This topic came up at alternityrpg.net, so I put up another post there, but expanded on it and thought I would repost it here.

Furthermore, the energy as well as the quantity and rate (summarized as fluence) of all these types of radiation all contribute to the effect of the radiation. The latter is measured as exposure in the units Sievert (Sv) or Rem. (The unit rad, or Gray [Gy] is amount of radiation absorbed with 1 Gy = 1 Joule / kilogram).

For example, a Sv of beta particles of low energy (say, 0.3 Mega Electron Volt [MeV]) might not even pentrate the very outer layer of skin, whereas a higher engery (6 MeV) will penetrate 3 cm deep. Even high energy (15 MeV) will go 8 cm deep, plus, will start to generate gamma rays themselves from the Bremsstrahlung effect http://en.wikipedia.org/wiki/Bremsstrahlung

However, those 15 MeV beta particles will not do much acute actual tissue damage if their rate is so slow (ie. 1 Gy every 24 hours) that tissue repair occurs as the damage is occuring, and the total exposure time is less than a week. On the other hand, a very high rate of 6 MeV beta particles (say, 10 Gray a minute over 5 minutes) will damage the skin so badly, that in 2 or 3 days, all the exposed skin will suddenly slough off extremely painfully and perhaps without the person even realizing that they'd been exposed to radiation at all!

As for how radiation "medicine" works, once the person is no longer exposed to the radiation, the main thing to do is reverse the symptoms of the damage. There are 3 main ways that an [B]instantaneous[/B] radiation exposure acutely injures people. That is, high dose of radiation exposure over a period of a few minutes to an hour.

1) After about 4 Gy of absorbed gamma rays, this is sufficient to wipe out the immune system in a few days such that 50% off people will die of infection - fortunately, this is very easy to cure with contemporary medicine. Basically, isolate them long enough that the body can replace its immune cells, or perform a bone marrow transplant.

2) After about 8 Gy of total body irradiation in a short time period, sloughing off (similar to the skin example above) of the mucosal throughout the gastro-intestinal tract occurs within a few days. The subject then has severe fluid and blood loss through the GI system and die - ie. severe bloody diarrhea leading to death. This is much harder to fix with contemporary medicine.

3) After about 12 Gy immediate total body irradiation, brain swelling occurs within a few hours. A subject will get back headaches, then lose conciousness and die. Currently, there's really no cure for this.

If radiation exposure is of a lower level over a period of several days or weeks however, the side effects can be very different - ie. Sunburn to the skin (bad enough to peel or burn and blister), sunburn to the lungs causing them to completely fail over a period of a few days, sunburn to the spinal cord causing it to stop working, etc.

Assuming there's not enough radiation to cause any of these, the main concern about radiation is the small chance that it'll cause a cancer 5, 10, 20, or even more years later (an increase of about 2% of the population per 0.1 Sv of exposure).

In advanced societies however, these shouldn't be too hard to fix. What's more difficult is if the radiation exposure is continuous (ie. radioactive substance that's eaten or injected). Currently, the only real anti-radiation medicine we have is called Ethoyl (amifostine) that works by "scavenging" or taking up the free radicals that radiation causes within cells. This works because typically - at least with gamma and beta radiation - the radiation doesn't usually interact with the DNA - it reacts with water, causing it to produce free radicals, when then go on to cause the DNA damage. In essence, Ethoyl is a super "anti-oxidant" which takes up the free radical before it can cause the DNA damage. I guess in the future, there could be Rad-Protect injected into an adventurer before deployment
into a radioactive field (as oppsed to Rad-away from the old Fallout game 8-).

Suffice it to say, radiation is complicated enough that rather than coming up with a system of rules, it may be simpler to approach it as a plot point instead. Ie. the PC's were exposed to a high amount of radioactivity during a mission. They must now race to find the bad guy before he releases it on an unsuspecting population all the while knowing that if they take too long, they will perish before he can even do that!
I figure, one day, I'll give a talk about radiation and its effects on biological systems some day at Gencon or Origins...