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Halftime for the AKD-PTB intercomparison of passive H*(10) dosemeters

15.01.2007

The intercomparison takes place between autumn 2005 and autumn 2007 and serves to investigate the long-term behaviour of passive H*(10) dosemeters (or active dosemeters used for the same purpose) which are in particular used for the supervision of nuclear plants in the radiation field of natural environmental radiation. The intercomparison is based on an agreement concluded between the Fachverband für Strahlenschutz e. V. (Professional Association for Radiation Protection) for its "Dosimetry" working group (AKD) and PTB to promote and intensify research and development in the field of environmental dosimetry. After some of the individual dosimetry services in Germany had merged, the number of participants amounted to 10, among them two institutions from Switzerland and one from Austria. In the first measurement period, which ended in April 2006, a total of 188 dosemeters were installed at three different PTB measuring sites. 57 dosemeters were additionally irradiated in PTB photon fields.

Mainly solid-state dosemeters are used to detect the dose of direct radiation by photons and neutrons from nuclear plants and accelerators (in research and medicine). If the dose fraction which comes from the plant is to be quantified in addition to the natural environmental radiation, the net dose must be calculated by subtracting the dose fraction of the natural radiation from the dose indication of the area dosemeter. For the measurement of the area dose per year, the German Emission-Immission Directive specifies a lower detection limit of 0.1 mSv for photon radiation and of 0.5 mSv for neutron radiation. It has, however, so far not been possible to furnish proof for routinely used dosimetry systems - and in particular not for the new H*(10) area dosemeters - that these values can be reached using solid-state dosemeters as the measurement uncertainty of the net dose is decisively influenced by long-term changes of the natural radiation (by cosmic and climatic influences), but also by changes in the dosimetry system itself (artificial increase in the value displayed by inherent activity of the detector material and, in contrast, decline in the display value by loss of the stored dose information).

In practical applications, no experience of the fundamental detector characteristics can be gained, because the total area dose and the net dose cannot be measured independently at the same place. In the case of the AKD-PTB intercomparison, however, PTB makes reference values available in the form of monthly mean values. The fraction of the natural environmental radiation is - broken down into all components - measured day and night with the aid of the detectors at the measuring site for environmental radiation. Additional irradiations of a portion of the dosemeters are performed at PTB under defined conditions so that the net dose of the irradiated dosemeters is known. For the investigation of the inherent background, some dosemeters are stored at the underground laboratory for dosimetry and spectrometry (UDO) during the exposure time. These dosemeters are stored in lead shields partially separated from one another to rule out unintentional irradiation during calibration measurements at UDO. These dosemeters are permanently exposed to an area dose rate of less than one nSv/h. The response to cosmic radiation is measured on a floating platform on a lake close to PTB. On the lake platform, the dosemeters are placed in a permanently installed plastic housing. An active H*(10) probe, which is permanently installed on the lake platform, is used as control instrument.

At all measuring sites, the dosemeters are each exposed for six months. PTB checks the intactness of the installation by regular controls. Moreover, PTB provides for an equal irradiation of all dosemeters. First evaluations of the photon dosemeters showed that for pure environmental radiation on the free-field, the average dose rates of the participants deviated from one another by maximally 30%; in the case of additional irradiations and pure cosmic irradiation, the differences amounted to almost 50%. Practical experience concerned the dosemeter dispatch (additional doses by x-raying of air freight, which cannot be neglected) and the stability of the holders (one type of suspension proved to be insufficiently weatherproof). From the results obtained, the measurement uncertainties and the lower detection limits of the systems involved shall be determined for both the area dose and the net dose, and a potential improvement of the measurement method for the surveillance of nuclear plants shall be elaborated.

Figure: PTB reference measurement surface for environmental radiation in winter. The dosimeters exposed within the scope of the AKD-PTB intercomparison hang on wooden slats which are exchanged cyclically. In foreground there is a measuring head of the area-wide German early warning system IMIS (for exposing nuclear accidents) which is operated by the BfS. Moreover, the small PTB meteorological station can be seen in the background. Additional PTB reference detectors are located in the immediate vicinity; most of them have been placed in a hut to be weatherproof.