alpha particle

alpha particle

alpha particle, one of the three types of radiation resulting from natural radioactivity. Alpha radiation (or alpha rays) was distinguished and named by E. R. Rutherford in 1909, who found by measuring the charge and mass of alpha particles that they are the nuclei of ordinary helium atoms. Alpha particles consist of two protons and two neutrons (see nucleus).
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.

Experimental methods

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.

Liquid scintillation

An alternative method is to use internal liquid scintillation, where the sample is mixed with a scintillation cocktail. When the light emissions are 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.

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).

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