LAMBDA (Large Area Medipix Based Detector Array)

LAMBDA (Large Area Medipix-Based Detector Array) – A photon-counting pixel detector system based on the Medipix3 readout chip, with a small pixel size (55µm) and high speed, deadtime-free readout. A single LAMBDA module has 1536 x 512 pixels, and multiple modules can be tiled to cover a large area.

DESY is a member of the Medipix3 collaboration.
Medipix is a hybrid pixel detector readout chip developed at CERN. Each pixel on a semiconductor sensor is bonded to a channel on the Medipix chip. The Medipix chip performs single photon counting, much like the Pilatus detector system; each X-ray hit on a pixel generates a signal pulse, and the pixel circuitry counts the number of hits over an adjustable energy threshold. This means that the detector can achieve single-photon sensitivity, and does not suffer from the “blooming” effects seen in integrating detectors like CCDs. Since the photons are directly converted to an electrical signal in the semiconductor, rather than relying on indirect conversion, the point spread function is small. The chip can also be read out at a high rate.

Lambda module with a 1536-by-512-pixel silicon sensor

The third generation of this chip, Medipix3, has small pixels (55μm), and operates in a “continuous read-write” mode with negligible dead time between images. Communication between adjacent pixels can reduce the effects of signal sharing, allowing the chip to distinguish between different photon energies more effectively – for example, to exclude fluorescence photons.

DESY is have developed larger-area Medipix3 modules, incorporating 12 Medipix3 chips to give an array of 1536 by 512 pixels (28mm by 85mm). These modules are being designed to have a small edge region, so that they can be tiled to cover a larger area. A high-frame-rate readout system has also being designed; the current firmware can operate in bursts of 400 fps, and firmware is being developed to allow continuous 1000 fps readout using 10 Gigabit Ethernet links. The Medipix3 detector system will be used for diffraction, imaging and other experiments on PETRA III beamlines.

(a) X-ray image of a sardine tin, taken with the Lambda detector. (b,c) Photographs of the tin.

At higher photon energies (above 20keV) the absorption efficiency of silicon is poor. So, DESY and other institutes are working to develop finely-pixellated sensors made from semiconductors with higher atomic number (high-Z), such as germanium, gallium arsenide or cadmium telluride. The Medipix3 chip and large-area module (see below) are designed to be compatible with a wide range of different materials.

Project Leader: David Pennicard

Design of a Lambda system with 3 modules (1536 by 1536 pixels)