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PTB’s thermal neutron reference field at the Research Reactor FRG-1 of GKSS ceases to exist


In 2008 - after an operating time of more than 50 years - it was decided to decommission the research reactor FRG-1 of the GKSS 2010 (GKSS News of October 2008). The reactor was officially closed down on June 28, 2010. The reactor was decommissioned in view of the restructuring of neutron research in Germany, focussed on the FRM II in Munich.

After the decommissioning of its own research reactor FRMB, PTB in Braunschweig has operated at the GKSS - for almost 10 years - a measuring set-up for irradiations with thermal neutrons, which were made available by the core piece of the Geesthachter Neutronenforschungseinrichtung (GeNF), the 5 MW pool reactor FRG-1. According to the agreement, the certified reference measuring set-up was placed at PTB’s disposal twice a year, each time for two weeks. 19 measurement campaigns were carried out; the last measurements were performed in April 2010.

The spectral neutron energy distribution, which could be described by a Maxwell distribution, was determined with the time-of-flight method, whereas the neutron flux was directly measured with a calibrated monitor. Prior to every irradiation, these field properties were determined anew. In the neutron beam, a fluence rate of almost 105 cm-2 s-1 was reached. However, due to the small beam cross section of 3 x 3 cm2, a scanner system was required for the irradiation of larger objects. In this way, larger irradiation areas could be realized and the fluence rate was reduced correspondingly. As the field was free from epithermal neutrons and - due to the long, curved neutron guide - also almost free from photons - the irradiation conditions were ideal.

This measuring set-up, which is unique in Germany, was used by a great number of customers from industry and research institutes at home and abroad for the calibration of personal dosemeters and area dosemeters and for the investigation of the properties of measuring systems for radiation protection in thermal neutron fields.

PTB regards the end of this important reference field for neutron metrology - which is the result of the reactor being decommissioned - with great regret.

To what extent the operation of a reference field for thermal neutrons will in future be necessary in Germany, can only be clarified by the future applications. The search for an alternative solution at short notice is difficult, the more so as the establishment of a new field would require considerable personnel resources which would only be justified in the case of a corresponding need.