Measurement of the emissivity of surface alpha particles based on the gaseous μTPC with optics in a low radioactive background

In underground astrophysics experiments such as, neutrino, dark matter, and double beta decay searches, it is important to use the ultra-low radioactive impurities in the material of the detectors. We have been developing the gaseous TPC with the $\mu$-PIC (or MPGD) and optics to measure the emissivity of alpha particles from the material surface in a low radioactive background. As a result, the sensitivity (the upper limit of detection) is achieved to a few $10^{-4}~\alpha/{\rm cm^2/hr}$. Further, by suppressing the radon gas in the chamber using the zeolite, we plan to improve the background level more. Until now, the TPC has been operated to measure samples for many other groups. In this presentation, we will detail the detector setup, the mechanism, the operation status, and the prospect.