Joint Cryogenic Radon Emanation Measurement Facility

Lead Research Organisation: University College London
Department Name: Physics and Astronomy


Two of the biggest challenges in physics today are to understand the nature of dark matter and the properties of the neutrino. Although dark matter is believed to make up an incredible 85% of the mass of the Universe, it has never been directly observed and so we do not know what it is. And though we do know neutrinos have mass, we do not know precisely how much or if they are their own anti-particles. If they are it could explain the tiny imbalance between matter and antimatter shortly after the Big Bang. Answers to these questions will profoundly impact our understanding of the Universe and its evolution.

Experiments that aim to detect interactions with galactic dark matter particles or observe the signature of neutrino-less double beta decay that will tell us about the neutrino share a common requirement: both processes are extremely rare and so detectors must be shielded from all sources of radiation that present background to the faint signals. The first line of defence is to place the detectors in deep underground laboratories, in mines or under mountains, to limit the rate of cosmic rays from space that bombard the Earth's surface. After this the detectors are shielded with passive materials like copper, lead and water to block radiation from the underground laboratory environment, particularly from the rock. The last step is the most difficult - the detectors themselves must be constructed from pure and exceptionally clean materials that are free from trace contaminations of radioactive isotopes.

The UK has internationally renowned expertise in rare-event underground physics, built up over several decades, and today we continue to hold major roles in the most sensitive experiments. We have advanced techniques to screen materials for radio-contaminants fixed within them, the traditional source of major backgrounds to-date, to limit their effect and achieve unprecedented experimental sensitivities. We may be on the edge of discovery with the next generation of dark matter and neutrino experiments. However, to meet the science reach of these future instruments, we must address an emerging background that cannot be screened with regular methods, nor easily rejected through analysis techniques. This background is the noble gas, radon. Radon is produced in materials as a decay product of trace uranium and thorium in materials, but unlike other progeny, it can diffuse out and populate entire target volumes.

To address this key challenge for future experiments we must perform R&D and highly sensitive radon emanation measurements as part of our assay campaigns to select suitable construction materials and to build models that characterize radon transport and expected backgrounds. Moreover, we must assay large amounts of materials and at different temperatures, since radon emanation is dependent on material type, exposed surface areas and on the temperature of the material. Of the few high sensitivity radon systems around the world, none meet requirements for future experiments for large samples and in conditions that mimic most experiments.
With this proposal we will deliver a unique radon emanation measurement facility to be located at the Rutherford Appleton Laboratory to support the UK's rare-event search research, particularly the dark matter and neutrino-less double beta decay communities. The system will enable entirely new capability for immediate R&D and for future experiments, regardless of the technology chosen for these detectors since all will need to address radon.

Such a facility would be useful to a wide variety of applications well beyond physics. It would allow improvements in commercial devices that are used to perform low-radiation measurements for the medical and nuclear monitoring sections, as well as enhancing low-cost radon detectors that are used to measure radon levels in homes and the workplace to ensure safe levels given its prominent role in causing lung cancer, second only to smoking.

Planned Impact

A dedicated facility to screen large materials for radon emanation to unprecedented sensitivity and through a range of temperatures has implications for several areas across a range of disciplines. With this new technical capability in the UK we will focus on two avenues with genuine potential for realization of socioeconomic impact on short timescales: development of low-background instrumentation and infrastructure for commercial and industrial application, and enhancement of low-cost environmental radon monitoring.

Advances in the development of low-level radiation detection equipment have led to commercial instruments that can easily reach sensitivity to only a few hundred parts per trillion of radioactive isotopes U-238 and Th-232 within materials, provided such assays are performed in radio-quiet environments; within lead and copper shields and in deep underground laboratories. The limiting factor in sensitivity reach is the radiation coming from the materials used to construct the detector and shield themselves. However, we must continue to improve these instruments if we are to be able to assay materials that will go into constructing the next generation of rare-event search experiments, particularly those searching for evidence of dark matter in the Universe, or signatures of neutrino-less double beta decay. However, the application of ultra-sensitive gamma spectroscopy devices extends well beyond science capability, for example, to medical and nuclear monitoring applications.

We are already working with commercial and industrial partners to perform radio-assays of materials from which they intend to build their radiation detection equipment. These radio-assays at present only include fixed contaminants that generate easily modelled and well-predicted backgrounds. Augmenting these assays with radon emanation measurements can result in a step change in sensitivity, particularly since we will characterise this major source of background at cryogenic temperatures identical to final operational conditions of the instruments. We will also perform measurements of the materials that go into making the shielding for these instruments, to reduce contributions there as well that in some cases may be significant. With this well-defined program of activity, in collaboration with our partners, we anticipate improvements in instruments and shields that would be rolled out for commercial application to the benefit of many sectors within only a few years.

The second area where we will seek to have impact is in the area of low-cost radon detectors for residential, commercial and industrial locations. Radon is the second largest cause of lung cancer after smoking, and is present in the air that we breathe in extremely variable concentrations depending on local conditions. Low-cost radon detectors, as used by government health agencies, are typically simple devices that are constructed from and housed within plastics that are themselves known to emit radon background that will degrade detector sensitivity. This is turn implies limited use in some settings or long counting times that may not be feasible and in any case do not account for short term variations in radon levels. We will explore partnership with the manufacturer of the leading devices to assess the existing plastics, explore alternative materials, and, most significantly, characterise the temperature dependence of the sensitivity.


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