Irradiation to decontaminate herbs and spices 69
irradiation chamber is shielded with concrete walls usually about 1.5–1.8 metres
thick. The goods in aluminum carriers or tote boxes are mechanically positioned
around the source rack and are turned around their own axis so that the contents are
irradiated from both the sides.
The absorbed dose is determined by the dwell time of the carrier or tote box in the
irradiation position. The dwell time can be preset after taking into consideration the
dose rate, which in turn would depend upon the source strength. The absorbed dose
is measured by placing dose meters at various positions in a tote box or a carrier. In
Fricke’s dose meter radiation-induced oxidation of ferrous ions in a 0.4 M sulfuric
acid solution to Ferric ions is measured at 304 nm.
Machine sources
Machine sources used in food irradiation include various types of electron accelerator.
The electron beam emerging from the accelerator can be either used directly or
converted into X-rays. Both DC (direct current) accelerators and microwave or radio-
frequency linear accelerators (LINAC) are used. In both types electrons are accelerated
close to the speed of light in an evacuated tube. Electrons emitted from an electron
source are pushed from the negative end of the tube and are attracted by the positive
end. The higher the potential difference, the higher the speed attained by the electrons.
A scanning magnet at the end of the accelerator tube deflects the mono-energetic
electron beam onto the material being irradiated. In LINACS, pulses of electrons
produced at the thermionic cathode are accelerated in an evacuated tube by driving
radio-frequency electromagnetic fields along the tube. The LINAC electrons are
mono-energetic but the beam is pulsed. As the electron beam can be directed at the
product, the efficiency of electron accelerators is about 20% higher then that of gamma
sources. Energy and current determine the output capacity of an electron accelerator.
Because of the lower depth of penetration (5 mm/MeV in water), electron beams
cannot be used for irradiation of thick chunks of food commodities or bulk packages.
Loading conveyorUnloading conveyorConveyor trackConcrete shieldControl roomSourceWater poolProduct carrierFig. 3.1 A typical gamma irradiation plant.