Development of a method to measure trace level of uranium and thorium in scintillation films

We have established a method to measure picograms-per-gram (pg g⁻¹) levels of ²³⁸U and ²³²Th in scintillation films by combining the dry ashing method and inductively coupled plasma mass spectrometry. Trace amounts of ²³⁸U and ²³²Th were measured in up to 2~g of the scintillation film with almost 100% collection efficiency. This paper details the experimental procedure, including the pretreatment of the samples and labware, detection limit of the method, collection efficiencies of ²³⁸U and ²³²Th, and measurement of ²³⁸U and ²³²Th in a polyethylene naphthalate film. This method is also applicable to ²³⁸U and ²³²Th measurements in other low-background organic materials for rare event search experiments.