How event-based detection works

ASI builds event-based hybrid pixel detectors. Instead of building up an image over a long exposure like a camera, each pixel works as its own detector and records individual particles as they arrive, capturing position, arrival time, and energy.

Direct detection and particle counting

When a particle (a photon, electron, X-ray, or ion) hits the sensor of a Timepix3 or Medipix3 chip, it creates a charge cloud of electron-hole pairs. A bias voltage collects this charge at the pixel contact, and the on-chip electronics turn it into a digital signal. With Timepix3, a threshold set just above the electronic noise removes intrinsic detector noise, so only real particle hits are registered and the detector is fully data-driven. Medipix3 chips can apply up to eight thresholds for energy-resolved detection.

Time of Arrival and Time over Threshold

Each pixel measures two things at once. Time-of-Arrival (ToA) is the timestamp of when a hit crosses the threshold, with nanosecond precision. Time-over-Threshold (ToT) is how long the signal stays above threshold, which relates to the deposited energy. Recording both per pixel gives time-resolved, energy-aware data from a single measurement.

Why it matters

Because data is produced only when a particle is detected, datasets stay small and meaningful. The result is single-particle sensitivity, noise-free imaging, nanosecond timing, and the ability to follow fast, dynamic, or beam-sensitive processes that frame-based detectors miss.

Timepix: data-driven detection

Timepix3 and Timepix4 are event-driven (data-driven) chips: they report each hit the instant it crosses threshold, each with its own timestamp. There are no frames, so the data stream contains only real events. This suits sparse, time-resolved, or asynchronous signals, and means data volume scales with the number of real hits rather than a fixed frame rate.

Timepix3

256 x 256 pixels, 55 µm pitch (about 14.1 x 14.1 mm active area per chip)
Simultaneous Time of Arrival and Time over Threshold per pixel
1.56 ns ToA resolution — down to 260 ps with on-chip TDCs, about 3 ps with the PicoTDC option
Event-driven readout, about 475 ns per-pixel dead time
Up to 80 Mhits/s single chip; up to 320 Mhits/s on a 2 x 2 (quad) assembly
3-side buttable; up to 40 Gbit/s data streaming

Powers CheeTah, FeliS, Hyperion, Phoebe, Chronos, Telesto, and LynX T3.

Timepix4

512 x 448 pixels, 55 µm pitch — one Timepix4 provides nearly the area of a full quad Timepix3
About 195 ps Time of Arrival binning for the finest timing
Four-side buttable — enables large, gap-free detector arrays
Supports both event-driven and frame-based modes (8- or 16-bit counters)
About 400 ns per-pixel dead time
130 Mhits/s standard, up to about 900 Mhits/s in high-rate mode

Powers LynX T4, the Timepix4-based Hyperion, and FeliS T4.

Precise timing and synchronisation

Every ASI pixel is a digital pixel: it counts and time-stamps hits with its own on-chip electronics, so data leaves the detector already digitised. To place those events in the context of a wider experiment, the detectors use Time-to-Digital Converters (TDCs).

A TDC synchronises any external signal — a laser or electron pulse, a chopper, a T-zero reference, or another instrument — with the detected events.
This turns each event's Time of Arrival into a true time-of-flight relative to your reference signal.
On-chip TDCs reach 260 ps timing; the optional PicoTDC brings synchronisation down to about 3 ps.

PicoTDC: picosecond timing, built in

For the most demanding timing, ASI integrates the PicoTDC — a Time-to-Digital Converter developed at CERN — directly inside the detector. It timestamps external triggers with sub-10 ps precision and injects them straight into the Timepix data stream, alongside each hit's position and arrival time, so there is no separate timing instrument, no stitching of data formats, and no synchronisation overhead.

Sub-10 ps resolution — far finer than the 260 ps on-chip TDC, with multiple input channels timestamped simultaneously.
One data stream — external signals such as a pump laser, chopper, bunch clock, or RF and gun signals are recorded together with the detector events.
Shot-by-shot correction — per-shot jitter can be tagged and removed in post-processing rather than averaged away.

Where it makes the difference:

Ultrafast electron microscopy (UTEM) — per-shot laser-to-electron jitter tagging and multi-pump, multi-probe synchronisation (CheeTah).
Quantum optics — coincidence windows below 20 ps for Hong-Ou-Mandel and g(2) measurements with spectrally broad photon pairs (Chronos).
Correlated X-ray science — XPCS, time-resolved diffraction, pump-probe, and ghost imaging tied to a single clock (LynX).
Ion and mass spectrometry — COLTRIMS and reaction microscopy, imaging mass spectrometry, and velocity map imaging.
High-repetition-rate pump-probe experiments at synchrotron and laboratory sources.

