One gamma radiation source comprises 75Selenium or a precursor thereof, wherein the 75Selenium is provided in the form of one or more thermally stable compounds, alloys or mixtures with one or more nonmetals which upon irradiation do not produce products capable of sustained emission of radiation which would unacceptably interfere with the gamma radiation of 75Selenium. A further gamma radiation source comprises 75Selenium or a precursor thereof, wherein the 75Selenium is provided in the form of one or more thermally stable compounds, alloys or mixtures with one or more metals or nonmetals, the neutron irradiation of which does produce products capable of sustained emission of radiation which would acceptably complement the gamma radiation of 75Selenium. Further, the gamma radiation source may have components that are separately irradiated before being combined and the components may be of natural isotopic composition or of isotopically modified composition so that the subsequent radiation peaks may also be adjusted in relative frequency.