Spurious TLD readings due to static electricity

Abstract

During a period of low relative humidity (<30%) about one in ten of the TLD-700 chips (7LiF extruded ribbons, 3*3*0.9 mm, manufactured by Harshaw Chemical Co.), which were read in the authors' laboratory directly after removal from glass annealing plate, had an increased reading equivalent to about 50 mu Sv (5 mrad). In the same period, spurious readings equivalent to 0.1 to 1 mSv were produced merely by inserting TLD chips into a Perspex holder. An investigation into the origin of high readings in these and other TL phosphors is described. It seems plausible that rubbing TL phosphors on insulators generates static electricity causing the migration of electrons and holes with the consequent filling of some TL traps. An examination of the glow curves shows that the effects of radiation and static electricity cannot be resolved by a suitable choice of read-out temperature cycle. Care must therefore be taken to keep the production of electrostatic charges to a minimum. For this reason TL detectors are now annealed on gold foil at Harwell, but for irradiation suitable alternative holders of tissue-equivalent composition have not yet been found. It it were possible to manufacture CaSO4:Dy in solid form without the addition of LiF then the effect of static electricity might be reduced.