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APPENDIX I
PRINCIPLE OF THE CALIBRATOR
GENERAL
The definition of activity, the basic principle of the calibrator, and the detailed discussion on
the calibration are presented in this section.
DEFINITION OF ACTIVITY
Activity
Activity is defined as:
The activity, A, of a quantity of a radioactive nuclide is the quotient of dN by dt, where dN is
the number of spontaneous nuclear transformations which occur in this quantity in time
interval dt.
dt
dN
A =
The special unit of activity is Curie (Ci):
1 Ci = 3.7 x 10
10
s
-1
(exactly)
Note: The term nuclear transformation is meant to designate a change of nuclide of an
isomeric transition. (ICRU REPORT 19, 1971)
The SI (International System of Units) unit for activity is the reciprocal second, s
-1
, and is
named the Becquerel (Bq), i.e.;
1 Bq = 1 Nuclear Transformation per second
1 Ci = 3.7 x 10
10
Bq
Types of Transformations
α-decay
The nucleus emits a helium nucleus (α-particle).
Electron Capture (ε-decay)
The nucleus captures one of its own orbital electrons, usually from the K shell, and a
neutrino is emitted.
June 09 APPENDIX I A1 - 1
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