What is the chemical process that used for irradiation of food?

Food irradiation is a process of treating food with specific dosage of ionizing radiation.
This treatment slows or halts spoilage by retarding enzymic action.
It controls the natural chemical changes or process temporarily Regards (Syed Burhanuddin Abdali) Principal Scientific Officer PCSIR Laboratories complex, Karachi Ministry of Science &Technology Government of Pakistan

Food irradiation is a method of treating food in order to make it safer to eat and have a longer shelf life.
This process is not very different from other treatments such as pesticide application, canning, freezing and drying.
The end result is that the growth of disease-causing microorganismns or those that cause spoilage are slowed or are eliminated altogether.
This makes food safer and also keeps it fresh longer.
Food irradiation is currently permitted by over50 countries, and the volume of food treated is estimated to exceed500,000 metric tons annually worldwide; however, the extent of clearances is varying significantly, from a single food category (dried herbs, spices and vegetable seasoningss) in Austria, Germany, and many other countries of the European Union to any food in Brazil.
Irradiation is a more general term of the exposure of materials to radiation to achieve a technical goal (in this context "ionizing radiation" is implied).
As such it is also used on non-food items, such as medical devices, plastics, tubes for gas pipelines, hoses for floor heating, shrink-foils for food packaging, automobile parts, wires and cables (isolation), tires, and even gemstones.