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Photon reference sources for the calibration of surface contamination monitors


Surface contamination monitors are a basic tool for practical radiation protection. Very often, gas-filled detectors or scintillation counters are used as mobile hand sensors or often also as hand-foot-clothing monitors in order to detect potential contaminations. The radionuclides to be proved can emit alpha radiation, beta radiation (electrons or positrons) or also photons which can be proved by the detectors. To receive reliable and significant results, regular testing and calibration with the aid of reference sources is necessary.

For a long time now at PTB, calibrations of large-area reference sources in terms of activity, emission rate as well as the alpha- and beta-particle flux have been carried out at the source surface. A possibility to characterize large-area photon-emitting sources in accordance with ISO standard 8769 has not been found yet.

Due to the increasing demand for corresponding calibrations, a new measuring set-up is currently being developed within the scope of a cooperation with Nuclitec GmbH (formerly QSA Global GmbH). The apparatus consists of a large Nal scintillation detector and a holder for radioactive sources which is mobile in automated operation. The measuring procedure to be developed shall allow the photon flux and activity determinations of large-area sources to be traceable to the activity standards of PTB. A diaphragm system is to furthermore enable the sources to be tested for the homogeneity of the activity distribution (uniformity). As supplement thereto, the homogeneity can also be tested with the aid of a digital autoradiographic system.

After completion of the project in April 2010, PTB will be the first national metrology institute to offer corresponding calibrations for photon reference sources. PTB would thus be the first institute at present which is able to carry out calibrations of the sources in accordance with class 1 of ISO standard 8769. The target is to establish calibration possibilities for sources with 57Co, 60Co, 129I, 137Cs, 238Pu and 241Am.