---
title: CCM200 Liquid Argon Detector
url: https://www.emergentmind.com/topics/ccm200-detector
type: topic
---

# CCM200 Liquid Argon Detector

Searching arXiv for the cited CCM papers to ground the article in the referenced literature.
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CCM200 is a liquid argon detector in the Coherent CAPTAIN-Mills program at Los Alamos National Laboratory, configured for MeV-scale neutrinos and beyond-Standard-Model searches and distinguished by selective wavelength-shifting instrumentation that enables event-by-event identification of Cherenkov radiation from sub-MeV electrons in a high scintillation light-yield medium [2507.08886]. The detector is located at the Lujan Center on the Los Alamos Neutron Science Center beamline, follows the CCM120 prototype, and improves physics reach with higher photocoverage, refined timing, and selective wavelength-shifting [2507.08886, 2105.14020]. In the reported 2025 result, gamma rays from a $^{22}$Na source were used to isolate prompt Cherenkov light with $>5\sigma$ confidence, establishing the first event-by-event observation of Cherenkov photons from sub-MeV electrons in a high-yield scintillator detector [2507.08886].

## 1. Detector configuration and optical architecture

CCM200 employs an upright cylindrical cryostat, $2.58 \ \mathrm{m}$ in diameter and $2.25 \ \mathrm{m}$ in height, containing $10$ tons of liquid argon. The inner detector fiducial region is $7$ tons, and a $3$-ton optically isolated veto region surrounds it [2507.08886]. The inner walls are lined with Mylar reflective foils evaporatively coated with tetraphenyl butadiene, which wavelength-shifts liquid argon ultraviolet scintillation into the visible [2507.08886].

The fiducial volume is instrumented with $200$ 8-inch Hamamatsu R5912-Y002 10-stage cryogenic photomultiplier tubes, providing approximately $50\%$ optical photocoverage. Of these, $160$ ($80\%$) are TPB-coated and $40$ ($20\%$) are uncoated [2507.08886]. PMT faces on the coated channels are directly TPB-coated, whereas uncoated channels lack a face coating and thus rely on visible photons, such as Cherenkov light, or ultraviolet photons that first wavelength-shift at other surfaces before being detected [2507.08886].

This selective coating pattern is central to the CCM200 concept. The coated channels are optimized for high scintillation-light collection, while the uncoated channels provide enhanced sensitivity to prompt visible Cherenkov photons [2507.08886]. A plausible implication is that the detector is not simply a high-yield scintillation instrument, but a deliberately hybrid optical system in which photocathode segmentation by wavelength-shifting treatment functions as a particle-identification handle.

CCM200 also inherits a broader programmatic role from CCM120. The earlier detector was a 10-ton liquid argon scintillation detector at the LANSCE Lujan Center designed to search simultaneously for sterile neutrinos via neutral-current disappearance and for light dark matter produced in the pulsed proton beam dump [2105.14020]. Lessons from CCM120 directly informed the CCM200 upgrade, which was engineered to improve sensitivity through increased optical coverage, cleaner liquid argon, enhanced vetoing, and new shielding [2105.14020].

## 2. Timing structure and the prompt-window method

The event start time is defined by a trigger that requires total charge exceeding $3$ photoelectrons in a sliding $2 \ \mathrm{ns}$ window across PMT digitized waveforms. Digitization uses CAEN V1730/V1730S boards at $500 \ \mathrm{MHz}$ with $14$-bit precision, and the detector’s effective timing resolution for the Cherenkov analysis is $2 \ \mathrm{ns}$ [2507.08886].

The analysis defines a prompt Cherenkov-enhanced window, $-6 \leq t \leq 0 \ \mathrm{ns}$, relative to the event start time $t=0$, and a subsequent scintillation-enhanced region for $t>0$ [2507.08886]. A characteristic small peak appears at $t \approx -3 \ \mathrm{ns}$ on uncoated PMTs for electromagnetic events from the $^{22}$Na source, before the reconstructed event start [2507.08886].

