Building Scientific Apparatus provides an overview of the physical principles that one must grasp to make useful and creative decisions in the design of scientific apparatus. We also describe skills, such as mechanical drawing, circuit analysis, and optical ray-tracing and matrix methods that are required to design an instrument. A large part of the text is devoted to components. For each class of components – electrical, optical, thermal and so on – the parameters used by manufacturers to specify their products are defined. Useful materials and components such as infrared detectors, metal alloys, optical materials, and operational amplifiers are discussed, and examples and performance specifications are given. Of course, having designed an apparatus and chosen the necessary components, one must build it. We deal in considerable detail with basic laboratory skills: soldering electrical components, glassblowing, brazing, polishing, and so on. Described in lesser detail are operations such as lathe turning, milling, casting, laser cutting, and printed-circuit production, which one might let out to an outside shop. Understanding the capabilities and limitations of shop processes is necessary to fully exploit them in designing and building an instrument. Overall, we recognize that there are many engineering and technical texts that cover every aspect of instrument design; our goal in Building Scientific Apparatus has been to winnow the available information down to the essentials required for practical work by the designer and builder of scientific instruments.

Physical, chemical, and biological phenomena are regularly studied or induced optically. Such experiments can involve light absorption, light emission, or light scattering. We can characterize any experimental arrangement where light is used, produced, measured, modified, or detected as an overall optical system. Any such optical system will always be reducible to three parts: a source of light, a detector of that light, and everything in between. We will frequently refer to this important and varied intermediary arrangement as the optical system. Consequently, our discussion of overall optical system design and construction will involve three key topics: sources, optical systems, and detectors (to be discussed in detail inChapter 7). The light source may be a laser, lamp, light-emitting diode, or the Sun. The detector may be a vacuum tube, solid-state device, or even the eye. Light intensity may vary from continuous wave (CW) to pulsed, and these pulses may have durations as short as a few femtoseconds. Passive elements in the system may transmit, reflect, combine, polarize, or separate light according to its spectral content. Nonlinear optical elements change the spectral content of light.

It is our aim in this chapter to explain the basic concepts that need to be understood by the experimentalist who uses optical techniques. In addition, we will provide examples of useful techniques for producing, controlling, analyzing, and modulating light.

OPTICAL TERMINOLOGY

Light is one form of electromagnetic radiation, the many categories of which make up the electromagnetic spectrum.

Fifty years ago devices employing charged-particle beams were confined to the purview of a small group of physicists studying elementary processes. Today chemists, biologists, and engineers employ beams of ions or electrons to probe various materials and to investigate discrete processes. Physicists are constructing beam machines to control the momentum of interacting particles with energies from a few tenths of an electron volt to trillions of electron volts. Chemists routinely use mass spectrometers as analytical tools and various electron spectrometers to probe molecular structures. The electron microscope is one of the primary tools of the modern biologist. Furthermore charged-particle beam technology has spread to industry, where electron-beam machines are used for cleaning surfaces and welding, and ion-beam devices are used in the preparation of semiconductors.

The properties of charged-particle beams are analogous in many respects to those of photon beams: hence the appellation charged-particle optics. In the following sections the laws of geometrical optics will be covered insofar as they apply to charged-particle beams. The consequences of the coulombic interaction of charged particles will be considered. In addition we shall discuss the design of electron and ion sources, as well as the design of electrodes that constitute optical elements for manipulating beams of charged particles. We shall consider primarily electrostatic focusing by elements of cylindrical symmetry and restrict discussion to particles of sufficiently low kinetic energies that relativistic effects can be ignored.

Glass has been called the miraculous material. The ubiquity of glass in the modern laboratory certainly confirms this. Because glass is chemically inert, most containers are made of it. Glass is transparent to many forms of radiation, and its transmission properties can be varied by controlling its composition; all sorts of windows and lenses are made of glass. Because glass can be polished to a high degree and is dimensionally stable, most mirrors are supported on glass surfaces. Glass is strong and stiff, and is often used as a structural material. Considering its mechanical rigidity and density, it is a reasonably good thermal insulator. It is an excellent electrical insulator. Perhaps the greatest virtue of this material is that many glasses are inexpensive and can be cut and shaped in the laboratory with inexpensive tools.

Fifty years ago, most glass laboratory apparatus was produced by the scientist or technician in situ by blowing molten glass or by grinding, cutting, and polishing hard glass. Today the glass industry has grown to such an extent that nearly all components of a glass apparatus are available from commercial sources at low cost. These include all sorts of containers, chemical labware, vacuum-system components, mirrors, windows, and lenses. It is often only necessary for laboratory scientists to acquaint themselves with the range of components available and to acquire the skills needed to assemble an apparatus from these components.

Every scientific apparatus requires a mechanical structure, even a device that is fundamentally electronic or optical in nature. The design of this structure determines to a large extent the usefulness of the apparatus. It follows that a successful scientist must acquire many of the skills of the mechanical engineer in order to proceed rapidly with an experimental investigation.

The designer of research apparatus must strike a balance between the makeshift and the permanent. Too little initial consideration of the expected performance of a machine may frustrate all attempts to get data. Too much time spent planning can also be an error, since the performance of a research apparatus is not entirely predictable. A new machine must be built and operated before all the shortcomings in its design are apparent.

The function of a machine should be specified in some detail before design work begins. One must be realistic in specifying the job of a particular device. The introduction of too much flexibility can hamper a machine in the performance of its primary function. On the other hand, it may be useful to allow space in an initial design for anticipated modifications. Problems of assembly and disassembly should be considered at the outset, since research equipment rarely functions properly at first and often must be taken apart and reassembled repeatedly.

