Multisite event discrimination for the majorana demonstrator

Abstract

The Majorana Demonstrator is searching for neutrinoless double-beta decay (0νββ) in 76 Ge using arrays of point-contact germanium detectors operating at the Sanford Underground Research Facility. Background results in the 0νββ region of interest from data taken during construction, commissioning, and the start of full operations have been recently published. A pulse shape analysis cut applied to achieve this result, named AvsE, is described in this paper. This cut is developed to remove events whose waveforms are typical of multi-site energy deposits while retaining (90 ±3.5)% of single-site events. This pulse shape discrimination is based on the relationship between the maximum current and energy, and tuned using 228 Th calibration source data. The efficiency uncertainty accounts for variation across detectors, energy, and time, as well as for the position distribution difference between calibration and 0νββ events, established using simulations.

ADC Nonlinearity Correction for the Majorana Demonstrator

“…Signal amplitudes are measured to estimate event energies using a trapezoidal filter with a 4 µs integration time, a flat-top of 2.5 µs, and employing a modified pole-zero adjustment to correct for charge trapping [15]. The smoothed derivative of the pulse is computed with a running linear fit over a ∼100 ns range to distinguish single-site signal events from multi-site background interactions [16]. A third pulse shape parameter looks for excess slope in the exponential tail of the pulse that indicates the presence of "delayed charge recovery" (DCR) following surface alpha background interactions [17].…”

Cosmogenic Activation in Double Beta Decay Experiments

“…limits for the half-life of 76 Ge and the effective mass (T 0ν 1/2 ≥ 1.9 × 10 25 y and m ββ ≤ 0.35 eV [65]), but a few members of the collaboration later claimed hints of a positive signal [66], with a 4.2σ confidence level, corresponding to T 0ν 1/2 = 1.2 × 10 25 y and m ββ = 0.44 eV. New strategies based on new detector designs (like Broad Energy germanium (BEGe) detectors [67]) were developed to further suppress the radioactive background of germanium experiments; discrimination techniques following different approaches are under consideration [68,69].…”

A low energy rare event search with the MAJORANA DEMONSTRATOR

“…The manufacture, processing, and storage of the enr Ge detectors used in the Demonstrator was done to minimize surface exposure time at all points during the manufacturing process [9]. We model the signals of interest from bosonic dark matter and (separately) a 14.4 keV solar axion as a Gaussian peak with a width derived by the ββ(0ν) analysis [4] and taking a constant 30% uncertainty over the energy range of interest. We perform an unbinned extended maximum likelihood fit of our background model against the data using the RooFit framework [10].…”

“…The initial commissioning data set of 478 kg-d enr Ge exposure was used to search for bosonic dark matter, solar axions, and other rare events [3]. Since then, the Demonstrator has amassed a larger exposure (>30 kgy) with roughly two-thirds of it blinded, and several significant hardware and software upgrades to the system have been made, most notably the installation of a second detector module and the completion of the passive shield [4]. Hence, the work presented here uses improved analysis techniques and nearly a factor of 10 more enriched exposure (11.17 kg-y) with backgrounds reduced by a factor of 4 (to ∼0.01 cts/kg-d/keV between 20-40 keV).…”