Environmental

Funded by STFC through an innovation partnership scheme award, this knowledge exchange project will provide Canberra with access to the wealth of experience in high resolution gamma systems available at Liverpool, allowing them to exploit academically developed advanced digital signal processing algorithms in real world industrial environments. Liverpool will gain access to key knowledge on the requirements of the UK nuclear industry customer base, access to large scale manufacturing facilities and the associate commercial expertise. We aim to optimise Canberra’s unique “point like contact” BEGe (Broad Energy Germanium) detector product and revolutionary new SAGe detector technology for the digital era. The work will not only deliver unrivalled energy resolution performance, but also world leading timing performance and the ability to filter events by their interaction signature. This will lead to:

• Improved spectral response, which would increase the sensitivity of detecting special nuclear material (SNM)
• Enhanced signal processing for detector systems operated at high count rates, for industrial applications

This work will derive from the measurement of the relationship between the gamma-ray interaction position in the crystal and the shape of the charge pulse. Simulations are used to model the electric field within the bulk of the germanium crystal, to understand the charge collection process and experimental data have been collected to validate the models.

Figure: (a) Experimental signals and (b) the modelled charge carrier paths for a Broad Energy Germanium detector.

This proposal has come from the expertise in gamma-ray detection and associated instrumentation developed with STFC funding, particularly the detector characterisation and gamma-ray tracking techniques developed for the AGATA project.

 “An experimental characterisation of a Broad Energy Germanium detector” NIM A 760 (2014) 28-39

Canberra has recently developed a new detector (SAGe) with a novel electrode geometry that is aimed at high efficiency measurements for environmental studies.  This detector has a well geometry and will offer a significant improvement in energy resolution, resulting in an improved sensitivity for environmental measurements. We have characterised the detector performance, immediately after its launch and are now in a position to derive the full benefits from its exciting specification. An example application that would benefit from the SAGe system is ²¹⁰Pb dating, a technique used internationally to track environmental change over many years, with the timescale defined by the 22 year half life of ²¹⁰Pb. It relies on the measurement of a gamma-ray peak at 47 keV, where conventional well detectors have an energy resolution limited by their capacitance dictated by their detector electrode geometry. The SAGe detector will have an energy resolution a factor of two to three better. We plan to characterise the pulse shape and interaction position response enabling us to develop an algorithm to identify useful low energy photopeak events that occur near the detector surface close to the sample and reject other low energy events in the crystal (from Compton scattering or background). This improvement, taken together with the improved energy resolution, should allow measurements to be extended to older samples and samples where the amount of material available is very limited.

As these are high efficiency detectors, the coincidence summing correction developed above would also be used for parts of the Uranium decay chain that help establish whether the samples are in equilibrium. This detector will then remain at Liverpool and be used in its internationally leading laboratory where Prof Peter Appleby will carry out world leading environmental measurements.

Funded by NERC through a Technologies Proof of Concept Programme award, this project aims to assist the clean-up operation following the accident at the Fukushima Daiichi nuclear power plant in Japan. A significant amount of ¹³⁷Cs was released into the atmosphere and surrounding areas were contaminated, the extent of which is not well understood. Systems that can locate and quantify radioactive sources are vital to ensure an efficient decontamination operation. There are several approaches to this; small-scale imaging of radiation transport through soils and uptake in plants and large-scale imaging of soil erosion across fields and radioisotope retention in tree canopies. There will also be investigations into monitoring sand filtration systems used to decontaminate liquid effluent at nuclear power facilities.

Current experiments are using the Compton camera system to monitor radioisotope flow through a column of quartz sand. Time lapse imaging of ¹³⁷Cs and ¹³⁹Ce liquid sources will provide a control comparison to future measurements using soil samples from fields surrounding the Fukushima Daiichi site.

This project is in association with the Departments of Environmental Engineering and Environmental Sciences at the University of Liverpool, the National Nuclear Laboratory and the University of Tsukuba. The work is funded by the Natural Environment Research Council.‌

The figure on the right column illustrates an example of the setup for an experiment that will use Compton camera time-lapse imaging to monitor ¹³⁷Cs flow through a column of quartz sand.