The goal of this project was to develop a new detection technique for reactor antineutrinos that is sensitive to neutrino direction on an event-by-event basis. This was motivated by (1) the interest of the particle physics community in confirming the existence of a new type of non-standard model neutrino, known in the community as a sterile neutrino, and (2) the requirement for detector technologies that would enable deployment of reactor-monitoring devices near a reactor core without the need for large volumes of shielding. If proven to work, the new detector technology would enable the deployment of detectors above ground and as close as physically possible to the reactor core in order to maximize flux and sensitivity. The ability to reconstruct the direction of an incoming antineutrino was identified as a new technique that would enable these new measurements. For oscillation physics, event-by-event directional sensitivity would permit the identification of the source of each antineutrino, particularly in situations where multiple reactors are used. Toward that end, we sought to test for the detection of an ionization signal from an organic liquid scintillator and combine the signal with readily detectable scintillation in a liquid-and-gas time-projection chamber. Such a detector could improve the energy, position, and particle-identification sensitivity of any type of detector based on an organic-liquid scintillator. Unfortunately, we were not able to confirm the detection of ionization drift in any of the three base-liquid scintillators tested. This result contrasts with the results of an earlier report of the successful detection of such ionization signals by another group (McConkey and Ramachers 2014).
The project had two broad goals. The first goal was to prove that we could identify a neutrino signature from the presence of drifting electrons in a liquid scintillator, then (through repeated testing) demonstrate a control of the electron mobility in each liquid by utilizing a set of scintillator-purification procedures that, when applied, influence neutrino mobility. The second goal was to build a liquid-and-gas time-projection chamber (TPC) capable of harnessing both the scintillation and ionization signals.
Reactor antineutrinos are generally detected via the inverse beta decay reaction,
where an electron antineutrino (νe) interacts with a proton (p) to produce a positron (e+) and a neutron (n). A proton-rich detector medium (such as an organic scintillator or water) is required to initiate the interaction. The observables are a correlated pair of particles: the prompt positron and the neutron, which is generally captured on a small amount of an isotope with a large neutron capture cross section (such as 6Li or Gd) to produce a second delayed signal. In the case of a liquid scintillator, both of these signals are detected via the scintillation light that they produce. In addition to the scintillation, both of the charged particles ionize the scintillator. The aim of this project was to detect this ionization signal.
Event-by-event detection of these antineutrinos requires very accurate measurement of the track direction, position, and energy of the positron-neutron pair, using a method first proposed by Dawson and Kryn (2014). The figure below illustrates the detection principle. The positron and the neutron (via a proton recoil) produce two energy deposits in a detector consisting of a liquid scintillator and a scintillating noble gas, such as xenon or argon. The scintillation light produced by the positron in the liquid is detected by photomultiplier tubes (PMTs) placed immediately below (or above) the liquid. The ionization signals from both the positron and the first neutron/proton recoil are induced by a strong vertical electric field to drift upward toward the surface of the liquid and are extracted into the xenon gas, where they are accelerated further, producing an avalanche in the gas detectable by PMTs placed above the gas. The ionization/luminescent signals in the gas reveal the exact position of both interactions in the horizontal plane, since they are directly above the original interaction point. The time difference between the scintillation signal and the subsequent ionization signals reveal the vertical position.
The key factors that impact the viability of this method were identified as the mobility and lifetimes of free electrons in the liquid, and the ability to extract them into the gas. Electron drift has been demonstrated for some organic liquids. Non-polar compact molecules (such as tetramethylpentane and tetramethylsilane) seem to work best (Dawson and Kryn 2014).
Some liquid scintillators (e.g., di-isopropyl naphthalene, mono-isopropyl naphthalene, and mono-isopropyl biphenyl) were previously shown to drift electrons (Dawson and Kryn 2014). However, these results were not confirmed. More common formulations currently being used for antineutrino experiments (such as linear alkyl benzene and phenyl xylyl ethane) were tested with wavelength-shifting elements included, but they have not yet undergone testing in their pure form. Complex chemistries (caused by the presence of wavelength shifters in the liquid) were proposed as the primary reason why they were found not to work. In our project, however, we proposed testing the pure base liquids with no wavelength-shifting molecules added. Our aim was to test three pure base-liquid scintillators with no wavelength shifters, purify them, and ensure that the liquids were free of common contaminants such as oxygen, water, and other electronegative molecules that might capture drifting electrons.
The motivation for this project was to enable a new detection technique capable of producing highly sensitive particle-interaction information from a liquid scintillator that could be used to reconstruct the direction of reactor antineutrinos event-by-event, in support of NNSA and Laboratory missions in nuclear threat reduction. If proven feasible, this kind of sensitivity could provide a safeguard technology for monitoring the production of plutonium in real time without the need for the bulky shielding normally required to protect detectors from cosmogenic neutrons. More generally, the technology could also enable dual scintillation and charge readout in liquid scintillator detectors for any application, including neutron detection and imaging. Unfortunately, we were unable to confirm that charge drift in a liquid scintillator is possible, a result that is inconsistent with the earlier results published by another group (McConkey and Ramachers 2013). One possible explanation is that the charge-buildup and discharge events that we observed might be common and could have been mistaken by past groups for a genuine detection of charge drift in a scintillator.
The way forward for research into ionization detection in a liquid organic scintillator is unclear. Our results show that electron drift is severely constrained in three of the most common types of pure-liquid organic scintillators without additional additives such as wavelength-shifting substances; typically, organic scintillators do contain additives that shift emitted light into a wavelength range aligned with PMT sensitivity. One possible way forward is to use the semiconductor approach: Dope the scintillators with an impurity that enables charge transport. Since organic chemistry is highly specialized, we suggest that consultations with organic chemists might provide other possibilities for future investigation.
Dawson, J. V. and D. Kryn. 2014. "Organic Liquid TPCs for Neutrino Physics." Journal of Instrumentation 9(07). doi: 10.1088/1748-0221/9/07/P07002.
McConkey, N. and Y. A. Ramachers. 2013. "Liquid Scintillator Time Projection Chamber Concept." Nuclear Instruments and Methods in Physics Research, Section A: Accelerators, Spectrometers, Detectors and Associated Equipment 718: 459–461. doi: 10.1016/j.nima.2012.11.047.
Dazeley, S. 2017. "Charge Drift in Organic Liquids at Room Temperature." IEEE Nuclear Science Symposium and Medical Imaging Conference, Atlanta, GA, October 2017. LLNL-PRES-740657.