Materials Research Activities

A short history of x-rays: sources and detectors

A short history of x-rays

by Arne Hessenbruch

Controlling and quantifying: Sources and detectors

Before WWII

As radiotherapy boomed in the 1920s, so did the development of x-ray tubes. It was of great importance to control the tube's x-ray output, and the traditional x-ray tube was fickle. Basically, an x-ray tube consists of two electrodes in a partial vacuum. When a sufficiently high voltage is applied, electrons will jump from one electrode (cathode) to the other (anti-cathode). When they impact upon the second electrode, they generate x-rays. Not all electrodes arrive with the same speed, for instance because they are slowed down by collisions with gas atoms on the way, and hence x-rays of differing wavelength will be produced. Changing the degree of vacuum will have an impact but a complex and seemingly idiosyncratic one. Much research was done on improving the materials of the electrodes, vacuum pumps, and high-voltage sources, but the most important breakthrough came with the insight, by Coolidge and Lilienfeld, that the production of x-rays was controllable by heating the cathode instead of varying the vacuum. This rendered the x-ray quality much more stable [23] .

Radiotherapy also drove the development of radiation detectors. Röntgen's original method of detection had been photography and this technique remained important because it was widely available. In the 1900s and the 1910s several rival techniques of chemical colouration evolved which were easier to use because no development was required. The discolouration of pastilles left on a body receiving radiotherapy would by comparison with a colour chart give a measure of the dose applied. Such techniques were good enough as long as the precision required was not great. The rational radiotherapy of the 1920s required greater precision, however, driving the development of instrumentation that required no judgment by the human eye. The instrument eventually chosen was an elaboration of that with which Ernest Rutherford and the Curies had conducted their experiments in radioactivity [24] . The idea was that the radiation ionized air in a chamber and the ions were counted (by measuring the current they produced across an electric potential). However, much depended on the size of the chamber and the material of the walls, and also on the relative positions of x-ray tube and chamber, and it was not until 1928 that an international x-ray intensity standard was generally accepted, a standard that specified the behaviour required to achieve the requisite precision. For the first time, x-ray researchers had confidence that numbers could be compared between labs. [25]

After WWII

In a synchrotron, a beam of electrons is accelerated almost to the speed of light within an evacuated doughnut-shaped ring and maintained by means of external magnets. In 1947, it was observed that the electrons lose energy by emitting x-rays tangentially to their orbit. At first, researchers were focused on the electrons, and the x-rays appeared to be just an annoying energy loss. But when it became clear that the x-ray spectrum was continuous with intensities 100000 times those from x-ray tubes, x-ray researchers began to take notice. At first, they worked parasitically alongside high-energy physicists at the synchrotrons and since 1980, many synchrotrons have been built specifically as x-ray sources. Synchrotron storage rings are not exactly cheap; they cost between £20 million and £500 million but nonetheless there are now more than 1000 operating worldwide (including those for high-energy physics).

Although synchrotron x-ray research has grown out of the big science culture of large accelerators, the type of science undertaken at storage rings is actually little science, with relatively small groups of researchers tapping the source at a unique place, each group with its own beamline leading tangentially from the storage ring. One form of such little science research is high-resolution imaging of thicker specimens (e.g. biological). For such work, synchrotron x-ray analysis outperforms the electron microscope which requires careful thinning of the specimen preparation because of electron scattering in matter. Much research across the old boundaries of physics, chemistry and biology takes place at the end of such beamlines. Indeed, three-dimensional structural information of complex biological macromolecules underpins much of molecular biology [26] .

 

New x-ray detectors were developed after World War II: proportional and scintillation counters and their associated electronics. The solid-state counters were cheap and had high collection efficiency and no need for moving parts. The smaller counters have enabled a couple of things: portability (e.g. x-ray spectrographs by the 1980s); new analytical procedures such as indirect excitation of x-ray spectra (x-ray fluorescence analysis) of bulk samples; and ultimately combinations of instruments in a small space [27] .


[23] Ruth and Edward Brecher, R. & E. (1969), The Rays: A History of Radiology in the United States and Canada, Baltimore: Williams and Wilkins.

[24] Hessenbruch, A. (2000), "Rutherford's 1901 experiment on radiation energy and his creation of a stable detector", Archives for the History of the Exact Sciences, 54, 403-420.

[25] Hessenbruch, A. (1995), "The origins of exact x-ray dosage", in Från moderna helgonkulter till självmord - Föredrag från Idé- och vetenskapshistorisk konferens 1995, edited by Thomas Kaiserfeld, Stockholm: KTH, 63-68, and "Die PTR und der Standard der Röntgenstrahlenintensität", in Emergence of modern physics, edited by Dieter Hoffmann, Fabio Bevilacqua, Roger H. Stuewer, Pavia: Università degli Studi di Pavia, 1996, 81-87.

[26] Munro, I. (1996), "Synchrotron Radiation", in X-RAYS: The First Hundred Years, edited by Alan Michette and Slawka Pfauntsch, Chichester: John Wiley & Sons, 131-154.

[27] Long, J. (1996), "X-ray Microanalysis", in ibid., 61-100.

This page was last updated on 28 October 2002 by Arne Hessenbruch.