Development of glass-based nuclear emulsion plate as an ultra-high precision tracking detector in the era of fully automated readout systems

The positional accuracy of a nuclear emulsion can reach a submicrometric resolution owing to the size of its detection elements, i.e., 200 nm sized AgBr crystals. However, when used as a film in an experimental environment, the film is deformed by temperature and various mechanical stresses, and it becomes difficult to maintain the accuracy of the submicrometric absolute position throughout the detector. In this study, we attempted to solve this problem by using a glass base instead of a plastic base, which has been widely used thus far. To evaluate the deformation effect, sets of nuclear emulsion plates with a size of 10cm×12.5cm using various base materials were exposed to 5GeV/c pion beams. The magnitude of the deformation between the two films was evaluated using the beam pattern discrepancy. For the commonly used 180 μm thick polystyrene base, the magnitude of the deformation within the 3cm×4cm region was evaluated to be 1.77 μm on average, whereas for the 500 μm thick glass base, it was improved to 0.31 μm. This was approximately the same level as the measurement accuracy of the fully automated nuclear emulsion readout system, and the effect of the film deformation can be ignored. Through an analysis of this experiment, it became clear that the angle of the track had a systematic correlation with the material of the base. Because it can be inferred that the origin is owing to the difference between the refractive index of the base and the designed refractive index of the optical system of the readout system, an equation for correcting this systematic error was proposed. Nuclear emulsions with glass supporting bases are suitable for applications that require submicrometric positional accuracy throughout the detector, such as the precise measurement of momentum using a magnetic field and multiple scattering, and for use in outdoor experiments where the temperature changes widely.