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Imaging system for freight inspection systems

14.12.2012

ACCIS is a binational joint research programme which is supported by BMBF and aims to investigate a screening system based on neutron- and gamma-radiation for medium-sized freight units (e.g. air cargo containers). Altogether, 7 German, 3 Israeli and 1 South-African partners from research, industry and the police are involved in the project. Besides coordinating the work of the German partners, PTB’s contribution focuses on developing the imaging systems and providing the testing infrastructure. In addition, PTB takes part in the activities aimed at generating intensive pulsed neutron- and gamma-radiation as well as in research dealing with the estimation of the applicability and marketability of such a system in the public domain.

In the period under report in which the "milestone" was due, the sub-projects mentioned below were completed or significant intermediate results were obtained.

One of the important goals of the milestone was to set up and characterize two different detection systems for simultaneous neutron- and gamma-radiography. These two systems, which are based on different optical technologies and are known under the acronyms "TRECOR" and "TRIOR", are nearly completed. Due to delays in the supply of a component for TRIOR, it was agreed with the funding agency that for the remaining project period, the work would be focused on the second system, TRECOR. TRECOR is based on organic scintillators for radiation conversion and on a special electro-optical read-out system which was developed in the working group, in collaboration with one of the project partners. This system is capable of detecting the impact position of fast neutrons and gamma quanta in the converter screen with a rate of currently up to approx. 1 MHz. Applying the Time-of-Flight technique, it measures the neutron energy and provides an estimate of the gamma energy via the measurement of the scintillation light in the converter screen. A schematic representation and detailed photos of the TRECOR imaging system are shown in Fig. 1.

Figure 1 : Schematic drawing of the set-up of the TRECOR imaging system (top) and detailed photos of the assembled module (bottom).

  1. TRECOR was used in a demonstration experiment in June 2012. The funds that have become available due to the fact that TRIOR was abandoned were passed onto the area of activity described in 3).

  2. At the ion accelerator facility of Division 6 - where the required pulsed neutron fields are generated - the infrastructure necessary to carry out experiments serving to assess the method was set up. To this end, besides facilitating access with large and heavy objects, also a "cargo scanner" for objects of up to approx. 90 kg has been set up and integrated to the control and data acquisition system.

  3. To develop a high-intensity pulsed neutron/gamma radiation source, a postgraduate position has been supplied together with the industrial partner RI Research Instrument. Hereby, an ion source and a pulsed injection into an RFQ accelerator are to be set up in collaboration with the accelerator group of the Helmholtz Zentrum Berlin (HZB) and, if possible, to be mounted into the existing high-power linear accelerator at NECSA, our South African partner. To this end, a concept was developed and optimized by means of beam transport calculations using CST and PAMELA. Fig. 2 shows a schematic representation of the concept which is currently being numerically optimized and will then be designed and set up at PTB for experimental evaluation and optimization.

  4. For the realization of the concept, it was very helpful to take over the components of the terminated "Ion accumulator" experiment of Department 1 in the late summer of 2012. This is currently being set up and supplemented with a new LEBT (low energy beam transport system) to meet the needs of the new application.

    To generate the required neutron and gamma beams, besides the high-power ion accelerator, also high-power-resistant 10B targets are required. To this end, a target scanning system was set up which distributes the high thermal load prevailing in the focal point onto a large cooled surface. In a first testing round performed in the spring of 2012, the target was, however, destroyed due to "thermal shock" of the brittle and poorly heat-conducting boron sinter layer. A second series of B4C layers, which were deposited onto a molybdenum carrier layer by plasma spraying, was purchased and will be tested in the autumn of 2012. Deposits of this kind were previously used in fusion plasma experiments under intense ion bombardment and showed high mechanical and thermal wear-resistance.

  5. In a further work package, the system was simulated with the programme package "GEANT", and algorithms for data analysis and element re-construction inside the sample were developed. A full GEANT model of an X-raying facility with an LD3 unit load device was elaborated and computed. Based on the simulated data and the data measured to date, algorithms for the re-construction of the spatial element distribution in the sample container were developed. Whereas re-constructing the element distribution in known samples was successful with the data simulated with GEANT and with those measured using the TRIOR system, the measurement data obtained with the more advanced TRECOR system did not yet allow the element distribution of the samples to be reproduced. The reasons for this are being investigated at present.

Figure 2 : Schematic representation of the low-energy ion transport and pulsing system for the RFQ linac.

All in all, the total project as well as PTB’s sub-project (apart from the task related to the TRIOR concept) is as scheduled. In the coming months (the project is currently planned to end in July 2013), work will focus on clarifying the reasons for the insufficient data for element re-construction with TRECOR; later, the experimental facility at PTB will be re-assessed, and the pulsed ion injection for the RFQ linac will be set up and optimized.