III.21.
It is possible to measure the dose equivalent rates both from beta radiation (or low energy X rays ) and from strongly penetrating photon radiation using a single detector. In this case, the detector (ion chamber) is fitted with a ‘window’ that can be opened or closed (see para. III.22). When it is closed, the strongly penetrating component (i.e. photons with energies above approximately 20 keV) can be measured. With the window open, both components are measured and the weakly penetrating component (beta particles and low energy photons) of the dose equivalent rate is estimated by subtraction.
III.22.
Most survey measurements for beta radiation and low energy photons are made with small, portable ion chambers that can also be used for X ray and gamma radiation surveys. One side of the chamber comprises a thin conducting plastic sheet that is covered when measuring photons (‘window closed’) with a piece of material equivalent to 1 cm of tissue. This cover is removed for measuring beta radiation [200]. Another type of beta survey meter is constructed entirely with thin walls. Such a chamber might not be appropriate for the measurement of directional dose equivalent.
III.23.
The walls of an ion chamber to be used for the measurement of beta radiation should be made of materials similar in composition to tissue. However, the exact composition is not as important for electrons as in the case of ion chambers for X rays or gamma radiation. With electrons, the function of the walls is merely to simulate absorption and backscattering by the body.