Nathan Shaman with Tom Shutt (project started in summer 2006 and finishing in fall 2006 )

Building a Radon Counter to Characterize Radon in the Case XENON Detector

We attempt to design and build a radon counter to characterize 222 Rn levels in the XENON detector. For XENON to operate effectively, extremely low background radiation levels are required. When natural 238 U decays, it eventually becomes 222 Rn. It happens that 222 Rn is a very radioactive isotope. 222 Rn eventually decays to 214 Pb, which ß-decays to 214 Bi. This creates a significant detection problem as the electrons created from the ß-Decay have the potential to have energies and detection characterisitics causing them to appear much like the events related to a WIMP interaction. As such, it is necessary to measure the 222 Rn background contained within the XENON detector chamber. By bringing to vacuum the chamber of the XENON detector, and allowing 222 Rn to diffuse into the chamber over time, the 222 Rn can be extracted and the amount present can be measured using a radon counter.

The radon counter for this purpose is based on the decay chain of 222 Rn, which will lead to positive daughter ions of 218 Po in 90% of a-decays. In addition, 218 Po a-decays into a positive ion of 214 Pb. The 218 Po ions are attracted to the surface of a PIN , so when their subsequent positive a-decays occur, these particles are attracted to the PIN diode and measured using the semiconductor properties of the diode. These events are used to construct the 222 Rn decay rate in the XENON vacuum.