Vintage radioactive wrist watches - Is Radium really nasty?

[Manolo Sobrino]

(01-21-2015 09:43 PM)walter b Wrote:  For sure. The only common material really working as a gamma shield is lead (Pb). Plastic and wood are mainly carbon - they don't shield gamma.

Yes, the half-value layer for 186 keV gammas in lead is ~0.4 mm (you need a 1.35 mm thick lead shield to reduce it to 10%... or about 8 cm of concrete). The idea for gammas is to use high Z materials. You'd use plastic (low Z) for betas, so you can reduce Bremsstrahlung. Usually beta sources are kept in plastic boxes (e.g. PMMA), to avoid having an X-ray source around. Plastics are rich in hydrogen, that's an interesting point for using them to stop (heavy) particles.

(But the trouble with jebem's radium are not the gamma rays. After all, those watches aren't really dangerous to wear if we look at the measured doses. The trouble is having radium in a battered paint laying around!)