Gamma cell – Mayapuri incident

Gamma cell (also called gamma irradiator) is used for irradiation of materials to study the effects of radiation. The cell generally contains high energy gamma radiation sources like cobalt-60. The cobal-60 is artificially produced in nuclear reactor by neutron irradiation of Cobalt-59 isotope. The Cobalt-59 is naturally occurring isotope and is not radioactive. Cobalt-60 has a half life of 5.3 years. This only means, in 5.3 years, the radioactive content of the source becomes one-half of the initial radioactivity. As per the decay law, in 2, 3, 4, half lives (in 10.6, 15.9 and 21.2 years), the radioactivity will reduce by one-fourth, one-ninth and one-sixteenth of the initial activity. It will take many more years for the radioactivity to come down to any acceptable, safe levels.

In some news papers, it was reported that the source found in a Mayapuri (Delhi, India) scrap dealer was showing radiation level of 1000 R/h (R is the unit of exposure rate, Roentgen per hour). It is also reported that the gamma cell was bought in 1968. That means the source has decayed by 8 half-lives. The initial dose rate must have been (8x8=64) in the range of 64,000 R/h.

Invariably the gamma cells are accompanied by the operating manuals and the design details. These documents, if available will give details about the number of source pencils and their configuration inside the cell which will vary depending on the required radiation dose for irradiation.

Sources of this high dose levels are never handled openly. They are handled remotely in specially designed and well-shielded enclosures called Hot Cells. The gamma cell is also a well-designed enclosure with the radiation source inside the lead or depleted uranium shield to prevent radiation leakage outside the cell. The design is such that only authorized persons with lock & key arrangement can remotely operate the source for irradiation of sample materials.