Hello all and thank you for your replies. I've had a couple people recommend the 'hourglass' shaped hole and the machine shop is looking into a close (and easier) alternative, a cone shaped pinhole. A corona would be fine as we are more interested in being able to find the center of the spot precisely than getting an image.
To answer some questions: We are using a flourecing screen, the light from which is amplified and picked up by what I'm guessing is a CCD camera, at very least it's an electronic image capturing device. It has been setup to optimize it's own exposure time as it was originally designed for real time x-ray imaging. The limiting value on contrast then is the difference in intensity between the gamma-rays penetrating the lead and the intensity through the pinhole. The 'Half-Value-Thickness' or the thickness of lead needed to stop half the gamma rays is 5.5mm. We're in the process of taking images with different thicknesses to find the thinest piece of lead that will still give us a good contrast. The most promising thicknesses are looking to be around 11 and 16mm. I also thought that an array of small holes as collimators might be a better option than the pinhole, however, talking to some of the more experienced medical physicists here it was determined that the source was too strong and needed to be positionned to close to be able to get good enough resolution. What we are imaging isn't so much the source itself as radiation which passes through the hole. The smaller the hole, the smaller the spot but the farther away we have to put the source for the same field of view. Thanks again for your time, Jenn -- __________________________________________________________ Sign-up for your own FREE Personalized E-mail at Mail.com http://www.mail.com/?sr=signup
