X-RAYS INDIVIDUAL DOSE ASSESSMENT USING TLD DOSIMETERS

Transcription

1 X-RAYS INDIVIDUAL DOSE ASSESSMENT USING TLD DOSIMETERS Carlos Salas Nucleoeléctrica Argentina SA - Embalse NPP Environmental Control and Dosimetry Division INTRODUCTION: This paper describes the methodology implemented in Embalse NPP for individual X-ray dose assessment in professionals, especially dentists and radiologists, who work in the surrounding areas of the plant This dosimetry is free of charge, and is performed upon request As a contribution to the community, the professionals are temporarily provided with TLD personal dosimeters for thoracic use, as well as TLD ring dosimeters to assess the doses during their job, as well as in their equipments Ring dosimeters are not considered in this paper The implemented methodology is based in several techniques described in the scientific bibliography (see Ref 1) The effective energy in the radiation field is of main importance in this kind of dosimetry, since dosimetry factors depend on that energy The response of the TLD crystals depends on this energy too This is fundamental when assessing the absorbed dose It is really convenient and necessary for the dosimeter to simultaneously determine the effective energy value as well as the corresponding dose value The basic principle for determining effective energy is by using at least two different TLD materials covered by filters of different thickness, in the same dosimeter The different TLD materials give respond differently to energy Therefore, different readouts from each of the crystals are obtained The ratio between both readouts provides a factor that depends on the effective energy, but that is independent from the exposure values irradiated to the dosimeter 1

2 EXPERIMENTAL DETAILS The Personal TLD dosimeter currently in use is Bicron-Harshaw It comprises a model 8807 card holder This holder contains a card model 2211 which groups two TLD 200 crystals and two TLD 100 crystals It has internal filters at each side of the TLD 200 crystals, which diminish energy dependence Using four crystals has the advantage of improving measurement statistics, and at the same time provides the necessary redundancy in case of crystal damage These dosimeters are evaluated by an automatic Bicron-Harshaw 6600 E TLD reader (see figure 1) Figure 1 The periodical calibration of these dosimeters consists in the irradiation of some dosimeters with different X-ray energy beams in the National Atomic Energy Commission (CNEA) Once Embalse NPP gets the corresponding readouts, different ratios are made, and then different graphics are made a) TLD 200/TLD 100 ratio versus X rays energy 2

3 b) Dosimetric factors for each material versus X rays energy Please see Charts 1 and 2 Chart #1 Calibracion de TLD 2211 para Rx Determinación de la energía efectiva 2 TLD 200/ TLD 100 1,5 1 0, ( Kev ) 3

4 Chart # 2 Factores Dosimétricos vs Energía ( msv/ Gu ) 0, , , , , , , , , ( Kev ) TLD 200 TLD 100 DOSIMETER EVALUATION When a professional working in the surroundings requires an evaluation of his personal dosimeter, the corresponding readout is made Once the ratio is determined, it is entried in chart #1 and the effective energy in the X ray field of the professional s working place is thus determined Finally, with the energy value obtained in Chart #1, the dosimetric factors needed for dose assessment in Chart # 2, are obtained Corrective actions due to TLD fading and to variation in the relative response of each crystal are performed in every step of the way Factors showed in Chart #2 allow us to obtain the equivalent personal dose HP(10) in msv for TLD 100 and for TLD 200; those values are averaged and the dose value for each person is therefore obtained The dosimeter referred to in this paper, was used by the National Regulatory Authority (ARN) in several comparisons, always getting satisfactory results The abovementioned comparisons consist in the irradiation of several dosimeters to doses and energies unknown for the participants Some examples of the results obtained are those of year 2006, when the average difference between the 4

5 assessed values and the real values was +14%; the largest difference was close to 15% CONCLUSIONS: This method is rather tedious, and demands lots of work, but it is necessary, considering the dependence on X Rays that TLD dosimeters and the dosimetric factors have It is similar to that performed by labs when using FILM dosimeters Both kinds of dosimeters (TLD and FILM) have advantages and disadvantages which are not being considered in this paper There s another factor that increases the uncertainties of the results It s the Angular dependence that happens in every personal dosimeter Angular dependence occurs because when personal dosimeters used on the body,are not placed facing the beam, they will assess less radiation than when positioned facing it Calibrations used for Charts 1 and 2 refer to latter position This is an unresolved problem International bibliography recommend the usage of two dosimeters instead of one, or the usage of corrective factors (see Bibliography, ref 2) Finally, this methodology for personal dose assessment is not so necessary for gamma ray fields, like Embalse NPP, or in other places using Gamma sources like CO-60 or Cs 137, for TLD effective energy dependence is lower than TLD X Rays dependence BIBLIOGRAPHY: 1HARSHAW- A personnel dosimeter having high x-ray sensitivity TP Radiation Protection Dosimetry, vol 93 Nº3 pp (2001) 5