Logo of the Physikalisch-Technische Bundesanstalt

AKD/PTB intercomparison of passive H*(10) dosemeters


The intercomparison serves to investigate the long-term behaviour of passive (or similarly used active) H*(10) dosemeters which are dedicated to the monitoring of the surroundings of nuclear plants and accelerators and used in the radiation field of the natural ambient radiation. Between autumn 2005 and 2007, approx. 600 photon and neutron dosemeters were exposed on three PTB reference measuring sites for ambient radiation. Some of the dosemeters were also subjected to an additional irradiation in PTB photon and neutron fields. Ten institutions, which are involved in ambient monitoring in Germany, Austria and Switzerland, took part in this intercomparison on the basis of a contract which was signed between the German Swiss Radiation Protection association (for the subgroup "Dosimetry") and PTB.

In order to supervise the radiation in the surroundings of nuclear plants and accelerators, mainly solid state dosemeters such as thermo-luminescence dosemeters or optically stimulated luminescence dosemeters are used. When such dosemeters are exposed at one location over weeks or even months, the following problem arises: The dose contribution of the natural radiation must be subtracted from the total measured value in order to obtain the fraction of the dose which was caused by the nearby facility. The natural ambient radiation, however, is subject to strong temporal and spatial variations. Furthermore, in the course of long-term measurements, the dosimetry system itself can have an influence on the measured value (increase of the indicated value due to the inherent activity of the detector material, or decrease of the indicated value due to losses of the dose information). The "Directive for the Surveillance of Emission and Immission from Nuclear Installations" (REI) sets lower detection limits for the measurement of the yearly area dose. For dosimetric systems used routinely, however, it could not be established before the REI was compiled whether these requirements could be kept when using solid state detectors. The intercomparison will provide essential contributions to clarifying this issue.

Figure 1 : Response of different dosimetric systems at PTB's reference site for cosmic radiation. The three dosimetric systems pictured on the right are neutron dosemeters; the other ones are photon dosemeters. Contrary to photon dosemeters, neutron dosemeters show an under-response.

Figure 2 : Like Fig. 1, but with the difference that the measurements were carried out at PTB's measuring site for ambient radiation.

After the dosemeters had been exposed for six months, the results of the measurements were compared with reference values. The reference values are provided by PTB in the form of monthly average dose values which are based on data routinely retrieved around the clock at the measuring site for ambient radiation. By means of standard procedures and by using the measuring sites for ambient and cosmic radiation, it is possible to determine the response of the dosemeters for the cosmic and the terrestrial component of the natural radiation. The inherent background was measured by storing part of the dosemeters inside the UDO underground laboratory.

First results are now available. Photon dosimetry by means of passive dosemeters seems to yield good results, whereas it becomes apparent that there is still a need for clarification in the area of neutron dosimetry. PTB will carry out further measurements to clarify the unsolved issues. The results of the intercomparison will contribute to improving measurement procedures for the surveillance of nuclear plants or accelerator facilities.