Alpha (α) Radioactivity


Alpha particles consist of two protons and two neutrons bound together into a particle identical to a helium nucleus. They are generally produced in the process of alpha decay, but may also be produced in other ways. The symbol for the alpha particle is α or α2+. Because they are identical to helium nuclei, they are also sometimes written as He2+ indicating a helium ion with a +2 charge (missing its two electrons). If the ion gains electrons from its environment, the alpha particle can be written as a normal (electrically neutral) helium atom He.

The best-known source of alpha particles is alpha decay of heavier (> 106 u atomic weight) atoms. When an atom emits an alpha particle in alpha decay, the atom’s mass number decreases by four due to the loss of the four nucleons in the alpha particle. The atomic number of the atom goes down by exactly two, as a result of the loss of two protons – the atom becomes a new element. Examples of this sort of nuclear transmutation are when uranium becomes thorium, or radium becomes radon gas, due to alpha decay.
Alpha particles are commonly emitted by all of the larger radioactive nuclei such as uranium, thorium, actinium, and radium, as well as the transuranic elements.  The process of alpha decay sometimes leaves the nucleus in an excited state, wherein the emission of a gamma ray then removes the excess energy.
Alpha rays, because of their electrical charge, strongly interact with matter and therefore are easily absorbed by materials and can travel for only a few centimeters in the air.

Experimental Setup

To detect alpha particles the following geiger tube has been used :


Sensor : Geiger tube LND-712
Sens,: 18 CPS/mR/h
Bkg: 0.10 CPS
Voltage: 500 volt
Resistence: 5.6 megaohm
Type of Radiation Alfa+Beta+Gamma

Setup image with sensor and source

Particles α emitted by Americium (1μCi 241Am)

Using the experimental setup described in the previous paragraphs we made the measurement of the alpha particle radioactivity  emitted by a 0,9μCi of americium source. Two measurements were made: the first keeping fixed the distance between the sensor and the source and putting a sheet of paper in front of the sensor so as to evaluate the absorption of alpha particles; in the second measurement the distance between the sensor and the source was varied in order to measure the distance traveled by the particles in the air. For this second measurement the source must be collimated so as to maintain the beam focused over the distance .

Effect of the interposition of a sheet of paper in front of the source : the alpha particles are blocked
Measure of the alpha particle range in air : 25mm

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