AugerPrime: UHECR Detector Upgrade

• AugerPrime is the Phase II upgrade of the Pierre Auger Observatory designed to separate electromagnetic and muonic signals for detailed UHECR analysis.
• It integrates surface scintillator detectors, enhanced water-Cherenkov detectors, advanced electronics, radio arrays, and underground muon counters to extend dynamic range and improve measurement precision.
• The upgrade enables precise studies of flux suppression, primary mass composition, and the muon discrepancy in air showers, advancing cosmic ray physics.

Searching arXiv for AugerPrime and core upgrade papers to ground the article in current literature. AugerPrime is the Phase II upgrade of the Pierre Auger Observatory, designed to convert the Observatory’s large-area surface array into an event-by-event mass-sensitive instrument for ultra-high-energy cosmic rays (UHECRs). Its central methodological principle is the separation of the electromagnetic and muonic components of extensive air showers at ground level by combining new Surface Scintillator Detectors (SSDs) with the pre-existing water-Cherenkov detectors (WCDs), while also extending dynamic range, timing capability, hybrid exposure, and direct muon measurements. The upgrade was conceived to address three coupled problems in UHECR physics: the origin of the flux suppression above $\sim 4\times 10^{19}$ eV, the possible survival of a light component at the highest energies, and the persistent discrepancy between measured and simulated muon content in air showers (Unger, 2017, Stasielak, 2021).

1. Scientific questions that motivated the upgrade

AugerPrime was formulated after the Pierre Auger Observatory had established three robust but mutually entangled observations. First, the all-sky energy spectrum shows a flux suppression around $40$ EeV, with a break at $E_s \simeq 4\times 10^{19}$ eV. Second, Fluorescence Detector (FD) measurements of $X_{\max}$ indicate a trend toward increasingly heavy composition with rising energy. Third, direct muon measurements at ground exceed predictions of hadronic-interaction models, with quoted discrepancies of $30$–$60\,\%$ and characteristic center-of-mass energies beyond the LHC domain (Stasielak, 2021).

These observations leave open whether the high-energy cutoff is dominated by propagation losses, including the Greisen–Zatsepin–Kuzmin effect, or by the finite maximum rigidity of the sources. The same ambiguity affects expectations for “charged-particle astronomy”: if a sufficient proton or light-nuclei fraction persists above $10^{19.5}$ eV or above $6\times 10^{19}$ eV, magnetic deflections may remain small enough for composition-selected anisotropy studies. AugerPrime was therefore defined around three interlocking goals: determining the origin of the flux suppression, deciding whether enough light nuclei remain for astronomy, and clarifying the muon discrepancy as a probe of hadronic physics at $\sqrt{s}$ well above accelerator energies (Unger, 2017).

A recurring limitation of the pre-upgrade Observatory was not exposure but composition sensitivity at the highest energies. The FD provides the key mass observable $X_{\max}$, yet its duty cycle is only $40$0 in clear, moonless conditions, or $40$1 in later summaries, so statistics above $40$2 EeV are scarce. The surface array, by contrast, operates with nearly $40$3 duty cycle but originally mixed electromagnetic and muonic signals in the WCD response. AugerPrime addresses precisely that bottleneck by moving composition inference from limited-statistics hybrid operation toward full-exposure surface detection (Stasielak, 2021, Schmidt, 11 Aug 2025).

2. Detector architecture and subsystem realization

AugerPrime upgrades each surface station of the $40$4 array by adding a scintillator detector above the WCD, an additional small-area PMT in the WCD, new station electronics, radio instrumentation, and—in the infill region—buried muon counters. It also extends FD operation into higher night-sky background conditions. In combination, these additions turn the Observatory into a multi-hybrid detector in which electromagnetic, muonic, radio, and fluorescence observables are recorded with substantially broader duty cycle and dynamic range (Stasielak, 2021, Schmidt, 11 Aug 2025).

Subsystem	Stated role	Representative specification
SSD	Electromagnetic-sensitive signal complementary to WCD	Active area $40$5 or $40$6; extruded polystyrene scintillator
WCD small PMT	Dynamic-range extension near shower core	Range extended from $40$7 VEM to $40$8 VEM; factor $40$9 extension
UUB electronics	Faster sampling, timing, trigger, calibration	$E_s \simeq 4\times 10^{19}$0 MHz, $E_s \simeq 4\times 10^{19}$1-bit ADC, GPS timing resolution $E_s \simeq 4\times 10^{19}$2 ns
Radio detector	Electromagnetic measurement for inclined showers	SALLA, $E_s \simeq 4\times 10^{19}$3–$E_s \simeq 4\times 10^{19}$4 MHz, dual polarization
UMD	Direct muon measurements and calibration of surface separation	Three buried scintillator modules per station at $E_s \simeq 4\times 10^{19}$5 m depth
Extended FD	Larger hybrid statistics	Duty cycle increased to $E_s \simeq 4\times 10^{19}$6 or by $E_s \simeq 4\times 10^{19}$7

The SSD is the most characteristic AugerPrime addition. In the upgrade literature it is described either as a $E_s \simeq 4\times 10^{19}$8 plastic-scintillator panel or, in finalized implementations, as a detector with active area $E_s \simeq 4\times 10^{19}$9. It is mounted on the roof of each WCD and built from extruded polystyrene scintillator bars with wavelength-shifting fibers and a Hamamatsu R9420 PMT. Because thin scintillator is chiefly sensitive to the electromagnetic component while the WCD has a relatively stronger muon response, the SSD–WCD combination provides the differential response required for component separation (Unger, 2017, Collaboration, 10 Jul 2025).

The WCD upgrade adds a small PMT, described as a $X_{\max}$0, $X_{\max}$1, or $X_{\max}$2-area photomultiplier depending on the source and implementation stage, to prevent saturation near the shower core. This channel extends the measurable signal from the large-PMT saturation regime to $X_{\max}$3 VEM and enables accurate particle-density measurements much closer to the core of high-energy showers. The upgrade literature consistently treats this dynamic-range extension as essential for stable lateral-distribution reconstruction in the highest-energy events (Stasielak, 2021, Anastasi et al., 2022).

The Upgraded Unified Board (UUB) is the electronic backbone of AugerPrime. It digitizes station signals at $X_{\max}$4 MHz with $X_{\max}$5-bit ADCs, provides GPS-based timing at the $X_{\max}$6 ns level, supports new local triggers, and integrates slow control, calibration, and communication. In later hardware summaries the UUB has $X_{\max}$7 ADC input channels, with split high- and low-gain readout for the large WCD PMTs and SSD, a dedicated small-PMT channel, and a spare channel. The same upgrade path also powers and reads radio detectors and monitors environmental and electronics health variables (Collaboration et al., 2023, Boháčová, 18 Sep 2025).

Beyond the WCD–SSD pair, AugerPrime adds radio antennas and buried muon counters. Each station receives a dual-polarized Short Aperiodic Loaded Loop Antenna (SALLA) operating in the $X_{\max}$8–$X_{\max}$9 MHz band, particularly valuable for very inclined showers whose radio footprint can span $30$0. In the infill region, the Underground Muon Detector (UMD) places segmented scintillator modules at $30$1 m depth, corresponding to an effective vertical-muon threshold of about $30$2 GeV, to provide direct muon measurements and a calibration reference for surface-based muon extraction (Stasielak, 2021, Jesus, 10 Sep 2025).

3. Measurement principle: electromagnetic–muonic disentanglement

The defining AugerPrime observable is the joint SSD–WCD response. In one formulation, the local signals are written as

$30$3

$30$4

where $30$5 is the local electromagnetic energy deposit and $30$6 the muon number. Because the response coefficients are known from calibration, the two linear equations can be inverted on an event-by-event basis to extract both $30$7 and $30$8 at ground level. The same principle is presented elsewhere in terms of $30$9 and detector response coefficients $60\,\%$0, with an explicit inversion formula for $60\,\%$1 (Unger, 2017, Stasielak, 2021).

In the 2021 overview, the response is written as

$60\,\%$2

leading to

$60\,\%$3

This algebraic inversion is the core of AugerPrime’s event-by-event mass sensitivity. The underlying rationale is explicit in the upgrade papers: $60\,\%$4 correlates strongly with primary mass, whereas the electromagnetic component and total shower size constrain the energy scale (Stasielak, 2021).

AugerPrime does not rely exclusively on amplitude ratios. The higher sampling rate of the UUB enables time-profile decomposition, exploiting the fact that the muonic component tends to arrive earlier and in narrower pulses than the broader delayed electromagnetic contribution. In the upgrade description this is written as

$60\,\%$5

with empirical templates

$60\,\%$6

and amplitudes extracted by a maximum-likelihood fit. For inclined showers, the radio detector adds an independent measurement of electromagnetic energy, which can be combined with the surface-derived muon number through

$60\,\%$7

with $60\,\%$8 (Stasielak, 2021).

