---
title: Long Imaging ModulE (LIME) Detector
url: https://www.emergentmind.com/topics/long-imaging-module-lime
type: topic
---

# Long Imaging ModulE (LIME) Detector

LIME, the **Long Imaging ModulE**, is a **50 L active-volume gaseous time projection chamber (TPC)** developed within the **CYGNO** program as the largest prototype of its R&D phase and as the reference module for subsequent demonstrator-scale detectors. It combines a **50 cm drift length**, a **\(33 \times 33~\mathrm{cm^2}\)** **triple-GEM** amplification stage, and **optical readout** with one **Hamamatsu ORCA Fusion sCMOS** camera and **4 PMTs**, operated with a **\(\mathrm{He:CF_4}=60{:}40\)** gas mixture at atmospheric pressure. Its scientific purpose is the directional detection of rare events, especially **low-mass Dark Matter** and **solar neutrino interactions**, through low-threshold imaging, 3D reconstruction, and fiducial background rejection [2306.16856][2305.06168][2510.01646].

## 1. Role within the CYGNO program

Within CYGNO, LIME is explicitly the **unit module** from which the future demonstrator is intended to be assembled, and its underground results are described as **paramount in the optimization of the CYGNO demonstrator**, which is foreseen to use **multiple modules with the same LIME dimensions and characteristics** [2306.16856]. The thesis literature places LIME at the end of **Phase-0 R&D**, after the smaller **ORANGE** and **LEMOn** detectors, and as the experimental bridge toward the approved **\(0.4~\mathrm{m^3}\)** **CYGNO-04** demonstrator and the longer-term **\(\mathcal O(30~\mathrm{m^3})\)** **CYGNO-30** detector [2510.01646].

The physics motivation is directional rare-event detection in gas. CYGNO targets **nuclear recoils below about 100 keV**, where directional information is especially valuable because, in a WIMP scenario, the recoil angular distribution is expected to point roughly toward the **Cygnus constellation**. The choice of a gaseous detector follows from the need to spatially resolve low-energy recoil tracks; the TPC architecture provides drift-time information and fiducialization, while optical readout supplies fine-grained imaging over large areas with relatively low channel count [2306.16856].

The overground characterization paper is explicit that LIME was **not yet a low-background dark-matter detector**. It used **non-radiopure materials** and was operated **overground at Laboratori Nazionali di Frascati (LNF)** with **no shielding against environmental radioactivity**; in that phase its function was technological validation, response characterization, and background diagnosis rather than a competitive search [2305.06168].

## 2. Detector architecture and readout chain

LIME is a room-temperature, atmospheric-pressure gaseous TPC housed in a **10 mm** thick transparent **PMMA** vessel kept at an internal overpressure of about **3 mbar** over atmosphere. The active drift region is bounded by a **0.5 mm copper cathode** and a **triple-GEM** amplification stage. The field cage is formed by **34 copper rings**, each **10 mm** wide and spaced by **4 mm**, connected by **\(100~\mathrm{M\Omega}\)** resistors to establish a uniform drift field orthogonal to the cathode [2510.01646].

The readout architecture is optical. The main imaging device is a **Hamamatsu ORCA-Fusion sCMOS** with **\(2304\times 2304\)** pixels of **\(6.5\times 6.5~\mu\mathrm m^2\)**, coupled to a **25 mm** Schneider Xenon lens with **\(f/\# \approx 0.95\)**. The camera is placed **623 mm** from the GEM plane and images about **\(34.9\times 34.9~\mathrm{cm^2}\)**, corresponding to an effective granularity of **\(152\times 152~\mu\mathrm m^2\)** per pixel. Four **Hamamatsu R7378 PMTs**, each with a **22 mm** diameter bialkali photocathode, are symmetrically arranged around the camera [2510.01646][2305.06168].

| Subsystem | Reported specification | Function |
|---|---|---|
| Active volume | **50 L** | Target and drift region |
| Drift length | **50 cm** | Electron transport and fiducial depth |
| Amplification area | **\(33 \times 33~\mathrm{cm^2}\)** | Triple-GEM avalanche stage |
| Gas mixture | **\(\mathrm{He:CF_4}=60{:}40\)** | Low threshold and scintillation yield |
| Optical imager | **ORCA Fusion sCMOS** | High-resolution 2D topology |
| Timing sensors | **4 PMTs** | Waveform timing and \(z\)-information |

