Postal Dose TLD Intercomparisons for Cobalt 60 Teletherapy Units in Bulgaria (1975-1980)

During the period 1975-1980 in Bulgaria 6 TLD postal dose intercomparisons including all teletherapy units have been performed. LiF powder TLD-100 and a Harshaw TL analyser model 2000 have been used. The fading effect and the accuracy of the dose measurements for 60 Co gamma rays have been investigated preliminarily. The fading is below 1% for 30 days. The sensitivity of the reading equipment has been investigated using the mounted check light source. By means of a statistical analysis it was determined that the relative uncertainty due to unstabilities in the sensitivity is ±0,24% at confidence level P = 0,95 for a set of 10 successive check source readings. In the same manner the relative uncertainty of the dose measurements using one TL detector (4 measurements) was determined to be ±1,6% at P = 0,95. The intercomparison performance method is described and the results from the six intercomparisons are presented.