Gamma-Ray Source

(medicine), or radium (cobalt) “gun,” an apparatus for remote gamma therapy, mainly for malignant tumors. The gamma-ray source uses a directed beam of gamma rays with variable cross section. The device is equipped with a shielding container (head) made of lead, tungsten, or uranium, which holds the radiation source (usually ⁶⁰Co, less frequently ¹³⁷Cs; radium was formerly used). A port in the head, equipped with a diaphragm, makes it possible to produce an exposure field of the required shape and size and to cut off the radiation beam when the source is not operating.

There are long-focus and short-focus types of gamma-ray source. The short-focus type, in which the distance from the source to the patient’s skin is less than 25 cm, is intended for the irradiation of tumors no deeper than 3-4 cm and usually uses sources with an activity of up to 100 curies. The long-focus type, in which the distance between source and skin is 70-100 cm, is used to irradiate deep-lying tumors. The radiation source in this type is usually ⁶⁰Co with an activity of several thousand curies; they create the necessary dosage distribution. A distinction is made between long-focus gamma-ray sources for stationary and moving irradiation; in the latter the radiation source can either rotate around one axis, making a complete turn or oscillating over a specified angle (rotary sources), or it can rotate simultaneously about three mutually perpendicular axes, describing a spherical surface (rotary-convergent sources). With moving irradiation the dosage absorbed is concentrated at the focus that requires therapy, while avoiding injury to healthy tissues.

Gamma-ray sources are installed in rooms whose walls are made of special materials that protect surrounding areas from the gamma radiation.