The amplification stage consists of **three 50 \(\mu\)m GEM foils**. The overground operating summary reports **\(\Ed = 0.9~\mathrm{kV/cm}\)**, **\(\Et \approx 2.5~\mathrm{kV/cm}\)**, and **\(\Vg = 440~\mathrm{V}\)** across each GEM, while the underground thesis reports typical drift fields of **\(0.8\)–\(0.9~\mathrm{kV/cm}\)** and notes that **Run1** underground was limited to **\(420~\mathrm V\)** per GEM because of discharges [2305.06168][2510.01646].

The chosen gas is motivated by CYGNO’s low-energy directional program. The papers emphasize the combination of **low energy threshold** and **high scintillation yield**, together with optical compatibility between the visible **CF\(_4\)** emission near **620 nm** and the camera’s high-QE band [2306.16856][2510.01646].

## 3. Operating principle and reconstruction workflow

LIME follows the standard TPC chain, extended with optical avalanche readout. Ionization electrons drift toward the **triple-GEM** stack, where avalanche multiplication produces both amplified charge and **secondary scintillation light**. The **sCMOS** records the spatial distribution of this light on the GEM plane, providing the transverse image; the **PMTs** register its time development, which contains information on the coordinate along the drift direction. The prototype papers therefore describe LIME operationally as a detector with **\(x\)-\(y\)** information from the optical image plane and **\(z\)** information from timing, enabling **3D track reconstruction** and fiducialization [2306.16856].

In the underground thesis, PMT-based waveform analysis is described as still under refinement, but the detector concept and dedicated studies support both absolute and relative \(z\) reconstruction. The same thesis also reports an **absolute-\(z\)** estimator built from camera images alone for **5.9 keV** spots, yielding an absolute \(z\) resolution from about **4 cm** at short drift to about **8 cm** at large drift distances [2510.01646].

The image-analysis chain became increasingly elaborate in the underground campaign. Raw **\(2304\times2304\)** sCMOS frames undergo pedestal subtraction, zero suppression, **\(4\times4\)** rebinning into macro-pixels, median filtering, and vignetting correction. Clustering is then performed with **iDBSCAN**, an intensity-based version of DBSCAN; dense seeds are merged into longer structures by **superclustering**, first with geodesic active contours and later with **Chan–Vese** segmentation, while **iDDBSCAN** adds repeated linear or polynomial **RANSAC** fits to merge long directional tracks such as cosmic rays [2510.01646].

The principal reconstructed observables include **\(sc\_integral\)** as the total light integral, **\(sc\_nhits\)** as the number of nonzero pixels, bounding-box coordinates, **\(sc\_length\)**, **\(sc\_width\)**, **\(sc\_tgausssigma\)** as the Gaussian width along the minor axis, and **\(sc\_rms\)** as the RMS of pixel intensities inside the cluster. Calibration spot selection uses the **slimness** variable \(s = sc\_width/sc\_length\), and fiducial and quality selections include
\[
sc\_xmin > 300,\quad sc\_ymin > 300,\quad sc\_xmax < 2000,\quad sc\_ymax < 2000,
\]
together with
\[
sc\_rms > 6,
\]
and
\[
sc\_tgausssigma \times 0.152~\mathrm{mm/pixel} > 0.5.
\]
For preliminary dark-matter counting analyses, the additional variable
\[
p = \frac{sc\_rms}{sc\_nhits}
\]
is used with the selection \(0.005 < p < 0.15\) to reject MIP-like backgrounds [2510.01646].

## 4. Calibration, stability, and measured response

The central calibration source throughout the LIME program is **\({}^{55}\mathrm{Fe}\)**, which provides **5.9 keV** X-rays. Overground, additional calibration lines were obtained from Ca, Ti, Cu, Rb, Mo, Ag, Ba, and in the large-prototype study also from **Tb**, spanning from **3.7 keV** up to **47 keV** [2306.16856][2305.06168]. The overground characterization reports that the detector achieved a **few-keV threshold** and an **energy resolution of 10–20%** over the studied range while running for several weeks continuously with **very high operational efficiency** [2305.06168].

Several complementary threshold statements appear in the literature. The prototype performance paper states that a threshold of **0.5 keV** was set such that the expected rate of fake \({}^{55}\mathrm{Fe}\)-like events due to noise would be at most **10 per year** [2306.16856]. The later thesis reports that fake clusters become negligible above **400 counts**, about **0.5 keV**, and that a threshold of **1 keV** corresponds to roughly **10 false events/year** [2510.01646]. The overground characterization also distinguishes between a low cluster-counting threshold corresponding to about **300 eV** and the practical spectroscopy range demonstrated down to the **Ca** and **Ti** lines near **3.7–4.9 keV** [2305.06168].

At **5.9 keV**, the commissioning study reports an energy resolution of **around 14%** across the full **50 cm drift length**, and notes that a multivariate regression analysis using **track position** and **various shape parameters** was under development with preliminary indications of **better than 10% energy resolution at 5.9 keV** [2306.16856]. The overground characterization resolves this more finely: after multivariate correction, the full-sample **RMS** at **5.9 keV** is about **12%** for favorable drift regions, while the best-cluster **\(\sigma_G\)** from the Crystal Ball fit is **smaller than 10%** for \(z \gtrsim 25\) cm [2305.06168]. The underground thesis then reports that, after equalisation across runs, the energy resolution of the \({}^{55}\mathrm{Fe}\) peak is about **12%** in every phase [2510.01646].

A persistent detector effect is the dependence of light yield on drift distance. Rather than decreasing with \(z\), the average \({}^{55}\mathrm{Fe}\) light yield increases away from the GEM plane because diffusion spreads the primary charge cloud, reducing local charge density in the GEM holes and mitigating **gain saturation**. The thesis quantifies this strongly near the amplification stage: at **4.75 cm** the \({}^{55}\mathrm{Fe}\) peak is reduced by about **35%**, at **14.75 cm** by about **15%**, and from **24.75** to **45.75 cm** it is near the plateau around **\(10^4\)** counts [2510.01646].

Environmental dependence is another central result. Overground at LNF, the normalized \({}^{55}\mathrm{Fe}\) response versus pressure was fitted with \(y = a + b/x\), showing a decrease of about **0.6% per mbar** as pressure increased [2305.06168]. Underground, this behavior was confirmed: **Run1** showed **\((0.57 \pm 0.08)\%\)/mbar** at **3 l/h** and **\((0.57 \pm 0.04)\%\)/mbar** at **20 l/h**, while **Run2** showed **\((0.68 \pm 0.04)\%\)/mbar** at **20 l/h** [2510.01646]. Once gas recirculation started, humidity became a dominant variable: the normalized \({}^{55}\mathrm{Fe}\) light yield decreased by about **20% per unit increase in RH** in **Run3**, and by **\((25 \pm 5)\%\)** per RH unit in **Run4** [2510.01646].

To merge long data periods taken under changing environmental conditions, the thesis introduces a run-by-run equalisation procedure based on the new variable **\(LY_{30}\)**, defined as the mean light integral of reconstructed clusters between **30 kcounts** and **300 kcounts**, using only fiducial cuts. The measured \({}^{55}\mathrm{Fe}\) peak and \(LY_{30}\) are reported as linearly correlated, and the relative difference between measured and inferred iron-peak positions has a Gaussian width of **13%**, which is then adopted as the equalisation uncertainty [2510.01646].

## 5. Underground deployment and background characterization

LIME was installed underground at **Laboratori Nazionali del Gran Sasso (LNGS)** in **February 2022**, in the **TIR gallery between Hall A and Hall B** [2510.01646]. The underground campaign was not merely a relocation. It was intended to validate detector operation in a **low radioactivity** and **low pile-up** environment, to test the gas system and the full 3D reconstruction and background-rejection chain, and to confront detector simulations with real underground data [2306.16856].

The shielding configuration evolved in stages. The thesis defines **Run1** as unshielded, **Run2** with **4 cm Cu**, **Run3** with **10 cm Cu**, and **Run4** with **10 cm Cu + 40 cm water** [2510.01646]. The broader prototype study had already emphasized the role of dedicated MC optimization for a shielding concept based on **copper** against gammas and **water tanks** against neutrons [2306.16856].

