Determination of surface dose rate from a 90Sr ophthalmic applicator

Surface dose rates from two 90Sr ophthalmic applicators were measured using thin thermoluminescent dosimeter (TLD) chips. The TLD's were calibrated against the 90Sr source itself. The calibration technique is described. The results were compared to those obtained by three other techniques. The calibration in terms of roentgen equivalent betas per second provided by the manufacturer was found to be approximately 30% lower when compared to our measured dose rates. The surface dose rates obtained with the TLD's calibrated against 6-MV x rays were 18% higher. One of the two sources was sent to both the National Institute of Standards and Technology (NIST) and Amersham International for calibration. NIST and Amersham use extrapolation chambers as a Bragg-Gray cavity to determine the surface dose rate. NIST results gave values higher than ours by approximately 5%. The Amersham results were approximately 35% lower than ours.