Ultra-high energy neutrinos are detectable through impulsive radio signals generated through interactions in dense media , such as ice .
Subsurface in-ice radio arrays are a promising way to advance the observation and measurement of astrophysical high-energy neutrinos with energies above those discovered by the IceCube detector ( \geq 1 PeV ) as well as cosmogenic neutrinos created in the GZK process ( \geq 100 PeV ) .
Here we describe the NuPhase detector , which is a compact receiving array of low-gain antennas deployed 185 m deep in glacial ice near the South Pole .
Signals from the antennas are digitized and coherently summed into multiple beams to form a low-threshold interferometric phased array trigger for radio impulses .
The NuPhase detector was installed at an Askaryan Radio Array ( ARA ) station during the 2017/18 Austral summer season . In situ measurements with an impulsive , point-source calibration instrument show a 50 % trigger efficiency on impulses with voltage signal-to-noise ratios ( SNR ) of \leq 2.0 , a factor of \sim 1.8 improvement in SNR over the standard ARA combinatoric trigger .
Hardware-level simulations , validated with in situ measurements , predict a trigger threshold of an SNR as low as 1.6 for neutrino interactions that are in the far field of the array .
With the already-achieved NuPhase trigger performance included in ARASim , a detector simulation for the ARA experiment , we find the trigger-level effective detector volume is increased by a factor of 1.8 at neutrino energies between 10 and 100 PeV compared to the currently used ARA combinatoric trigger .
We also discuss an achievable near term path toward lowering the trigger threshold further to an SNR of 1.0 , which would increase the effective single-station volume by more than a factor of 3 in the same range of neutrino energies .