HEALTH PHYSICS INSTRUMENTATION

RADIATION DETECTORS Humans do not possess any sense organs that can detect ionizing radiation. As a consequence, they must rely entirely on instruments for the detection and measurement of radiation. Instruments used in the practice of health physics serve a wide variety of purposes. It is logical, therefore, to find a wide variety of instrument types. We have, for example, instruments such as the Geiger-M‥uller counter and scintillation counter, which measure particles; film badges, pocket dosimeters, and thermolu-minescent dosimeters, which measure accumulated doses; and ionization-chambertype instruments, which measure dose and dose rate. In each of these categories, one finds instruments designed specifically for the measurement of a certain type of radiation, such as low-energy X-rays, high-energy gamma rays, fast neutrons, and so on.

The basic requirement of any such instrument is that its detector interacts with the radiation in such a manner that the magnitude of the instrument’s response is proportional to the radiation effect or radiation property being measured. Some of the physical and chemical radiation effects that apply to radiation detection and measurement for health physics purposes are listed in THE following table.

PARTICLE-COUNTING INSTRUMENTS Particle-counting instruments are frequently used by health physicists to determine the radioactivity of a sample taken from the environment, such as an air sample, or to measure the activity of a biological fluid from someone suspected of being internally contaminated. Another important application of particle-counting instruments is in portable radiation-survey instruments. Particle-counting instruments may be very sensitive—they literally respond to a single ionizing particle. They are, accordingly, widely used in searching for unknown radiation sources, leaks in shielding, and areas of contamination. The detector in particle-counting instruments may be either a gas or a solid. In either case, the passage of an ionizing particle through the detector results in energy dissipation by a burst of ionization. This burst of ionization is converted into an electrical pulse that actuates a readout device, such as a scaler or a ratemeter, to register a count.

Gas-Filled Particle Counters Consider a gas detector system such as is shown in the following Figure. This system consists of a variable voltage source V , a high-valued resistor R, and a gas-filled counting chamber D, which has two coaxial electrodes that are very well insulated from each other. All the capacitance associated with the circuit is indicated by the capacitor C. Because of the production of ions within the detector when it is exposed to radiation, the gas within the detector becomes electrically conducting. If the time constant RC of the detector circuit is much greater than the time required for the collection of all the ions resulting from the passage of a single particle through the detector, then a voltage pulse of magnitude

where Q is the total charge collected and C is the capacitance of the circuit, and of the shape shown by the top curve in the following Figure , appears across the output of the detector circuit.