mode of operation of the accelerator, whilst the second is triggered for the low energy mode, i.e. every other pulse of the accelerator beam. The images collected by the eight cameras are reconstructed by computer to provide a high/low energy contrast image. The added advantage of this system is that it is able to reconstruct an image of a moving target, thus it is possible to scan a large container moving through the field of view of the detector. Scintillator Image intensifier Mirror CCD Cameras Figure 5. Dynamic imaging system, showing the scintillator, pyramid mirror and CCD camera systems for collecting high energy and low energy images. 7

8 Although this system was developed for dynamic imaging of rock samples on a moving conveyor belt, the principle can be equally applied to dynamically scan a cargo container. This area of research will be pursued in the near future. Extending the neutron detection capabilities not only by operating the accelerator over a broad range of beam energies but also by adding a beam pulser system, such that nano-second beam pulses are generated, has been proposed by V Dangendorf of PTB. This will enable time-of-flight techniques to be used in monitoring the scattered neutrons, further enhancing the neutron resonance scattering technique. A collaboration project with PTB is presently being formulated. Conclusions Although we have not yet been successful in demonstrating the ability of our two RFQ accelerator systems to detect illicit material and explosives, we have however been successful in implementing the infrastructure to undertake the task in the very near future. This infrastructure stretches from the establishment of a team of scientists, engineers, technicians and students, with the fundamental knowledge and experience to undertake the tasks at hand. We feel that the project has been a success, although very little experimental data has been generated so far, since we now have a research and development program in place for the developing various accelerator based techniques for the detection of illicit material and explosives. References 1. H.S. Uhm, W.M. Lee. J. Appl. Phys. 69(1991) J. Guzek, K Richardson, C.B. Franklyn, A. Waites, W.R. McMurray, J.I.W. Watterson, U.A.S. Tapper. Nucl. Instr. and Meth. B152(1999) R.M. Ambrosi, N.P. Bannister, J.I.W. Watterson. Nucl. Instr. and Meth. A566(2006)663. 8