Introduction

This chapter describes the use in X-ray astronomy of semiconductor ionisation detectors as non-dispersive spectrometers of high energy resolution. Semiconductor detectors which operate on a calorimeteric principle are described separately, in Chapter 6.

The early history of semiconductor radiation detectors is comprehensively described by McKenzie (1979). The first practical ‘solid state’ detectors – the term is usually taken to exclude scintillation counters (Chapter 5) – were small germanium surface barrier devices with gold electrodes (section 4.2.1), fabricated in the late 1950s. Such devices could be regarded as solid state analogues of the gas-filled ionisation chamber (Wilkinson, 1950), in that the primary ionisation produced in the dielectric and collected, without multiplication, by its internal electric field, was found to be proportional to the energy of an incident particle.

Over the past 30 years, improvements in material purity (Eichinger, 1987) and advances in microelectronic process technology have given rise to an array of detector types based on electron-hole pair creation in cooled silicon or germanium or in a number of ‘room temperature’ materials, of which the most developed is currently mercuric iodide. As in the case of gas-filled detectors (Chapter 2) much of the impetus for the new semiconductor detectors comes from the particle physics community (Kemmer and Lutz, 1987). Trends in modern particle physics include the development of both large (1 m2) silicon detector arrays (Borer et al. 1987) and of integrated detectors in which some at least of the signal processing is embodied ‘on chip’.

In X-ray astronomy, however, solid state detector research is still driven primarily by the desire for high spectral resolution.

The purpose of this monograph is to provide a bridge between “two cultures.” On one side are mathematically inclined relativists such as J. M. Stewart (Stewart, 1971) and J. Ehlers (Ehlers, 1971), who have constructed the formal mathematics for kinetic theory in general relativity and on the other, the practical cosmologists who are concerned with physically interesting results obtained with a minimum of formalism. A partial bridge between these communities was erected by Weinberg (Weinberg, 1971, 1972). However, Weinberg's discussion focuses on macrophysics, ignoring for the most part the underlying microphysics. Our discussion will emphasize the microphysics. We will therefore be concerned with the solutions of the Boltzmann equations in the expanding universe for various cosmological processes. We will derive these equations from first principles and solve them as explicitly as possible. Many of the results will be familiar to practicing cosmologists but are presented here with more pedagogical rigor than is generally found in the literature. The treatment is meant to be essentially self-contained, but someone not familiar with the cosmological implications of general relativity would be advised to consult a book like that of Weinberg (Weinberg, 1972).

Now, by way of acknowledgment: My interest in this subject was first aroused by a series of informal lectures given by my Stevens Institute colleague J. L. Anderson (Anderson, 1970). I recall how surprised I was to learn that a Robertson–Walker universe did not, in general, admit equilibrium solutions to the Boltzmann equations (details are given later in the book).