14.2.6 Units of radioactivityThe Syste`me International d’Unite ́s (SI system) uses the becquerel (Bq) as the
unit of radioactivity. This is defined as onedisintegration per second(1 d.p.s.).
However, an older unit, not in the SI system and still frequently used, is thecurie
(Ci). This is defined as the quantity of radioactive material in which the number
of nuclear disintegrations per second is the same as that in 1 g of radium, namely
3.7 1010 (or 37 GBq). For biological purposes this unit is too large and the micro-
curie (mCi) and millicurie (mCi) are used. It is important to realise that the units Bq
and Ci refer to the number of disintegrations actually occurring in a sample not to
the disintegrations detected, which generally will be only a proportion of the disin-
tegrations occurring. Detected decays are referred to ascounts(i.e. counts per second
or c.p.s.).Example 1THE EFFECT OF HALF-LIFE
Question The half-life of^32 P is 14.2 days. How long would it take a solution containing 42 000
d.p.m. to decay to 500 d.p.m.?
Answer Use equation 14.3 to calculate the value ofl. This gives a value of 0.0488 days^1.
Then use equation 14.1 to calculate the time taken for the counts to decrease. In this
equationN 0 ¼42 000 andNt¼500. This gives a value fortof 90.8 days. (You can
check that this is right by doing an estimate calculation in your head; it is roughly
6 half-lives to get from 42 000 to 500.)6.25%10
23412.5%Number of half-lives0255075100% of original activity remaining25%50%Fig. 14.1Demonstration of the exponential nature of radioactive decay.558 Radioisotope techniques