An illuminator comprising a generally cylindrical body (14) along which extends a beta light cavity (54) and a sleeve (16) which can be slid onto the cylindrical body (14) after a beta light has been inserted into the beta light cavity (54) thereof. The sleeve (16) has a slot (56) in it which corresponds in position to the beta light cavity (54). The slot (56) is narrower than the beta light cavity (54), so that light from the beta light within the cavity can pass through the slot (56), but not the beta light itself The sleeve (16) may be translucent, in which case it does not necessarily need a slot in it.
