Characterisation of a cobalt-60 small-beam animal irradiator using a realtime silicon pixelated detector

The paper presents a study performed by the Centre for Medical Radiation Physics (CMRP) using a high spatial and temporal resolution silicon pixelated detector named MagicPlate-512. The study focuses on the characterisation of three pencil beams from a low-dose rate, 6 TBq, cobalt-60 source, in terms of percentage depth dose, beam profiles, output factor and shutter timing. Where applicable, the findings were verified against radiochromic EBT3 film and ionization chambers. It was found that the results of the MagicPlate-512 and film agreed within 0.9 mm for penumbra and full-width at half-maximum measurements of the beam profiles, and within 0.75% for percentage depth dose study. The dose rate of the cobalt-60 source was determined to be (10.65 + or - 0.03) cGy/min at 1.5 cm depth in Solid Water. A significant asymmetry of the small pencil beam profile was found, which is due to the irregular machining of the small collimator. The average source shutter speed was calculated to be 26 cm/s. The study demonstrates that the MagicPlate-512 dosimetry system, developed at CMRP, is capable of beam characterisation even in cases of very low dose rate sources.