The UMD supplies a direct muon reference rather than an indirect proxy. In binary mode it identifies a muon hit as at least four consecutive ones in a strip’s bit stream and reconstructs a shower-level muon-density parameter through a likelihood fit. In calorimetric mode, suitable for higher densities, the total charge is converted to muon number via

$60\,\%$9

A later recalibration replaced the constant single-vertical-muon denominator by a density-dependent quantity $10^{19.5}$0, yielding

$10^{19.5}$1

with the stated goal of removing density-dependent biases below the $10^{19.5}$2 level (Jesus, 10 Sep 2025, Scornavacche, 11 Jul 2025).

4. Electronics, calibration, and operational stability

AugerPrime’s detector concept depends on continuous cross-calibration. In the WCDs, the standard unit remains the Vertical Equivalent Muon (VEM), while the SSDs are calibrated in Minimum Ionizing Particle (MIP) units using atmospheric muons. In the later electronics and SSD calibration papers, the WCD VEM hump is quoted at $10^{19.5}$3 ADC counts on the high-gain channel, while the SSD MIP peak is $10^{19.5}$4 ADC counts. Charge histograms are produced every minute in the WCD system, and SSD muon histograms are built every $10^{19.5}$5 s using a WCD muon tag to suppress electromagnetic background (Boháčová, 18 Sep 2025, Conte, 12 Jul 2025).

For the SSDs, the calibration chain is more detailed than a simple peak fit. The single-muon charge histogram is modeled as

$10^{19.5}$6

and the raw omnidirectional muon peak is corrected to a vertical-equivalent MIP through

$10^{19.5}$7

An online rate-based algorithm is also under development, with threshold update

$10^{19.5}$8

and the relation

$10^{19.5}$9

once the loop has converged near $6\times 10^{19}$0 Hz (Conte, 12 Jul 2025).

The small-PMT chain required dedicated infrastructure because it had to remain stable in the environmental conditions of the Argentinian pampa. Its high-voltage supply is an external single-channel module based on CAEN’s A7501 $6\times 10^{19}$1–$6\times 10^{19}$2 V/$6\times 10^{19}$3A DC-DC converter, with $6\times 10^{19}$4–$6\times 10^{19}$5 V corresponding to $6\times 10^{19}$6–$6\times 10^{19}$7 V and a linear slope of $6\times 10^{19}$8 V/V. Validation campaigns tested $6\times 10^{19}$9 modules between $\sqrt{s}$0 and $\sqrt{s}$1; $\sqrt{s}$2 passed all tests on first pass, with $\sqrt{s}$3 requiring vendor replacement after final thermal cycling. The reported performance metrics include ripple at the $\sqrt{s}$4 level, power absorption $\sqrt{s}$5 mW at $\sqrt{s}$6, and thermal stability distributions peaked at $\sqrt{s}$7C (Anastasi et al., 2022).

The UUB itself underwent laboratory verification and environmental stress screening. Design targets included $\sqrt{s}$8 MHz sampling, linearly matched gain chains, field-verified inter-station timing better than $\sqrt{s}$9 ns, and power consumption of about $X_{\max}$0 W. In field and laboratory reports, high-gain noise remained below $X_{\max}$1 LSB, low-gain noise below $X_{\max}$2 LSB, and dynamic-range verification showed the large-PMT high/low ratio at $X_{\max}$3 in the laboratory and $X_{\max}$4 in the field, with the SSD high/low ratio near its nominal $X_{\max}$5 value. These hardware metrics are not peripheral: they define the precision with which waveform shape, risetime, curvature, and station-level electromagnetic–muonic decomposition can be reconstructed (Collaboration et al., 2023, Boháčová, 18 Sep 2025).

Long-term monitoring is treated as part of the AugerPrime measurement system rather than a maintenance afterthought. The monitoring framework tracks station-level health metrics every few seconds and physics summaries from every minute to daily cadence. Over the first four years of Phase II, the WCD event rate per active hexagon remained constant within $X_{\max}$6 over five years, SSD MIP charge showed a $X_{\max}$7–$X_{\max}$8 seasonal modulation, radio RMS noise was stable to better than $X_{\max}$9 outside thunderstorm periods, and UMD muon-pattern rates remained flat to within $40$00 after thermal compensation (Andrada, 11 Jul 2025).

5. Performance targets, commissioning, and first physics results

Before full deployment, an Engineering Array of twelve upgraded stations was used to validate the design. In that prototype, the reconstructed muon fraction

$40$01

had a statistical resolution of $40$02 per $40$03 eV shower in single stations and systematic stability below $40$04 over seasonal atmospheric variations. Combining eight to ten stations in a shower footprint reduced the event-by-event muon uncertainty to $40$05, corresponding to a quoted mass resolution $40$06 above $40$07 eV. The same engineering studies reported hybrid energy resolution maintained at $40$08–$40$09 and angular resolution for SD-only events improved to $40$10 at the highest energies (Unger, 2017).

