In general, all whole body counters must have (1) a mechanism that
reacts to the energy emitted by some kinds of disintegrating, or
radioactive, atoms; (2) a device that displays or records these
reactions; and (3) adequate shielding to exclude unwanted rays from
other sources.
[Illustration: Figure 1 _Types of whole body counters._]
[Illustration: A _The subject may be seated in a chair in an
iron-shielded room and under a scintillation detecting crystal._]
[Illustration: B _The subject may lie in a bed that slides into the end
of a hollow cylindrical tank filled with scintillation fluid._]
[Illustration: C _The subject may stand in a semicylindrical
double-walled tank filled with scintillation fluid._ (_See Figure 2._)]
[Illustration: D _The subject may lie on a wheeled cart and be wheeled
beneath a shielded detecting crystal._]
One of the first whole body counters was shown at an atomic science
conference in Geneva, Switzerland, in 1955 (Figures 1D and 2). While it
was on display, 4258 visitors to the meeting climbed a set of stairs to
enter a 10-ton lead-walled chamber. Here they stood still for 40 seconds
while the radioactive atoms in their bodies were being “counted”, or
recorded. This device, because it was the first one persons could walk
into, aroused great interest.
[Illustration: Figure 2 _How a “walk-in” whole body counter, such as the
one demonstrated at Geneva, works._]
Showing:
Signal lights
Lead glass windows
Lead shield
Scintillation shield
Photomultiplier tube
Solution storage tank under platform
Shielding for the Geneva counter consisted of 3 inches of lead. Only the
most energetic background gamma rays and cosmic rays can penetrate this
amount of shielding, and the number that do so remain almost constant
during successive counting periods. This constant remaining “background”
radiation level, once determined, could be subtracted from the recorded
number of emissions to provide the correct radiation total from the body
of each person examined.
[Illustration: Figure 3 _Typical crystals and liquid materials used to
produce scintillations for whole body counters and other
radiation-detecting instruments. Scintillation counters provide much
faster recording of radiation than Geiger counters, and are widely used
in experiments with high-energy particle accelerators, as well as in
whole body counters._]
To detect the gamma rays emitted by radioactive atoms disintegrating
within the body, whole body counters take advantage of a property of
radiation that has been known since 1896. In that year the English
physicist William Crookes discovered that X rays react with certain
chemicals to produce fluorescence. A few years later a New Zealand-born
physicist, Ernest Rutherford (later Lord Rutherford), found that this
glow consisted of many tiny individual flashes or scintillations, each
caused by the emission of a single alpha particle. He laboriously
counted individual flashes by observing them through a magnifying glass.
If you examine a luminous watch with a hand lens in a dark room, you can
see these fascinating scintillations, just as Rutherford saw them long
ago.
Today, scientists have found several crystals, liquids, and plastics
that are especially effective in showing scintillations caused by
nuclear radiations. One of these substances, with the challenging name
2,2′-_p_-phenylene bis [5-phenyloxazole], often shortened to POPOP, was
used in the scintillating liquid of the Geneva counter. How the flashes
are detected can be appreciated by considering the infinitely small
world of individual atoms and following a single atom as it
disintegrates. (For a more complete explanation of radioactivity, see
the companion booklet _Our Atomic World_ in this series.)
Let us assume that we are looking at a single potassium-40 atom in the
body of the person to be examined and that it is about to disintegrate.
(Potassium-40 is naturally radioactive. It is the most abundant
radioisotope in our bodies.) In any sizable portion of potassium-40, we
know that half of the atoms will disintegrate over a period of 1.3
billion years, but, since this process is random, there is no way for us
to know when any particular atom will do so. However, when it does, one
of two alternative events will occur: either a beta particle (that is,
an electron) will be ejected from the nucleus, creating an atom of
nonradioactive calcium-40, or the nucleus will capture one of its own
orbital electrons, resulting in creation of an atom of stable argon-40
and the emission of a gamma ray. (The beta emission process occurs in 89
out of every 100 disintegrations. See Figure 4.)
[Illustration: Figure 4 _Comparison of potassium-40 disintegration
methods._]
Assume that the particular gamma ray is traveling in the direction of
the scintillating liquid in the counter. Remember that the gamma ray is
tiny in comparison with an atom, which is mostly empty space. Therefore,
any one gamma ray probably will miss all the material part of the atoms
in the body of the person being studied. Nor will it collide with
anything as it passes through his clothes and the stainless steel tank.
It also may fail to collide with any of the atoms in the molecules of
scintillation liquid, of course. But let us assume that the one we are
watching does make a hit there. Its total energy will be converted
instantaneously to a flash of many bits or photons of light.
These photons radiate from the collision scene and strike a
light-sensitive surface in one or more of the counter’s photomultiplier
tubes, which have been placed where they can “see” the scintillation
liquid. Energy transformations result, and a tiny pulse of electricity
is originated. These photomultiplier devices are similar to the
equipment in the familiar “electric eye” door openers. As their name
suggests, photomultiplier tubes (see Figures 6 and 9) do more than
merely respond to the light flashes produced in the scintillation
liquid. They also amplify the weak electron disturbances into electrical
pulses to operate meters that record each scintillation and count the
total.
The Geneva counter recorded about 25% of the total gamma rays emitted by
each subject. Since this sample was a constant proportion of the total
body radiation, it could be converted to whole body measurements with
about 97% reliability.
In addition to finding persons with actual body contamination among
those counted at Geneva, the 1955 counter revealed some interesting
sideline information. People who failed to remove radium-dial watches
were soon spotted. And one small boy who had picked up a sample of
uranium ore at a nearby exhibit “jammed” the instrument.
Each of the 25 persons who were found to have above-normal levels of
radiation could recall having worked with radium or some other
radioactive substance at some time in the past.