THERMOLUMINESCENT DOSIMETRIC SYSTEM USED FOR CARACTERIZATION OF THE UNDERGROUND ENVIRONMENT

Transcription

1 THERMOLUMINESCENT DOSIMETRIC SYSTEM USED FOR CARACTERIZATION OF THE UNDERGROUND ENVIRONMENT A. Stochioiu*, M. Sahagia, R. Margineanu, S. Bercea, I.Tudor Horia Hulubei National Institute of R&D for Physics and Nuclear Engineering, IFIN-HH * stoc@ifin.nipne.ro

2 INTRODUCTION The main objective of environmental radiation monitoring is to assess the dose to the public from the manmade sources and from natural radiation sources. Environmental radiation monitoring with Thermoluminescent Dosimeters (TLD), measures directly the penetrating gamma radiations. The TLD system provides measurements of the dose integrated during long exposition time interval (daymonths) what means that only an average dose rate value for this period can be estimated.

3 DESCRIPTION AND TECHNICAL BASIS The dosimetric system, uses thermoluminescent detectors. It is calibrated in terms of ambiental dose equivalent H(10)*, at 1 m distance from the ground level. It is composed from three parts 3. TLD Reader Analyser with Nitrogen-generator; 4. Dosimeter containing TL Detectors; 5. Oven and accessories

4 1.TLD Reader Analyser TL-RA-04 and nitrogen generator The model TL-RA-04 is a portable Reader Analyser TLD system for measurement of the light emitted by TL detectors or other thermoluminescent materials. The reading is performed in nitrogen atmosfere, to eliminate chemiluminescence and control temperature The dedicated software package is fully PC compatible. The unit incorporates analysis and measurement data and a database of luminescence curves, visualization and transfer in ASCII format. The following RA-04 parameters can be set by the user or downloaded from the host PC: - heating times and temperatures; - temperature limits; - sensitivity; - speed of linear heating acoustic signals.

5 TLD Reader Analyser TL-RA-04

6 2. TL Dosimeter The dosimeter contains : an external case, provided with 3-7 mini cassettes, which contain inside, the TL detectors. The external case body, made from PVC (Poly Vynil Chloride), is cylindrical in shape and consists from two component parts: case body and cover; the external dimensions are: diameter: 50.0± 0.2 mm, height:10.0±0.1 mm. All components are marked with identification data..

7 3. Thermoluminescent detectors; Vacuum tweezers and annealing dishes. The main component of the dosimeter is the TL detector. It is of the type LiF:Mg, Cu, P, commercially known as GR-200A, supplied by Solid Dosimetric Detector & Method Laboratory, Beijing- China, having an extremely low detectable threshold. LiF has an effective atomic number similar to tissue and air. Tweezers and dishes are accessories for manipulation and annealing of detectors

8 4. Annealing oven The Oven is used for the thermal treatment of detectors: - Annealing of TLD s before irradiation (program 1) - Preheating after irradiation and before reading by a TLD - reader (program 2). The annealing oven is controlled by a programmable microprocessor. Reproducible heating procedures for thermoluminescent dosimeters are essential to maintain constant the sensitivity and low background readings.

9 Main technico-functional parameters of the TL system Referential: Norm IEC 1066/91, defining the basic parameters for the whole TL system, TL reader, and TL detectors Parameter Required Obtained Batch non-homogeneity 30% 11% Response Nonlinearity within the 10% interval of ambiental dose equivalent: 5E-3 msv 1E2 msv 7% Repeatibility for 10 dosimeters, 10 identical irradiations at defined values: 5E-2 msv and 2E-1 msv 5.5% 7.5% Stability of response for dosimeters 5% irradiated with 10mSv and kept for a 30 days interval, in normal testing condition 2%

10 Measurement conditions in the underground laboratory A number of four interest points, were chosen inside the salt mine Each placement contained fifteen dosimeters, which were exposed for time intervals of 37 d, 66 d, and 93 d, five dosimeters for each time interval. Sets of fifteen dosimeters were placed also at the mine entrance, in surrounding area, at 10 km and 120 km (our Laboratory) distances.

11 Processing of data The ambiental equivalent dose values,h(10)* [µsv], registered during exposition intervals, were corrected for supplementary background irradiation, and calculated as mean values of five dosimeters measurement; the individual dosimeter values are means from three detectors.

12 Measurements in the Salt mine Unirea. A. Stochioiu, M. Sahagia, F. Mihai, I.Tudor, H. Lupescu Application of the Thermoluminescent Dosemeters for the Measurement of Low Level Background, BPU6, 2006, AIP Conf. Proc.899, pp , Print ISSN X, Online ISSN Dosimeter placement Sanatorium Genesis Hall Statues Hall Mine Center Mine Entrance 10 km from Mine 120 km from Mine (Laboratory) Mean ambiental equivalent dose rate [nsv h-1] ± 0.21 ± 0.81 ± 1.2 ± ± ± ± 3.0

13 Discussion of results The ground level ambiental equivalent dose rate values are within the interval ( ) nsv h-1, confirming the previously reported values. The ambiental equivalent dose rate mean values, registered in the mine, are situated within the interval ( ) nsv h-1. The values registered during various exposition intervals are in good agreement. A consistent monitoring of the radiation background levels, expressed as equivalent dose rates, in the monitored mine, was performed.