A very senior faculty member comes by your office and bring

. A very senior faculty member comes by your office and brings you a small disk that is labeled Ra226 – 10 Ci – 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?

Solution

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