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

[ludwik kowalski <kowalskil@mail.montclair.edu>]

P.S.

1) The range-energy relation for alpha particles in air (at NTP, and  
for E=>2 MeV) is E(MeV)=2.12*R(cm)^2/3. In other words, R=1 cm when  
E=1.2 MeV.

2) Mike's table shows that the detector stops counting alpha particles  
near 2 cm of air. Suppose the cutoff distance is 1.5 cm. This  
corresponds to E=2.7 MeV. The R=2 cm would correspond to E=3.4 MeV.  
This seems  to indicate that the energy of most alphas escaping the  
source somewhere between 2.5 and 3 MeV. That is already very useful to  
me. But I am not correcting for the unknown thickness of the window of  
Mike's counter.

3) By the way,  R=0.4 cm at E=0.8 MeV and R=0.6 cm at E=1.5 MeV. But  
that is not relevant.

Ludwik


On Sep 21, 2009, at 1:39 PM, ludwik kowalski wrote:

> 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...
> >
> > www.bluffton.edu/~edmistonm/AlphaISL.pdf
>
> 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?
>
> - - - - - - - - - - - - - -
> Ludwik Kowalski, a retired physics teacher and an amateur journalist.
> Updated links  to selected publications and reviews are at:
>
> http://csam.montclair.edu/~kowalski/cf/   http://csam.montclair.edu/~kowalski/my_opeds.html
>    http://csam.montclair.edu/~kowalski/revcom.html
>
> Also an ESSAY ON ECONOMICS at: http://csam.montclair.edu/~kowalski/economy/essay9.html
>
>
>
>
>
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>
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- - - - - - - - - - - - - -
Ludwik Kowalski, a retired physics teacher and an amateur journalist.  
Updated links  to selected publications and reviews are at:

http://csam.montclair.edu/~kowalski/cf/   http://csam.montclair.edu/~kowalski/my_opeds.html 
   http://csam.montclair.edu/~kowalski/revcom.html

Also an ESSAY ON ECONOMICS at: http://csam.montclair.edu/~kowalski/economy/essay9.html