The physical basis of the method is the different transport and emission timescales of Cherenkov and scintillation light in liquid argon. Ultraviolet scintillation in liquid argon is produced near $128 \ \mathrm{nm}$ and must be wavelength-shifted to be detected efficiently by visible-sensitive PMTs. On uncoated PMTs, such ultraviolet scintillation photons generally cannot be detected without first traveling to a TPB-coated surface, being re-emitted in the visible, and then propagating back, introducing path-length and re-emission delays [2507.08886]. By contrast, visible Cherenkov photons produced at the interaction site propagate directly to uncoated PMTs and arrive in the prompt window [2507.08886].

Liquid argon scintillation itself arises from excited $\mathrm{Ar}_2^\ast$ dimers decaying via singlet and triplet channels. For lightly ionizing electrons, about $20$--$30\%$ of the scintillation yield is fast singlet and $70$--$80\%$ is triplet. The singlet decay constant is $O(5 \ \mathrm{ns})$, and the triplet is $O(1000 \ \mathrm{ns})$ [2507.08886]. These timescales, together with wavelength-shifting-induced propagation delays, permit temporal separation in which visible Cherenkov dominates the prompt window on uncoated PMTs, while scintillation dominates for $t>0$ [2507.08886].

This timing structure differs from the beam timing logic emphasized in CCM120, where time-of-flight separation was used to define a beam region of interest and to isolate prompt neutrino or light-dark-matter signals from later neutron backgrounds [2105.14020]. In CCM200, the same general emphasis on nanosecond-scale timing is applied inside individual events to separate optical channels rather than beam species.

## 3. Cherenkov production in liquid argon

The Cherenkov condition used in the CCM200 analysis is
$$
\cos\theta_C = \frac{1}{\beta n},
$$
where $\beta = v/c$ and $n$ is the refractive index at the photon’s wavelength [2507.08886]. The spectral yield per unit path length is
$$
\frac{\mathrm{d}^2 N}{\mathrm{d}x\,\mathrm{d}\lambda} = \frac{2\pi \alpha}{\lambda^2}\left(1 - \frac{1}{\beta^2 n^2(\lambda)}\right),
$$
which emphasizes the $1/\lambda^2$ preference for shorter wavelengths [2507.08886].

The threshold occurs at $\beta_{\min} = 1/n$, equivalently
$$
\gamma_\text{min} = \frac{1}{\sqrt{1 - 1/n^2}},
$$
with total-energy threshold
$$
E_\text{th} = \frac{m_e c^2}{\sqrt{1 - 1/n^2}},
$$
and corresponding kinetic threshold
$$
K_\text{th} = m_e c^2\left(\gamma_\text{min} - 1\right).
$$
The paper cites $n \approx 1.22$ for visible light in liquid argon [2507.08886]. For $\beta$ near $1$ and $n \approx 1.22$, $\cos\theta_C \approx 1/n \approx 0.82$, so $\theta_C \approx \arccos(0.82) \approx 35^\circ$, consistent with the observed preference for $0.8 < \cos\theta < 0.9$ in the prompt hit-pair angles [2507.08886].

The paper notes that sub-MeV electrons in liquid argon emit Cherenkov light only above a kinetic threshold of roughly $0.2 \ \mathrm{MeV}$, and that the detected sample is dominated by electrons with $0.5 \lesssim \mathrm{KE} \lesssim 1.0 \ \mathrm{MeV}$, produced by Compton scattering of the $^{22}$Na gamma rays [2507.08886]. This places the CCM200 result in a parameter regime where Cherenkov emission is kinematically allowed but sparse, making timing-based isolation essential.

A common misconception is that high scintillation yield necessarily obscures Cherenkov emission on an event-by-event basis. The CCM200 result shows that this is not generally true in liquid argon when selective wavelength-shifting and nanosecond timing are combined [2507.08886]. This suggests that the limiting factor is not only total scintillation yield, but the joint structure of emission times, propagation delays, and channel-specific optical sensitivity.

## 4. Source deployment, data selection, and prompt-hit tagging

The demonstration of sub-MeV Cherenkov identification used a $3 \ \mu\mathrm{Ci}$ $^{22}$Na source encapsulated in approximately $1 \ \mathrm{mm}$ of stainless steel, deployed on a stainless steel bayonet through a central flange and positioned at the detector midline [2507.08886]. The dominant $\beta^+$ branch, approximately $90\%$, emits a $0.546 \ \mathrm{MeV}$ positron and a $1.275 \ \mathrm{MeV}$ gamma from $^{22}\mathrm{Ne}^\ast$ de-excitation; the positron annihilation yields two $0.511 \ \mathrm{MeV}$ gammas. The electron-capture branch, approximately $10\%$, emits only the $1.275 \ \mathrm{MeV}$ gamma [2507.08886].

