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Further development of radon emanation sources for temporally stable Rn-222 activity concentrations in the outdoor air activity concentration range of 1 Bq/m³ to 100 Bq/m³


Within the scope of the traceRadon project, radon detectors are being developed that can traceably measure the radon activity concentration in outdoor air. Such measurements are necessary to enable the use of the radon tracer method (RTM) in establishing a correlation with greenhouse gases (EMPIR 19ENV01 traceRadon).

It is essential here that the detectors be calibrated traceably, and PTB has further developed its radon emanation sources to meet this need. Generating such low radon activity concentrations requires sources with a very low Ra‑226 content but exactly known absolute activity and very well known emanation behavior. For the exact determination of the low levels of Ra‑226 activity involved, highly efficient defined solid angle alpha spectrometry is necessary. The online determination of source emanation variation likewise demands the very efficient detection of Ra‑226 and of the progenies of Rn‑222. This is achieved by directly combining the source with an alpha‑spectrometric detector, something that has now been successfully accomplished for the first time through the physical vapor deposition (PVD) of Ra‑226 onto an ion‑implanted silicon detector.

The process involved evaporating 150 Bq (Ra‑226)Cl2 directly onto the dead layer of the detector. This means that the detector itself becomes the source of the Rn‑222, most of which emanates due to recoil from the decay of the Ra‑226, while simultaneously serving as the spectrometric detector of the alpha radiation produced both by the Ra‑226 and by the Rn‑222 and its progenies. The detection efficiency of this technique is approximately 50 %. The first ever alpha spectrum recorded with a combined source‑detector system is shown in Fig. 1. The graph plots the pulse height distribution against the energy of the α‑particles. Ra‑226, Rn‑222, Po‑218 and Po‑214 can be identified here by their energy levels.

Fig. 1:   Alpha spectrum of a source‑detector combination for determining the progress over time of very low‑level Rn‑222 emanations. The black histogram plots the frequency distribution against the energy of the α‑particles. The red line shows the optimized fitting of a Gaussian peak extended by an exponential tail per line. The attained full width at half maximum of the Gaussian distributions is FWHM 40 keV.



Opens local program for sending emailF. Mertes, Department 6.1, Working Group 6.13

Opens local program for sending emailS. Röttger, Department 6.1, Working Group 6.13