Make a habit of studying the design and operation of machines. Learn to visualize in three dimensions the size and positions of the parts of an instrument in relation to one another.

Phenomena that are studied experimentally often manifest themselves as sources of electromagnetic radiation or particles. To be useful, the radiation or particles that are involved in the experiment must be detected. In this chapter we will consider the operating characteristics, selection criteria, and performance of various types of radiation and particle detectors. We will focus primarily on detectors of optical radiation, from X-rays to far infrared, and on charged-particle detectors. For a discussion of detectors of high-energy photons, such as γ rays, and elementary particles, such as neutrons, neutrinos, and the many particles involved in high-energy nuclear physics, we refer the reader to the more specialized treatises on these subjects.–

OPTICAL DETECTORS

Optical detectors of various kinds detect electromagnetic radiation from the X-ray to the far-infrared region of the electromagnetic spectrum. In common with other branches of electronics, the development of optical detectors has occurred by a series of advances through the use of gas-filled tubes and vacuum tubes to various semiconductor devices. Gas-filled and vacuum-tube devices still retain some niche electronics applications, such as high-voltage switching, microwave power amplification, and specialized audio. Gas-filled photodetectors, which offer some degree of signal amplification in low-cost consumer applications, have essentially disappeared. Vacuum-tube photodetectors, especially photomultiplier tubes, remain in wide-spread use for detection of radiation below about 1 μm. This is especially true for low-light-level signal detection (photon counting), and in applications where a large detector area is required (as in scintillation counters).

There are two levels of concern with temperature in scientific experiments. One is the control of temperature to achieve some secondary (but essential) aim in the apparatus. Examples are the use of a cold trap in a vacuum line, and the use of heaters and coolants in a distillation column. For such needs, the temperature needs only to be known and kept constant to within a few kelvins. In the second case, the measurement of the dependence of physical parameters on temperature is a primary aim of the experiment. A physicist learns about the nature of a material by measuring such properties as density or heat capacity as a function of temperature. An organic chemist studies the kinetics of a chemical reaction by measuring its rate of reaction as a function of temperature. For these experiments, the temperature must be varied over a range and controlled at any point in that range, at resolutions better than 1 K.

Sometimes the temperature must be known accurately. That is, the measurement must be closely calibrated to the International Temperature Scale. Accurate measurements are necessary if the new data are to be used with other measurements on the system under study. If measurements of the density of water are to be combined with measurements of its kinematic viscosity to calculate its shear viscosity as a function of temperature, then the temperature must be measured with the same accuracy in both the density measurements and the kinematic viscosity measurements.

This chapter discusses electronics at a level somewhere between that of a handbook, which consists essentially of charts, tables, and graphs, and a textbook, where the interesting, important, and useful conclusions come only after well-developed discussions with examples. The aim here is a presentation that has sufficient continuity and readability that individual sections can be profitably read without having to refer to preceding sections or other texts. On the other hand, it is important to have useful and frequently referenced material in the form of readily accessible tables, graphs, and diagrams that are sufficiently self-explanatory that very little reference to the text material is necessary. Another important goal is vocabulary. A large amount of jargon in electronics is meaningless to the uninitiated, but when it is necessary to understand the properties of an electronic device from a written technical description, when writing the specifications for electronic equipment, or when talking to an electronics engineer, salesman, or technician, this vocabulary is essential. With this in mind, terms not current outside of electronics are italicized.

To be used to best advantage, this chapter should be supplemented with manufacturers' catalogs, data books, applications texts, handbooks, and more specialized texts that treat the topic of interest in depth. Manufacturers of laboratory electronic equipment, discrete devices, and integrated circuits have publications that describe, in clear practical terms, the properties of their products and their applications to a wide variety of tasks.

In the modern laboratory, there are many occasions when a gas-filled container must be emptied. Evacuation may simply be the first step in creating a new gaseous environment. In a distillation process, there may be a continuing requirement to remove gas as it evolves. Often it is necessary to evacuate a container to prevent air from contaminating a clean surface or interfering with a chemical reaction. Beams of atomic particles must be handled in vacuo to prevent loss of momentum through collisions with air molecules. Many forms of radiation are absorbed by air and thus can propagate over large distances only in a vacuum. A vacuum system is an essential part of laboratory instruments such as the mass spectrometer and the electron microscope. Far infrared and far ultraviolet spectrometers are operated within vacuum containers. Simple vacuum systems are used for vacuum dehydration and freeze-drying. Nuclear particle accelerators and thermonuclear devices require huge, sophisticated vacuum systems. Many modern industrial processes, most notably semiconductor device fabrication, require carefully controlled vacuum environments.

GASES

The pressure and composition of residual gases in a vacuum system vary considerably with its design and history. For some applications a residual gas density of tens of billions of molecules per cubic centimeter is tolerable. In other cases no more than a few hundred thousand molecules per cubic centimeter constitutes an acceptable vacuum: “One man's vacuum is another man's sewer”.

Introduction

How did the first stars in the universe form, how did they evolve and die and what was their impact on cosmic history (Woosley et al. 2002; Bromm & Larson 2004; Ciardi & Ferrara 2005)? The first stars formed at the end of the cosmic dark ages beyond the current horizon of observability (Couchman & Rees 1986; Haiman et al. 1996; Tegmark et al. 1997). These so-called Population III (Pop III) stars ionized (Kitayama et al. 2004; Whalen et al. 2004; Alvarez et al. 2006; Johnson et al. 2007) and metal-enriched (Furlanetto & Loeb 2003; Tornatore et al. 2007) the intergalactic medium (IGM) and consequently had important effects on subsequent galaxy formation (Barkana & Loeb 2001; Mackey et al. 2003).