Answer to Question #9892 Submitted to "Ask the Experts"

Category: Accelerators — Radiation fields

The following question was answered by an expert in the appropriate field:


I have a Monitor 4 radiation survey meter with a 9.14 mm diameter mica end window GM (Geiger Mueller) tube. I placed a 241Am smoke detector button in a wrap of aluminum foil, then placed a 3.18 mm paraffin disk atop the wrapped button. Are the four times above background readings atop the paraffin from proton emission or some other particle or radiation?


You are repeating one of the famous experiments that Enrico Fermi and his group in Rome conducted many years ago. See the following link for historical details:

241Am is naturally radioactive. It decays by emitting alpha particles of about 5 MeV (8.01 x 10-13 Joules) energy. It also emits some low-energy beta particles and gamma rays in the keV (10-16 Joules) range, which can also be detected by Monitor 4.

The alpha particles interact with your aluminum foil and produce 30P and a neutron. 30P has a half life of about 2.5 minutes and decays to the stable 30Si by positron emission. The positron will annihilate with an electron producing two 0.511 MeV (8.19 x 10-14) gamma rays that are easily detectable. Note the decay of 30P to 30Si will not necessarily be to the ground state of the nucleus, but to one of the several excited states of 30Si, which will quickly decay by gamma emission. The more probable of these gamma rays have energies between 1.2 MeV (1.92 x 10-13 Joules) to 2.2 MeV (3.52 x 10-13 Joules). These are also detectable with your detector.

Last but not least, is the neutron which was emitted from the interaction alphas with the aluminum. Addition of the paraffin disk, since it contains lots of hydrogen atoms, will slow down the neutrons. Slow neutrons can be captured by nuclei in the paraffin, in the mica window, and in the body of the detector itself. This results in several other nuclides that can decay by emitting gamma rays and other radiation.

This is how you have increased the radiation from your 241Am source.

Kamran Vaziri, PhD

Ask the Experts is posting answers using only SI (the International System of Units) in accordance with international practice. To convert these to traditional units we have prepared a conversion table. You can also view a diagram to help put the radiation information presented in this question and answer in perspective. Explanations of radiation terms can be found here.
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