-12methods and can be measured only by instruments that are
designed to count nuclear radiations.
For example,
the
amount of Sr9° that has fallen out of the stratosphere
and onto the continental United States has been less than
a pound and a half or approximately one gram per 5,000
square miles (calculated on the basis of 30 millicuries
per square mile and 3 x 106 square miles in the continental
United States).
be found,
sr? from this pound and a half source can
by radiological methods,
in milk,
wheat,
bone samples collected throughout the nation,
plant and
but the amount
in any one sample is too small to be weighed even by the
most sensitive balance.
In biological work,
a curie is often too large an
amount of radioactivity to be expressed simply,
so the
unit of radioactivity is often expressed as a fraction of
a curie, e. g., a millicurie (mc), one one-thousandth of a
curie;
or a microcurie (uc),
one one-millionth of a curie;
or even a micro-microcurie (uuc), which is equal to 2.2
Gisintegrations per minute.
The curie is the rate at which energy is being
released from nuclei of atoms regardless of whether the
energy is being released as alpha or beta particles,
gamma