12.1.4 - Caesium 137

Caesium 137 replaced radium 226 for temporary interstitial / intracavity low dose rate brachytherapy due to its stable daughter product and improved radiation protection features. It is 'readily' obtained from fission of uranium-235 in a nuclear reactor. Caesium 137 is a salt and is stored in a doubly encapsulated steel structure to prevent leakage; all caesium salts react violently with water.

Physical Properties

Caesium decays by beta emission to metastable 137mBarium (95% of the time), emitting an electron with energy of 0.512 MeV. Metastable 137mBa releases a gamma photon as it returns to its ground energy state, with a usual energy of 0.662 MeV.
The half life of 137Cs is 30.1 years. The much higher energy of 137Cs leads to a much higher HVL of 0.65 cm (when compared with 192Ir/103Pd/125I), which is still less than that of 226Ra and 60Co.

Source Construction

Caesium sources are manufactured either as an insoluble powder, ceramic microspheres or a solid ceramic rod. The radioactive substance is then doubly encapsulated in stainless steel, with each layer welded shut.

Special Handling Issues

It is important to regularly wipe test caesium sources to detect leaks. Storage is also a problem due to the higher photon energies, requiring lead shielding at least 6.5 mm thick. 137Cs is typically stored in a lead safe, with sapces

Manufacture, Supply and Disposal

137Cs is a fission product with a long half life. This has benefits, as a source may be used for many years without needing replacement. Disposal of 137Cs sources is complicated due to the long half life. Typical disposal involves sealing of the source within concrete and lead, followed by placement of the source in a deep mine shaft.



  • Long half life means sources need to be replaced less often
  • Stable daughter product
  • Most photons released with identical energy (662 keV)
  • Low dose rate more beneficial radiobiologically


  • Long half life makes disposal difficult (this has resulted in radiation accidents overseas where 137Cs sources were left behind in radiotherapy clinics)
  • Penetrating photons make radiation protection of staff difficult
  • Patients need to remain in hospital during treatments, often over several days due to the low dose rate.

Decay Scheme