Gamma-rays in low-background facilities : fundamentals of spectra measurement and energy calibration at Callio Lab
|Author:||Puputti, Hannah J.1|
1University of Oulu, Faculty of Science, Physics
|Online Access:||PDF Full Text (PDF, 6.9 MB)|
|Persistent link:|| http://urn.fi/URN:NBN:fi:oulu-202201231099
Oulu : H. Puputti,
|Publish Date:|| 2022-01-24
|Thesis type:||Master's thesis
Low-background gamma spectroscopy is an essential tool in designing and building low- and ultra-low-background experiments. Low-background experiments study extremely rare phenomena, where a detector may expect to see one signal a day. The experiment must be designed and constructed so that the true signals can be separated from the background. The background consists of three sources: cosmic-ray-induced, environmental, and instrumental radiation.
The background from cosmic-rays can be mitigated by situating the experiment deeper underground and by using veto detectors. The natural background radiation can be mitigated by choosing the appropriate location according to preliminary NBR characterisation and taking the necessary steps to shield the detector from the remaining radioactivity. The third, and perhaps most challenging, source of background is the gamma-ray background from the radioactivity of the detector instrumentation and shielding itself.
Gamma-rays are high-energy electromagnetic radiation that carry energy in discrete units called photons. Gamma-rays interact through the photoelectric effect, Compton scattering, and pair production within the detector and detector shielding. These interactions create the signature gamma-ray peaks and other gamma spectrum features, which allow the identification of radionuclides present in the measurement chamber. Radionuclides produce gamma-rays of various energies, which have been well-documented and can be used as references. Of the three most common types of gamma detectors (gas-filled, scintillation, and semiconductor detectors), semiconductor germanium-based HPGe detectors are the most suited for low-background radionuclide analysis due to their usability and good resolution.
Low-background experiments are situated in deep underground laboratories because the overburden effectively shields from the cosmic-ray flux. One potential reuse application for underground facilities is in the field of low-background experiments. Such facilities already exist at, for example, the HADES and Felsenkeller underground laboratories. SNOLAB was developed as a dedicated low-background facility to support the activities at the Sudbury Neutrino Observatory. Low-background gamma spectra measurements can require weeks or months to complete, so more DULs with material assaying capabilities are needed.
The possibility of using Callio Lab at the Pyhäsalmi Mine in Finland as a site for low-background gamma activities was piloted and evaluated during the BSUIN project, providing extensive characterisation of the natural background environment. Low-background gamma spectroscopy was practiced in Lab 2 and Lab 5 at Callio Lab. A dedicated gamma spectrometer setup was used for the characterisation of natural background radiation and for low-background materials sample measurement. This provided practice on the transport, start-up, calibration, and operation of a HPGe spectrometer setup. Lab 5 was deemed a viable location for further low-background activities in regard to usability, characterisation, and trained personnel familiar with gamma spectrometry.
The HPGe spectrometer was recalibrated after transport from Lab 2 to Lab 5. Preliminary calibrations included energy and full-width half maximum calibrations with dedicated software SpectraLine. For the purposes of becoming familiar with the use of the gamma spectrometer and evaluating possible experiment changes, these primary calibrations were sufficient. Preliminary comparison of acquired spectra of background and the high-purity samples showed that the background present was as high or higher than the sample. The HPGe instrument used is defined as a low-background device fitted with standard industrial shielding. In practice, the limitations of this setup and shielding were reached with the measurement of high-purity samples. The current limits are defined by the radioactive background, and further study should be conducted to ascertain whether it is of environmental or instrumental origin.
An understanding of gamma-ray sources and interactions is essential for the evaluation and development of better low-background gamma spectrometry setups. Before focusing on the instrument background, which should be low already due to the use of radiopurity-certified materials, other aspects should be considered. Further study and development of the experiment setup will improve the low-background measurement capabilities at Callio Lab to an even higher-level.
© Hannah J. Puputti, 2022. Except otherwise noted, the reuse of this document is authorised under a Creative Commons Attribution 4.0 International (CC-BY 4.0) licence (https://creativecommons.org/licenses/by/4.0/). This means that reuse is allowed provided appropriate credit is given and any changes are indicated. For any use or reproduction of elements that are not owned by the author(s), permission may need to be directly from the respective right holders.