intensifying screens

Wed Jun 19 14:44:04 EST 1996

On 19 Jun 1996 daj at 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". 
Interesting understanding indeed! 

> (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.
Good, but the beta emission from 35S is weaker than that from 32P. In 
fact, very little of it actually can cross the film to cause any 
excitation in the intensifier. Even a plastic wrap like 'SARAN' blocks 
the beta from 35S...

> 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

Yeah! It should be read again.
> 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

			  Hiranya S. Roychowdhury
   			  Plant Genetic Engineering Lab.
			  Box 3GL, NM State Univ.
			  Las Cruces, NM 88003
			  Phone: (505) 646-5785
			  hroychow at

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