Free-field Co-60 gamma source

Three self-contained, shielded cobalt-60 gamma irradiators are operated at Risø High Dose Reference Laboratory (HDRL) and used for irradiation of dosimeters and samples over a wide dose-rate range.

Two of the irradiators were developed at Risø and are now operated as low-dose-rate facilities, while a third system, an MDS Nordion Gammacell 220 Excel, is maintained at high dose rates (> 5 kGy/h) to avoid dose-rate dependent effects in dosimetry systems such as alanine. Together, the facilities enable irradiation from low to very high dose rates under well-defined and reproducible conditions.

Two irradiators, one representing low dose rate and one high dose rate, are calibrated with dose rates traceable to the SI through the National Physical Laboratory (NPL, UK) and constitute the Danish National Standard for absorbed dose in this range. All industrial dosimetry performed at Risø HDRL is traceable through these reference facilities.

The gamma cells allow irradiation in well-defined geometries over exposure times ranging from seconds to several hours, with temperature control typically between 15 °C and 55 °C for dosimeter irradiation. Larger items can also be irradiated, although without active temperature control. All irradiators are in a dedicated, secure irradiation facility.