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Why freeze autorads?

Dr. Peter Gegenheimer PGegen at RNAWorld.bio.UKans.edu
Sat Nov 23 03:22:59 EST 1996

In <wgallin.1198981441C at news.srv.ualberta.ca>, wgallin at gpu.srv.ualberta.ca (Warren Gallin) writes:
>In Article <5702q5$3s4 at sjx-ixn6.ix.netcom.com>, geneco at ix.netcom.com(The
>Great American Gene Company ) wrote:
>>Dima Responds:
>>>I don't think so. Intensifying screens emit light upon absorption of
>>>high-energy particles. Light excites silver on film. Freezer is to 
>>>increase sensitivity of latter. 
>>Dead wrong!  The emission of light is quite temperature dependent.  Get
>>hold of DuPont (a leading maker of intensification screens), they have
>>data showing the effect of temperature on emission.
>As suggested, I contacted DuPont Technical Services and asked.  They stated
>quite unequivocally that the enhancing screens are not temperature sensitive
>and that the film response is.  Fluorescence is actually a pretty fast
>process, so it isn't too surprising that the thermal effect would be small.
>    Could you perhaps post a sourec for your information so I can look it up
>and figure out what is going on.

The original reference for the use of autoradiography screens is:
   Swanstrom, R. and Shank, R. (1978) X-ray intensifying screens greatly enhance 
   the detection by autoradiography of the radioisotopes {32}P and {121}I.
   Anal. Biochem. 72, 433-446.  
     (The authors were post-docs in the Varmus & Bishop labs at UCSF.)

The paper demonstrates that sensitivity of the X-ray film is constant, or decreases
slightly, as the temperature of exposure is decreased from 20C to -70C. However, 
the sensitivity of the film/screen combination increases about 8-fold over the same range.
One would conclude that the lower temperature either enhances energy capture by,
or prolongs phosphorescent emission from, the screen. 

The process is not fluorescence but phosphorescence, and both processes 
are prolonged at low temperature. 

DuPont's claim that there is no temperature effect on the Ca-W screen is 
irrelevant, because the manufacturer's experience is only with the screen's
response to X-rays. Swanstrom & Shank demonstrated that different screen 
phosphors respond differently to beta-radiation; this result suggests that the
interaction of electrons with the phosphor is at least partially different from the 
interaction of X-rays. 

|  Dr. Peter Gegenheimer       |  Vox: 913-864-3939  FAX: 913-864-5321  |
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