Event reconstruction again uses the sliding $2 \ \mathrm{ns}$ window with a $3$ PE threshold to set $t=0$ [2507.08886]. Data-quality cuts remove high-charge cosmic muons observed within the $16 \ \mu\mathrm{s}$ DAQ window, events with less than $2.2 \ \mu\mathrm{s}$ separation from neighboring triggers, and events with reconstructed radial positions more than $25 \ \mathrm{cm}$ from the detector center, where position is estimated by PMT-weighted charge in the first $20 \ \mathrm{ns}$ [2507.08886].

After these cuts, the total charge in the first $90 \ \mathrm{ns}$ shows a lower peak, approximately $50$ PE, associated with electron capture, and a higher peak, approximately $100$ PE, associated with the $\beta^+$ branch [2507.08886]. Events within $\pm 8$ PE around the $\beta^+$ peak center are selected to increase statistics and reduce contamination, with expected random background contamination after cuts of $\leq 0.57\%$ [2507.08886]. The dataset used for the early-time study consists of $43{,}522$ selected $^{22}$Na events [2507.08886].

A simple per-event selection requires at least one hit on uncoated PMTs in $-6 \leq t \leq 0 \ \mathrm{ns}$ [2507.08886]. In this Cherenkov-enhanced window, the Cherenkov purity reaches $94.8\%$ for events with at least one prompt hit. The selection efficiency with all cuts plus at least one prompt hit is $9.78\%$ in data and $9.10\%$ in simulation [2507.08886].

| Quantity | Value | Context |
|---|---:|---|
| Selected $^{22}$Na events | $43{,}522$ | Early-time light study |
| Prompt window | $-6 \leq t \leq 0 \ \mathrm{ns}$ | Cherenkov-enhanced region |
| Cherenkov purity | $94.8\%$ | Events with $\geq 1$ prompt hit |
| Selection efficiency in data | $9.78\%$ | All cuts plus $\geq 1$ prompt hit |
| Selection efficiency in simulation | $9.10\%$ | All cuts plus $\geq 1$ prompt hit |
| Random background contamination | $\leq 0.57\%$ | After charge and quality cuts |

Since uncoated PMTs represent only $10\%$ of the total photocathode coverage, this simple requirement already yields a high-purity Cherenkov tag with useful efficiency for sub-MeV electromagnetic identification [2507.08886]. The statement refers to photocathode coverage rather than PMT count, and it underscores that selective optical instrumentation, rather than maximal uniform coating, is essential to the CCM200 strategy.

## 5. Simulation, validation, and statistical demonstration

Optical modeling uses GEANT4 to simulate scintillation and Cherenkov photon production, transport, wavelength shifting at TPB-coated surfaces, PMT response, dark noise, and post-pulsing, with measured background spectra from source-removed data included [2507.08886]. Offline reconstruction converts digitized voltage traces to PE-level hits using fits that account for PMT response, dark noise, and post-pulsing time structure [2507.08886].

For a typical uncoated PMT, the summed hit-time distribution in data and simulation agrees within $\pm 15\%$ as a $1\sigma$ systematic band across the prompt region and beyond out to $2 \ \mu\mathrm{s}$ [2507.08886]. The early $t \approx -3 \ \mathrm{ns}$ peak is reproduced by the Cherenkov component, while the simulation decomposes the total rate into Cherenkov, scintillation, and random-background contributions [2507.08886]. The number-of-hits distribution in the prompt window on uncoated PMTs shows that scintillation plus random backgrounds alone are two orders of magnitude lower than the total data rate in the Cherenkov-enhanced window [2507.08886].

Angular validation provides the principal statistical test. For events with at least two prompt hits, the angle between PMT-hit direction vectors, defined from PMT centers to the source location at the origin, shows a clear preference for $0.8 < \cos\theta < 0.9$ in data, consistent with visible Cherenkov in liquid argon and electron kinetic energies $0.7$--$1.0 \ \mathrm{MeV}$ [2507.08886]. The total simulation, including Cherenkov, scintillation, and random backgrounds, matches the data with $\chi^2 = 30.12$ for $20$ degrees of freedom [2507.08886]. A $\Delta\chi^2$ test rejects the background-only hypothesis at $>5\sigma$ confidence, establishing event-by-event Cherenkov identification [2507.08886].

