Back to Main
These definitions are for the lab exercise Natural Occurring Radioactivity
N = number of nuclides
D = disintegration rate (Bq, Ci, dimension s
-1)
A = activity is disintegration rate D per unit of volume or unit of mass (dimension s
-1)
We distinguish between two notions:
As(i) = specific activity is activity per unit of mass of that particular element the actual
radionuclide i belongs to. This notion is also used to express the activity of a
particular radiolabelled molecule in relation to the total mass of the same molecule
(labelled and unlabelled) in the sample (for instance when the inactive molecule is present
as a carrier).
Ac(i) = activity concentration is the activity of a special radionuclide
i or all present
radionuclides (AcT = A
c(∑i) = total activity) per unit of volume or unit of weight of a radioactive
sample.
N,D and A may, in addition, have an index t (i.e. N
t, D
t and A
t). This denotes the parameter values at time t relative to a starting time t = 0.
R = counting rate (cps, cpm or generally cpt where t should be defined, dimension s
-1)
S = number of counts
N, D, A, R and S may, in addition, have indexes B = background, T = total or N = net. The relations between them are: R
N=R
T - R
B
and
S
N =S
T - S
B. Other indexes may also be used, for instance S
Cr-51 in order to separate from S
Fe-59.
Examples:
We consider the following three samples:
P1: 1 kg pure iron (Fe). This sample contains10 kBq of the radionuclide 59Fe.
P2: 1 kg steel consisting of 70 weight% iron and 10 weight% of each of the elements Cr, Ni and Mo. The sample contains 10 kBq of
59Fe and 10 kBq of
51Cr.
P3: A 1L aqueous solution with 100 g dissolved FeCl
3 and 100 g dissolved CrCl
3. This corresponds to 35 g dissolved Fe and 31.6 g dissolved Cr. The solution contains 10 kBq
51Cr and 10 kBq
59Fe.
For sample P1 we then have:
As(
59Fe) = 10 kBq/kg Fe
Ac(
59Fe) = 10 kBq/kg sample
For sample P2:
A
s(
59Fe) = 10 kBq/0.7kg = 14.3 kBq/kg Fe
A
s(
51Cr) = 10 kBq/0.1kg = 100 kBq/kg Cr
A
c(
59Fe) = 10 kBq/kg sample
A
c(
51Cr) = 10 kBq/kg sample
AcT = 20 kBq/kg sample
For sample P3:
As(
59Fe) = 10 kBq/0.035 kg Fe = 285.7 kBq/kg Fe
As(
51Cr) = 10 kBq/0.0316 kg Cr = 216.5 kBq/kg Cr
Ac(
59Fe) = 10 kBq/1.2 kg sample = 8.33 kBq/kg sample and/or 10 kBq/l sample.
Ac(
51Cr) = 10 kBq/1.2 kg sample = 8.33 kBq/kg sample and/or 10 kBq/l sample
AcT = 20 kBq/1.2 kg sample = 16.66 kBq/kg sample and/or 20 kBq/l sample.
Indexes may be omitted when the meaning of the parameters cannot be misunderstood.