SAXS/WAXS GEM Project

GEMs (Gas Electron Multipliers) consist of a thin sheet of insulating material metallised on both sides and perforated with small holes in a regular pattern. A high voltage is applied between the top and bottom surfaces so that the electric field inside the holes exceeds 15kV/cm. Free electrons produced in the gas space above the GEM holes as a result of photo-induced ionisation processes are channelled into the holes, accelerated and produce additional electrons in an avalanche process. Each hole in the GEM plate therefore acts as an individual, independent electron multiplier structure. GEMs achieve near noiseless, linear amplification of photon-induced electrons which allows accurate resolution of the incident photon energy. At the same time, the spatial origins of incident photons can be determined by using a suitable pattern of read-out electrodes beneath the GEM structure. A very attractive feature of GEMs is that they can be fabricated using standard, inexpensive printed circuit board (PCB) technology and materials.

The aim of this project is to design and construct a 200*200mm GEM-based low energy X-ray detector with pixel or strip readout for SAXS (Small Angle X-ray Scattering) or WAXS (Wide Angle X-ray Scattering) experiments. SAXS detectors require high count rate capability and good spatial resolution, both of which can be achieved using GEMs. For this purpose, a small-scale prototype detector has been designed and is currently under evaluation. Figure 1 below shows the experimental setup

Figure 1: Experimental setup of the small scale prototype detector. The X-ray source is located behind the lead shield and in front of the detector.

Figures 2 show the inside views of the detector. When sealed, the detector is filled with an Argon/CO2 gas mix (ratio 70:30). The X-ray photons enter the Mylar window and strip electrons off the Argon atoms. The cathode (top electrode) is biased at high negative voltage, therefore the electrons drift towards the first GEM plate, get channelled into the holes and multiplied. After passing through a second GEM plate, they are finally collected at the anode which is near ground potential. A charge amplifier then extracts the brief current pulse.

Figure 2: Inside the detector. Drift cathode is at the top, GEM plates (yellow) and readout PCB near the bottom.

Figure 3 above shows an enlarged view of the GEM plate (actual size is 40*48mm). Hole diameter is 0.3mm and plate thickness 0.3mm. (GEM plates courtesy of A. Breskin, Weizmann Institute of Science, Israel).

The use of thick GEM plates has several advantages over "standard" thin GEMS  used at many labs around the world. They are electrically and mechanically more robust, easier, cheaper and quicker to manufacture and do not require complex installation procedures.

Project status

The prototype HV biasing scheme is currently being re-evaluated to allow operation at higher voltages (and therefore, much higher gains). CAD versions of the full-sized GEM boards are complete.

For further information on this project contact George Jung (george.jung@sync.monash.edu.au ) .