# Cobalt-60

#### Background

Cobalt-60 (60Co) is a synthetic radioisotope that has been used in both external beam radiation therapy and brachytherapy. 60Co decays via $$\beta$$ decay to an excited state of Nickel-60 which then emits two high energy (1.17MeV and 1.33MeV) gamma-rays. It is these energetic gamma rays which are primarily important for use in radiation therapy. The $$\beta$$ particle emitted is absorbed by the cobalt metal and the sources encapsulation such that it doesn’t contribute meaningfully to the dose distribution. Cobalt 60 has been largely replaced by other Iridium-192 in high dose rate brachytherapy and by linear accelerators in external beam radiation therapy. Because of it’s high activity and simple gamma spectrum, it is used in the calibration of radiation detectors. Today it is also used in some MRI guided external beam therapy systems Cobalt-60 is able to generate an MV beam without strongly influencing a magnetic field.

#### Quick Facts

• Half life: 5.2714 years
• Decay Mode: $$\beta ^-$$
• $$^{60}_{27}Co \rightarrow ^{60}_{28}Ni + e^- + \bar{v}_e + \textrm{gamma rays}$$
• Mean gamma energy: 1.25MeV (1.17MeV and 1.33MeV)
• ~10% of the beam is resultant of scattered photons.
• Exposure rate constant: $$\Gamma_{\delta} = 13.07 \frac{R \ cm^2}{mCi \ hr}$$