A compact CZT γ-ray detector for nuclear applications in medicine

Abstract

This thesis presents two case studies for the use of a compact CZT γ-ray detector (manufactured by Kromek) in a medical context. The aim is to evaluate the potential of this type of detector for common nuclear applications in medicine. CZT γ-ray detectors are a type of semiconductor detectors, made of Cd$_{0.9}$Zn$_{0.1}$Te. Owing to the particular band structure of the material (Eg = 1.57 eV), CZT detectors can operate at room temperature. This is a major benefit compared to, for example, HPGe detectors, which must be cooled to limit thermal excitation. Moreover, the CZT detectors of Kromek’s GR family, which are considered in this thesis, are compact: the detection crystal and electronics are contained in a 2.5 cm × 2.5 cm × 6.3 cm casing. Therefore, these detectors can be used in remote places that might be inaccessible to other detection systems. In both case studies, the CZT detector is used for γ-spectroscopy. In this technique, energy spectra of γ-ray emitting samples are investigated to identify and quantify the present radioactive isotopes. The first case study for the use of a compact Kromek CZT detector involves the production of terbium radioisotopes for medical applications at CERN MEDICIS (Geneva, CH). The Tb radionuclide family is very promising for nuclear medicine, because it contains four radioisotopes with complementary decay characteristics suitable for both imaging and therapy. For the neutron-rich radioisotope, $^{161}$Tb, large-scale production has been established at SCK CEN (Mol, BE) by neutron irradiation of Gd targets. The neutrondeficient radioisotopes, $^{149}$Tb, $^{152}$Tb and $^{155}$Tb, are predominantly produced at MEDICIS. In this case, a heavy target of Gd or Ta is irradiated by energetic protons up to 1.4 GeV, and the Tb radioisotopes are separated from the other species by the ISOL (Isotope Separation On-Line) method. Finally, the separated isotopes of interest are collected on implantation foils. At MEDICIS, the Tb activity can be monitored during the collection using the Kromek GR1 CZT detector installed in front of the collection chamber. In this thesis, the γ-spectra acquired by the detector during the $^{155}$Tb collections in 2021 are analysed. They show the presence of $^{139}$Ce$^{16}$O as isobaric contaminant that is collected together with $^{155}$Tb. Furthermore, they indicate that (self-)sputtering occurs in the collection chamber, limiting the amount of $^{155}$Tb that stays implanted on the foil. Finally, combined with ion current measurements, the γ-spectra suggest that target conditioning prior to the collection is useful to prevent saturation of the ion source. In the second case study, a Kromek CZT detector is used in the context of radioactive waste management at the proton therapy facility PARTICLE (Leuven, BE). Due to direct irradiation by the proton beam of 230MeV or indirect irradiation by scattered protons, nuclear reactions are induced in the construction elements of the beam-line. A variety of radioisotopes is produced, depending on the composition of the construction material and the energy of the protons. In this manner, radioactive waste is created. To decide on the proper treatment for specific components, the radioisotopes generated in them must be identified and quantified. In this thesis, that is done with a Kromek GR05 CZT detector for 12 parts of the accelerator at PARTICLE. $^{22}$Na, $^{54}$Mn, $^{57}$Co, $^{60}$Co and $^{65}$Zn are identified as common radioisotopes. This is in agreement with FLUKA simulations of the proton irradiation of conventional construction materials. Furthermore, it is observed that higher activities are produced in objects that are directly irradiated. Consequently, it takes longer for these objects to decay to activities below the appropriate clearance level for disposal. In both case studies discussed in this thesis, the Kromek CZT detector presents itself as a suitable device to apply γ-spectroscopy in a medical context. Given its ability to operate at room temperature as well as its compact size and mobility, the detector can be used in remote locations