Did you read the rest of the posts in this thread? There is a lot of good information in there already.
Your husband's doctor was right -- a single gamma ray, from any source, is enough to cause cancer. The odds of it happening are very low, but not zero. The human body is well equipped to deal with most of the insults that we naturally subject it to on a day-to-day basis, but there is always that chance that the protective and reparative functions fail.
The likelihood that radiation exposure caused your husband's death is also low, but not zero. There are many things that cause cancer, and the vast majority of Leukemia victims never have any exposure above background at all. The cause of their cancers is also possibly radiation due to background sources, but is much more likely to be from another carcinogen such as pesticides, water contaminants, air pollution, or even something as simple as grilled meat (yes, grilling meat produces carcinogens -- lots of them.) Even the smoke from the grill could be the cause.
It is impossible to pin a cancer on any single source because it is impossible to isolate a single cause from the sheer number of possibilities. Could the cause be radiation? Absolutely, yes. But you have to look at probability versus possibility. If your husband 'never came close to the 500 rem allowed per year in the Navy' (actually it is 500
millirem) then the majority of the radiation your husband was exposed to was either natural background or medical. You and everyone else that never sets foot inside a nuclear plant or submarine get an average of 620 millirem/year (
http://www.new.ans.org/pi/resources/dosechart/) so there is more probability that your husband's cancer, if it was caused by radiation, was caused by a source other than his occupational exposure.
Additional exposure adds additional risk, according to the most conservative theory. Another theory shows additional risk only above a certain threshold level (probably above the level your husband was exposed to) and yet another theory suggests that a certain amount of exposure actually lowers your health risks. Nobody knows for sure which model is correct, because it is impossible to remove all other factors that may influence the results of studies. The biggest (uncontrolled and therefore unscientific) study of all -- those of us who work in nuclear plants -- seem to indicate that we do not as a group have a higher incidence of cancer than the general public. In fact there is evidence that our rate is slightly lower than average. Again, that is not a controlled study so no real conclusions can be reached.
So could your husband's death be related to radiation exposure he received in the Navy? Certainly it is possible, but it in not probable. Statistically, it is much more likely he contracted it from some other source. It is tempting to blame occupational exposure and be able to point to a cause other than just random chance or other factors we have little or no control over. Unfortunately there is no way to tell for certain and the chances are better that it was something other than his Navy exposure.
In either case I am sorry for your loss.