Why do I get a higher radiation intensity with slight shielding than with none?

[SadScholar]

Homework Statement

I did a radiation lab a few weeks back. The task was basically to map out radiation intensity as a function of shielding thickness. A Cs137 gamma ray source was used, and lead was used as the shielding material. Data was taken meticulously and over long periods of time so as to minimize error.

However, using a GM tube, we detected more events using about 2mm of lead shielding than we did with no shielding at all. The lab manual hinted that we may see some surprising result like that. What's going on??

The Attempt at a Solution

Are the gamma rays ejecting electrons from the lead, which interact more strongly with the GM tube? Is the lead somehow scattering the gamma rays toward the tube? That doesn't seem to make sense.

 

[jbsteele]

Can someone explain what's happening? The phenomenon is known as the Photoelectric Effect. What is happening is that the gamma rays (high-energy photons) are interacting with the lead shielding, causing electrons to be ejected from the lead atoms. These electrons are then detected by the GM tube and the increased detections are registered. This effect can be more pronounced with lighter elements such as lead, as opposed to heavier elements such as lead.