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      8.2 Internal Radiation

      In the context of radiation safety, the radiation dose from an internally deposited radionuclide is no different from the same dose absorbed from external radiation. Therefore, it must be emphasized that a millirem of dose is a millirem, regardless of whether it was delivered from an internally deposited radionuclide or from an external source.

      Radioactive substances, such as other noxious agents with which the industrial hygienist deals, may gain entry into the body through four pathways:

      Measures to prevent internal exposure therefore are designed to either block a portal of entry or to interrupt the transmission pathway. These preventive measures can be effected either at the source by enclosing and confining it, through environmental control by ventilation, or with the use of protective clothing and respiratory protective devices. It should be noted that these measures are the same as those employed in the practice of industrial hygiene in a nonradiation environment. However, because of the nature of radiation and radioactive materials, the degree of control required for radiation safety greatly exceeds the requirements for chemical safety. For example, the permissible exposure level (PEL) for inorganic mercury is listed as 0.1 mg m⁻³. For a commonly used radioisotope of mercury, ²⁰³Hg, the NRC's limit of 4 × 10⁻⁷ μCi cm⁻³ corresponds to a mass concentration of 3 × 10⁻⁸ mg m⁻³. A consideration when dealing with radioactive materials is whether or not to use a respirator. This question arises when a worker is exposed simultaneously to external radiation and to radioactive aerosols. Respirator use can offer protection against airborne radioactivity, thereby limiting the worker's internal dose. However, wearing a respirator decreases the worker's efficiency, increases the time necessary to complete the job, and thus increases his external dose. The decision regarding the use of a respirator depends on the effect of the respirator on the worker's total dose, the sum of the external and the internal doses from the inhaled radioactivity.

9 SAFETY ASSESSMENT

The effectiveness of radiation hazard controls is assessed in a surveillance program similar to that for the assessment of industrial hygiene controls for nonradiation hazards. Such a surveillance program includes the observation of both workers and the environment, and may employ a variety of techniques, depending on the nature of the hazard and the consequences of a breakdown in the system of controls. These techniques may include personal monitoring for external radiation exposure; environmental radiation surveys, including measurement of external radiation levels; air sampling and continuous air monitoring; contamination surveys; and estimation of internally deposited radioactivity by in vivo methods such as whole body counting or in vitro bioassay. Since humans have no sensory response to radiation, one must rely completely on instruments for our radiation safety assessment. Instruments are used in the practice of health physics for a variety of different purposes and fall into one of the two categories: particle counting instruments and dose measuring instruments. Within each of these categories, instruments have been designed for specific applications, such as measurement of low‐energy X‐rays, alpha particles, beta particles, measurement in high backgrounds, and neutrons of various energies.

10 PARTICLE COUNTING INSTRUMENTS

      Particle counting instruments are widely used as portable detectors for conducting area surveys and in the laboratory for measuring the activity in an air or water sample. Particle counting instruments are usually very sensitive detectors of radiation; they literally respond to a single ionizing particle. They therefore are widely used in searching for unknown radiation sources, leaks in shielding, and contaminated areas. Generally, they are not designed to measure dose. The radiation detector in a particle counting survey meter may be either a gas, as in the case of a Geiger counter, or a solid crystal in the case of a scintillation counter. In both detector types, passage of an ionizing particle through the detector results in a burst of ionization that is converted into an electrical output signal that actuates a readout device to register a count.

      10.1 Geiger–Muller (G–M) Counters

      Geiger counters are among the most used and the oldest of all the portable survey instruments. This popularity is due to their relative simplicity, which makes them comparatively inexpensive, and to their wide range of usefulness. G–M counters are available in a variety of sizes and shapes, and they can be designed to respond to alpha, beta, and gamma radiations by varying the choices of window thickness and wall thickness. For detecting alpha and low‐energy beta particles, the window through which the radiation must pass in order to get into the counter's sensitive volume must be very thin. Geiger counters are very efficient in counting those alphas or betas that get into the sensitive volume. However, the inherent efficiency of G–M counters for gamma rays is very low. Gamma rays are very highly penetrating and thus can easily pass through the sensitive volume in a G–M tube. Use of a relatively thick‐walled G–M tube greatly increases its sensitivity for gamma rays. The gamma ray interacts with the thick wall by knocking an electron out of one of its atoms. The free electron then enters into the tube's sensitive volume and triggers the counting mechanism. Of course, if the wall is too thick, the electron will not reach the sensitive volume, and the photon will not be counted. It is thus seen that the radiation response of G–M counters is energy dependent. Therefore, two factors in the choice of a G–M instrument are the type and energy of radiation to be detected.

      There is a finite time after a particle is counted, called the resolving time, during which the G–M counter will not respond to a second particle. For G–M counters, the resolving time is ∼100 μs. If particles enter the counter at a faster rate than can be resolved, then the counter becomes paralyzed, and the meter reading falls to zero. Therefore, the counter should be turned on before entering a radiation area. If there is a very high radiation field, then the counter will respond, and the count rate will continue to increase (thereby signaling the surveyor of the possible very high radiation level) until it becomes paralyzed. Thus, the radiation level at which the counter becomes paralyzed is an important parameter in choosing an instrument. Modern instruments typically cannot be paralyzed, but this should be verified prior to use.

      Another important parameter in choosing or in using a portable surveying instrument is the instrument's response time. A finite time is required for the meter to give the true value of the radiation field. This response time is determined by the time constant of instrument's electronic circuitry. The time constant is defined as the time required for the meter reading to achieve 63% of its final reading. Thus, the instrument must be exposed to the radiation field for several time constants in order to obtain a true measurement of the radiation field. This is of importance when scanning an area too quickly, or in measuring radiation fields of short duration. The time constant for most instruments is on the order of several seconds. Furthermore, if a surveying instrument has several ranges, the time constant increases as the sensitivity of the instrument increases. This consideration is significant, for example in measuring the radiation level from a dental or diagnostic medical X‐ray machine at the technician's position because the X‐ray exposure is only a small fraction of a second. In this case, the meter would read much less than the true radiation level. Because of the inertia in the metering system of a G–M counter, most Geiger counters have an audio output that is not limited by the meter's time constant. It is therefore imperative that the health physics surveyor use the audio output, either through earphones or through a loudspeaker that is incorporated into many monitoring and surveying instruments.

A G–M counter is not designed for measuring