-illcurrent pulse is produced.
The pulses,
recorded by a
scaling unit, are a measure of the radioactivity of the
sample.
Radioactivity is measured in terms of the number
of disintegrations per unit of time.
The disintegration
rate of radium was arbitrarily selected as the standard.
Tne unit is called a curie and is defined as the quantity
of any radioactive materialhaving associated with 1t
3.7 x 101° disintegrations per second (or 2.2 x 1012 disintegrations per minute).
activity of one curie.
at different rates;
One gram of radium has an
Other radioisotopes disintegrate
therefore the number of curies per
gram varies from isotope to isotope in proportion to the
half life and the number of atoms per unit weight.
One
gram of strontium-90 (Sr9°) which has a half life of about
28 years has an activity of 147 curies.
A radioisotope
with a shorter half life and/or a lesser atomic weight,
would have an even greater specific activity.
It is impor-
tant to note that the radioactivity per gram is large and
that the amount of a radioisotope necessary to be of concern
as a potential health hazard is essentially weightless.
The
amount is usually too small to be determined by gravimetric