Photon Effects

Photon is produced by the interaction of electrons with atoms.

Compton Scatter

Photon has much greater energy than the binding energy of electron. Photon hits the electron and it gives some energy to electron and electron moves to higher energy orbit. The photon has lower energy after this interaction. Compton scatter is the most dominant interaction in tissue at the photon energies in radiotherapy.

Photoelectric effect

Photon has a little greater energy than electron, so it gives all its energy to electron and electron can escape from the atom. The vacancy left in atom is filled by an outer shell electron. As it falls from higher energy level, it releases x-ray photon, which has the energy as the difference between the orbits. These x-rays can be seen in the spectra of diagnostic x-ray sets which have energies in the range of 30-70kV.

Pair production

In the field of the atomic nucleus, when the photon energy is greater than 1.022MeV which is the combined energy of the positron and electron, it produces a pair of electron and positron.

Radiotherapy

Radiotherapy aims to kill the cancerous tissue and reduce the damage of normal tissue. There are two purposes of using radiotherapy, cure the patient and relieve the pain. There are three treatment types: teletherapy, intracavitary and interstitial therapy. Teletherapy uses external photon and electron, intracavitary treatment inserts radioactive sources into body cavities, interstitial therapy involves inserting radioactive sources into tumours surgically.

Linear accelerator is used for teletherapy, which generates photon and electron beams. The source is an electron gun.