Handbook of Herbs and Spices - Volume 3

68 Handbook of herbs and spices

3.3.2 Sources of ionizing radiations

The sources of ionizing radiations can be classified into two broad categories, namely,

radioisotopes, and machines.

Radioisotope sources

It is a general practice to use cobalt-60, however, cesium-137 can also be used. The

broad characteristics of the two sources are given in Table 3.15. While cobalt-60 is

produced in nuclear power reactors by bombardment of cobalt-59 with neutrons,

cesium-137 is a fission product and has to be extracted from the spent fuel of a

nuclear reactor through reprocessing. Though cobalt-60 is the preferred choice, cesium-

137 offers advantage in building portable or modular types of irradiators.

In the case of radioisotopes, emission of radiation results in conversion of the

isotope into a stable atom. This results in reduction in the number of radioactive

atoms over a period of time. The time required by a set of radioactive atoms to

display half of its original activity is called half-life. The energy of radiation emitted

by a radioisotope is fixed, however, in the case of machine sources variable energies

can be obtained. Radioisotopes also provide much lower dose rates compared to

machine sources.

With a half-life of 5.27 years, an annual replenishment of 12.3% is needed to

maintain the source strength. A basic design of a gamma irradiation facility is shown

in Fig. 3.1. For use as a radiation source, cobalt-60 pellets are encapsulated in stainless

steel and these pellets or slugs are loaded in stainless tubes to form a pencil. Several

such pencils are then mounted on a rack to make the final source of radiation in a

radiation processing facility. Goods to be irradiated are conveyed to the irradiation

chamber through a labyrinth, which prevents radiation from reaching the work area

and operator room. When the facility is not in operation, cobalt-60 is stored in the

source rack under water at a depth of about six metres. The water column thus

absorbs the radiation and acts as a shield to prevent radiation being present in the cell

area when the source is idle. During the processing of a commodity, the source rack

is brought up to the irradiation position after activation of all safety devices. The

Table 3.15 Characteristics of different types of ionizing radiation

Radioisotope sources

Characteristics Radionuclide

Co-60 Cs-137

Typical source form Metal Cesium chloride pellets
Half-life 5.3 years 30 years
Specific activity 1–400 Ci/g 1–25 Ci/g
Gamma energy 1.17–1.33 MeV 0.66 MeV
Dose rate* (10 kCi) 0.953 kGy/h 0.221 kGy/h

Machine sources

Characteristics Mode

EB X-rays

Power Variable Variable
Energy** 10 MeV (max.) 7.5 MeV (max.)
Penetration 3–4 cm (water equivalent) 30–40 cm

*At a distance of 30 cm in a material of 20 cm thickness.
**Machine to be operated at the energy level permitted.