PicoTDC White Paper

Medipix3: frame-based photon counting

Medipix3, also developed at CERN, takes the complementary approach to Timepix. Instead of time-stamping individual hits, each pixel counts photons above one or more energy thresholds during an exposure and reads out complete frames. It suits steady, high-flux imaging where energy thresholding matters, and shares the same 55 µm, 256 x 256 geometry as Timepix3.

Medipix3 at a glance

256 x 256 pixels, 55 µm pitch, 3-side buttable (about 14.1 x 14.1 mm active area per chip)
Charge-summing mode — recovers charge shared between neighbouring pixels, sharpening spatial and energy response
Continuous read-write — near-zero dead time between frames
Up to 2000 frames/s at full depth; up to about 10,000 frames/s at reduced bit depth
Noise-free through per-pixel energy thresholding

Used in LynX M3, CheeTah M3, FeliS M3.

Choosing the chip

Frame-based and event-driven detection answer different experimental needs.

Chip	Readout	Best for
Medipix3	Frame-based photon counting with energy thresholds	High-flux, steady-state imaging where energy discrimination matters.
Timepix3	Event-driven, 1.56 ns timestamps	Time-resolved, single-particle work; the most mature, widely configured ecosystem.
Timepix4	Event-driven or frame-based, about 195 ps timestamps	The finest timing (below 200 ps), the largest gap-free area, and the highest hit rates.

Software: control, acquisition, and analysis

Every ASI detector runs through one connected software ecosystem, built around an open HTTP API so a detector drops straight into an existing beamline, microscope, or instrument-control setup. Three tools cover the whole workflow: Serval controls the hardware and streams the data, Accos provides a full graphical cockpit, and Luna turns the raw event stream into analysis-ready datasets.

Serval

Serval is ASI's control and data-acquisition server: a single HTTP API that drives every detector in the portfolio, so any language or tool that can speak HTTP can run an acquisition with no proprietary client software. It is built for high-throughput streaming, exposing configurable output channels for raw events, processed images, and live previews that can all run at the same time to disk, TCP, or HTTP.

One interface for all ASI detectors
Raw events (.tpx3/.tpx4), processed images, and live preview at once
Full hardware control: bias, triggers, exposure, DACs, masking, TDC
Leader/follower multi-detector sync
Applications: scriptable acquisition sequences, multi-instrument synchronisation over triggers and TDC inputs, high-throughput recording to disk or network, remote beamline operation, and custom interfaces built on the open API.

Serval Timepix3 Manual Serval Medipix3 Manual

Accos

Accos is the graphical interface for the whole ASI detector portfolio, designed to make advanced hybrid pixel detectors approachable without hiding their depth. Real-time image preview with live intensity and Time-of-Flight histograms gives immediate feedback during acquisition, while guided per-pixel equalisation and threshold optimisation keep each detector performing at its best. Because it talks to the hardware through the Serval API, Accos fits straight into larger experimental workflows, and a dedicated 4D-STEM mode adds synchronised scanning for electron microscopy.

Timepix3, Timepix4, and Medipix3 (single and multi-chip)
Live preview with intensity and Time-of-Flight histograms
Guided per-pixel equalisation and pixel masking
Dedicated 4D-STEM mode
Applications: detector setup, commissioning and per-pixel equalisation, plus real-time acquisition and experiment monitoring across every ASI product line.

Accos Starting Guide Trigger manual

Engineered for demanding environments

ASI detectors are built to keep performing where experiments are hardest.

Vacuum compatible: robust vacuum-compatible construction and mounts for in-vacuum operation in beamlines and microscope chambers.
X-ray shielding: heavily shielded designs protect the readout electronics and keep data clean under intense beams.
Stable cooling: an integrated cooling system holds the detector at a stable temperature for long, drift-free acquisitions.

Integration and mechanical know-how

A detector only helps if it fits your instrument. ASI's mechanical expertise means our detectors can be adapted to almost any experimental setup — custom flanges and vacuum-compatible mounts, instrument-specific geometries, and integration with existing beamlines, microscopes, and chambers.

If you have an unusual geometry or a tricky integration, tell us about your setup — chances are we can make it fit. Get in touch with our team.