A visit to this type of counter recalls the first glimmers of scientific
insight that the radiation in the human body could be counted. In the
early 1950s, Frederick Reines and Clyde L. Cowan, two scientists at the
Los Alamos Scientific Laboratory, Los Alamos, New Mexico, built a large
liquid scintillation counter hoping to prove or disprove that neutrinos
really existed. Neutrinos are elusive, uncharged particles with
essentially no mass. They had been predicted in theory nearly 20 years
earlier to explain how beta particles of different energy levels can be
emitted from atoms with apparently identical nuclei.
[Illustration: Figure 5]
[Illustration: _Dr. Frederick Reines (left) and Dr. Clyde L. Cowan
(right), co-discoverers of the neutrino, lower a fellow worker into the
first “whole body counter”, the scintillation assembly used in their
experiment. Below, Dr. Wright Lanham, inside the counter, peers from the
opening._]
[Illustration: Figure 5, below]
According to theory, neutrinos are created whenever negative
beta-emitting atoms are produced. On this basis, Drs. Reines and Cowan
were convinced that the fission of the nuclear fuel in the reactors of
the Hanford atomic plant at Richland, Washington, should create high
densities of neutrinos. They went to Hanford and set up an elegant
neutrino-catching experiment that hinged on detecting and counting gamma
rays of definite energy. To accomplish this, they built a large liquid
scintillation detector and shielded it from stray gamma radiation. As
their work progressed, someone realized that the equipment was large
enough to allow a person to crawl inside. After further research at the
Atomic Energy Commission’s Savannah River Plant in South Carolina, they
were successful in finding their long-sought neutrino. In doing so, they
also developed the sort of instrument that can study the human body.
Figure 6 shows a person about to enter one version of a Los Alamos
counter, the instrument’s 140-gallon tank of scintillation fluid, and 45
of its 108 photomultiplier tubes. When the instrument is in use, the
tank slides into, and is shielded by, a 20-ton barrier of 5-inch lead.
[Illustration: Figure 6 _A liquid scintillation whole body counter at
Los Alamos National Laboratory, showing (left) a subject in the chute
before it is slid into the shielded detector chamber. Below, the same
instrument’s detecting assembly, showing the photomultiplier tubes,
removed from the shielding._]
The loading chute will hold a person 6 feet 4 inches tall and weighing
up to 260 pounds. The subject lies in the chute as it slides into the
counter. A lead plug behind his head closes the end of the cylinder to
add shielding. Counters of this type have “panic buttons” with which
subjects may signal if they become uneasy on being confined. Most counts
are completed in less than 5 minutes, however, so the buttons are rarely
used.
Since the detector fluid almost completely surrounds the subject when
the chute is in place, this type of counter captures twice as large a
fraction of the emitted gamma rays as does the Geneva type.
Each radioactive substance emits gamma rays with an energy level
characteristic of that substance. Whole body counters are able to
measure this specific energy spectrum, or “fingerprint”, and so identify
the kind of atom producing the radiation.
The number of light photons produced in the scintillation fluid is
proportional to the energy transferred by the incoming gamma rays. For
example, gamma rays emitted by potassium-40 have 1.46 million electron
volts (Mev) energy; those of cesium-137 have 0.660 Mev energy. When both
these radionuclides are producing flashes of light in the scintillation
fluid at once, the photomultiplier tubes produce two different strengths
of electrical pulses. Electronic devices called multichannel
pulse-height analyzers sort and record the number of each.