In this chapter some historical particle detectors will be briefly described. These are mainly optical devices that have been used in the early days of cosmic rays and particle physics. Even though some of these detectors have been ‘recycled’ for recent elementary particle physics experiments, like nuclear emulsions for the discovery of the tau neutrino (vT) or bubble chambers with holographic readout for the measurement of shortlived hadrons, these optical devices are nowadays mainly integrated into demonstration experiments in exhibitions or employed as eye-catchers in lobbies of physics institutes (like spark chambers or diffusion cloud chambers).

Cloud chambers

The cloud chamber (‘Wilson chamber’) is one of the oldest detectors for track and ionisation measurement [1–4]. In 1932 Anderson discovered the positron in cosmic rays by operating a cloud chamber in a strong magnetic field (2.5 T). Five years later Anderson, together with Neddermeyer, discovered the muon again in a cosmic-ray experiment with cloud chambers.

A cloud chamber is a container filled with a gas–vapour mixture (e.g. air–water vapour, argon–alcohol) at the vapour saturation pressure. If a charged particle traverses the cloud chamber, it produces an ionisation trail.

Ageing effects in gaseous detectors

Ageing processes in gaseous detectors are about as complicated and unpredictable as in humans.

Avalanche formation in multiwire proportional or drift chambers can be considered as a microplasma discharge. In the plasma of an electron avalanche, chamber gases, vapour additions and possible contaminants are partially decomposed, with the consequence that aggressive radicals may be formed (molecule fragments). These free radicals can then form long chains of molecules, i.e., polymerisation can set in. These polymers may be attached to the electrodes of the wire chamber, thereby reducing the gas amplification for a fixed applied voltage: the chamber ages. After a certain amount of charge deposited on the anodes or cathodes, the chamber properties deteriorate so much that the detector can no longer be used for accurate measurements (e.g. energy-loss measurements for particle identification).

Ageing phenomena represent serious problems for the uses of gaseous detectors especially in harsh radiation environments, such as at future high-intensity experiments at the Large Hadron Collider at CERN. It is not only that gas mixtures for detectors have to be properly chosen, also all other components and construction materials of the detector systems have to be selected for extraordinary radiation hardness.

The book on Particle Detectors was originally published in German (‘Teilchendetektoren’) with the Bibliographisches Institut Mannheim in 1993. In 1996, it was translated and substantially updated by one of us (Claus Grupen) and published with Cambridge University Press. Since then many new detectors and substantial improvements of existing detectors have surfaced. In particular, the new proton collider under construction at CERN (the Large Hadron Collider LHC), the planning for new detectors at a future electron–positron linear collider, and experiments in astroparticle physics research require a further sophistication of existing and construction of novel particle detectors. With an ever increasing pace of development, the properties of modern detectors allow for high-precision measurements in fields like timing, spatial resolution, energy and momentum resolution, and particle identification.

Already in the past, electron–positron storage rings, like LEP at CERN, have studied electroweak physics and quantum chromodynamics at energies around the electroweak scale (≈ 100 GeV). The measurement of lifetimes in the region of picoseconds required high spatial resolutions on the order of a few microns. The Large Hadron Collider and the Tevatron at Fermilab will hopefully be able to solve the long-standing question of the generation of masses by finding evidence for particles in the Higgs sector.

Introduction

The analysis of data and extraction of relevant results are goals of particle physics and astroparticle physics experiments. This involves the processing of raw detector data to yield a variety of final-state physics objects followed by the application of selection criteria designed to extract and study a signal process of interest while rejecting (or reducing to a known and manageable level) background processes which may mimic it. Collectively, this is referred to as an analysis. Physics analyses are performed either to measure a known physical quantity (e.g. the lifetime of an unstable particle), or to determine if the data are compatible with a physics hypothesis (e.g. the existence of a Higgs boson). At each stage, however, a variety of ‘higher-order’ issues separate particle physics and astroparticle physics analyses from a brute-force application of signal-processing techniques.

Reconstruction of raw detector data

All physics analysis starts from the information supplied from the dataacquisition system, the raw detector data.

There is a large number of applications for radiation detectors. They cover the field from medicine to space experiments, high energy physics and archaeology [1–4].

In medicine and, in particular, in nuclear medicine, imaging devices are usually employed where the size and function of the inner organs can be determined, e.g. by registering γ rays from radioactive tracers introduced into the body.

In geophysics it is possible to search for minerals by means of natural and induced γ radioactivity.

