Long term stability studies of the Harshaw Tld 6600 system used for personal dosimetry monitoring in Ghana

A dosimetry system made up of the Harshaw TLD 6600 has been used in the monitoring of occupationally exposed workers and for research purposes in Ghana since 1998. Continuous use, calibration and maintenance have been done periodically on the equipment at the Personal Monitoring Service (PMS) laboratory since its installation at the PMS laboratory of the Radiation Protection Institute in Ghana. The Daily Electronics Quality Control (DEQC) test carried out to ensure that the Harshaw 6600 TLD Reader was in good working condition prior to the reading of TLD cards. The present work was done to assess the long term stability of the system with respect to the DEQC parameter in order to ensure quality control and accurate measurements for personal dosimetry in Ghana. The parameters under investigation included the high voltage, temperature, reference light, plus and minus 15 volts, ground, digital to analog conversion and the photomultiplier tube noise. WINREMS software is incorporated into a computer to monitor these parameters in order to verify whether they conform to performance standards. The annual mean reference light values in nano coulomb for chip (ii) were 471.0±49.5, 77.0±109.2, 18.5±7.9, 31.0±81.1 and 18.7±3.2; for chip (iii) were 475.1±45.7, 102.4±132.8, 74.3±107.4, 83.7±125.3 and 8.6±3.1 respectively. These values are outside the reference ranges indicating instability for the period of five (5) years (2008 to 2012). Measurement of exposed TLDs to known doses at the SSDL confirmed the PMT gain stability of system. The instability in the reference light readings was due to ageing and yellowing of the optical portion of the reference light. The high voltage and the reference light were unstable. The temperature, plus and minus 15 volts, ground and digital to analog conversion and PMT noise were all stable. The study indicates the PMS therefore provides a reliable and accurate dosimetry in the long term. (au)