LiF:Mg,CU,P and TLD-100 based Environmental Dosimeter Design

Research project P 14248 LiF:Mg,Cu,P and TLD-100 based Environmental Dosimeter Design Hannes STADTMANN 06.03.2000 Recent international and European regulations recommend the use of the so called operational quantities, especially ambient dose equivalent H*(10), for monitoring external radiation fields and providing reasonable estimates of the protection quantity effective dose. For low dose measurements during area monitoring over longer periods of time the combination of small and robust LiF thermoluminescence (TL) detectors and various filter layers shall yield a passive dosimeter for environmental monitoring. Due to their high photon sensitivities and their near tissue equivalent energy responses the standard T detector material LiF:Mg,Ti (TLD-100) and the new hyper-sensitive TL material LiF:Mg,Cu,P will be employed in this project. Optimisation of their reproducibility and re-usability properties and improvement in low dose measurement performance will need the development of thermal handling procedures and- an appropriate glow curve analysis concept. The use of Monte Carlo (MC) simulations as a computational tool for radiation transport gives us the possibility to observe interaction events and to determine macroscopic quantities, such as the absorbed dose, by sampling from probability distributions governing the physical processes involved. An important point of view of this project is the combination of experiments and Monte Carlo calculations which shall improve the search for appropriate TL and filter materials and their geometrical arrangements. The necessary energy response of a dosimeter measuring the ambient dose equivalent prescribed by the definition of this radiation protection related quantity, and the, required isotropic response shall be achieved by the balanced combination of experiments in well defined standard radiation fields providing input data for simulations, computer aided optimisations and succeeding experimental verifications. The primary aim of this project is designing a passive energy and directional independent H*(10) reference dosimeter for environmental monitoring making a visible progress in performance of passive dosimeters in radiation protection and dosimetry.

Within the framework of the FWF research project "LiF:Mg,Cu,P and TLD-100 based Environmental Dosimeter Design" the prototype of a new radiation protection dosemeter for area monitoring was developed using computer simulation techniques. The applied Monte Carlo simulation programme MCNP allowed the close to reality modelling of radiation fields and dosemeter components, such as the aluminium card holders of the detector chips. The optimisation of the dosemeter design was carried out by varying the dosemeter form, dosemeter dimensions, dosemeter materials, and the positioning of the detector chips within the dosemeter. The effective employment of the computer aided optimisation method was based on the successful identification of a physical function which relates the results of a simulation to the measurement results of an equivalent experiment. The development of the area dosemeter is based on the international recommendations for new operational quantities in the field of radiation protection. Those measuring quantities are also included in the new regulations in Austria and the European Union providing also new limiting levels for the impact of ionising radiation to the population and working people in radiation areas. Our dosemeter was optimised for detecting X- and gamma radiation providing the correct indication of the new measuring quanitiy H*(10), the ambient dose equivalent. For measuring this quantity independently of the energy and independently of the direction of incident radiation fields cylindrical dosemeter designs and triangular detector card positioning concepts were optimised. The simple and robust construction of the dosemeter and the usage of special thermolunescence detector chips allow the detection of the integrated dose without on-the-spot support or any power supply. Measuring time intervalls up to several months at various measuring locations, e.g. nearby industrial, medical or research facilities are possible. Besides the reference detector material, called TLD-100, a new and hyper-sensitive detector material LiF:Mg,Cu,P was studied. A consistent readout procedure and analysing method for determinig the measuring signal of the individual detector chips was developed for both detector materials. For the application of the ambient dosemeter in routine radiation protection measurements some fundamental dosimetric properties, such as the long time stability and the reuseability of the detector chips were also investigated within the framework of this project.