Degradation Studies of LaBr3 Detector Units in High-Radiation Scenarios for Nuclear Fuel Inspection

Sammanfattning

Nondestructive assaying (NDA) is an important step in ensuring the safe handling of spent nuclear fuel and radioactive waste. The assaying techniques require the use of ionizing radiation detectors to determine the activity of said fuel. One candidate detector for such use is the cerium-activated LaBr3 inorganic crystal scintillator. This scintillator offers excellent resolution, decay times and light yield, making for an excellent scintillator detector. During NDA the detector will be exposed to large doses of radiation. To establish whether the detector is a good candidate or not, it is important to investigate how the detector holds up against these large doses and to what degree they cause degradation of its scintillation qualities. In this thesis a comparison is done between three different crystal scintillators; a LaBr-scintillator irradiated with 9.8 kGy in γ- and neutron radiation, a NaI(Tl)-scintillator and a reference undamaged LaBr-scintillator. By measuring the main peaks of 137Cs, 22Na and 60Co and calculating the resolution of these peaks, the degradation in resolution and Fano factor in LaBr γ-detection units due to large doses of radiation are discussed. The study measures 6.193(1)% resolution peaks at 661.7 keV using the irradiated LaBr-scintillator, compared to a resolution of 3.260(1)% using the undamaged LaBr. The average relative degradation in resolution across all the measured γ energies measured to 74.1% with the peak for 60Co measuring as high as a 92.3(13)% degradation. Despite the high dose of radiation incurred, the irradiated LaBr still outperforms the NaI(Tl)-scintillator in terms of energy resolution. The results presented in this thesis may serve as a guide in determining how reliable the LaBr-scintillator is in use for high-radiation scenarios, as well as valuable data for improving detector performance simulation is such scenarios. Further characterization to determine the degradation of light yield and how the crystal structure is affected by the heating and cooling during the irradiation process in future studies would give greater insight of the radiation hardness of the LaBr-scintillator. Its use in NDA may be limited to scenarios with smaller effective doses to ensure the longevity of the detector.