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Re: [Phys-l] Alpha decay intensity against distance

On Sep 21, 2009, at 10:04 AM, Edmiston, Mike wrote:

I was not available over the weekend when much of this discussion took place. I actually do this experiment in a home-made vacuum chamber and using an alpha source from a smoke detector. It works extremely well and points out several interesting facts about detection of radiation with Geiger-Mueller tubes.

A photograph of the home-made apparatus, and a fairly detailed explanation, and some actual data and graphs are available on my web space. If you are at all interested in this topic, I recommend that you take a look at this document. It is two pages of text plus two graphs, and is in PDF format.

Here is the link...

Dear Mike,
1) Thanks for sharing a useful document. Referring to high counting rate, you wrote: "The dead time is so high for the 1-cm separation that this data point is not reliable and cannot be used." I suspect that even if a much faster counter were used (much smaller dead time correction), the data point for r=1 cm would still not fit the 1/r^2 line as accurately as your other data points. The diameter of your Am-241 is probably close to 0.5 cm. A point-like approximation for the N(d) is not good at r=1 cm. A realistic N(d) relation, for a source that is not point-like, can be obtained via Monte Carlo simulation; some students might be able to do this. I am only guessing what the result would be.

2) I would like to see the energy spectrum of alpha particles from a smoke detector source. Such sources are probably coated with thick layers of something (to prevent contamination and to maximize the ionization density). My guess is that the initial energy of 5.5 MeV is reduced to something like 4 or 3 MeV. Unfortunately, I do not have an energy calibration system. What do you know about the energy spectrum of alphas from a smoke detector?

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Ludwik Kowalski, a retired physics teacher and an amateur journalist. Updated links to selected publications and reviews are at: