Question

. A very senior faculty member comes by your office and brings you a small disk that is labeled Ra226 – 10 μCi – 2 Feb 56. He says he has been carrying it in his pocket for 40 years. How much radioactivity is left in the disk? (A question for later on: Has been exposed, and is his risk for cancer increased?)

A new source has just arrived in an electron capture device for a gas chromatograph. It says it is 20 mCi of Ni63. For your inventory report however you need the activity in bequerels. How many bequerels of Ni63 do you have?

Answer 1

Ra 226 has half life of 1600 yr

k = ln2 / ( t1/2) = 0.693 / ( 1600 yr) = 0.000433 yr-1

we use decay kinetcis formula to calculate activity left after 40 years

t = ( 1/k) ln [Ao] /[At] where [Ao] = 10uCi , t = 40 yrs , we calculate [At]

40yr = 1/ ( 0.000433yr-1) ln ( 10ui) / [At]

10ui / [At] = exp ( 0.000433yr-1 x 40yr)

[At] = 9.828 uCi is left

2) 1Ci = 3.7 x 10^ 10 Bq is relation

20mCi = 20 x 10^ -3 Ci = 20 x 10^ -3 x ( 3.7 x 10^ 10 Bq) / ( 1Ci) =7.4 x 10^ 8 Bq

Thus activity in Bq is 7.4 x 10 ^ 8 Bq