Low background screening facility at Boulby for rare event search experiments

Scientists around the world are closing in on two of the biggest challenges in physics: understanding the nature of dark matter and the properties of the neutrino. Dark matter accounts for 85% of the mass of the Universe yet we do not know what it is since it has never been observed, and although we do know neutrinos have mass we do not know precisely how much nor 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. These questions have profound impact on our understanding of the Universe and its evolution - answering them is amongst the highest priorities in science.

Experiments that hope to observe dark matter particle interactions or neutrinoless double beta decay events that will tell us about the neutrino share a common requirement. Since both of these processes are extremely rare the detectors need to be shielded from all sources of background radiation that might mask the signal. The first line of defence is to place the detectors deep underground in mines or under mountains - this reduces the rate of cosmic rays from space that bombard the surface of the Earth. Next the detectors are surrounded with copper, lead, plastics and water to block the radiation emitted by the underground rock. Although this is natural low-level radiation that causes no harm to humans, it is catastrophic for our extremely sensitive rare-event searches! The final and most difficult step is to construct the detectors themselves from only the purest materials that are exceptionally 'clean' in terms of trace contaminants that may emit radiation.

The UK has a very strong track record and international standing in rare-event underground physics, going back several decades, and we continue to take leading roles in the most advanced experiments. We now stand on the edge of discovery with the next generation of dark matter and neutrino experiments. Unfortunately, here in the UK, our capability to screen materials to check their radio-purity before using them to build detectors is no longer sufficient. As detectors have become ever more sensitive, so has the requirement for the materials to be ever cleaner and free from even the smallest amount of radiation. Whereas previously we could rely on a germanium detector located at the Boulby Underground Laboratory to perform our screenings, it can no longer keep up with the sensitivity requirements of the dark matter and neutrino-less double beta decay experiments. This is doubly problematic because in addition to vetting material before accepting it in detector construction, we must also understand precisely the amount of radiation expected from each and every component. Only if we know what we expect from all of these materials can we hope to observe an excess and claim discovery of an exceptional signal. Very few facilities exist worldwide with germanium detectors with sufficient sensitivity to satisfy these needs.

We will install a new facility at Boulby with a state-of-the art germanium detector that will be used by 15 institutes across the UK in their low background screening campaigns for the leading dark matter and neutrino-less double beta decay experiments. With this much needed capability to deliver amongst the world's most sensitive material screening tests, the Boulby facility will be reinstated as a leader in the field, and we will be able to construct the next generation of experiments that will help unravel the mysteries of the Universe.

Such a facility would be very useful to a wide variety of applications well beyond particle physics and cosmology. It would significantly improve environmental radioactivity studies already in progress at Boulby, and would provide capability for studying the climate with aerosol growth and cloud formation, and next generation electronics. Internationally there is an industrial and commercial demand for such instruments with the sensitivity we would provide.

Across the world, the ability to assess the uranium and thorium content of materials at the part-per-trillion level is becoming ever more important. In particular, such measurements are vital for determining acceptable components to be incorporated in key next-generation science projects, most critically the ultra-rare event searches of direct dark matter detection and neutrinoless double beta decay. These are the breakthrough projects that will take us beyond the Higgs, beyond the standard model, and into scientific terra incognita.

The 15 UK institutions supporting this proposal are seeking to establish major roles in such upcoming projects, including responsibilities for radioactive purity of construction materials. However, at present, the UK has no facility with sufficient sensitivity to conduct materials screening at the requisite level, and instead we have to rely on our international partners.

Funding of this project will enable UK leadership in this important area. Moreover, without sufficient control of radioactive backgrounds, these projects become critically compromised, and as a consequence, leadership and responsibility for material screening provides a gateway to scientific leadership of the projects as a whole. A world-class screening facility is an enabler of scientific leadership in the breakthrough physics projects of the next decade.

The impact of such a facility goes well beyond particle physics and cosmology. The UK has expertise and world recognition in environmental gamma ray spectroscopy research. Indeed, the Boulby facility is already being used for environmental radioactivity studies with dedicated detectors. A new BEGe facility would significantly enhance this capability with well over an order of magnitude improvement in sensitivity. The screening facility will also be used for a variety of further applications including, but not limited to, climate (aerosol growth and cloud formation), next generation electronics (single event upsets) and environmental research (radio-ecology, landscape evolution). Similar modes of operation and exploitation already exist at other international underground laboratories. There is a high industrial and commercial demand for such measurements and therefore running the facility may become self-funding on the timescale of a year.