The principles of irradiation

Processing by ionising radiation is a particular kind of energy transfer: the portion of energy transferred per transaction is high enough to cause ionisation. This kind

374 The nutrition handbook for food processors Table 17.1 Types of particle

Particle Description electron An elementary corpuscle carrying one unit of positive or negative electrical charge. The positively charged electron is called a positron.

alpha A charged particle, identical to the nucleus of a helium atom, composed of two neutrons and two protons. It carries two positive elementary units of charge.

beta A charged particle, identical to an electron or a positron but emitted from a radioactive nucleus.

gamma A particle or photon emitted from a radioactive nucleus.

X Fast-moving charged particles in an electric or magnetic field, usually generated by high-energy electrons impinging on a high-atomic-number absorber (e.g. tungsten); also called Rontgen-rays. They are generated by braking radiation (bremsstrahlung).

Wavelength [cm]

100 10-2

Radio waves

  • Infrared
  • Visible light —|- Ultraviolet

X-radiation -

Gamma radiation -

Cosmic radiation -

ionising radiation

102 104

Photon energy [eV]

1011

Fig. 17.1 Range of energies (electromagnetic spectrum): ionising radiation is characterised by the ability to split molecular bonds and to transfer electrons; this energy limit is indicated by the vertical, dashed line beginning in the range of ultraviolet light.

of radiation was discovered because the emitting radioactive material caused ionisation in the surrounding air. From the multitude of atomic particles known, only gamma rays from nuclear disintegration and accelerated electrons are useful for food processing (Table 17.1). Electrons may be converted into X-rays by stopping them in a converter or target (Fig. 17.1). Other particles such as neutrons

Electrons

Photons

  1. 17.2 Interaction with matter (photon versus electron): 1) primary incident radiation, 2) Compton electrons caused by photon interaction, 3) secondary electrons and final energy transfer, 4) irradiated medium, 5) finite depth of penetration for electrons.
  2. 17.2 Interaction with matter (photon versus electron): 1) primary incident radiation, 2) Compton electrons caused by photon interaction, 3) secondary electrons and final energy transfer, 4) irradiated medium, 5) finite depth of penetration for electrons.

are unsuitable because induced radioactivity is produced. The same may occur at elevated energy levels with electrons and X-rays; for this reason the electron energy is limited to a maximum of 10MeV and the nominal energy of X-rays is limited to 5MeV. Gamma rays of cobalt-60 have photon energies of 1.17MeV and 1.33 MeV and cannot induce radioactivity; caesium-137 is not available in commercial quantities but gamma rays of 0.66 MeV are emitted from it. This means that gamma rays from available isotope sources are incapable of inducing radioactivity.

Whether in the form of particles or as electromagnetic waves, the primary high energy is broken into smaller portions and converted into a 'shower' of secondary electrons (Fig. 17.2). These electrons finally interact with other atoms and molecules knocking out electrons from their orbits or transferring them to other positions (Fig. 17.3). This means that an elementary negative charge is removed and a positively charged atom or molecule, i.e. an ion, is left behind. If an electron has been transferred then orbital electrons are no longer paired and free radicals are created. Both ions and free radicals are very reactive, in particular in an aqueous medium such as in food, leading finally to chemical reaction products that are stable. The effects caused by corpuscular or electromagnetic radiation are essentially equal; the difference is in the dose distribution along the penetration line into matter. Corpuscles have a definite physical range in matter, they are slowed down by several processes of collision and finally stopped. They have no energy beyond their range. Electromagnetic waves are attenuated exponentially and do not have a defined physical range.

A schematic diagramme of irradiation facilities (Fig. 17.4) helps to understand the simplicity of the irradiation process: the goods are brought by a transport system into the irradiation cell which essentially is a concrete bunker shielding

Incident

Scattered

Scattered

Secondary electron

Orbital electrons

Fig. 17.3 Principal diagram of 'ionisation': whether photon or electron, the incident particles interact with the orbital electrons and are scattered, an orbital electron is removed gaining kinetic energy as a secondary electron; in this way an ionised atom/molecule is left behind and a cascade of secondary electrons causes further ionisation or formation of free radicals.

Secondary electron

Orbital electrons

Fig. 17.3 Principal diagram of 'ionisation': whether photon or electron, the incident particles interact with the orbital electrons and are scattered, an orbital electron is removed gaining kinetic energy as a secondary electron; in this way an ionised atom/molecule is left behind and a cascade of secondary electrons causes further ionisation or formation of free radicals.

Beam handling system (10 MeV electrons)

Radioactive source (Co 60)

Beam handling system (10 MeV electrons)

Radioactive source (Co 60)

  1. 17.4 Schematic diagram of irradiation facilities: the product to be irradiated has to pass through the irradiation zone; the design details largely depend on the physical properties of the type of radiation used and may be adapted to the packaging and handling requirements of the goods.
  2. 17.4 Schematic diagram of irradiation facilities: the product to be irradiated has to pass through the irradiation zone; the design details largely depend on the physical properties of the type of radiation used and may be adapted to the packaging and handling requirements of the goods.

the environment and the workers from the radiation. A tunnel system allows free access for the goods but prevents radiation leakage; fences and detectors prevent unintentional access of anything or anyone when the radiation is 'on'. Machine sources (accelerators) emit the radiation uni-directionally, gamma sources (radioactive isotopes) emit it in all directions. This means that for electron and X-ray processing the goods pass just before the beam exit window and for gamma processing the goods are piled and moved around the source to absorb as much as possible of the emitted energy. When it is not needed a machine source is simply switched off; for radioactive isotopes the frame with the source must be moved to a safe position which is usually a deep water pool. The design of irradiation facilities is widely standardised; the safety-features are offically approved and authoritative control is well established.

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