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The measurement of environmental radiation doses and dose rates

Objective


The free-field and shadow-shield calibration facilities established at Risø during a previous phase of the EC project were commissioned in the present programme by determining the scattered gamma photon contribution to the detector responses in different calibration geometries by Monte Carlo calculations.

Measurement data obtained from experiments carried out at the Hinkley Point Nuclear Power Station in the UK with the aim of comparing the responses from different types of dose rate meters and TL dosemeters were further evaluated and analysed. An attempt was made to determine the separate dose rate contributions from the different ambient radiation components present at this special location including that from the periodic release of Ar-41.

Two natural environment measurement stations were established and commissioned at Risø in 1994-95.

Several manufacturers provided a variety of electronic dosemeters to be tested under environmental conditions.

In the week 12-18 June 1994 an international intercalibration experiment was performed at Risø with participants from USA, Eastern Europe (PECO), and EU.

The optically stimulated luminescence (OSL) properties of the highly sensitive TL material Al(2)O(3):C were investigated at Risø in relation to environmental radiation dosimetry.

The PTB mainly used their underground laboratory (UDO) in the Asse mine for investigations of detector characteristics at ultra-low radiation dose rates. In the present phase emphasis has been put on 1) Accurate determination of background dose-rate levels, 2) Experimental determination of the scattered radiation from different calibration set-ups, 3) Investigation of the inherent background and of the linearity of newly developed electronic dosemeters and dose rate meters, 4) Determination of the inherent radioactivity of the construction materials, e.g. batteries, of electronic dosemeters.

At CIEMAT the main achievement of the project has been the demonstration of practical measurement capabilities of new hypersensitive TL materials, especially LiF:Mg,Cu,P, for environmental radiation dosimetry. This was possible after an intensive study of the TL material characteristics and by proposing innovative solutions for the problems that these materials present when evaluated by conventional procedures.

The Institute of Radiation Dosimetry (IRD), Prague, started as an associated PECO partner of the project in November 1993 and the main items of IRD's research work has been 1) implementation of computerized glow curve analysis methods of TLD systems for environmental radiation measurements at IRD, 2) theoretical and experimental studies of environmental radiation fields and their variations and 3) unfolding of NaI photon spectra obtained from measurements of environmental photon radiation.

The Institute of Nuclear Physics (INP) Krakow, joined the project in January 1994 as an associated PECO partner with a formal agreement signed in April 1994. The main objective of INPs contribution to the project was to improve the accuracy and sensitivity of TL-based methods of assessing environmental dose rates. Over the period April 94 - May 95, ultra-sensitive LiF:Mg,Cu,P detectors of different Mg, Cu, and P compositions were synthesized, developed, investigated, and optimised at INP. Furthermore, a new model of a compact TL reader-analyzer was designed and tested at INP. Intercomparison measurements and field measurements have demonstrated the suitability of the INP developed LiF material, TLD equipment and readout techniques for environmental monitoring with superior sensitivity, permitting daily or even hourly dose assessment. Work in progress at INP concerns a systematic survey of the dosimetric properties of LiF:Mg,Cu,P phosphor with respect to dopant concentration, in which a new promising LiF:Mg,Cu,P phosphor may have been discovered. Also in progress is the development of an automatic TL system (TL cards and reader) designed specifically for large-scale environmental monitoring using LiF:Mg,Cu,P.

Both the Institute of Isotopes (I.I.) and the Atomic Energy Research Institute (AERI), Budapest, joined the project under one PECO contract (with I.I. as principal investigator) in January 1994 with a formal agreement signed in April 1994.

Much effort has been devoted to studying the TL properties of the newly developed Al(2)O(3) single crystal doped with carbon prepared in Russia. One of the important properties of this material is that it exhibits an increased sensitivity compared with common TL dosemeters (e.g. TLD-100).

The following TL properties were investigated: 1) Annealing procedure in order to optimise reproducibility, 2) inherent background of dosemeters during long term storage, 3) fading characteristics at various temperatures, and 4) light and UV sensitivities of dosemeters.
Studies of calibration methods for environmental monitoring systems and the determination of different detector characteristics were carried out as a successful cooperation between RISOE, PTB and CIEMAT in 1990/92 CEC contrac Bi7-027). It is the intention to continue this work by studying further demanding aspects of environmental monitoring such as the effect of radon daughters. Electronic dosemeters will be included in the environmental programme to see how they respond to ambient doses. Experiments will be undertaken to determine the cosmic radiation response of different detector used for measuring doses received e.g. by flight crews.

It is proposed to establish Natural Environmental Monitoring Stations at RISOE for both cosmic and terrestrial photon radiations.It is intended that these facilities would be made available to CEC member states for assessing and intercomparing their environmental monitoring systems including TLDs.
The new UDO low-level measurement laboratory in the Asse salt mine(PTB)will play an important role in future activities for the determination of sensitivity, linearity, inherent background and dose rate dependence of a variety of new detectors suitable for measuring the ambient radiation.
TL glow curve analysis techniques introduced by CIEMAT will be further developed to assess the dose rate dependence and self-dose properties of a variety of new TL materials suitable for environmental monitoring. The techniques will be tested against experimental measurements carried out in the Asse facilities (PTB) and at the RISOE measurement stations.
The incorporation of NPL into the project will further establish a link between the UK and other European standards.

With the experience gained from these studies it is proposed that technical recommendations on environmental measurement techniques will be provided to member states.The above studies together with these recommendations should greatly assist in improving the standard of measurements and the level of harmonisation in the field of ambient photon monitoring within the CEC.

Funding Scheme

CSC - Cost-sharing contracts

Coordinator

Risø National Laboratory
Address

4000 Roskilde
Denmark

Participants (2)

Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas
Spain
Address
22,Avenida Complutense
28040 Madrid
PHYSIKALISCH-TECHNISCHE BUNDESANSTALT
Germany
Address
Bundesallee 100
Braunschweig