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Re-start of alpha spectrometry


The determination of the activity of alpha emitters is on offer as one of PTB’s services again. The method, which had already been established previously, has now been further developed towards α-spectrometry under a defined solid angle. Thereby, two aims have been set which represent an increased metrological challenge: On the one hand, the activity should be disseminated with the smallest possible uncertainties, and on the other hand, it should be possible to detect radioactive impurities and to analyze isotopic mixtures.

Alphaspectrometry with defined solid angle

The emission rates and the activities of 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. Currently, relative standard uncertainties of 0.3 % are achieved. This kind of activity determination is an absolute method in the sense that – for the determination of all required calibration factors – only measurements which are based on the base units of our system of units must be performed. These are, in this case, time and length measurements.

The measurement uncertainty which can be achieved for an activity determination with this measurement procedure is, thus, essentially determined by the accuracy with which the geometry factor of the spectrometer that is being used can be calculated. The calculation of this factor also requires quantitative knowledge of the relative activity distribution as a function of the place on the source surface. This surface is measured with a digital radiography system. The geometry factor is then calculated with the aid of a Monte Carlo simulation. The dimensions of the diaphragm system used as well as the measured activity distribution enter into this simulation.

The measuring equipment contains energy-dispersive detector systems. From the measured pulse height distributions, information on the quality of the source with regard to the energy loss of the alpha particles in the source, and on the radionuclide composition can then be gained with suitable computer programs.