Q on intensifying screens --correction of fallacies!

Dr. Peter Gegenheimer PGegen at UKans.nolospamare.edu
Fri Jun 5 18:59:17 EST 1998


Some of this dialogue contains serious errors, which might dissuade people
from trying two screens even when it might help them, and will certain confuse
anyone who is trying to separate fact from fantasy!

The original question was simple:

Does one need one or two intensifying screens, and if two are used, how should
they be placed?

The answer is equally simple: one screen works fine for most purposes. The
placement is sample:film:screen (active side against film).

Greater sensitivity can be obtained with two screens, placed on either side of
the film with the active side of each screen in direct contact with the film.
This will work for film with emulsion of one or two sides, although
double-sided film will be somewhat more sensitive. This is the only placement
of two screens that has ever been suggested, and its benefits were clearly
documented in the original paper describing the use of intensifying screens
for 32P samples. The sensitivity isn't twice that of a single screen, for
obvious reasons, but it is clearly higher (at least as reported).

Two screens are expected to yield a less-sharp image, for the obvious reason
that the beta particles will spread out from their source as they travel
through the first screen to the film. Those which travel through the film will
be spread more, and the light they produce from the second screen will spread
out, too. However, we are not talking Ansel Adams here! If you're looking for
a few bands or spots, and your choice is between exposures of 1 week vs 2
weeks, or 1 month vs 3-4 months (because of reciprocity failure), you aren't
in a position to complain about image quality.

I am troubled because one poster disparaged the use of two screens, and in
doing so made several erroneous statements which were not based in physical
reality. Edited portions follow (... represent my deletions):

> This surprisingly common notion of using two intensifying in routine
> autoradiography baffles me.
 ...
> There are certainly a large number of other variations (some bizarre) on
> how you could use two screens. One would be to place the film between two
> screens with phosphors facing in, then set that onto the blot or gel.

 -- This is the correct way to use two intensifying screens. There is nothing
bizarre or outre' about it at all.

> That might seem to be a way of improving sensitivity, but first consider how
> thick an intensifying screen is,
 ...

>... there will be some small attenuation of the 32P passing through the
plastic.
 -- True, the attenuation will be small.

> The biggest problem, however, would be ... the size of the 32P image" by the time
> it hit the nearest screen would obviously be degraded, and because it is
> now spread over a larger area, the intended increase in sensitivity will be
> less than predicted.

 -- There will be a spread of the beta radiation away from the original
source, but not as much as the above statement suggests.

> Another factor in this 2 screen "sandwich" is that the
> screen nearest the 32P actually blocks (either by absorption or reflection)
> close to half of the 32P beta hitting it (from behind, technically).

 -- This statement has no basis in fact. The screen does _NOT_ "block" beta
particles: they either interact with the screen to produce light emission, or
they pass through it! There are no other physical possibilities. "Converting
an electron to a photon" is not the same as "blocking an electron."

 -- The notion that electrons could be "reflected" is simply absurd. The
closest phenomenon is Compton scattering, which is not "reflection" -- it more
resembles refraction.

...
> The beta that does make it to the second screen can result in light emission, or
> pass through the screen (and this is lost), or some will reflect
> off and hit the first screen,

 -- As before, it is physically meaningless to speak of electrons as
"reflecting".

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| Dr. Peter Gegenheimer       | Vox:785-864-3939  FAX:785-864-5321 |
| Department of Biochemistry, |   PGegen at UKans.removethis.edu      |
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