---
title: 'BEACON: Neutrino Observatory Design'
url: https://www.emergentmind.com/topics/beacon
type: topic
---

# BEACON: Neutrino Observatory Design

BEACON is the **Beamforming Elevated Array for COsmic Neutrinos**, a planned ultra-high-energy neutrino observatory that uses phased radio arrays on mountains to detect radio emission from upgoing air showers produced by Earth-skimming tau neutrinos. In its instrument concept, small elevated stations view the horizon, where long radio-propagation distances and large target mass in the Earth make the \(\nu_\tau\) channel attractive. A prototype program established the basic instrument architecture and radio-frequency operating environment, while later sensitivity studies recast BEACON as a scalable mountaintop array for diffuse and transient neutrino searches [2206.09660] [2504.13271].

## 1. Scientific scope and observational objective

BEACON is designed for neutrinos with energies \(E_\nu \gtrsim 10^{17}\,\mathrm{eV}\), where fluxes are expected to be extremely low but the science case is unusually strong. The relevant signals include neutrinos produced inside extreme accelerators, cosmogenic neutrinos generated during ultra-high-energy cosmic-ray propagation, and transient emissions from source classes such as short gamma-ray bursts, newly born magnetars, and flat-spectrum radio quasars [2504.13271].

The instrument’s stated objective is to detect radio emission from **upgoing extensive air showers** initiated by **Earth-skimming \(\nu_\tau\)**. In this channel, a tau neutrino enters the Earth at a shallow angle, undergoes a charged-current interaction in rock, produces a tau lepton, and the tau exits the Earth before decaying in the atmosphere. The decay products then generate an upward-going air shower whose radio pulse can be observed from high-elevation stations. The radio emission is modeled as being dominated by the geomagnetic mechanism [2504.13271].

A central consequence of this design is that BEACON is optimized simultaneously for diffuse searches and for time-dependent point-source searches. The transient case is especially prominent in the sensitivity study, which treats both short events and long-duration episodes and derives point-source effective area as the basic observable for source-dependent sensitivity [2504.13271].

## 2. Detection principle and measurement formalism

The sensitivity study defines the point-source effective area in the ANITA-style form
\[
A(t,E_\nu,\hat{r}) = \int_{A_g} dA_g \:(\hat{r} \cdot \hat{u}) \:\Theta(\hat{r} \cdot \hat{u}) \: P_\text{obs}(t, E_\nu,\hat{r}, \hat{u}),
\]
where \(A_g\) is the geometric area on the Earth surface containing potentially detectable tau exits, \(\hat r\) is the source direction, \(\hat u\) is the local surface normal, and \(P_{\rm obs}\) is the detection probability [2504.13271].

Its Monte Carlo estimator is written as
\[
A(E_\nu, \alpha, \delta)
 = \frac{A_g}{N} \sum_{i=1}^{N} (\hat{r}_i \cdot \hat{u}_{i})\:\Theta(\hat{r}_i \cdot \hat{u}_i) \: P_{i,\text{exit}} \: P_{i,\text{detect}},
\]
with \(P_{i,\text{exit}}\) the tau emergence probability and \(P_{i,\text{detect}}\) the probability that the resulting shower radio signal triggers at least one station [2504.13271].

The radio-electric-field scaling used in the simulation is
\[
E_\text{peak}(f_c)=E_\text{interp}(f_c)\cdot \frac{\mathcal{E}}{\mathcal{E}_\text{LUT}} \cdot \frac{d_\text{LUT}}{d} \cdot \frac{|\hat{v}\times\vec{B}|}{|\hat{v}_\text{LUT}\times\vec{B}_\text{LUT}|},
\]
and the phased-array trigger metric is
\[
\text{SNR} = \sqrt{N_A}\left(\frac{V_\text{peak}}{V_\text{RMS}}\right),
\]
so the signal-to-noise ratio grows as \(\sqrt{N_A}\) for \(N_A\) phased antennas [2504.13271].

