What's this polluting my background

[Jim Kovalchick]

Im trying to figure our what this peak in my background is? Around 400 keV. I have my detector surrounded by a pile of lead bricks. Maybe one is contaminated. Any ideas?

[Rob Tayloe]

An obvious first question concerns the energy calibration of you system. What isotopes and energies were used? What was the fitted equation and some measure of goodness of fit? Also, what is the measurement system? It appears that there are only 50 (counts?) beneath the peak shown at 400; the counting statistics for such a low count would not be very good.

[Jim Kovalchick]

Rob,
2" NaI. 512 channels. Calibrated with Eu 152, radium, and Cs 137. Third order polynomial with decent fit across the range in the spectrum. By decent, I mean less than 5 keV.

After I posted, I started investigating my shield. Outside my lead bricks, I stacked some steel plates. There are some spots where the lead doesn't overlap and there is lines into the chamber. The background total is higher without the plates, but I'm going to take a spectrum to see if the peak is still there. Maybe a thorium peak?

The peak is relatively small, but it's in a bad spot for some work Im doing using a fusor to do neutron activation. The isotopes Im looking for will be very low activity and most won't last long.

Jim K

[Jim Kovalchick]

Because of Rob's questions, I double checked my energy calibration and found it way off. I re-performed the cal and found a second order polynomial with a good fit. My peak turned out to be cesium contamination. I tore down the shield and cleaned it successfully.