Intensifying screens

Paul Digard pd1 at mole.bio.cam.ac.uk
Wed Jun 19 06:11:55 EST 1996


(Dave Johnston) wrote:
> 
> Robyn, as I understand it:
> 
> (1) screen will have no effect with P32, they are designed for weak
> emittors like 35S -if a P32 emission gets past the X ray film without
> being trapped, it is likely to keep on going through the screen as well
> without "touching the sides".  

> (2) for 35S, the weak emission is trapped on the screen and excites an
> electron. As the electron drops down again, you get a photon released
> which will affect the film. 

SNIP

> If your sample is opaque, then any fluorescence from a screen on the side
> away from the film will not reach the film. Smilarly, if the sample is
> not homogeneous (eg if it is a gel dried onto glass or paper or plastic),
> then chances are that the weak emission will never get as far as the screen
> in the first place. So, for most purposes, one screen is fine
> 
> Amersham produce (or used to) an excellent free booklet on autoradiographic
> detection which explains everything very clearly
> 
> DAJ
> 
> David A. Johnston


This is backwards (as I understand it at least, but also from the 
Amersham booklet).  Screens are aimed specifically at strong 
emissions from 32P or 125I which have a good chance of zipping 
through the film without effect.  The screen is denser than the 
film, so has a better chance of absorbing the energy.  Electrons 
from 35S aren't penetrating enough to get through film to reach the 
screen in any great amount; they're not even that great at getting 
out of the gel or through the outer layer of emulsion on the film, 
which is why fluorography is used for this isotope.

As DAJ says, you don't need two screens, but they do up the 
sensitivity; sandwich the film between the screens with the 
(32P/125I) gel on the outside.  Higher sensitivity because two 
screens trap a higher proportion of the radiation, but lower 
resolution because the decay emissions have a longer distance to fan 
out in before hitting something.  

There's also preflashing of the film to consider; increases the 
sensitivity a bit more, and gives a more linear response with faint 
bands.  Also to do with the grain structure of the film needing more 
than one hit to go black when developed I think.  The Amersham book 
also talks about sensitizing the film by baking it in an N2/H2 mix, 
but I've never tried that, or seen it used.

Cheers


Paul Digard

Division of Virology
Department of Pathology
University of Cambridge



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