2. Since there will usually be a mixture of elements in a target, and
many of these will be made radioactive, how can we tell one from
another?
3. Since radioactive atoms are constantly “disappearing” by
radioactive decay, how do we obtain the number of atoms created from a
measurement made some time after the bombardment has taken place? And
what of those atoms disintegrating while others are still being
created in the reactor?
Radioactive atoms almost always decay by emitting negatively charged
beta particles usually accompanied by gamma rays. Instruments can detect
these kinds of radiation, and it is by measuring the radiation that we
determine how many radioactive atoms are present. To do this we have to
know the types of radiation emitted by the radioactive atoms we are
trying to measure. Fortunately each kind of radioactive atom decays with
a unique “pattern” scientists call a “decay scheme”. The figure on the
next page shows a simplified decay scheme for manganese-56, which is
produced by activation of manganese, and a diagram showing what the
decay scheme means.
Until a few years ago, it was difficult to measure the number of gamma
rays of a particular energy that were being emitted by a mixture of
radioactive isotopes unless there were only a few such gamma rays with
very different energies. Today instruments are available that can really
pick them out of a complex mixture. Thus it is usually possible to
“separate” with electronic instruments the radioactive element we are
interested in measuring. Some of the examples below will show how this
might be accomplished.
Each radioactive nuclide[4] also has a characteristic half-life,[5]
which is a measure of how fast the radioactive atoms change (transmute)
to atoms of another element. In a reactor, even while they are being
produced in the target, atoms of the radioactive nuclide are decaying
with the particular half-life of the nuclide. The mathematical laws that
govern this process tell us that the number of atoms determines the
amount of decay; i.e., the more atoms there are, the greater the amount
of decay in a given period of time. (The fraction that decays in that
time is constant.) As a result, the target eventually becomes
“saturated”, that is, the rate of production equals the rate of decay.
When the irradiation is first begun, the number of radioactive atoms
increases steadily. But eventually, this rate of increase slows down
until, at saturation, further irradiation no longer increases the number
of radioactive atoms present in the target.
[Illustration: The energy of a gamma ray is equal to the energy
difference between the two levels involved in the gamma-ray emission.]
An _energy level diagram_. The slanted arrows indicate radioactive
decay by beta-particle emission. In each case, manganese-56 decays to
a certain energy level of iron-56. On the right the energy of each
level is indicated. Following a beta emission to a high-energy
(excited) state in iron-56, one or more gamma rays are emitted until
the nucleus is de-excited to the level marked zero. The vertical
arrows indicate gamma rays emitted during the de-excitation process.
The energy of each gamma ray is the difference between the levels
involved in the change. The numbers above the vertical arrows indicate
the relative proportions of gamma rays of different energies emitted
from that level.
The mathematical relationship that describes the irradiation process
exactly is: