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

[Bernard Cleyet <bernardcleyet@redshift.com>]

interleaved
On 2009, Sep 19, , at 06:22, Savinainen Antti wrote:

> Hi,
>
> my colleague showed me an exam question on alpha decay intensity  
> against distance in the air. The data was supposedly taken with and  
> without
> placing a paper between the source and the detector. Both graphs  
> looked
> like exponential decrease. Well, alpha particles do not generally  
> reach distances approaching 0.5 m in the air and not many (if any?)  
> alpha
> particles can penetrate through a piece of paper. So I concluded that
> the data was fake.

Yes!

Others will correct me:  All Alphas have a line spectrum, and no  
natural ones will penetrate thick paper (except for some possible  
straggling, which would require a sensitive detector (no window,  
maybe ina vacuum and near 100% efficiency.
> However, I got curious on what would a real data look like? What  
> might the relationship between distance and intensity be in the  
> case of alpha particles? One may assume that all alphas have the  
> same energy (say 5 MeV).
> Their range is within a few centimeters (< 5 cm) in the air as  
> indicated
> by the Bragg curve:
> <http://en.wikipedia.org/wiki/File:Bragg_Curve_for_Alphas_in_Air.png>

The range detection curve is like a step function in a weak absorber  
(air).   If one has a thin window G-M detector (typically mica 1.4 mg/ 
cm^2) and manually moves it to the source there is mostly only  
background (including concomitant [only?] gammas).  Then quite  
suddenly** the counting rate increases, and follows the function  
expected from the geometry (depending on the areas of the detector  
and the source).   This again is because of the monoenergenity, and,  
also, it's functional absorption, i.e. Z^2 + high mass (more nearly  
matching the absorber than betas).


** not if one uses a fine motion app. because of straggling.

> BTW, what radioactive source could be used for this experiment as
> there usually are daughter nuclei present emitting gamma and/or
> beta radiation?

Am-241 is an inexpensive (free) source from ionization type smoke  
detectors.  Illegal to recover in the US, how about in Suomi?   It  
has a coupala weak gammas that are convenient for fluorescence of  
high Z nuclei.  However, requires a dense detector, e.g. NaI, which  
is convenient for alphas.   Of course depending on the energy G-M  
tubes are about several percent efficient for gammas (x-rays)  and  
about 100% for Alphas (once they get past the window.  A common  
experiment is to use a SS detector and do the expt.  ina variable  
vacuum.  Also Alpha spectroscopy is done ina vacuum.
> Best regards,
>
> Antti

bc caught the train in Helsingfors for Moscow, 1989

> Antti Savinainen, Ph.D., B.Ed.
> Adjunct Professor (University of Jyväskylä)
> Senior Lecturer in Physics and Mathematics
> Kuopion Lyseo High School
> Finland
> E-mail: <antti.savinainen@kuopio.fi>
> Website: <http://kotisivu.dnainternet.net/savant/>
>
>
> --
>
> ********************************************************************** 
> **********************
> Tämä viesti on tarkoitettu ainoastaan henkilölle tai yhteisölle,  
> jolle se on osoitettu.
> Viesti saattaa sisältää luottamuksellista tietoa. Jos olet saanut  
> viestin erehdyksessä,
> ilmoita siitä lähettäjälle ja tuhoa viesti postilaatikostasi
>
> Viesti on tarkastettu roskapostisuodatus- ja  
> virustorjuntaohjelmistolla.
> Tarkastuksesta huolimatta noudata asianmukaista varovaisuutta  
> liitteitä avatessasi.
>
> Kuopion kaupunki / ATK-keskus
> ********************************************************************** 
> ***********************
>
> _______________________________________________
> Forum for Physics Educators
> Phys-l@carnot.physics.buffalo.edu
> https://carnot.physics.buffalo.edu/mailman/listinfo/phys-l