Gamma Spectrometric Measurements Of Nuclear Material For Nuclear Forensic Purposes

Licentiate thesis, 2016

Nuclear forensics is a scientific discipline that aims to aid in criminal investigations concerning illicit use of nuclear and other radioactive material. The discipline aims to find signatures within the nuclear material that can help to find the attribution, or the origin and intended use of the seized material. Examples of signatures are isotopic composition, age and trace element content. The work presented in this thesis sheds some light upon limitations and possibilities in measurements of radioactive material using hand-held gamma spectrometric instruments commonly used by first-responders or in early stages of a nuclear forensic investigation, i.e. in the detection and identification of the material at e.g. the finding place. One part of the thesis evaluates the capability of categorizing uranium using low-resolution gamma spectrometry and gives a plausible explanation to why the categorization may become erroneous. The second part investigates what kind of information that can be extracted from a simple high-resolution gamma spectrometric measurement of a strong 241Am source. The results show that categorizing uranium using low-resolution gamma spectrometry is a challenging task, due to the characteristics of these instruments. To make trustworthy categorizations of uranium based on low-resolution measurements, the properties of the sample as well as possible shielding needs to be known. The results of the investigation of 241Am sources show that there are a number of promising signatures, such as radioactive and stable impurities and age, which may be used to identify a source when other information about the source is unavailable.