intensifying screens

Pamela Norton pnorton at lac.jci.tju.edu
Wed Jun 19 18:04:19 EST 1996


In article <4q8a8u$caa at mserv1.dl.ac.uk>, (Dave Johnston) <daj at nhm.ac.uk> 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 
...
> (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 
...
> in the first place. So, for most purposes, one screen is fine

Some stuff edited out...

Sorry Dave, but you have number 1 wrong. Intensifying screens are _only_
useful for high energy emitters. S35 never makes it to the screen. P32
reaches it and is reflected back as higher intensity fluorescent light.
Other methods are needed to employ fluorography with low energy emitters. 

I agree that one screen is fine, although you can get a slight enhancement
if you sandwich screen-film-screen-gel (or filter).  

The appendix to Current Protocols by Ausubel et al has a treatment of
these issues, and many manufacturers prepare booklets, as Dave pointed out
(but I had to edit out as this news reader will not let me quote too much
material.)

Pam Norton

-- 
Pamela A. Norton, Ph.D.          Assistant Professor of Medicine
Thomas Jefferson University
Philadelphia, PA 19107           p_norton at lac.jci.tju.edu



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