For diffuse sensitivity, the acceptance is
\[
\left<A\Omega\right>(E_\nu)
 = \int_0^{2\pi} \int_{-\pi/2}^{\pi/2} A(E_\nu, \alpha, \delta) \cos\delta \; d\delta \, d\alpha,
\]
and the corresponding all-flavor \(E^{-2}\) diffuse-flux sensitivity is
\[
E_\nu^2 \Phi_\nu
 = \frac{2.44}{\ln(10)} \frac{3 \: E_\nu}{\left<A\Omega\right>T}.
\]
For transient fluence, the analogous all-flavor \(E^{-2}\) point-source sensitivity is
\[
E_\nu^2 \phi_\nu
 = \frac{2.44}{\ln(10)} \frac{3 \: E_\nu}{A(E_\nu)}.
\]
These expressions make explicit that BEACON’s source reach is controlled by effective area for transients and by sky-integrated acceptance for diffuse searches [2504.13271].

## 3. Instrument architecture and prototype implementation

The prototype paper describes an **8-channel prototype instrument** installed at high elevation at **Barcroft Field Station**, operating since 2018. It consists of **4 dual-polarized antennas** sensitive between **30–80 MHz**; the signals are **filtered, amplified, digitized, and saved to disk using a custom data acquisition system**. The prototype site is high elevation “to maximize effective volume” and uses a **directional beamforming trigger** to improve rejection of anthropogenic background noise at the trigger level. The prototype report discusses design, construction, calibration, the radio-frequency environment, event categories, and includes a likely cosmic ray candidate event [2206.09660].

The later sensitivity study elevates this prototype concept into a phased-station architecture. Its benchmark station contains **10 phased antennas per station**, implemented as low-cost short dipoles whose signals are digitally delayed and summed into multiple trigger beams. The benchmark detector assumes **30–80 MHz** operation, a **trigger threshold of SNR = 5**, and a **one-station trigger** as sufficient for event detection [2504.13271].

The benchmark 100-station array is placed at **3 km altitude**, arranged along a **single line of longitude**, centered near the prototype region at **latitude \(37.589^\circ\) N** and **longitude \(118.238^\circ\) W**, with **3 km spacing** between stations. The stations face **East** and each has a **\(120^\circ\)** azimuthal field of view. A 1000-station array is estimated by scaling the 100-station result by a factor of 10; the study argues that, for the assumed 3 km spacing, overlap effects are sufficiently linear between 100 and 1000 stations to justify that extrapolation [2504.13271].

The modeled antenna/noise chain gives a single-antenna RMS noise voltage
\[
V_\text{RMS}=3.8\,\mu\mathrm{V},
\]
for the prototype antenna model with \(s=0.5\), where \(s\) is the sky fraction in the field of view [2504.13271].

## 4. Simulation framework, environment modeling, and array scaling

The sensitivity calculations are performed with **MARMOTS** (“Multiple Antenna Arrays on Mountains Tau Simulation”), a point-source effective-area framework based conceptually on the ANITA code **Tapioca** but adapted to many stationary arrays. The simulation chain includes **NuTauSim** for neutrino propagation and tau exit, **PYTHIA** for tau decay and shower-energy fraction, **ZHAireS-RASPASS** lookup tables for radio emission from up-going showers on a spherical Earth, **IGRF** for the geomagnetic field, **XFdtd** for the prototype antenna model, **Dulk** parameterization for galactic and extragalactic radio noise, and **PREM** plus **SRTM** for grammage and topographic modeling [2504.13271].

The geometric setup is horizon-centered. At any instant, BEACON’s effective area is concentrated in a **narrow band just below the horizon**, where shallow Earth-skimming trajectories combine large visible area with favorable tau-exit probability. Above the horizon the effective area is zero, and farther below the horizon both geometric area and exit probability decrease rapidly. As the Earth rotates, this band sweeps in right ascension, yielding day-averaged sky coverage of about **\(\sim 70\%\)** [2504.13271].

Station overlap is energy dependent. At \(10^{17}\,\mathrm{eV}\), stations are nearly independent for spacing of about **2 km**, whereas at higher energies spacing greater than **5 km** would be needed for true independence. Even so, for the benchmark geometry, the study concludes that **1000 stations have \(\sim 10\times\) the effective area of 100 stations** despite overlap [2504.13271].

Topography is treated separately from the smooth-Earth baseline. For a single station at the prototype site, local topography increases the **maximum instantaneous effective area at 1 EeV by about 15%**, but narrows sky coverage because nearby terrain blocks steeper lines of sight. Below \(10^{17}\) eV, the same topographic treatment can improve maximum instantaneous effective area by **more than a factor of 3**. This indicates that site selection and local terrain are not perturbative details but part of the array design space [2504.13271].

