Alpha-particle spectroscopy

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One method for testing for (and measuring) many alpha emitters is to use alpha-particle spectroscopy. For methods for gamma rays and beta particles please see Gamma spectroscopy and Liquid scintillation counting respectively.

[edit] Experimental methods

[edit] Counting using a source in a metal disk

It is common to place a drop of the test solution on a metal disk which is then dried out to give a uniform coating on the disk. This is then used as the test sample. If the thickness of the layer formed on the disk is too thick then the lines of the spectrum are broadened, this is because some of the energy of the alpha particles is lost during their movement through the layer of active material.

[edit] Liquid scintillation

An alternative method is to use internal liquid scintillation where the sample is mixed with a scintillation cocktail. When the light emitted is then counted, some machines will record the amount of light energy per radioactive decay event. Due to the imperfections of the liquid scintillation method (such as a failure for all the photons to be detected, cloudy or coloured samples can be difficult to count) and the fact that random quenching can reduce the number of photons generated per radioactive decay it is possible to get a broadening of the alpha spectra obtained through liquid scintillation. It is likely that these liquid scintillation spectra will be subject to a Gaussian broadening rather than the distortion exhibited when the layer of an active material on a disk is too thick.

[edit] Alpha spectra

From left to right the peaks are due to ²⁰⁹Po, ²¹⁰Po, ²³⁹Pu and ²⁴¹Am. The fact that isotopes such as ²³⁹Pu and ²⁴¹Am have more than one alpha line indicates that the nucleus has the ability to be in different discrete energy levels (like a molecule can).

Intensity against alpha energy for four isotopes, note that the line width is narrow and the fine details can be seen

Intensity against alpha energy for four isotopes, note that the line width is wide and some of the fine details can not be seen. This is for liquid sintillation counting where random effects cause a variation in the number of visible photons generated per alpha decay

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