10.2 Scintillation Counters
A scintillation detector is a transducer that converts the energy of the radiation into a flash of light, which is then converted by a photomultiplier into a voltage pulse. Whereas the inherent gamma ray detection efficiency of G–M counters is very low, usually less than 1%, the gamma ray detection efficiency of solid scintillation detectors is very high. Thus, a scintillation detector is extremely sensitive for detecting gamma radiation as compared to a gas filled G–M detector. Furthermore, since the intensity of the flash of light in the scintillation detector is proportional to the energy of the incident radiation, a scintillation counter can, with the aid of the proper electronics, be used to distinguish among various gamma ray energies. Using this principle, scintillation survey meters are available that can be used to identify the gamma‐emitting isotope. The resolving time of a scintillation detector is much less than that of a G–M counter. Scintillation detectors therefore can be used in higher radiation fields than G–M counters. However, in return for these advantages, scintillation detectors are relatively expensive and relatively fragile as well as being temperature dependent. Because of its high gamma ray sensitivity, the typical scintillation survey meter has a full‐scale response of 0.02 mrad h−1 (from 137Cs or 60Co). Like G–M counters, scintillation detectors generally are not designed for measuring dose. Table 4 identifies the best applications for some of the more common scintillation detectors and for G–M detectors in general. Typical portable scintillation detectors for contamination control include ZnS(Ag) and NaI(Tl) crystals.
11 DOSE MEASURING INSTRUMENTS
The response of dose measuring instruments is proportional to the amount of energy absorbed from the radiation, rather than to the radiation flux. Dose measuring instruments used in the practice of health physics fall into two categories: portable survey instruments and personal monitoring devices.
11.1 Portable Survey Meters
The most commonly used survey meter is the ion chamber in which the ionization current is measured. Since a specific amount of energy is expended in creating a single ion (34 eV per ion in air), the ion current generated by the radiation within the ion chamber is proportional to the rate of energy transfer to the gas within the chamber. The same caveats regarding energy dependence and response time that applied to G–M counters also apply to ionization chambers. Generally, survey meters are relatively energy independent over a wide range of energies, from about 50 keV to about 3 MeV. The energy range can be extended downward by incorporating several windows of varying thickness into the instrument. For example, a window thickness of 10 mg cm−2 enables an ionization chamber to measure dose from 10 keV X‐rays. Ordinarily, ionization chambers are sensitive down to about 0.5 mrad h−1. However, the sensitivity can be greatly increased by increasing the gas pressure within the ion chamber. This presents the radiation with more atoms that can be ionized, thereby increasing the ion current from a given radiation field.
The measurement of dose rates using G–M and scintillation counters should be performed with caution, as these instruments measure individual ionizing events, rather than the number of charges created by the radiation. Both detectors need to be calibrated with a radiation source.
11.2 Personal Dosimeters
The ICRP recommends monitoring individual workers whose annual external doses are likely to exceed 1 mSv (100 mrem), and those workers whose annual doses are likely to exceed 5 mSv (500 mrem) “should certainly be monitored.” The USNRC requires personal monitoring if a worker's annual external dose is likely to exceed 10% of the dose limit. Despite the legal requirement for monitoring only those workers whose dose is likely to exceed 10% of the limit, it is reasonable to monitor all radiation workers in case a question should arise in the future about the possible relationship between the worker's ill health and his occupational radiation dose. In addition to monitoring for legal reasons, knowledge of personal doses and dose distribution can contribute to the control of operating procedures and to the design of facilities. Personal monitoring for external radiation is accomplished by personal dosimeters that are worn by individual workers. These dosimeters are discussed in the following sections, and a summary is provided in Table 5.
TABLE 4 Detector selection for particle counting and contamination control.
| Detector type | Application | Ability to detect α | β | γ |
|---|---|---|---|---|
| G–M | Surface scanning, measurement of smear samples | Good (>4 MeV) | Good (>70 keV) | Poor |
| ZnS(Ag) | Surface scanning, measurement of smear samples | Good (>4 MeV) | Poor | Poor |
| NaI(Tl) | Surface scanning, measurement of smear samples | Poor | Poor | Good |
TABLE 5 Advantages and disadvantages of common dosimeters.
| Dosimeter | Advantages | Disadvantages |
|---|---|---|
| Film | Permanent record, tissue equivalent | Requires wet chemical developing |
| OSL | Can be reread, tissue equivalent, low detection threshold, high detection possible | Insensitive to neutrons (neutron sensitive OSL dosimeters are in development) |
| TLD | Variety of sensitivities, tissue equivalent | Cannot be reread |
| Electronic dosimeters | Can be read at any time by user, self‐contained alarm. Can be used as dosimetry of record | Directional, expensive, can be energy dependent, usually sensitive only to gamma rays |
| Pocket dosimeters | Can be read at any time by user | Easily fail if dropped, not suitable for dosimetry of record |
11.2.1 Film Badges
Exposure to penetrating ionizing radiation of a film wrapped in light‐tight paper darkens the film; the degree of darkening is proportional to the radiation dose. After developing the film, the degree of darkening is measured and the dose is read from a calibration curve that relates the degree of darkening to the dose. The lower limit of quantitative dose measurement with film is considered to be 10 mrem. Film badges are worn for periods ranging from one week to