Page 94 - Radiochemistry and nuclear chemistry

Unstable Nuclei and Radioactive Decay 83

shorter-lived species to have died. The decay line related to this species can be extrapolated
back to t = 0 and this line subtracted from the observed decay curve. The resulting curve
should be the linear decay due to the shorter-lived species in the sample (line B). For more
complex mixtures, this process may be repeated until the curve is completely resolved into
linear components.
Sometimes it is possible in mixed decay to observe preferentially the decay of one species
by proper choice of detection technique. For example, a proportional counter may be used
at an operating voltage that allows detection of u-decay (~ > 0) but excludes detection
of B-decay (fib = 0). By contrast a typical Geiger counter can be used for B-decay but does
not detect a-radiation since the c~-particles do not penetrate the window of the Geiger tube.
These problems are discussed more extensively m Ch. 8.

4.13. Radioactive decay units

The SI unit for radioactivity is the Becquerel (Bq), and the activity is given in reciprocal
seconds, s- 1.

1 Becquerel (Bq) = 1 (disintegration)s-1 (4.48a)

The measured count rate (R, in (4.45)) is given in counts per second (cps) or per minute
(cpm); the abbreviations are in German Impulse pro minute (Ipm), and in French coups de
minute (c/m).
An earlier unit, still in some use, is the Curie unit (abbreviated Ci) defined as:

1 Curie (Ci) = 3.7 x 1010 S -1 (Bq) (4.48b)
The Curie unit was originally defined as the number of decays per unit time and gram of
226Ra, assuming its half-life to be 1580 y.
The specific radioactivity S is defined as the decay rate A per unit amount w of an element
or compound,

S = AIw (4.49)

The SI unit of specific radioactivity is Bq kg- 1. For practical purposes it is sometimes also
,,
I
,,
I
defined in dpm g- or @m mole- . Activity concentration (or radioactive concentration )
3
1
is given in Bq m- or Bq 1- . With the half-life of 1599 +4 y the specific activity per gram
12
10
of 226 Ra is 0.988 Ci or 3.65 x 10 Bq or 2.19 x 10 dpm. The specific activities of some
of the longer-lived naturally occurring radioactive species are: K, 31.3 kBq kg-1; 232Th '
4.05 MBq kg - 1; 2380, 12.4 MBq kg- I.