So my knowledge of radioactive isotopes is limited to a series of youtube videos. I assume this idea is very dangerous and should never be attempted, but my curiosity still thrives. I own a tritium keyfob light, I've read it poses almost no threat to keep on me and it's a cool piece of science. I wonder if a handwarmer can work in the same idea.
I would like to know if a radioactive handwarmer is possible. The design would be simple, assume 5cm by 8cm by 2cm object (fits well in most hands). I assume for it to work one would need to put a radioactive core in the center, some material around that to turn the radiation into heat, and a protective shielding around the outside. Dimensions can change if needed, but that's the target. I don't know what materials, dimensions, or proportions would work best to ensure warmth and safety.
My questions are as follows:
1- What materials are recommended for this construction?
2- How hot can the hand warmer become without pumping out harmful/lethal radiation? (If specifics are needed, the hand warmer is at arms length from vital organs in the users hand)
3- How harmful/lethal would this device be if it were to maintain a warm heat during a cold day. (if temperature is needed, assume the air temp is 15c, the hand warmer should get up to 38c with some insulation such as a hand wrapped around it [EDIT: As suggested, temperature isn't as good a measurement as knowing exactly what the power output should be. If possible, try to aim for 3-4 thermal Watts] )
4- For any answers to the above, how long would it last? Where would the half-life stand?
If this is not the right place to ask this question, can someone point me in the right direction to find these answers?