How GammaTPC
Detects Gamma Rays
A step-by-step journey through Compton telescope physics — from cosmic photon to scientific measurement. Scroll to explore each stage.
Liquid Argon Time Projection Chamber
GammaTPC is built around a spherical Liquid Argon TPC. The detector volume is filled with ultra-pure liquid argon at −186 °C. A strong electric field permeates the entire volume.
The uniform electric field is the key that turns the passive argon into an active 3D camera — ionisation electrons drift along field lines with millimetre precision.
Gamma Ray Enters the Detector
A high-energy photon arrives from a distant cosmic source — a gamma-ray burst, a neutron star merger, or dark matter annihilation. Travelling at the speed of light, it passes through the detector wall and into the LAr volume without leaving any trace.
Unlike charged particles, photons produce no ionisation while travelling through matter. The detector is completely blind to them — until the first interaction.
Compton Scattering
The gamma ray collides with an atomic electron in the argon. This Compton scatter transfers part of the photon's energy to the electron, which recoils — while the photon continues in a new direction with reduced energy.
The scatter angle θ and the deposited energy E − E′ are geometrically linked by the Compton formula. This is the mathematical key GammaTPC uses to work backward to the source.
The Recoil Electron Track
The kicked electron tears through the liquid argon, losing energy through collisions and ionising the medium along its path. It leaves a dense trail of electron–ion pairs — a 3D signature of its passage.
At 0.5 kV/cm, recombination of ions and electrons reduces the collected charge from the ~42,000 pairs created to ~21,000. The remaining pairs drift freely to the anode. Measuring the initial electron direction is GammaTPC's primary challenge.
Ionisation Electrons Drift to the Anode
The applied electric field sweeps the freed ionisation electrons toward the pixel anode at the base of the detector. They travel in straight, parallel lines — preserving the original 3D geometry of the ionisation track.
The time each electron takes to arrive at the anode directly encodes its depth (z-coordinate) in the detector — turning the 2D pixel map into a full 3D image of the event.
GAMPix Pixel Readout
The arriving electrons are collected by the GAMPix (Grid Activated Multi-Scale Pixel) readout — a two-stage system: ~200 μm pixel pads on CMOS ASICs collect charge, preceded by coarse 1 cm pitch x–y induction wires that trigger only the chips expected to collect charge.
Power is managed by duty cycling: only 10⁻³–10⁻⁴ of all pixel chips are powered on at any moment — those triggered by the coarse induction wires above them. This meets the strict power budget of a satellite without sacrificing spatial resolution.
Track Reconstruction & Source Direction
Offline software combines the pixel hit pattern with arrival timing to reconstruct the full 3D track. From the Compton kinematics — the electron's initial direction and the energy deposits — the original gamma-ray direction is recovered.
Each gamma ray constrains the source to a circle on the sky (an "event ring"). Many events from the same source, combined statistically, pinpoint its location with sub-degree precision.
GammaTPC at a Glance
The complete detection chain — from cosmic photon to reconstructed direction — enables unprecedented MeV gamma-ray science.