There is a need for truly portable source of gamma rays to scan large volumes for radiological or other contraband materials. Euclid’s compact gamma ray source, based on microwave accelerated electrons can reach energies of a few MeV and produce gamma rays that are able to penetrate large cargo containers or suspicious vehicles for scanning purposes. Currently the systems available commercially are rather large, truck mounted systems. Because of the lack of portability people resort to radioisotope-based density monitors, which are inherently hazardous and have indirect costs related to safety, security and disposal of radionuclides.
Euclid BeamLabs LLC is developing an X-ray source based on an ultra-compact accelerator. This is a portable, virtually hand held device that produces high energy gamma rays that can penetrate deeply into materials to satisfy inspection needs.
Modern mobile cargo inspection systems utilize a similar, few – MeV electron linear accelerator (linac) and are truck mounted: the RF power system (magnetron, modulator and cooler) for a conventional accelerator weighs at least 500kg and the accelerating structure with shielding and X-ray target weighs at least another 500kg. The ultimate goal is to make the system almost two orders of magnitude lighter at the expense of beam power. This corresponds to going down from the truck mounted system’s 1 kW electron beam to about 20 W beam power. The kinetic energy of the beam has to stay at 1-4 MeV level to allow production of high energy x-rays (hence deep penetration), so the current is allowed to be low, 10-50 μA. To achieve 1-4 MeV beam energy relatively high peak power RF is needed. Going to high frequency, the weight and volume of the structure can be reduced, scaled by approximately 1/f2. There are ~ 30 GHz compact magnetrons with reasonably high peak power (~90 kW). While we do not discard this option, we are focusing on a light weight solution for a ~ 10 GHz source because much higher peak power is available, manufacturing tolerances are reasonable, cell to cell coupling is not a problem with the tuning procedures developed, and our proposed solution is truly compact.
We have identified an X-band 200 – 250 kW peak air traffic control radar magnetron as the power source for our compact accelerator. The average power of this magnetron is only ~220 W due to its low duty cycle. Magnetron efficiency is on the order of 40% which means that modulator will consume ~ 600W. Including the overhead for cooling etc, the 1 kW total power budget can be provided for 1 hour with a compact ~7 kg Li-ion battery. The parameters of a minimum weight 1 MeV accelerator seem to be in-line with the desired 1 ft3 / 50 lb target, provided that a lightweight accelerating structure can be demonstrated.
Recently Euclid designed and patented an inexpensive ultra-compact accelerating structure based on a waveguide partially loaded with dielectric – dielectric loaded accelerator (DLA) and a hybrid dielectric – iris – loaded waveguide (Hybrid DLA – HDLA). The use of high permittivity ceramics reduces the transverse size of the cavity significantly (Figure 1A), making the accelerating structure thickness comparable to the thickness of a pencil. A typical X-band cavity has OD ~ 5 cm (Figure 1C), while DLA ~ 1 cm (Figure 1A), transverse size is reduced ~ 25 times at the expense of small length reduction. This results not only in a sizeable weight reduction of the structure itself, but even more importantly, a reduction of the volume required for the lead shielding.
Euclid Beamlabs LLC proposes to use this approach for a portable lightweight and cost-effective accelerator.
Dielectric loaded accelerating structures were proposed years ago, but the idea did not gain traction because of concerns that electron beam would charge the ceramic structure. The enabling factor for the proposed approach is that for the case of a minimum weight accelerator, the beam current is a miniscule 10-50 μA, manageable for ceramic structures.