The more mature performance projections in later overview papers are broadly consistent with those prototype results. Mock-data studies distinguished benchmark composition scenarios above $40$11 eV and found that with five years of AugerPrime WCD+SSD data the significance for distinguishing a $40$12 difference in proton content exceeds $40$13. Hybrid reconstruction was projected to reach $40$14 at $40$15 EeV and $40$16 at $40$17 EeV, while the arrival-direction resolution was projected to improve from $40$18 to $40$19 at the highest energies. The same upgrade summary states that doubling the FD duty cycle to $40$20 should yield $40$21 high-quality $40$22 measurements above $40$23 EeV per year (Stasielak, 2021).

By 2025, AugerPrime reports had shifted from projected performance to initial Phase II measurements. The full $40$24 array was stated to be instrumented and UUB triggers fully commissioned by mid-2025, with timing resolution $40$25 ns, SSD linearity from $40$26 MIP to $40$27 MIPs with $40$28 non-linearity, and WCD dynamic range extended to $40$29 VEM. Preliminary systematic uncertainties were quoted as $40$30 on $40$31, $40$32 on $40$33, and $40$34 on the radio-plus-WCD energy scale (Schmidt, 11 Aug 2025).

The first physics results also address the longstanding muon problem directly. In a sample of $40$35 showers above $40$36 eV, reconstructed muon fractions were reported as consistent with intermediate-mass expectations, $40$37–$40$38. Direct comparisons between UMD measurements and surface-derived muon estimates confirmed that real showers contain $40$39 more muons than QGSJet II-04 predictions at $40$40 eV. Preliminary $40$41 versus energy trends lie above EPOS-LHC and QGSJet II-04 by $40$42–$40$43 in $40$44, extending the earlier Phase I muon-deficit tension into the AugerPrime era (Schmidt, 11 Aug 2025).

6. Physics reach, controversies, and outlook

The scientific significance of AugerPrime lies in the combination of full-exposure composition sensitivity and direct muon benchmarking. The upgrade was explicitly designed so that, once fully commissioned, it would measure the evolution of the mean logarithmic mass across the cutoff with $40$45, determine the fraction of protons and light nuclei above $40$46 eV at the $40$47 level, and compare absolute muon yields with hadronic-model predictions at the $40$48 level for $40$49 TeV (Unger, 2017).

One common misconception is to treat AugerPrime as merely a dynamic-range or electronics refresh. The upgrade literature instead presents it as a systematic reconfiguration of the Observatory into a precision multi-hybrid detector: WCD+SSD for electromagnetic–muonic separation, radio detection for inclined showers, UMD for ground-truth muons, and extended-uptime FD measurements for $40$50. Its purpose is not only better reconstruction of known observables but the promotion of primary mass to a shower-by-shower observable over the full surface-array exposure (Stasielak, 2021, Schmidt, 11 Aug 2025).

Another central controversy concerns the interpretation of the flux suppression. The existence of the suppression itself is not disputed, but whether it is dominated by GZK-like propagation losses or by the exhaustion of source acceleration power remains unresolved. AugerPrime was designed precisely because the existing FD-based composition trend toward heavier primaries and the surface-based muon anomaly complicate any single-cause interpretation. This suggests that the key discriminant is not a more precise spectrum alone, but composition-resolved and muon-resolved measurements across the suppression region (Unger, 2017, Castellina, 2019).

The longer-term outlook described in the recent status papers is correspondingly broad. Phase II exposure had already reached $40$51 of Phase I and is projected to accumulate $40$52 by 2035. Expected mass resolution is stated as $40$53 per event at $40$54 eV, compared with $40$55 previously, enabling composition-tagged anisotropy studies with $40$56 events above $40$57 eV. New full-bandwidth station triggers are also being developed for searches for neutral primaries $40$58 and for signatures beyond the Standard Model (Schmidt, 11 Aug 2025).

In that sense, AugerPrime is best understood as the stage of the Pierre Auger Observatory in which the mass of individual UHECR primaries becomes an operational observable at the scale of a $40$59 array. Its importance follows from that transition: once electromagnetic, muonic, radio, and fluorescence measurements are integrated at full exposure, source interpretation, anisotropy analysis, and hadronic-model tests are no longer separable programs but different projections of the same detector system (Stasielak, 2021).