THERMOLUMINESCENT CHARACTERISTICS OF NEW PRE-CALIBRATED DOSIMETERS (TLD) IN COMMERCIALLY AVAILABLE READERS FOR SELECTED APPLICATIONS

This paper reports the performance of newly developed pre-calibrated T hermoluminescent Dosimeters (TLD) with a description and performance r esults for two commercially available TLD readers. Introduced for use in basic research, medical diagnostics and therapy applications, the H arshaw Model 5500 Automatic TLD Reader evaluates up to 50 dosimeters ( rods, chips, disks, mu-cubes) per loading; the Harshaw Model 3500 Manu al TLD Reader evaluates single dosimeters and powder. Tests were condu cted using the International Electrotechnical Commission (IEC) guideli nes for TLD readers to determine system performance and compliance. Bo th instruments passed the compliance standards in all tests establishe d by the IEC: detection thresholds for both instruments were less than 10 mu Gy, reproducibility was better than 0.7%, sensitivity to ambien t light was less than 0.2H(max) (where H-max is the maximum detection threshold), TL residue was less than 1% of the total integral charge, stability less than 1% deviation after a 24 hour warm-up period, linea rity was within specification, test light stability was under 1%, and power leakage requirements set forth by UL-544 (less than 500 mu A AC RMS leakage) was less than 42 mu A AC RMS in all cases. The characteri stics of newly introduced, factory calibrated, extruded TLD-100 rods w ere investigated by studying the consistency of the Element Correction Coefficients (ECCs) under a variety of conditions. The ECC ([x]/x(i)) relates an individual dosimeter's response, x(i), to the mean respons e, [x], of the group. Supralinearity effects and the validity of ECCs generated at 1 cGy for use in the 50-900 cGy dose range were character ized and reported. The results for two specific applications, phantom studies with Co-60 and at four Orthovoltage energies are also reported . Portability of Element Correction Coefficients from reader to reader was achieved with a relative standard deviation of less than 1.7%. Th e relative standard deviation of five TLDs exposed to various doses in the range of 50-900 cGy, with ECCs applied using a 1 cGy dose, yielde d results under 2%. At Orthovoltage energies ranging from 100 kV(p) to 300 kV(p), the corresponding relative standard deviations were less t han 3%.