Alpha-particle spectrometry

In the PDF-files below you find comprehensive technical information on modern alpha-particle spectrometry using semiconductor detectors such as PIPS or ULTRA, or gridded ionisation chambers. Our information is based on almost 40 years of experience in alpha measurements which we provide for your applications in research, environmental supervision and industry.

Alpha spectrometry with surface barrier detectors, ion implanted semiconductor detectors or gridded ionisation chambers is a proven method since decades for identification of heavy nuclides in research and prospection as well as in environmental supervision. The mass of alpha particles is very large and therefore the energy-loss in each interaction of an alpha particle with matter is high and its range is short (just a few centimeters in air). For that reason the detector and sample must be contained in vacuum when high-resolution measurements shall be made.
In a gridded ionisation chamber (GIC) the short range of alpha particles in gas is used for measurement of the total alpha particle energy in the counting gas. As the GIC operates as a proportional counter and the ionisation energy of the counting gas is high one does not reach the same good resolution in a GIC as is possible with semiconductor detectors. The best resolution achieved with small ion-implanted semiconductor detectors having 25 mm² active area is below 10 keV FWHM whereas the typical resolution of a GIC is around 50 keV FWHM or a little below.
The limiting factor which determines the resolution achieved in alpha-particle spectrometry is normally not the quality of the detector employed but rather the thickness of the sample and the distance between sample and detector. The ideal sample thickness is “massless”, which for practical applications means that the sample should be thinner than 1 µg/cm².

As alpha-particle spectra measured from weak sources do not exhibit any background under the peaks one can quantify a few mBq of activity from a thin source in relatively short measuring time.

Very high sensitivity and low detection limits are particularly significant for GIC measurements where the sample area can exceed 300 cm² and efficiency is almost 50%. The total counting efficiency of a GIC is more than a factor of 100 higher than that of a semiconductor detector setup.

For improved identification in nuclide-specific measurements from environmental samples a chemical separation of the elements Radium, Uranium, Thorium, Plutonium, Americium, and Curium into individual fractions or into element groups is often made. Very thin samples are then produced from the fractions by electrodeposition, by spray procedures or through adsorption. These procedures yield very “clean” spectra in which the chemical yield is easily controlled via tracer nuclides. However, the amount of laboratory work for sample preparation and the associated cost is significant.
Thin samples of Polonium are easily produced by electrochemical deposition on a silver plate.

Alpha-particle spectrometry without chemistry
Dr. Westmeier GmbH is developing since years new algorithms and strategies to improve the analysis of Gamma-ray and Alpha-particle spectra. A significant part of improved analysis is the definition of the peakshape according to the physics of interaction of the alpha particle between emission and registration and the associated energy loss. Interactions can happen in the sample, on the sample carrier, with residual air in the measuring chamber and in the dead layer on the detector.
In GIC detectors one also has to consider wall interactions and other effects.

With the help of our new software developments, the consideration of interactions of alpha particles with matter and our special fit-procedures you can now quantitatively analyze also those spectra which were measured from thick samples. You even can deconvolute spectra which were measured without vacuum chamber.
Direct measurement of powder samples without chemical processing or of filter cake (air filter or filtrated precipitates) provides completely new applications of alpha-particle spectrometry. One can now make in-situ measurements and spectrum analysis in order to get immediate information about nuclides and contamination levels (see PDF-file “In-situ alpha spectrometry“).
This new development is highly interesting for environmental supervision as well as for decommissioning of nuclear facilities.

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Alpha Spectrometry
Fitting of solid state detector spectra
In-situ alpha Spectrometry