Low Background Techniques
The particle astrophysics programs all require ultra-low backgrounds. Typical event rates in dark matter detectors may be as low as one event per tonne per year, and hence removing all potential sources of backgrounds to levels below this is essential to achieve the necessary sensitivity.
During SNO, techniques were developed to reach activity levels as low as 10-15 gU/g. This was considered remarkable. In SNO+, two orders of magnitude better are required, and future experiments will be even more demanding. Novel techniques need to be developed to purify detector materials to ultra-low levels, and tools need to be developed with sufficient sensitivity to measure the level of residual activity in detector and construction materials. The selection often involves co-operation with industry to have components built from low-activity materials and handled in ways that do not introduce contaminants.
In support of the scientific work on ultra-low backgrounds, Queen's University plans to hire both an analytical radiochemist and an analytical geochemist, to be located in the Departments of Chemistry and Geological Sciences at Queen's University, respectively. The radiochemist will be cross-appointed to physics, and help with the challenging radiochemical work, developing new tools and chemical procedures related to the CPARC program. These will include analytical tools and procedures to measure radioactivity at extremely low levels and to purify complex materials of radio-impurities and other sources of backgrounds, and methods to handle and prepare samples for further analysis. The geochemist will develop a new research thrust within the context of the Queen's Facility for Isotope Research (QFIR) where existing infrastructure will be employed for new purposes in the development of low background techniques, particularly ultra-sensitive mass spectrometry tools for measurements of low-level background radiation, and in partnership with the analytical radiochemist, the development of the chemical processing steps required to treat samples for mass spectroscopy.
The techniques to be developed by the analytical geochemist at QFIR are of great interest to various industries. Commercial analytical laboratories in Canada have international facilities that offer their clients ultra-low level analyses, the novel techniques and protocols of which will be developed at the QFIR. These commercial analytical companies require extremely low detection limits to serve the agricultural, mining, environmental, and health care sectors better. Commercial labs are end users of the protocols developed at QFIR and the focus on low-level detection of radiation and the associated elements will enhance this by developing new analytical techniques and protocols. Technology transfer of novel ICP-MS protocols for low-level analyses is also attractive to pharmaceutical companies and the health industry for determining the sources and levels of contaminants and toxins in their products and extremely small samples.
The CPARC members have unique access to both the clean underground lab and (at Laurentian) a facility where radioactive samples may be tested. Research and testing using these facilities will expand on the broad spectrum of skills in place to qualify and quantify backgrounds, and will focus on bringing numerous technologies to bear on the issue of measuring radiation in complex materials to screen them for use in experiments. The research program will include:
- the development and full exploitation of ultra-low background germanium counters
- enhanced capability to measure and mitigate against radon isotopes at unprecedented low levels
- establishing full background models
- high sensitivity beta-alpha counting devices
- commissioning and operating specialized analytical instrumentation and spectrometers
- radiochemical techniques to concentrate samples to levels of detectability
This program is ideal for the training of graduate students. The additional resources required to bring these systems online will be drawn from personnel provided by the CPARC program.
A related activity is the need for precise calibration methods to understand the detector response. This requires exotic radioactive sources that are sufficiently radioactive to enable statistically meaningful data sets to be collected in short calibration runs, but safe to take underground without risk of contamination. The use of particle beams— for example, mono-energetic neutron beams— is particularly useful for the calibration of dark matter detectors as neutrons produce recoil nuclei with energies similar to those expected from dark matter. To this end, Queen's University will hire a faculty member, located in the Department of Mechanical and Materials Engineering at Queen's and working with the Reactor Materials Testing Group where there is access to an 8 MeV proton tandem Van de Graaff, microscopy, and x-ray testing laboratory. This facility will be used to develop the calibration tools, specialized sources, and radiation detectors required for the astroparticle physics program. This activity complements that at the Université de Montréal where there is a 12 MeV proton tandem Van de Graaff allowing access to higher energies and where a few sophisticated targets have already been implemented to produce a number of mono-energetic neutron beams.
One of the major challenges in searches for neutrinoless double beta decay will be to push the sensitivity by several orders of magnitude in tonne-scale detectors. To reach such a sensitivity, the sophisticated approach of barium tagging has been proposed. With CPARC support, the progress at Carleton, McGill and TRIUMF to extract Ba ions successfully from high-pressure xenon gas will be extended to enable sensitivity to single Ba ions in high pressure targets at the one tonne scale.