Storage phosphor is not very sensitive to tritium-labeled molecules in gels, since the gel matrix prevents the weak emissions from reaching the screen. It's better to use a fluor, such as Amersham Amplify TM , and a low temperature exposure to sensitive film for this application. Special tritium screens are required for this application. A. PhosphorImager systems are linear over a wide dynamic range so exposure time doesn't matter as much as it does with x-ray film. Typically, an exposure of one-tenth of the time you would expose the sample to film gives a useful image for quantitative analysis.
Longer exposures give the best-looking images.
Check by drawing a line that goes through the darkest part of the spot and creating an intensity profile for it. If the top of the intensity profile is flat, you've over-exposed. Erase the screen for the maximum erasure time and do a shorter exposure. The exception is tritium, which because of its low energy, requires a special uncoated screen. But take care not to contaminate your storage phosphor screen with 14 C. Make sure that your TLC plate is absolutely dry before starting your exposure, and place a sheet of Mylar between the TLC plate and the storage phosphor screen.