The application of neutron emitting radioisotopes in brachytherapy facilitates the use of the higher biological effectiveness of neutrons compared to photons in treating some cancers. Different types of high intensity sources are in use for the treatment of different cancers. To improve the therapy of bulky tumors the dose can be augmented by the additional use of the boron capture reaction of thermal neutrons. This requires information about the thermal neutron dose component around the Cf source. In this work, a Mg/Ar‐ionization chamber internally coated with was used to measure the thermal neutrons. These measurements were performed on two different sources, one in use in the Gershenson Radiation Oncology Center at Harper Hospital in Detroit, MI, and one at the University Hospital of Chiang Mai in Chiang Mai, Thailand. The results of these measurements are compared and indicate that the differences in the construction of the sources influence the thermal dose component.