Novel approaches to the analysis of nuclear and other radioactive materials - Improving detection capability through alpha-gamma coincidence, alpha-induced optical fluorescence and advanced spectrum analysis

Nuclear and other radioactive materials pose a special concern in the proliferation of nuclear weapons, reactor accidents or through criminal acts. To prevent the adverse effects of the use of these materials, novel approaches for their detection and analysis are required. The objective of the research in this thesis was to improve the detection and characterisation of nuclear and other radioactive materials with radiometric methods. Radioactive sources can be detected and identified based on their radiation, such as alpha particles or gamma rays, emitted in the decay of unstable atoms. In practice, background radiation and attenuation impair the minimum detectable activity. In this thesis, the simultaneous detection of both gamma rays and alpha particles originating from the same decay was shown to increase the detection sensitivity of certain radionuclides compared to the equivalent separate measurements. Unfortunately, the measurement geometry in this direct alphagamma coincidence approach can be limited by the short range of alpha particles in air. This deficiency was overcome for the first time by the use of coincidences between gamma rays and secondary optical photons. The secondary optical photons are produced during the flight of alpha particles in air. In addition to the measurement techniques, statistical uncertainties were investigated both in spectrum fitting and in the interpretation of the results. The techniques were applied to the analysis of gamma-ray and alpha-particle spectra. Emphasis was placed on the correlated variables and analysis of data with limited statistics. The techniques presented improve the analysis of radioactive materials in several applications. The alpha-gamma coincidence technique is especially suited to the analysis of plutonium samples, which is important for nuclear safeguards. Nuclear decommissioning and crime scene investigation would greatly benefit from the capability to detect alpha-particle emitters at a stand-off distance. The reliability of the analysis algorithms is particularly crucial in portal monitors and other applications where weak signals from a large number of spectra are automatically inquired without expert support.; Radioaktiivisten aineiden ja ydinmateriaalien kaytto vaatii viranomaisvalvontaa. Radioaktiivisten aineiden lahettama sateily aiheuttaa vakavan uhan seka onnettomuustilanteissa etta lahteiden joutuessa rikollisten kasiin. Ydinmateriaaleja voidaan lisaksi kayttaa ydinaseiden valmistukseen. Jotta naita materiaaleja voitaisiin hyodyntaa turvallisesti ja estaa niiden lainvastainen kaytto, tarvitaan tehokkaita havaitsemis- ja tunnistamismenetelmia. Tassa vaitoskirjassa keskitytaan radiometrisiin analyysimenetelmiin. Sateilylahteen tunnistus perustuu radioaktiivisissa hajoamisissa syntyvaan sateilyyn, kuten alfahiukkasiin tai suurienergisiin fotoneihin. Menetelmien haasteena on erottaa tutkittavan kohteen lahettama sateily muista lahteista johtuvasta taustasateilysta. Tassa tyossa osoitetaan tunnistamisen helpottuvan, kun hyodynnetaan…