Radiation processing technology

Radiation processing technology

 

Gamma ray is a kind of electromagnetic wave with very short wavelength emitted from radioactive isotope transformation. There are three types of interaction between gamma ray and matter: (1) photoelectric effect; (2) Compton Wu Youxun effect; (3) electron pair effect. Photoelectric effect was first discovered by Hertz in 1887. When the light beam irradiates the metal surface, some electrons escape from the metal surface, In 1905, Einstein explained the photoelectric effect with quantum theory. Light has wave and particle duality, so it can be called photon. When the light irradiates the metal surface, the photon will collide with the electron in the metal. The electron can absorb a photon, obtain the photon energy, increase the kinetic energy and escape from the metal surface, And it has a certain escape velocity. The escaped electrons can be measured in the experiment

 

In the X-ray experiment, Compton et al. Projected monochromatic X-rays onto graphite, and found that X-rays scattered along different directions contain two different wavelength components: one wavelength is the same as the incident X-ray wavelength; the other wavelength is longer than the incident X-ray wavelength, The change of wavelength is related to the scattering angle. Compton et al. Explained this phenomenon with the light quantum theory. In light atoms, the electron is not strongly combined with the nucleus, so it can be regarded as free compared with the photon with higher energy (such as energy greater than 0.2MeV). When the photon is injected into the material, it collides with the electron and transfers part of the energy to the electron, thus increasing the kinetic energy of the electron, The Compton Wu Youxun effect is one of the main ways of energy loss when photons with energy of 0.2-2 MeV pass through the material. When the photon energy is low, the energy loss is mainly ionization absorption

 

When the high energy gamma photons pass near the nucleus, under the Coulomb field of the nucleus, y photons will be converted into positive and negative electron pairs, satisfying the charge conservation. This process is called electron pair production process. The electron pair effect of photons satisfies the energy conservation. Part of the photon energy is converted into the static mass of the positive and negative electron pairs, and part of the photon energy is converted into the recoil energy of the nucleus or electron, In the process of electron pair generation, the nucleus provides a strong Coulomb field and absorbs the momentum of photons, so that the momentum of the whole process is conserved. The average energy of y-ray of CO source is 1.25 MeV. The atomic number of water and organic components is relatively low. Therefore, when the co ray interacts with low atomic number matter, the atomic number of CO source is low, The Compton Wu Youxun effect mainly occurs. In this process, the intermediate energy y-photon collides with the outer electrons of the atom to form recoil electrons and scattered photons. The recoil electrons move in the matter at a high speed, The interaction between ionizing radiation and matter first produces excited molecules and ions, which are then redistributed by the so-called secondary process to form free radicals or stable products

 

The common gamma radiation sources of radionuclide are Co60 and Cs137. At present, Co source is the most widely used in industry. Two kinds of y-rays with energy of 1.17mev and 1.33mev are produced during the nuclear decay of Co. the half-life is 5.25 years, and the monthly radioactivity decreases by about 10%, which makes the radiation sources need to be replenished and updated frequently, Therefore, it is not necessary to replace the source as often as the CO source. Due to the defects of low utilization rate of radiation and easy to cause radioactive pollution, the 3CS source is not as popular in industry as the CO source. The high reliability, simple manufacturing and strong penetrating ability of gamma ray are its advantages. Now, the status of large-scale gamma source for industrial treatment has been established

 

accelerator

Accelerator is a device that uses electromagnetic field to accelerate charged particles to obtain high-energy rays. Compared with gamma radiation source, it has its own advantages, such as high yield, high energy utilization and good controllability. However, the penetration ability of charged particle beam generated by accelerator is weaker than that of y-ray. However, from the long-term development point of view, accelerator as the main radiation source is the future development trend, At present, its high price limits its application in industry.