Detectors for cosmic particles, neutrinos and exotic matter

Astroparticle physics deals with the investigation of cosmic radiation using similar detection methods as in particle physics, however, mostly with quite different detector arrangements. In this chapter the detection principles for the different radiation types with cosmic origin are presented, this includes charged particles, gamma radiation, neutrinos and possibly existing Dark Matter. In the case of neutrinos also experiments at accelerators and reactors are included. Examples, which are typical for the different areas, are given for detectors and their properties. For cosmic ray detection apparatuses are deployed above the atmosphere with balloons or satellites or on the ground using the atmosphere as calorimeter in which high-energy cosmic rays develop showers or in underground areas including in water and ice.

Gas-filled detectors

Detectors that record charged particles through their ionisation of gases are found in many experiments of nuclear and particle physics. By conversion of the charges created along a track into electrical signals, particle trajectories can be measured with these detectors in large volumes, also inside magnetic fields. The operation principles of gaseous detectors are explained, which include charge generation, gas amplification, operation modes and gas mixtures. Different detector types are described in some detail, starting with ionisation chambers without gas amplification, proceeding to those with gas amplification like spark and streamer chambers, parallel plate arrangements, multi-wire proportional chambers, chambers with microstructured electrodes, drift chambers, and ending with time-projection chambers. The chapter closes with an overview of aging effects in gaseous detectors which cause negative alterations of the detector performance.

Signal formation by moving charges

Normally modern detectors are read out electronically. The signals that are induced on the detector electrodes are generated by the movement of charges relative to the electrodes. The general principle for the calculation of the signals is introduced on the basis of the Shockley-Ramo theorem applying the concept of weighting fields to an arbitrary number of electrodes in field volumes with and without space charge. Examples of the time development of signals are calculated for electrode arrangements with plate and cylinder geometry and for electrodes with strip or pixel segmentation.

Interactions of particles with matter

Particles are sensed through their interactions with matter. To begin with, the chapter introduces the terms cross section and absorption. Then successively the most important interactions that are employed for the detection of the various particle types are discussed: energy loss of charged particles by ionisation and bremsstrahlung, multiple Coulomb scattering of charged particles, interactions of photons and hadrons with matter. The interactions leading to the development of electromagnetic and hadronic showers are treated in more detail in chapter 15 (Calorimeters), while energy loss by Cherenkov and transition radiation are discussed in chapters 11 and 12, respectively. When describing the interaction processes an attempt is made to address the theoretical background in a way that the derivations ought to be comprehensible.

Calorimeters

The determination of the energy of particles is called ‘calorimetry’ and the corresponding detectors are called calorimeters. The particle energy is deposited in a calorimeter through inelastic reactions leading to the formation of particle showers. The deposited energy is measured either through the charge generated by ionisation or through scintillation or Cherenkov light. Depending on the particle type initiating a shower one distinguishes electromagnetic calorimeters from hadronic calorimeters. In this chapter the formation of showers for both cases is explained and the corresponding construction principles are discussed. For hadron calorimeters special attention is given to the different response to electromagnetically and hadronically deposited energy and the possible compensation of invisible energy. This is followed by a description of typical implementations of electromagnetic and hadronic calorimeters as well as of systems combining both types. Special emphasis is given to the discussion of the energy resolution of the different detectors and detector systems.

Particle identification

The identity of a particle is fixed by its mass, lifetime and quantum numbers such as charge, spin, parity and flavour. A particle’s identity can be inferred by observing its interactions in matter, as for example the shower development of an electron or a photon, the specific energy loss of charged particles, the emission of radiation by a particle or the penetration capability of a muon. The mass of a particle can be determined by measurements of specific energy loss, time-of-flight or Cherenkov radiation when combined with a momentum measurement. High energy electrons can be separated from heavier particles through transition radiation. For particles which decay in the detector the mass can often be kinematically reconstructed from the decay products and the lifetime can be determined by the reconstruction of secondary vertices.

Scintillation detectors

The detection of scintillation light, which is generated when an ionising particle passes certain media or when radiation is absorbed, belongs to the oldest detection techniques. Scintillation detectors are read out electronically by employing the photon detectors described in a previous chapter. Scintillators are either made of organic or of inorganic materials (crystals) with essential differences of their properties and application field. For both scintillation mechanisms, the light yield and the time dependence of the signals are explained and the specific application areas pointed out. Typical assemblies of scintillation detectors are presented which include organic scintillators as trigger and timing counters, scintillating fibres for tracking and calorimetry and inorganic crystal arrangements for calorimetry.

Semiconductor detectors

Already since the early 1960s semiconductor detectors have been employed in nuclear physics, in particular for gamma ray energy measurement. This chapter concentrates on position sensitive semiconductor detectors which have been developed in particle physics since the 1980s and which feature position resolutions in the range of 50–100 μ‎m by structuring the electrodes, thus reaching the best position resolutions of electronic detectors. For the first time this made the electronic measurement of secondary vertices and therewith the lifetime of heavy fermions possible. The chapter first conveys the basics of semiconductor physics, of semiconductor and metal-semiconductor junctions used in electronics and detector applications as well as particle detection with semiconductor detectors. It follows the description of different detector types, like strip and pixel detectors, silicon drift chambers and charged-coupled devices. New developments are addressed in the sections on ‘Monolithic pixel detectors’ and on ‘Precision timing with silicon detectors’. In the last sections detector deterioration by radiation damage is described and an overview of other semiconductor detector materials but silicon is given.

Movement of charge carriers in electric and magnetic fields

For the detection of charged particles many detector principles exploit the ionisation in sensing layers and the collection of the generated charges by electrical fields on electrodes, from where the signals can be deduced. In gases and liquids the charge carriers are electrons and ions, in semiconductors they are electrons and holes. To describe the ordered and unordered movement of the charge carriers in electric and magnetic fields the Boltzmann transport equation is introduced and approximate solutions are derived. On the basis of the transport equation drift and diffusion are discussed, first in general and then for applications to gases and semiconductors. It turns out that, at least for the simple approximations, the treatment for both media is very similar, for example also for the description of the movement in magnetic fields (Lorentz angle and Hall effect) or of the critical energy (Nernst-Townsend-Einstein relation).

Overview, history and concepts

The progress in nuclear and particle physics is based on the development of detectors that allow us to observe particles and radiation. This chapter gives an historic overview of the development and the employment of detectors. It is pointed out how this led to scientific discoveries and how, on the other hand, the developments in other fields, in particular in electronics, widened the potential of today’s detectors. Examples of typical detector concepts for experiments in particle and astroparticle physics are given and applications in other areas are pointed out. In a short section the ‘natural units’ (ℏ = c = 1), often used in particle physics, are defined and relativistic particle kinematics is introduced. The chapter finishes with an overview of the content of the book.