Radium 225 emits ALPHA particles, i.e. a Helium nuclei, two protons-two neutrons. These heavy particles have little ability to penetrate solid matter and as such the glass of the dial would stop 99% of them and the remainder would not travel far in the air.
Radium 225 emitting an alpha particle now becomes Radon 222 (4 days) a gas, the minuscule amount produced should easily be contained by the dial enclosure. Radon 222 emits another alpha particle to become Polonium 218. With a half-life of 3 minutes it (Polonium) almost immediately emits another alpha, a beta particle (high energy electron) and a gamma ray. Beta and gamma are penetrating radiations, gamma much more so. So your detector needs must react to all three types and anything you detect outside the dial will invariably be either beta or gamma from the Polonium decay.