Background simulation and measurement became a major component of LIME’s scientific role. The underground MC includes both **intrinsic radioactivity of detector materials** and **natural ambient gamma and neutron flux**, and the 50 L prototype paper reports that **fiducial cuts** can reduce radioactivity-induced backgrounds by **96%** [2306.16856]. The thesis provides more detailed rates for the unshielded and shielded phases: about **\(1.14\times10^9\)** ER events/year and **\(8.0\times10^4\)** NR events/year without shielding; **\(3.45\times10^7\)** ER events/year with **4 cm** copper; **\(9.37\times10^6\)** ER events/year with **10 cm** copper; and **\(8.38\times10^6\)** ER events/year with **10 cm Cu + 40 cm water**, at which point internal backgrounds dominate. With full water shielding, the environmental neutron recoil contribution is reduced to about **2/year** [2510.01646].

The overground campaign provided the reference point for why underground operation was necessary. In source-free LNF data, with **50 ms** exposure and a threshold corresponding to about **300 eV**, the average detected interaction rate was about **250 Hz**, and the average thresholded energy deposition rate was about **\(6.3~\mathrm{MeV/s}\)** [2305.06168]. For cosmic-ray-like events, the overground paper reports a measured rate of about **15 Hz**, to be compared with a predicted maximum active-volume interaction rate of about **24 Hz** from standard sea-level flux estimates [2305.06168]. The underground move was therefore directly tied to reducing pile-up and isolating internal detector backgrounds.

Data/MC comparison in the underground thesis shows **good agreement in Run1 above 10 keV** and a clear **Cu fluorescence peak near 8 keV**. In **Run2** the shapes remain similar. In **Run3**, however, the MC underestimates the low-energy data, which is interpreted as evidence for internal background sources not fully simulated [2510.01646].

## 6. Dark-matter relevance, limitations, and legacy

LIME’s first dark-matter study is described as explicitly **preliminary** and as a proof of analysis chain rather than a competitive search. The thesis uses **17 days** of **Run4** exposure with **10 cm Cu + 40 cm water** shielding, approximately constant pressure around **0.907 bar**, and thresholds of **1** and **1.5 keVee**. A Bayesian treatment with Poisson likelihood is then used to derive **90% credible** upper limits; the author states explicitly that the resulting SI and SD exclusions are **not competitive with world-leading experiments**, although the SD result is compared qualitatively to **DRIFT** as encouraging given the modest exposure and prototype status [2510.01646].

The detector nevertheless establishes several features that are directly relevant to future directional searches. It demonstrates atmospheric-pressure operation of a large optical TPC, low-keV calibration, long-term gain monitoring, run equalisation, and underground background characterization [2510.01646]. The large-prototype paper also notes that the detector-response MC already reproduces key observables such as **light integral** and **track dimension** for **5.9 keV** ERs with preliminary agreement **within 10%** in the linearity study, and that **ER/NR discrimination** at LIME scale is under active study [2306.16856].

The limitations are equally clear. LIME was not built with radiopure components, so once external backgrounds are shielded, **internal radioactivity dominates** [2510.01646]. Gain saturation near the amplification plane produces a strong \(z\)-dependent response below about **15 cm** from the GEMs [2510.01646]. PMT-based 3D reconstruction was still under refinement in the underground analysis chain [2510.01646]. Overground, the detector suffered from unshielded environmental radioactivity, cosmic-ray pile-up, and the fact that the **50 ms** exposure made overlap of tracks unavoidable in a high-rate environment [2305.06168].

In spite of those limitations, LIME functions as a technically mature module-scale demonstrator. The collected literature supports a consistent interpretation: it is the first CYGNO prototype that matches the scale and architecture of a demonstrator module; it validates stable operation of a **50-liter**, **50-cm-drift**, **triple-GEM optical TPC**; it quantifies pressure, humidity, impurity, and saturation effects in detail; and it provides the experimental basis for scaling to **CYGNO-04** and, ultimately, to the modular **CYGNO-30** concept [2306.16856][2510.01646].

Source: https://www.emergentmind.com/topics/long-imaging-module-lime