As well as being important in particle physics, multiwire detectors have been applied in other fields: X-rays for medical imaging, ultraviolet and single-photon detection, neutron and crystal diffraction studies and so on. Now their major limitation of modest rate capability has been overcome through the introduction of micropattern devices.
The pioneering work at the beginning of the 20th century by Thomson, Rutherford and Geiger, just after the discovery of electromagnetic radiation, focused attention on the development of tools to detect this radiation. The single-wire proportional counter (Geiger counter) became an essential physics tool.
Georges Charpak’s 1968 invention of the multiwire proportional chamber (MWPC) ushered in a new era, with a major impact on high-energy physics. The main performance features of the MWPC are a space resolution of few hundred micrometres, the two- and three-dimensional localization of incident radiation, excellent energy resolution and rates of a few kilohertz per square millimetre.
Other MWPC applications include crystal diffraction, beta chromatography and dual-energy angiography. A low dose X-ray digital radiography scanner based on the MWPC developed at Novosibirsk is currently being used routinely in hospitals in Russia and France.
Despite this success, some basic limitations of MWPCs restrict their use at high rates. The wire spacing defines the best achievable position accuracy and gives two-track resolution to about 1 mm. Electrostatic instability limits the stable wire lengths. The widths of the induced charges define the pad response function and, at high rates the accumulation of positive ions spoils rate capability.
The advent of high-luminosity colliders demands fast, high-performance position-sensitive detectors. Key requirements are unsurpassed position localization; good two-track, two-dimensional and time resolutions; and the ability to withstand hostile radiation over a considerable period of time.
The microstrip generation
The invention of the microstrip gas chamber (MSGC) by Anton Oed marked another era of gaseous detectors. An MSGC comprises a pattern of thin anode and cathode strips on an insulating substrate with a pitch of a few hundred micrometres. With a drift electrode and with appropriate potentials applied, the electric field is such that positive ions are removed immediately from the avalanches, increasing rate capability by some two orders of magnitude.
The salient features are localization accuracy of some 30 µm, double-track resolution of 400 µm and good energy resolution. Long-term and magnetic field operations have been demonstrated, and these devices have found applications in many fields of X-ray spectrometry digital radiography and high-energy physics.
Difficulties began when MSGCs were exposed to the highly ionizing particles that are usually present in a high luminosity machine. These particles deposit in the detection volume almost three orders of magnitude as much charge as a minimum ionizing particle.
In the case of microstrip detectors, the anode-cathode distance is small compared with that in a wire chamber, and, with electric fields at the tip of the streamer and along the surface being high, the streamer is likely to be followed by a voltage- and ionization density-dependent discharge. The charging up of surface defects, long-lived excited states and overlapping avalanches seems to be the culprit, lowering the discharge limits of operation. With this insight, several novel designs appeared.
The detection of micropatterns
Advances in photolithography and the application of silicon foundry techniques heralded a new era in the design and fabrication of “micropattern detectors”. The microdot (introduced by Biagi) is the ultimate gaseous pixel device, with anode dots surrounded by cathode rings. Although achieving gains of about 1 million, it does not discharge, probably because the field emulates the 1/r field of an anode wire.
A very asymmetric parallel plate chamber, the MICROMEGAS detector invented by Charpak and Giomataris, takes advantage of the behaviour at high fields (100 kV/cm) in several gas mixtures, thus achieving stable operation with the minimum of ionizing particles at high gains and rates. Large MICROMEGAS detectors are being made and tested for the COMPASS experiment at CERN.
A new detector invented by Lemonnier is the CAT (compteur à trous). This comprises a narrow hole micromachined into an insulator metallized on the surface, which acts as the cathode, while the metal at the bottom of the hole constitutes the anode. With appropriate potentials and a drift electrode, this scheme acts as a focusing lens for the drifting electrons left in the wake of ionizing radiation.
Removing the insulator leaves the cathode as a micromesh, which, with a thin gap between it and the read-out electrode, emulates CAT operation (hence microCAT or µCAT). This structure offers gains of several 104. Another option uses ingenious read-out from “virtual pixels” made by current sharing, giving 20 times as fine resolution compared with the read-out cell and 400 times as many virtual pixels. The µCAT combined with the pixels is called the VIP (figure 1).
A new concept of gas amplification introduced by Sauli in 1996 is the gas electron multiplier (GEM) manufactured using printed circuit wet etching techniques. A thin (50 µm) Kapton foil clad on both sides with copper is perforated and the two surfaces maintained at a potential gradient, thus providing the necessary field for electron amplification (figure 2).
Coupled with a drift electrode above and a read-out electrode below, it acts as a high-performance electron amplifier. The essential features of this detector are that amplification and detection are decoupled and the read out is at zero potential. Charge transfer to a second amplification device opens up the possibility of using a GEM in tandem with an MSGC or a second GEM.
With these developments and a better understanding of the discharge phenomena, new detectors have appeared: Micro-Wire, an extension of the µDOT in the third dimension, Micro-Pin Array, Micro-Tube, Micro Well, Micro Trench and Micro Groove. All aim for minimal insulator between the anode and cathode to reduce discharges. The Micro-Tube uses a combination of laser micromachining and nickel electroplating, and gives an electric field that increases rapidly at the anode, similar to the µDOT. However, there is no insulating material on the direct line of sight from cathode to anode. These features are predicted to lead to higher gas gains, better stability with fewer discharges and the reduction of charging effects.