When we visit a crystal counter, shown under construction in Figure 8,
walls of battleship steel 6 to 8 inches thick are the first things we
see. Rather than using shielding only around the detecting instrument,
as was done in the Geneva counter and early versions of the Los Alamos
counter, crystal counters have shielding around the entire counting
room. With this arrangement the instruments are available for adjustment
and servicing.
This type of counter also uses a different detection device: a solid,
rather than a liquid scintillator. A large crystal, usually of sodium
iodide sensitized with thallium, is used to convert gamma rays to light
photons.
Let us return to the shielding problem for a moment. Tanks of water,
bricks, stone, and lead have been tried by scientists seeking effective,
cheap, and convenient shielding. Some early counters were built deep
underground in the hope of avoiding cosmic radiation. Radioactive
elements are so widely distributed in the rocks, soil, water, and air,
however, that there is no place where background radiation does not
exist. Not even the crystals, glass, or metals used in the detection
system are free of radioactivity.
Pre-World War II surplus armor plate came to be the preferred shielding
material. Thick slabs of battleship steel were available after the war
at low cost. Furthermore, steel produced since the war may contain
unwanted radioactivity originating in fallout from nuclear tests and
make it undesirable for shielding. Sometimes cobalt-60 used as a tracer
to measure deterioration of blast furnace walls causes problems in
postwar steel, too, so old warship armor is used when possible.
Some whole body counters have additional shielding. In the counter at
the Brookhaven National Laboratory, Upton, New York, the steel room is
lined with ¼-inch lead sheets, covered by thin layers of cadmium and
copper. The lead is intended to absorb the secondary X rays produced in
the iron by the interaction of high-energy gamma and cosmic rays. The
cadmium and copper absorb the secondary radiation that is similarly
produced in the lead.
The doors of these rooms often weigh 6 tons or more. A special escape
hatch was built into the counter room at the University of California at
Los Angeles, to be used if the main door should be jammed by an
earthquake. In newer whole body counting laboratories, such as the one
at the National Institutes of Health, Bethesda, Md., the steel rooms are
concealed in the interior design and are so pleasantly furnished that
the patient scarcely is aware of the thick walls around him.
[Illustration: Figure 9 _A sodium iodide crystal, right, and a cluster
of 7 photomultiplier tubes that fit under it to record its
scintillations._]
Figure 9 shows a sodium iodide crystal used to react with the gamma rays
that traverse it. To the left of the crystal is a cluster of seven
photomultiplier tubes that “watch” for the scintillations, convert them
to electrical pulses, and amplify them so they can be sorted, counted,
and recorded. A trace of thallium added to the sodium iodide improves
its scintillation properties.
In addition to being of convenient size and easy to maintain, crystal
detectors have another advantage over liquid systems. The energy of the
incident gamma rays from crystals is more accurately indicated by the
quality of the flashes of light impinging on the photomultiplier tubes.
If two or more radionuclides are emitting gamma rays, a crystal detector
distinguishes between their energy levels with much more precision and
sensitivity than does a liquid system. Crystal instruments separate
gamma rays differing by no more than 0.05 Mev.
The energies of the gamma rays emitted by nuclides have all been
determined and are listed in handbooks. A scientist can thus identify
the data delivered by a multichannel pulse-height analyzer as coming
from potassium-40, zinc-65, or any other nuclide.
Counters using sodium iodide crystals intercept, and therefore count, a
much smaller fraction of the gammas emitted by the subject’s body than
liquid systems, but they also pick up a smaller amount of background.
When speed is important, the liquid counter is more effective, but the
crystal counter is preferred when radionuclides emitting gammas of
nearly the same energies are to be separated and counted.