## 5. Transient and diffuse sensitivity

For short transients, the study uses the **maximum instantaneous effective area**. Its headline claim is that **with just 100 stations, BEACON achieves sensitivity to short-duration transients such as nearby short gamma-ray bursts**, while **with 1000 stations** it reaches much deeper transient sensitivity [2504.13271].

A representative short-burst benchmark is an on-axis **short gamma-ray burst at 40 Mpc**. For the **1000-station** array, the maximum expected detections for the **extended emission** model are **35 neutrinos**, and the maximum distance for at least one detected neutrino is **237 Mpc**. The same study emphasizes that such favorable geometry is rare instantaneously: only about **\(\sim 2.4\%\)** of the sky contains a detectable 40 Mpc short gamma-ray burst at a given instant for the benchmark setup [2504.13271].

The source-time dependence is also explicit. For a source at \((\alpha,\delta)=(0^\circ,0^\circ)\), sensitivity peaks when it is viewed at roughly **\(\sim -3^\circ\)** elevation. Under that geometry, a short gamma-ray burst is detectable for about **30 minutes** with **BEACON-100** and roughly **1 hour** with **BEACON-1000**. Lowering the trigger threshold materially improves performance: for the 40 Mpc short-burst benchmark, the maximum expected neutrino counts change from **3, 35** at SNR 5 to **5, 56** at SNR 4 and **9, 98** at SNR 3 for the 100-station and 1000-station arrays, respectively [2504.13271].

For long-duration transients, the study uses the **day-averaged effective area**, which is lower than the instantaneous peak by about **two orders of magnitude** but applies over large sky fractions. A **2-day** newly born magnetar at **1 Mpc** yields a maximum of **11 neutrinos** in **BEACON-1000**, and the maximum distance for at least one detected neutrino is **3.4 Mpc**. The declination band used to summarize the most favorable day-averaged sensitivity is
\[
-45^\circ < \delta < 35^\circ.
\]
The same long-duration comparison includes stacked all-flavor fluence from **10 FSRQs over 10 years** [2504.13271].

For diffuse sensitivity, the study adopts a **5-year exposure**. Its principal conclusion is that **BEACON-100** would roughly reach existing **IceCube** and **Auger** limits near **1 EeV**, whereas **BEACON-1000** would improve on existing experiments by about **a factor of 10 at 1 EeV** and begin probing a broad class of cosmogenic models. This factor-of-ten statement is the central diffuse-performance claim of the paper [2504.13271].

## 6. Development trajectory, design tradeoffs, and limitations

The two BEACON papers together outline a staged program. The prototype establishes that a mountaintop, low-frequency, beamforming radio instrument can be built, calibrated, and operated in a real radio environment, and that its event stream can be classified at least to the point of identifying a likely cosmic-ray candidate [2206.09660]. The sensitivity study then generalizes that prototype into a distributed array concept in which phased stations on mountains become a horizon-viewing neutrino observatory [2504.13271]. This suggests a development path from local trigger and environment characterization to a geographically extended neutrino array.

The concept involves an explicit geometric tradeoff. A single mountain chain yields **deep, narrow** instantaneous sensitivity, especially for short transients, whereas distributing stations across multiple ranges would produce **wider, shallower** coverage. The study also notes that final station locations have not yet been chosen, that the 1000-station performance is largely an extrapolation from the 100-station simulation, and that local topography and geomagnetic conditions can change the sensitivity by factors of a few [2504.13271].

Another limitation is methodological rather than instrumental. The baseline benchmark uses a **smooth Earth** and a highly regular station layout, while the topographic branch shows that real terrain can substantially reshape both instantaneous effective area and sky visibility. The exact balance between optimized source discovery, cosmogenic-flux reach, and practical siting therefore remains open [2504.13271].

Within those constraints, BEACON is best understood as a mountaintop phased-radio implementation of the Earth-skimming \(\nu_\tau\) technique: a system whose defining elements are high-elevation deployment, directional beamforming, low-frequency radio triggering, and a science program that links nearby short transients, longer-duration source classes, and the diffuse cosmogenic frontier in a single architecture [2206.09660] [2504.13271].

Source: https://www.emergentmind.com/topics/beacon