Modelling Alpha Particles in a Negative Ion Proportional Chamber.

Abstract

For a variety of reasons, it is thought that Dark Matter constitutes up to 90% of the mass in the universe. We believe that Weakly Interacting Massive Particles (WIMPs) are a promising candidate for Dark Matter. Definitive identification of WIMPs has yet to occur. The DRIFT project is hoping to use a Negative Ion Proportional Chamber (NITPC) chamber to observe WIMPs. The low expected WIMP event rate places a special importance on the identification of background events which can be mistaken for WIMPs. One worrisome source of background are alpha particles from nuclear decay of Uranium and Thorium found in the materials of the NITPC. However, WIMP recoils and alphas which create similar amounts of ionization travel different distances. In this way, we hope to distinguish the background from real events. Nevertheless, this doesn't always occur, and sometimes alphas are incorrectly identified as WIMPs. For a given range of ionization produced, the ratio of these alphas mistaken for WIMPs to the total number of alphas is called the misidentification probability. Last summer, using neutrons to mimic the WIMPs, we found a misidentification probability ranging between 0.2% and 10% depending on the ionization window. This summer, I put together a computer program to simulate alphas in the chamber. Preliminary comparisons show that the real and theoretical data match, which means that the program works. Once fully verified, we can use the program to make predictions of other types of background, which is useful in the long-term goals of the DRIFT collaboration.