Film badge dosimeter

The film badge dosimeter, or film badge, is a dosimeter used for monitoring exposure to ionizing radiation. The badge requires two parts to be effective: photographic film, and a holder.

Description

The film is sensitive to radiation and, once developed, exposed areas increase in optical density (i.e. blacken) in response to incident radiation. One badge may contain several films of different sensitivities or, more usually, a single film with multiple emulsion coatings. The combination of a low-sensitivity and high-sensitivity emulsion extends the exposure latitude to several orders of magnitude. Wide latitude is highly desirable as it allows quantitation of very large accidental exposures without degrading sensitivity to more usual low level exposure.

Holder

The holder may contain a number of filters that attenuate certain types of radiation, such that only the target radiation is monitored. To monitor gamma rays or x-rays, the filters are metal, usually tin or lead. To monitor beta particle emission, the filters use various densities of plastic. It is typical for a single badge to contain a series of filters of different thicknesses and of different materials, the precise choice may be determined by the environment to be monitored. The use of several different thicknesses allows an estimation of the energy/wavelength of the incident radiation.

Filters are usually placed on both the back and front of the holder, to ensure operation regardless of orientation. Additionally, the filters need to be sufficiently large (typically 5 mm or more) to minimize the effect of radiation incident at oblique angles causing exposure of the film under an adjacent filter. This places a minimum useful size on the holder and badge.

Usage

The badge is typically worn on the outside of clothing, around the chest or torso. This location monitors exposure of most vital organs and represents the bulk of body mass. If protective clothing is worn (e.g. lead aprons or gloves) then the badge is worn underneath the protection to monitor the dose actually received by the operator.

Under special circumstances, where an operator may receive a high dose to one area of the body (e.g. the hands of a radiopharmacist) then additional monitoring of the area is required. For monitoring of hands, a miniature Thermoluminescent Dosimeter worn as a finger ring is usually used.

The film badge is not widely used in the USA, having been replaced by multi-chip thermoluminescent dosimetry. However, the FBD is the dominant dosimeter used in the rest of the world.

Special properties

The film badge has several advantages over other types of dosimetry:

Permanent record of exposure: The developed film is physical evidence of the radiation exposure. The film can be stored after developing and reading, and could be reviewed at a later date if there is a query over exposure.

Exposure pattern discrimination: A film badge offers limited discrimination between different patterns of exposure. A single exposure tends to leave sharp shadows on the film from the filters, whereas multiple small exposures at different angles will leave a rim of blurring around the filters. This may allow the linking of a dose with a specific incident and provides a degree of protection against tampering (e.g. deliberate exposure to a radiation source).

Radiation type detection: Use of multiple filters allows estimation of type of radiation (beta / gamma), and energy spectra. Additional filters can be added to detect neutron radiation (e.g. Cadmium). The sensitivity to low energy (<20keV) gamma/x radiation of film can be better than electronic dosimeters.

Further reading

[http://www.nap.edu/openbook/0309040795/html/ Film Badge Dosimetry in Atmospheric Nuclear Tests (1989)] at the [http://www.nap.edu National Academies Press]