Abstract
The xenon time projection chamber (TPC) promises a novel detection method for neutrinoless double-beta decay (0 nu beta beta) experiments. The TPC is capable of discovering the rare 0 nu beta beta ionization signal of a distinct topological signature, with a decay energy Q(beta beta) = 2.458 MeV. However, more frequent internal (within TPC) and external events are also capable of depositing energy in the range of the Q(beta beta)-value inside the chamber, thus mimicking 0 nu beta beta or interfering with its direct observation. In the following paper, we illustrate a methodology for background radiation evaluation, assuming a basic cylindrical design for a toy titanium TPC that is capable of containing 100 kg of xenon gas at 20 atm pressure; we estimate the background budget and analyze the most prominent problematic events via theoretical calculation. Gamma rays emitted from nuclei of Bi-214 and Tl-208 present in the outer-shell titanium housing of the TPC are an example of such events for which we calculate probabilities of occurrences. We also study the effect of alpha-neutron (alpha-n)-induced neutrons and calculate their rate. Alpha particles which are created by the decay of naturally occurring uranium and thorium present in most materials, can react with the nucleus of low Z elements, prompting the release of neutrons and leading to thermal neutron capture. Our calculations suggest that the typical polytetrafluoroethylene (PTFE) inner coating of the chamber would constitute the primary material for neutron production, specifically; we find that the fluorine component of Teflon is much more likely to undergo an (alpha-n) reaction. From known contamination, we calculate an alpha production rate to be 5.5 x 10(7) alpha/year for the highest-purity titanium vessel with a Teflon lining. Lastly, using measurements of neutron flux from alpha bombardment, we estimate the expected neutron flux from the materials of the proposed toy TPC and identify all gamma rays (prompt or delayed, of energies comparable to the Q(beta beta)-value) originating from thermal neutron capture with all stable elemental isotopes present in the TPC. We show that to limit the most probable reactions to a rate of one event per year or less, the neutron flux would have to be reduced to (3-6) x 10(-10) cm(-2).s(-1). The predictions of our crude theoretical calculation are in good agreement with full simulation of TPC radiation background by existing experimental collaboration using xenon for 0 nu beta beta experiment.