Advanced Scintillator Material for High Energy X-ray Imaging.

The overarching project aims to develop a revolutionary analysis technique with a particular focus on monitoring of nuclear waste drums as a collaborative project between the Interface Analysis Center (IAC), Sellafield, Queen's University Belfast (QUB) and the STFC Central Laser Facility (CLF). By firing an extremely high-energy laser for a very short duration, an intense spot of x-ray radiation is generated and projected towards detector plates. In a similar manner to medical x-rays, any object placed between the bright source of x-rays and either photographic film, or digital image plates, is captured in detail. However, because a very high energy source is used, imaging of uranium waste, one of the densest materials on earth, is possible.

Since 1952 Sellafield has been responsible for safe storage and reprocessing of all the UK's nuclear waste. Decades of research and development have resulted in more manageable forms of nuclear waste. However, a whole host of problems remain, particularly with the ageing nuclear waste that has been stockpiled since the 1960's. Before long term storage in a geological disposal facility is considered, the composition and degradation state of the waste material and containment vessel needs to be established. Due to the radioactivity and dangerous corrosion products formed during storage, a destructive investigation of the waste containers is considered too hazardous to be performed. Therefore, a non-destructive evaluation of the containers is proposed.

For a visual inspection of the waste, high-energy x-rays are desired to create an image of the sample in storage. Typical means of producing x-rays do not achieve both the resolution required to identify corrosion products nor the energy needed to penetrate through the large, dense, waste containers. Therefore, it has been proposed that the Vulcan laser at the central laser facility (CLF) is utilised to generate the necessary high-energy x-rays required for this analysis.

This particular research proposal focuses on the development of the detection methods used to capture the x-ray images of nuclear waste containers. Specifically, we aim to deploy a novel transparent scintillator (referred to as an acronym of its elemental constituents as GLO - gadolinium lutetium oxide) capable of converting the high-energy x-rays into visible light for imaging. The key property of the high-energy x-rays that we require is the ability to travel through objects unimpeded. However, the advantage they present is also the reason why it is difficult to collect an image from them - they pass through most detector materials too easily! Therefore, the use of a high density, high atomic number material (which is the GLO scintillator) greatly increases the probability that x-rays will interact and convert to visible light photons. These are then captured on low-light CCD cameras providing a picture of the container contents.

The final stage of the overarching project involves replacing the single-shot Vulcan laser with a rapid-fire DiPOLE laser capable of 10 shots a second (or more). This, combined with our proposed new detector arrangement, will enable far better image acquisition and even 3D tomography (like a medical CT scan) if the drums are rotated during acquisition.