In space experiments one is frequently concerned with measuring solar and galactic particles and γ rays. In particular, the scanning of the radiation belts of the Earth (Van Allen belts) is of great importance for manned space missions. Many open questions of astrophysical interest can only be answered by experiments in space.

In the field of nuclear physics, methods of α-, β- and γ-ray spectroscopy with semiconductor detectors and scintillation counters are dominant [5]. High energy and cosmic-ray physics are the main fields of application of particle detectors [6–11]. On the one hand, one explores elementary particles down to dimensions of 10-17 cm, and on the other, one tries by the measurement of PeV γ rays (1015 eV) to obtain information on the sources of cosmic rays.

Momentum measurement and, in particular, muon detection is an important aspect of any experiment of particle physics, astronomy or astrophysics. Ultra-high-energy cosmic rays are currently at the forefront of astroparticle physics searching for the accelerators in the sky. These questions can be studied by the detection of extensive air showers at ground level by measuring secondary electrons, muons and hadrons produced by primary cosmic rays which initiate hadronic cascades in the Earth's atmosphere. The detectors have to operate for many years in order to map the galactic sources of high-energy cosmic rays which may be visible at the experimental sites. There are several experiments dedicated to studying these air showers that employ large detector arrays for electron and muon detection. Apart from water Cherenkov and scintillation counters, typical detectors such as limited streamer tubes [1] and resistive-plate chambers are also used [2].

In the field of high energy physics, over the last several decades many outstanding discoveries have been made from the studies of muons along with other precision measurements of leptons and hadrons.

The measurement of particle trajectories is a very important issue for any experiment in high energy physics. This provides information about the interaction point, the decay path of unstable particles, angular distributions and, when the particle travels in a magnetic field, its momentum. Track detectors, used intensively in particle physics up to the early seventies, have been described in Chap. 6.

A new epoch was opened by the invention of the multiwire proportional chamber [1, 2]. Now gaseous wire chambers and micropattern detectors almost play a dominant rôle in the class of track detectors.

The fast progress of semiconductor detectors has resulted in a growth of the number of high energy physics experiments using tracking systems based on semiconductor microstrip or pixel detectors, especially in areas where extremely high spatial accuracy is required.

Multiwire proportional chambers

A multiwire proportional chamber (MWPC) [1–4] is essentially a planar layer of proportional counters without separating walls (Fig. 7.1).

The development of particle detectors practically starts with the discovery of radioactivity by Henri Becquerel in the year 1896. He noticed that the radiation emanating from uranium salts could blacken photosensitive paper. Almost at the same time X rays, which originated from materials after the bombardment by energetic electrons, were discovered by Wilhelm Conrad Röntgen.

The first nuclear particle detectors (X-ray films) were thus extremely simple. Also the zinc-sulfide scintillators in use at the beginning of the last century were very primitive. Studies of scattering processes – e.g. of α particles – required tedious and tiresome optical registration of scintillation light with the human eye. In this context, it is interesting to note that Sir William Crookes experimenting in 1903 in total darkness with a very expensive radioactive material, radium bromide, first saw flashes of light emitted from the radium salt. He had accidentally spilled a small quantity of this expensive material on a thin layer of activated zinc sulfide (ZnS). To make sure he had recovered every single speck of it, he used a magnifying glass when he noticed emissions of light occurring around each tiny grain of the radioactive material.

Accelerators are in use in many different fields, such as particle accelerators in nuclear and elementary particle physics, in nuclear medicine for tumour treatment, in material science, e.g. in the study of elemental composition of alloys, and in food preservation. Here we will be mainly concerned with accelerators for particle physics experiments [1–5]. Other applications of particle accelerators are discussed in Chapter 16.

Historically Röntgen's X-ray cathode-ray tube was an accelerator for electrons which were accelerated in a static electric field up to several keV. With electrostatic fields one can accelerate charged particles up to the several-MeV range.

In present-day accelerators for particle physics experiments much higher energies are required. The particles which are accelerated must be charged, such as electrons, protons or heavier ions. In some cases – in particular for colliders – also antiparticles are required. Such particles like positrons or antiprotons can be produced in interactions of electrons or protons. After identification and momentum selection they are then transferred into the accelerator system [6].

Accelerators can be linear or circular. Linear accelerators (Fig. 4.1) are mostly used as injectors for synchrotrons, where the magnetic guiding field is increased in a synchronous fashion with the increasing momentum so that the particle can stay on the same orbit.