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PTB's new service offer for activity standards

Alpha spectrometry under a defined solid angle

PTB-News 2.2016
Especially interesting for

radiation protection

semiconductor industry

radionuclide industry

metrology institutes

So far, the metrological characterization of alpha emitters has only been offered by a small number of national metrology institutes, as the manufacturing of the sources and characterizing measurement technology are very laborious. The determination of the activity of open alpha particle emitting sources has now been included in PTB’s range of services. As of now, such activity standards can be transferred to customers with an improved characterization. To characterize the sources, an alpha spectrometry under a defined solid angle – an absolute method – is used. Currently, relative standard measurement uncertainties of 0.3 % are achieved.

Present and future of alpha spectrometry under a defined solid angle: On the right, the current setup including source holder, diaphragm system and detector. In measuring operation, the detector is flush at the outlet of the diaphragm system. On the left, the future detector can be seen. The appurtenant diaphragm system along with the source holder and the vacuum chamber are still under construction.

Pulse height spectrum (counts over channels) in logarithmic scale of an Americium-241 source. The attribution of the individual transitions of the alpha decay of americium (Peak 1 to Peak 7) is to be seen, but also the impurities by Pu-238, Pu-239 and Pu-240 (Peak 11 to Peak 18). Since this is an absolute method, each event must be attributed to a radioactive decay. The lower part shows the residuum. By means of the spectrometric evaluation, both the activities of isotopic mixtures and radioactive impurities (here: the plutonium isotopes) can be quantitatively determined.

The emission rates and the activities of the radioactive sources of alpha-emitting nuclides are determined by measuring the alpha particles which impact per time on a detector with a known solid angle. This kind of activity determination is an absolute method in the sense that – for the determination of all required calibration factors – only measurements are necessary which are based on the base units of our system of units. In this case, these are time and length measurements.

The measurement uncertainty which can be achieved for an activity determination with the aid of this measurement procedure, is, thus, essentially determined by the accuracy with which the geometry factor of the spectrometer used can be calculated. The calculation of this factor also requires quantitative knowledge of the relative activity distribution as a function of the position on the source surface. This distribution is measured by means of a digital radiography system. The geometry factor is then calculated using a Monte Carlo simulation, into which the dimensions of the diaphragm system used and of the activity distribution measured enter. The measuring device contains energy-dispersive detector systems. From the measured pulse height distributions, information can then be gained, with the aid of suitable computer programs, on the quality of the source as regards the energy loss of the alpha particles in the source, as well as on the radionuclide composition.

Currently, the existing equipment can be further developed and the energy-resolving power of the detector systems can be optimized. Work will be focused on the reduction of the measurement uncertainty. This measurement uncertainty is mainly determined by the uncertainty of length measurements. By enlarging the equipment, the relative uncertainty of these length measurements can be clearly reduced. Due to the design, this procedure is limited by the maximum size of the detector. In contrast to the previous systems having a maximum detector size of 450 mm2, the new equipment will comprise a detector with an active surface of 5000 mm2 and will thus allow the measurement uncertainty to be reduced. In addition, the new equipment will comprise measures for the reduction of the so-called zero effect and make a rapid exchange of sources possible. This opens up the possibility to determine – besides the activity – also the half-lives in the field of short-lived nuclides. Here, the general aim is not only to be able to disseminate the becquerel for alpha-emitting nuclides with smallest possible uncertainties, but at the same time also to be able to detect radioactive impurities and to analyze isotopic mixtures. This requires an improved spectrometric resolution and complex analytical algorithms.


Annette Röttger
Department 6.1 Radioactivity
+49 (0)531 592-6130