j. Soc. Cosmet. Chem. 22 711-723 (1971) ¸ 1971 Society of Cosmetic Chemists of Great Britain Gamma radiation for product sterilization F. J. LEY, Presented on 18th November 1970 in Bournemouth, Hants. at the Symposium on "Cleansing" organised by the Society 9f Cosmetic Chemists of Great Britain. Synopsis--The high penetrating power of GAMMA RADIATION makes it particularly suitable for the STERILIZATION of products in their final pack. The choice of radiation DOSE will depend on the number and types of CONTAMINANTS involved and on the influence of the environment on their radiation RESISTANCE. Radiation may have an adverse effect on some materials and products and these should be tested individually. The operation of radi.•tion planls is briefly d•scribed with emphasis on the methods of PROCESS CONTROL. INTRODUCTION It is just over 10 years since gamma radiation sterilization was introduced in the U.K. for the treatment of commercial products. The source of the radiation is the radioisotope cobalt-60 familiar through its use in medicine in the field of tumour therapy and it is the same properties of the emitted gamma rays. i.e. high penetrating power and lethal effect on living cells, which are used to advantage in sterilization. The rapid development of nuclear power has given the capacity to produce sufficient quantities of cobalt-60 for use in industrial radiation plants in which high radiation doses are required for the treatment of large volumes of material. Apart from its ease of production in reactors by bombardment of the inactive cobalt-59 with neutrons, cobalt-60 has a suitably long half-life of 5.3 years. Each disintegration of cobalt-60 involves the emission of a *Irradiated Products Ltd., Denchworth Road, Wantage, Berks. 711

712 JOURNAL OF THE SOCIETY OF COSMETIC CHEMISTS beta particle of energy up to 0.3 MeV and two gamma photons of 1.17 and 1.33 MeV, leaving an atom of stable nickel-60. The beta particles are absorbed within the cobalt-60 rod or its capsule and in practice products are treated with only gamma radiation with a mean energy of 1.25 MeV. This photon energy is well below the threshold for the production of radio- activity in elements in the material being irradiated and therefore no haz- ards exist in handling products no matter how high the dose applied. In the context of radiation processing the unit of dose is the rad,which is a unit of absorbed dose,and it is convenient to use the unit Mrad for 106 rad. A dose of 1 rad is obtained when 0.01J of radiation energy is absorbed per kilogram of material. A typical sterilizing dose is 2.5 Mrad and if it is assumed that all the absorbed energy, i.e. 2.5 X 104Jkg-1 is converted to heat, then this would amount to only 25kJ kg-1 giving a temperature rise of 6øC in water. In practice a temperature rise of only a few øC is observed in products treated at this dose and hence the sterilizing process is essentially 'cold'. Gamma radiation is ionizing and most of the absorbed energy is used up in interaction with electrons in the orbits of atoms in the material. Some electrons may be ejected producing positive ions and free electrons which may become attached to other atoms forming negative ions. Other electrons may receive energy sufficient only to cause displacement into a different orbit, a process known as excitation. Molecules containing atoms so affected become very reactive, free radicals may be formed and complex chemical changes ensue. It is the chemical changes initiated in this way which lead on the one hand to the inactivation of micro-organisms, our concern in radiation sterilization, and on the other to the possibility of undesirable effects on products. Electrons from electrical machines may also be used for product sterilization. A description of such a machine and discussion of the proper- ties of the emitted electrons will not be included in this paper but may be found elsewhere (1). INACTIVATION OF MICRO-ORGANISMS Lethal effect In spite of considerable progress towards identification of the mechanism of inactivation of micro-organisms by ionising radiation, there still remains considerable doubt as to the nature of the critical lesions involved. It seems certain that the lethality is primarily the consequence of genetic damage