Improvement of radon detector performance by using a large-sized PIN-photodiode

Radioactive noble gas radon (${}^{222}\mathrm{R}\mathrm{n}$) is one of the major background sources below the MeV region in rare event search experiments. To precisely measure radon concentration in purified gases, a radon detector with an electrostatic collection method is widely used. In this paper, we discussed the improvements of a radon detector by installing a new PIN-photodiode ($28\times 28$ $\mathrm{m}\mathrm{m}$) whose surface area is $2.5$ times larger than that used previously ($18\times 18$ $\mathrm{m}\mathrm{m}$). We evaluated the detector’s performance by serially connecting two radon detectors equipped with two types of PIN-photodiodes. As a result of the calibrations, we found an improvement of $(3.8\pm 2.4)\mathrm{\%}$ in the detection efficiencies below $1.0$ $\mathrm{g}\mathrm{/}{\mathrm{m}}^3$, while a $10$-$20\mathrm{\%}$ improvement occurred above this level. The intrinsic background of the detector equipped with the large PIN-photodiode was measured as $0.24_{-0.05}^{+0.09}$ $\mathrm{m}\mathrm{B}\mathrm{q}\mathrm{/}{\mathrm{m}}^3$. This background level is consistent with the radon detector with the small PIN-photodiode, although we installed the large one. This improvement is useful for applications in radon emanation measurements from a material, which also emits water from its surface.