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Development of detectors sensitive to coherent neutrino-nucleus scattering

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That is some neutron source! Two detector technologies in principle able to evidence the sub-keV recoils expected from this mode of neutrino scattering have been developed and several others are under consideration. One utilizes the novel micropatterned gaseous detectors mass-produced for the first time by the group in collaboration with 3M, see [GEMs Article]. By means of a quadruple Gas Electron Multiplier (GEM) the group has been able to demonstrate sensitivity to single electrons in a gaseous device (Figure 1). A two-phase detector (i.e., a cryogenic device containing both the liquid and gaseous phase of a noble gas) is presently being considered towards the goal of packing enough target mass to make a reactor neutrino experiment feasible. Another new detector technology, cooled Large Area Avalanche Photodiodes (LAAPDs), is being tested for the same purpose. A cryostat has been built to cool 1.3x1.3 cm LAAPDs to liquid nitrogen temperature, where it is possible to reach gains of a few thousand in very low-noise conditions (Figure 1). The group has been able to demonstrate sensitivity to single photons at this temperature with a high quantum efficiency (Figure 1). The cryostat is used to house ~1 c.c. inorganic scintillator samples, monitored by the LAAPD for very faint light emissions accompanying low-energy recoils. Scintillation efficiencies of O (10%) for recoils below 1 keV (relative to electrons of the same energy) are expected from some scintillators. This is based on an analysis of recent measurements by other groups at higher recoil energies, using a modified Birks-Lindhard model of the light yield to fit their data. If this insight is proven correct, cooled LAAPDs should be sufficient to exploit this faint scintillation as a novel and advantageous method of neutrino detection, profiting from the large coherent enhancement to the nuclear scattering cross section that is expected. A second cryostat able to house two LAAPDs operating in coincidence (a measure against phosphorescent backgrounds) and at lower temperatures is being constructed.


Figure 1. Figure 1.
Left: Single electron spectrum (showing a typical Polya distribution) in P-5 gas using a quadruple University of Chicago-3M Gas Electron Multiplier.
Right: Single photon pulses from a 10-12 filtered LED on a cooled Large Area Avalanche Photodiode (1.6 cm diameter, Advanced Photonix). The signals were obtained using a low-noise Ortec 142AH preamp and Ortec 672 spectroscopy amplifier. Cryogenic operation of LAAPDs produces a substantial reduction in their noise (inset) and gains of a few thousand while still in proportional mode, facilitating single-photon detection with a very high quantum efficiency (>70%).


Emphasis during the next year will be in demonstrating that the two mentioned technologies are indeed sensitive to actual sub-keV recoils, identical to those expected from reactor antineutrinos. Towards this end two neutron beam experiments are being built. The first exploits the monochromatic energy of the recoiling daughter that accompanies the (nth,gamma) capture reaction, which for some transitions can be in the few hundreds of eV. Preliminary measurements at the Intense Pulsed Neutron Source (Figure 2) show that the measurement should be possible in that facility, after improvements to data acquisition and coincidence electronics are made. The second experiment is planned towards the end of 2004 at the Triga Mark-II reactor in Kansas State University, using an Al+Fe filter able to produce highly monochromatic 24 keV neutrons (the filter has been constructed at the University of Chicago and will remain onsite at KSU, enabling other future calibrations there). This measurement will be the first ever to quantify the scintillation from <5 keV recoils in a dozen different inorganic scintillators (Figure 2). If sufficient light yield is found in any of the samples, a first measurement of this exciting neutrino cross section in a US reactor would soon follow.


Figure 2. Figure 2.
Left: Data collected at the Intense Pulsed Neutron Source, showing gamma lines in a Ge detector originating from the (nth,gamma) reaction in a ZnSe[Te] scintillator. The peaks are labeled according to gamma energy, branching ratio and energy of the recoil that accompanies gamma emission. A cooled high-quantum efficiency LAAPD will be used to detect any faint light emission accompanying these low-energy recoils in the scintillator.
Right: MCNP-Polimi simulation of recoil energies in a NaI crystal, produced by a monochromatic 24(±)2 keV neutron beam from an Al+Fe filter installed at the Triga Mark-II reactor. A large (2''x3'') enriched 6LiI detector determines the recoil energy by capturing the scattered neutrons. This experiment will measure, for the first time, the light yield from recoils below 5 keV in a dozen inorganic scintillators.


Figure 3. Figure 3.
Left: The "dip" in the Fe neutron cross-sections at the heart of the monochromatic filtered neutron technique.
Right: Simulated beam composition. First measurements at KSU indicate >6,000 24keV neutrons/cm2 s with a very low (0.2 mRem/hr) on-axis gamma contamination. Precise proton-recoil spectrometer measurements of beam composition are planned during fall of 2004.


Figure 4. Figure 4.
Top: Spectrum of recoil energies expected from reactor (anti)neutrinos in different materials
Bottom: A 24 keV highly monochromatic neutron beam is able to closely mimic these, providing an ideal calibration for these detectors.


Figure 5. Figure 5.
Left: Simulated experimental geometry
Right: The real thing, after some modifications (a 2"x3" enriched 6LiI detector is used).


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March 20, 2005