into account. Therefore, every time a new batch of food is to be irradiated, the operator must establish the dose and dose distribution by strategically placing dose meters into and between the food packages and evaluating the dose meter reading. Once the process is running smoothly, it is usually not necessary to carry out dosimetry on all the product. Monitoring the process parameters and making occasional dosimetric checks is now sufficient (23). In most countries government regulations require that food irradiation proces­ sors maintain records that describe for each food lot the radiation source, source calibration, dosimetry, dose distribution in the product, and certain other process parameters (see Chapter 11). A short introduction to the interaction of ionizing radiation with matter is appro­ priate at this point, although the effects of ionizing radiation on food components will be described in more detail in Chapter 3. When high-energy electrons are absorbed by a medium they lose their kinetic energy by interacting with electrons of the medium. (At very high energy, far above that allowed for food irradiation, accelerated electrons can also interact with nuclei of the medium.) The interaction with orbital electrons of the atoms of the medium (the absorber) causes ionizations and excitations. Ionization means that orbital electrons are ejected from atoms of the medium; excitation means that orbital electrons move to an orbit of higher energy. Ejected electrons (secondary electrons), carrying a large portion of the energy of the incident electron, also lose energy through interaction with orbital electrons of the absorber. Electrons at low velocities (subexcitation energy level) can cause molecular vibrations on their way to becoming thermalized. As a result of the collisions with atoms of the absorber material the incident electrons can change direction. Repeated collisions cause multiple changes of direction. The result is a scattering of electrons in all directions. This is shown schematically in Figure 12a. When gamma or x-ray photons interact with the absorber, three types of interaction can occur: The photoelectric effect The Compton effect, and Pair production (i.e., formation of pairs of electrons and positrons) Photoelectric absorption occurs largely with photons of energies below 0.1 MeV and pair production primarily with photons of energies above 10 MeV. Both are of minor importance in food irradiation, where the Compton effect predominates. As portrayed in Figure 13, in the Compton effect an incident photon interacts with an absorber atom in such a way that an orbital electron is ejected. The incident photon continues after the collision in a changed direction and with less