question_answer

The counting rate observed from a radioactive source at t = 0 was 1600 counts , and t = 8 s, it was 100 counts . The counting rate observed as counts  at t = 6 s will be [JEE ONLINE 26-05-2012]

A) 250

B) 400

C) 300

D) 200

Correct Answer: D

Solution :

[d] As we know,

n = no. of half life

N - no. of atoms left

initial no.. of atoms

By radioactive decay law,

k - disintegration constant

(ii)

From (i) and (ii) we get

or,

w = 4, Therefore, in 8 seconds 4 half life had occurred in which counting rate reduces to 100 counts .

Half life,

In 6 sec, 3 half life will occur

counts