The key empirical signatures are therefore mutually reinforcing: a distinct early-time peak on uncoated PMTs, a prompt-hit multiplicity excess over scintillation-plus-background expectations, and an angular distribution aligned with the visible-index Cherenkov angle in liquid argon [2507.08886]. This suggests that CCM200’s result is not a single-observable excess but a coherent optical signature supported by temporal, topological, and simulation-based validation.

## 6. Programmatic role within the CCM experiment

CCM200 is part of a broader experimental program aimed at MeV-scale electromagnetic final states and coherent scattering signatures at $O(100 \ \mathrm{keV})$ without electromagnetic activity [2507.08886]. Event-by-event Cherenkov identification directly strengthens both categories [2507.08886]. The detector therefore occupies a dual role: it is both a low-energy rare-event search instrument and a testbed for hybrid optical discrimination in liquid argon.

The CCM120 predecessor established the experimental setting. It was located $20$ meters from a high-flux neutron/neutrino source, designed to search for sterile neutrinos and light dark matter, and performed an engineering run in Fall 2019 [2105.14020]. The lessons learned from that run guided the development and construction of CCM200, which was engineered to improve sensitivity through increased optical coverage, cleaner liquid argon, enhanced vetoing, and new shielding [2105.14020]. CCM200 was described as having $200$ 8-inch PMTs, twice as many veto PMTs, highly efficient evaporative coated foils, and filtered and recirculated liquid argon [2105.14020].

The broader improvements were motivated by CCM120 limitations. In the earlier configuration, contaminated liquid argon reduced the $128 \ \mathrm{nm}$ attenuation length, and shielding and veto upgrades were introduced to mitigate beam-related gamma backgrounds and improve prompt-window usability [2105.14020]. Within that development arc, the 2025 Cherenkov result represents a detector-capability milestone rather than a standalone source-calibration exercise. It demonstrates that the upgraded optical layout can support low-energy topology and particle-identification information in addition to scintillation calorimetry [2507.08886].

## 7. Scientific significance and prospective applications

The 2025 CCM200 result establishes the first event-by-event observation of Cherenkov light from sub-MeV electrons in a high scintillation-yield liquid argon detector [2507.08886]. The physics impact stated in the paper is that hybrid liquid argon detectors can combine excellent energy resolution, from large scintillation yield of approximately $40{,}000$ photons/MeV, with directional and particle-identification information from Cherenkov light [2507.08886].

Event-by-event Cherenkov tagging can enhance background rejection in rare-event searches, provide directional information for low-energy electrons such as solar neutrinos, and enable separation of electromagnetic and highly ionizing low-energy signals in the CCM program’s two primary categories [2507.08886]. Pure liquid argon is transparent to visible light and does not intrinsically absorb above approximately $113.2 \ \mathrm{nm}$, allowing ultraviolet photons to propagate and be shifted at instrumented surfaces rather than quenched in bulk dopants; its relatively slow scintillation components and geometry-induced wavelength-shifting delays strengthen temporal separation from Cherenkov [2507.08886]. Cryogenic operation also supports future SiPM readout with dramatically reduced dark noise [2507.08886].

The result further shows that even with only $10\%$ photocathode coverage using uncoated PMTs, simple timing selections provide clean, per-event Cherenkov tags at approximately $10\%$ efficiency, with scope to improve by incorporating early portions of the scintillation-enhanced region and coated-PMT waveform information [2507.08886]. This suggests a broader detector-design principle: in low-energy liquid argon systems, selective wavelength shifting and channel-specific timing may be as consequential as absolute light yield for extracting event topology.

Within the CCM program’s larger objectives, such capabilities are relevant to MeV-scale neutrino measurements, beyond-Standard-Model searches, and any analysis in which discrimination between electromagnetic and non-relativistic energy deposition is limiting [2507.08886]. The CCM200 detector is therefore significant both as a specific experimental apparatus and as a concrete realization of hybrid scintillation-Cherenkov detection in liquid argon.

Source: https://www.emergentmind.com/topics/ccm200-detector