Particle experiment: Cold radon emanation for Dark Matter

Supervisors: Prof Chamkaur Ghag (UCL) and Dr M. van der Grinten (RAL)

All rare event search experiments must be constructed from ultra-pure materials following comprehensive radio-assay and screening programmes. Furthermore, expected background levels must be accurately characterised in experimental background models; it is against such high precision models that any hint of signal in dark matter or neutrino-less double beta experiments will be evaluated and signal confidence assigned. Radon emanation has emerged as the most important background limiting science capability in leading experiments such as LZ, where is it already suppressed to unprecedented levels. Future experiments, such as a planned LXe G3 dark matter experiment, will need to control radon backgrounds by a further order of magnitude. Facilities that provide high sensitivity radon emanation assay are rare, with only a handful across the world. None of these, however, are capable of assessing radon emanation at cryogenic temperatures important to experiments such as LZ or future LXe G3, given expected reduction of radon diffusion at low temperatures in many materials. The RAL cold radon emanation facility has been designed with low-temperature capability. This allows, for the first time, systematic measurements of emanation via recoil and diffusion components as a function of temperature for all Xe-wetted materials. The facility also allows testing of novel radon diffusion suppression techniques, from physical barriers using novel materials to chemical treatment of surfaces. The PhD work would focus on developing an understanding of radon transport and its temperature dependence through materials and subsequent recoil and diffusive emanation in a range between 77K to room temperature and above. The PhD would also include measurements of novel barrier materials, chemical treatment of surfaces, and radon filtering techniques. Finally, the PhD would assess potential for fast in-line radon removal using low-temperature vacuum swing absorption. This PhD work will generate the research required for the future of both direct dark matter and underground neutrinoless double beta decay experiments, otherwise limited by radon. The PhD student will have access to the LZ experimental data for testing and validation of radon transport models at xenon temperature. The research has immediate benefit to experiments such as LZ, informing the background model and specifically radon backgrounds. The development of the cold radon emanation facility and the research proposed, supporting high priority UK science in astroparticle physics, is timely and necessary for the future of rare event searches.

For more details please contact c.ghag at ucl.ac.uk