UC Berkeley

High pressure xenon gas detectors have the potential to significantly contribute to searches for rare physics processes such as neutrinoless double beta (0νββ) decay and interactions of dark matter with ordinary matter. We summarize the physics and implications of these two phenomena and discuss experimental developments conducted with an electroluminescent high pressure xenon time projection chamber (TPC).

The detector was constructed as a prototype for the NEXT experiment - Neutrino Experiment with a Xenon TPC - and was intended to demonstrate an ability to detect electron recoils with an energy resolution sufficient for a strong neutrinoless double beta decay search. Using the low-gain ionization signal amplification process of electroluminescence, we are able to obtain an energy resolution of approximately 1% FWHM in the detection of 662 keV gamma rays. In addition, we demonstrate basic particle tracking capabilities using a silicon photomultiplier-based tracking plane, a technique also relevant to the NEXT approach in searching for 0νββ decay.

To investigate the possibility of a gaseous xenon dark matter detector, we also characterize the ionization and scintillation signals produced by nuclear recoils in the TPC and compare them to the corresponding signals produced by electron recoils. The nuclear recoils are produced using radioisotope neutron sources. Our measurements demonstrate the ability to discriminate between electron and nuclear recoils using the ratio of ionization to scintillation produced on an event-by-event basis and provide an estimate of the ionization and scintillation yields of nuclear recoils in gaseous xenon.

We end with a discussion of the physical principles behind another approach to obtaining nearly-intrinsic energy resolution in a noble gas detector. This approach, called negative ion drift, consists of drifting negative ions produced by intentionally introducing impurities into a gaseous medium that capture ionization electrons. These negative ions are then counted individually by detaching and amplifying the electron they carry. We develop a basic formalism in an attempt to understand the ion drift and detachment processes that are critical to the implementation of this technique.