How can we able to calculate the distance of radiation detector ?

Question

which is used for determining the existence of DIRTY BBOMBS AND OTHER NUCLEAR AND RADIOLOGICAL BOMBS

Answer 1

You can't, because it all depends on how well the bomb is shielded as well as just what the bomb contains.

What you are really asking is at what distance from the bomb will the radiation level be 75mR/hr.

In general, you'll have a certain amount of material generating radiation at a certain rate R. Of this only a fraction X gets through to the outside.

As with all radiation, its intensity falls off with the square of distance. So if you know the initial radiation level R and the fraction X, then you could compute the distance. But, of course, you don't.

http://en.wikipedia.org/wiki/Dirty_bomb