intensifying screens

Jonathan B. Marder MARDER at agri.huji.ac.il
Sun Jun 30 01:45:15 EST 1996


In article 
<Pine.A32.3.91.960626133354.45748C-100000-100000-100000 at umabnet.ab.umd.e
du>,
   Stephanie Tucker <stucker at umabnet.ab.umd.edu> wrote:
>My best answer - 
>
>The intensifying screens are activated when exposed to -70C 
>temperatures.  

It's not a question of activation, rather efficiency of the 
scintillation process. Low temp also slightly increases the sensitivity 
of the film.


>You DO NOT need two of them, just put one on one side of 
>the gel, and a piece of film on the other side.  This procedure can be 
>substituted for merely developing 32P gels at room temp., with no 
>intensifying screen.  

I believe this to be wrong. The screen turns fast beta emissions to 
light, thus (for 32P) is most effectively used to trap radiation which 
has passed through the film. This implies putting gel and screen on 
opposite sides of the film. A second screen can be used on the other 
side of the gel (to trap radiation going away from the film). However,  
light emitted is a gel thickness away from the film leading to fuzziness 
of the image. 


>This may suffice if you have a screaming hot gel.  
If the gel is really hot, plain autoradiography gives the sharpest image 
- any fluorograsphic technique, including screens, reduces resolution.


>INtensifying screens are good if you are looking for something barely 
>there, or if you are using 35S.  Just remember, take the saran wrap OFF 
>when exposing 35S gels!

Intensifying screens have little value for the low-energy beta emissions 
from 35S. The radiation which makes it out of the gel is so low energy 
as to be completely absorbed by the film. Also, any radiation in the 
opposite direction would be blocked by the medium used to mount the 
(dried) gel.  

A nice trick for double-labelled (35S and 32P) gels is to arrange the 
following: gel-film-film-screen. The film nearest the gel has both 
images. The film nearest the screen has a strong image from the 32P only 
(enhanced by the screen).

The most common means of amplifying 35S and other weak beta emitters is 
to impregnate the gel with scintillant (PPO, salicylate several 
procedures and/or commercial preparations available) before drying. This 
traps beta particles within the gel matrix and turns them into flshes of 
light which can then migrate out to the film.



Jonathan B. Marder             ,      Department of Agricultural Botany
E-mail: MARDER at agri.huji.ac.il |      The Hebrew University of Jerusalem
Phone: (08 or +9728) 9481918   | /\/  Faculty of Agriculture
Fax:   (08 or +9728) 9467763   |/  \  P.O.Box 12, Rehovot 76100, ISRAEL
     http://www.agri.huji.ac.il/~marder



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