*2.3.2 Irradiation sources*

Ionizing radiation is mainly known to high-energy photons of radio nuclides (gamma rays) and X-rays from machine sources with energies up to 5 MeV and accelerated electrons with energies up to 10 MeV generated by electron accelerating machines [34]. Gamma ray occurs in the short wavelength, with high-energy region of the spectrum, and has the greatest penetrating power. Gamma rays come from spontaneous breakdown of radionuclides including the radionuclide's cobalt-60 (60Co) with a half-life of 5.3 years or cesium-137 (137Cs) with a half-life of 30 years [35]. X-ray is similar to gamma radiation based on radioactive isotopic sources. Although their effects on materials are generally similar, these kinds of radiations differ in their energy spectra, angular distributions, and absorbed dose rates [36, 37]. Electrons can be produced from machines capable of accelerating electrons as light speed by means of a linear accelerator. In comparison with gamma ray and X-rays, electrons cannot penetrate very far into materials and do not have deep penetrating power. So, electrons as beta particles are usually chosen to treat the surface of materials [32, 34]. Generally, the strength of the source and the length of time a material is exposed to the ionizing energy determine the irradiation dose, measured in grays (Gy) or kilo grays (1 kGy = 1000 Gy). One gray is equal with one joule of energy absorbed in a mass of one kilogram [38]. Based on available evidence, the safety of the irradiation technology in food industry was considered and judged acceptable. This has developed in international bodies such as the World Health Organization (WHO), the Food and Agriculture Organization (FAO), the International Atomic Energy Agency (IAEA), the and Codex Alimentarius Commission [35, 39].
