That is interesting, I looked up the decay chain to see if I could spot the daughter isotope that would be responsible for the browning.
Thorium-232 (14 billion years) alpha
Radium-228 (5.6 years) (most likely culprit) beta
Actinium-228* (6.1 hours) beta
Thorium-228 (1.9 years) beta
Radium-224 (3.7 days) alpha
Radon-220 (56 seconds) alpha
Polonium-216 (.15 seconds) alpha
Lead-212* (11 hours) beta
Bismuth-212* (61 minutes) alpha
Thallium-208* (3.1 minutes) beta
Looking into the Radium, we find that discoloration of glass is common.
When first prepared, nearly all radium compounds are white, but they discolor on standing because of intense radiation. Radiation causes a purple or brown coloration in glass on long contact with radium compounds. Eventually the glass crystallizes and becomes crazed.
What is happening is the Beta and Gamma radiation are knocking atoms around in the compound, and they are ending up in a more ordered state (crystallized) which is creating some sort of change in the index of refraction (as best I can tell).