Q on intensifying screens

David F. Spencer dspencer at is.dal.ca
Wed Jun 3 12:01:48 EST 1998


In article <3560A955.3A13 at nospam.ipk-gatersleben.de>,
boernke at nospam.ipk-gatersleben.de wrote:

> Hi folks,
> 
> we just had this discussion in the lab about the use of intesifying
> screens with 32P. And the question basicly was wether two screens per
> cassette are really necesarry or will only one do the same job. And how
> exactly is the best way to built the whole thing up, is it first the
> screen then the filter and last the film or any other order?
> Any hints will be appreciated.


This surprisingly common notion of using two intensifying in routine
autoradiography baffles me. It is true that in medical X-ray applications
the X-ray film is surrounded on both sides by intensifying screens but the
goal there is quite different, namely to double the odds of capturing an
X-ray, which, being a high energy photon, is not stopped easily. Fine
resolution is also clearly not an issue for that use.

In autoradiography, normally specifically for 32P, but also useable for
iodine and probably some more exotic isotopes, the traditional method is to
put the blot or gel on the "bottom", then the X-ray film, either double
emulsion or if single-sided, with the emulsion "up", then on top the
intensifying screen with phosphor facing down. Note that this setup is
different if you use Kodak's new TranScreen, because the film is actually
sandwiched in the special screen "packet".

If you take the traditional setup and add a second screen, phosphor side
up, under the nylon membrane or gel, think of what happens. 32P beta going
"down" (i.e., away from the film) strikes this second screen and light is
emitted. Where does this light go? To get to the film it must pass through
either the nylon blot (which is opaque white) or through your frozen gel
(which is also white). When that light reaches the film it is a diffused
blur which adds nothing and just creates a haze around any band or spot,
hardly a desirable goal. This arrangement would only make any sense if the
object containing the 32P were perfectly clear, and then you would still
lose sharpness because of the combined pathlength (and angles) of the beta
particle and the light resulting from it's striking the screen.

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. That
might seem to be a way of improving sensitivity, but first consider how
thick an intensifying screen is, most of the screen actually being the
plastic backing that supports the quite thin phosphor layer (this backing
is about 0.5 mm, in contrast X-ray film is about 0.2 mm thick). The path
between the P32 containing material and the closest screen phosphor will
now be about 0.5 mm, and there will be some small attenuation of the 32P
passing through the plastic.The biggest problem, however, would be the
result of the inverse square law, 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. 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). Thus
the second screen can't possibly improve the sensitivity by two. 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, resulting in some light plus some more reflection,
etc. And the longer the distance the beta moves (and particularly after
reflection) the broader the area over which it will be distributed, the
more degraded the image becomes and the poorer the enhancement because the
area of exposure increases. Given all of these factors the overall
enhancement is probably about 10-15%, with the best values being if the 32P
on the original source were spread over a relatively large area to start
with.

So if your autorad exposure times are 30 days or longer and if image
quality is a minor concern then using the "sandwich" technique will save 3
to 5 days, maybe. You'd be far better off to either go to manual processing
of the film or to do the modifications to an automated processor that
lengthen the processing time of the film in the machine.

Clearly you're wiser to use two screens for two autoradiographies.

Dave

--------------------------------------------
David F. Spencer, PhD
Dept. Of Biochemistry
Dalhousie University
Halifax, Nova Scotia
Canada

dspencer at is.dal.ca
dspencer at rsu.biochem.dal.ca



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