BνL - McGill

The Nature of the Neutrino

Recent observations of neutrino oscillations proved that neutrinos have mass. This finding was awarded the 2015 Nobel Prize in Physics. However, the masses of the neutrinos are much smaller than the other Standard Model particles. In fact, the mass of the electron is at least 10 million times larger than the mass of the lightest neutrino. The mechanism by which neutrinos acquire these tiny masses remains a mystery, but they could arise from new physics beyond the Standard Model. The fundamental nature of neutrinos is also still unknown (i.e. whether they are Dirac or Majorana particles). Many of the mechanisms proposed for how neutrinos acquire mass also require neutrinos to be Majorana particles.

The EXO-200 experiment

The EXO-200 experiment is searching for lepton-number violating decays in Xenon-136, in particular for neutrinoless double beta decays (0νββ). A positive observation of this decay mode would confirm that neutrinos are Majorana particles and could allow the determination of the absolute neutrino mass scale from the half-life of the decay. EXO-200 is currently searching for the existence of 0νββ decays in ~175 kg liquid Xe enriched to ~80.6% in the isotope ¹³⁶Xe and is operated at the WIPP underground facility in New Mexico, USA. EXO-200 has provided one of the most sensitive limits on the half-life of this decay (T_1/2>1.1 × 10²⁵ yr at 90% C.L.). It is planned to operate the detector for at least two more years to increase the sensitivity by a factor of 2-3.

The nEXO experiment

In parallel to the operation of EXO-200 I am involved in the development of the next-phase ββ-decay experiment which is called nEXO. This proposed detector will consist of a monolithic liquid Xe (LXe) time projection chamber (TPC) instrumented with charge-sensitive tiles and Silicon Photo Multipliers (SiPM) to detect the energy of events in the detectors in the charge and light channels respectively. The TPC will be filled with approximately 5000 kg of LXe enriched in ¹³⁶Xe. This detector is planned to be operated at the SNOLAB facility in Sudbury, Ontario, Canada with a projected half-life sensitivity to the ¹³⁶Xe 0νββ decay of T_1/2 > 10²⁸yr at 90% C.L. after 10 years of operation. This lab focuses on development and characterization of the SiPMs as well as a technique to observe the ¹³⁶Ba daugter of the decay for nearly complete background rejection.

Nuclear physics measurements

At TRIUMF's ion trap setup TITAN I am involved in nuclear physics measurements that will provide vital input to theoretical models describing 0νββ decays. We store the intermediate nucleus in a ββ decays in a Penning trap and measure the electron-capture branching ratio to the ground state of the ββ decay mother isotope. This measurement provides part of the 2νββ nuclear matrix element and helps benchmark theoretical frameworks describing 0νββ.

Research at BνL

Barium-tagging	Light Detection
A unique feature in the search for 0νββ in xenon is the possibility to extract and identify the ¹³⁶Xe decay daughter, ¹³⁶Ba. If a 0νββ-like event occurs inside the detector, the decay volume is extracted and probed for Ba. If Ba is found, the event can be identified as a ββ-decay of ¹³⁶Xe, while backgrounds that do not produce Ba can be rejected. This "Ba-tagging" technique will greatly enhance the sensitivity of the detector compared to a conventional, background limited detector. Ba-tagging is possible in liquid and gaseous TPC detectors and both possibilities are being pursued by the nEXO collaboration using several techniques. The lab focuses on the development of a proof-of-principle experiment to demonstrate the feasibility of Ba-ion extraction from high-pressure xenon gas of up to 10 bar.	Liquid-noble experiments, such as nEXO, measure the scintillation light resulting from particle interactions in the detector. This scintillation light is emitted at a characteristic wavelengths depending on the detector medium. For xenon, the scintillation emission peaks in the vacuum ultraviolet (VUV), around 175 nm; the relevant wavelength for nEXO. At BνL we have developed various cryogenic-apparatuses for studying SiPMs as a part of an nEXO R&D program: an environmental test stand for studying large-area SiPM tiles and developing protocols for SiPM mass-testing, an environmental test chamber for mass-testing SiPM tiles and the copper staves to which they will be mounted, and a LXe test chamber that uses SiPMs to precisely study the light emission from LXe. In addition, we have developed two gaseous VUV light sources where radioactive decay energy and accelerated electrons excite the xenon, creating a flash of 172 nm light.

Barium-tagging

Light Detection

A unique feature in the search for 0νββ in xenon is the possibility to extract and identify the ¹³⁶Xe decay daughter, ¹³⁶Ba. If a 0νββ-like event occurs inside the detector, the decay volume is extracted and probed for Ba. If Ba is found, the event can be identified as a ββ-decay of ¹³⁶Xe, while backgrounds that do not produce Ba can be rejected. This "Ba-tagging" technique will greatly enhance the sensitivity of the detector compared to a conventional, background limited detector. Ba-tagging is possible in liquid and gaseous TPC detectors and both possibilities are being pursued by the nEXO collaboration using several techniques. The lab focuses on the development of a proof-of-principle experiment to demonstrate the feasibility of Ba-ion extraction from high-pressure xenon gas of up to 10 bar.

Liquid-noble experiments, such as nEXO, measure the scintillation light resulting from particle interactions in the detector. This scintillation light is emitted at a characteristic wavelengths depending on the detector medium. For xenon, the scintillation emission peaks in the vacuum ultraviolet (VUV), around 175 nm; the relevant wavelength for nEXO. At BνL we have developed various cryogenic-apparatuses for studying SiPMs as a part of an nEXO R&D program: an environmental test stand for studying large-area SiPM tiles and developing protocols for SiPM mass-testing, an environmental test chamber for mass-testing SiPM tiles and the copper staves to which they will be mounted, and a LXe test chamber that uses SiPMs to precisely study the light emission from LXe. In addition, we have developed two gaseous VUV light sources where radioactive decay energy and accelerated electrons excite the xenon, creating a flash of 172 nm light.