Question:
The other day I search under my bed directly 5" below my pillow is a 1980s smoke detector place their since 2002 of Dec. Would I develop a malignant brain tumor since I noticed an ice pick headache on the left back of the head? I've heard of people having stroke due to radiation exposure. Is that true?

Answer: Smoke detectors are not a big source of radiation unless you decide to eat the little Americium disk inside, and even then it isn't clear it would hurt you. Assuming you are in the United States and the smoke detector is the usual Americium kind, here is what the EPA says: How much radiation is in smoke detectors? The radiation source in an ionization chamber detector is a very small disc, about 3 to 5 millimeters in diameter, weighing about 0.5 gram. It is a composite of americium-241 in a gold matrix. The average activity in a smoke detector source is about one microcurie, 1 millionth of a curie.

Americium emits alpha particles and low energy gamma rays. It has a half-life of about 432 years. The long half-life means that americium decays very slowly, emitting very little radiation. At the end of the 10 year useful life of the smoke detector, it retains essentially all its original activity.

How much radiation exposure will I get from a smoke detector? As long as the radiation source stays in the detector, exposures would be negligible (less than about 1/100 of a millirem per year), since alpha particles cannot travel very far or penetrate even a single sheet of paper, and the gamma rays emitted by americium are relatively weak. If the source were removed, it would be very easy for a small child to swallow, but even then exposures would be very low because the source would pass through the body fairly rapidly (by contrast, the same amount of americium in a loose powdered form would give a significant dose if swallowed or inhaled). Still, its not a good idea to separate the source from the detector apparatus.

http://www.epa.gov/radiation/sources/smoke_alarm.htm

The skull is excellent protection against external radiation sources and as Jeff says, even a sheet of paper will stop alpha particles. You get more gammas and betas from the natural radioactivity in your house than you will from this smoke detector. The cinder blocks in an average basement produce something like 75 millirems a year of radiation from the natural materials in them, so the hundredth of one millirem from the smoke detector is nothing in comparison. You get more irradiation to your brain from the natural potassium and carbon radioisotopes in your own body than you do from a smoke detector: Small traces of many radioactive materials are present in the human body. These come mainly from radioactivity present in minute quantities in the food we eat. The only radionuclide which gives a significant dose is K. A person weighing contains of potassium, most of which is located in the muscle. About 0.01% of this is K and this delivers an equivalent dose of about a year. C which is produced through nuclear transmutation by cosmic rays in the upper atmosphere, delivers about a year.

http://www.triumf.ca/safety/rpt/rpt_4/node7.html

( 0.2 mSV a year = 20 millirems a year)

Even people who were injected with the radioactive contrast material Thorotrast and had the misfortune to have some of it seep outside their blood vessels and lodge in their brains did not always develop brain tumors, and if they did, it usually took between 20 and 40 years for the tumors to develop, so you wouldn't have gotten one this quickly in any case.

As for strokes from radiation exposure, the only cases I could find of this were in people who had such large doses of radiotherapy in the head or neck region that it damaged the arteries there, leading to an eventual blockage and stroke.

So try to sleep peacefully despite the smoke detector (why was it hidden under the bed? From the 5", it sounds as if it must have been attached to the bottom of the boxspring or to the bed slats. This seems very odd, since smoke rises and one would think it should have been placed on the ceiling......).