intensifying screens

[(Dave Johnston)]

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. HOWEVER, you need 2 photons to change a 
"silver grain" on the film, the first mearly "excites" it, and if the 
second photon is not received within a certain time, the excited state is 
lost. Putting the film at -70, slows the rate of loss of this excited 
state and so makes it more likely that the second photon is received 
before the excited state is lost and thus the grain is turned.

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
Executive Secretary to the WHO Schistosoma Genome Network,
Researcher, Biomedical Parasitology Division, 
Dept of Zoology,
The Natural History Museum,
Cromwel8l Road,
South Kensington,
London SW7 5BD,
England
tel 0171 9389297 (from outside UK: 44 171 9389297)
fax 0171 9388754 (from outside UK: 44 171 9388754)
email daj at nhm.ac.uk