The X-ray spectrometer

WLB found the general parsimony and the 'string and sealing wax' tradition of the Cavendish Laboratory frustrating. He records in The Development of X-ray Analysis(7) published posthumously in 1975 that: "I could never have exploited my ideas about X-ray diffraction under such circumstances". In comparison, his father's laboratory in Leeds was far better equipped. Of major importance was WHB's design and the construction, in the winter of 1912 - 1913, in the physics workshop, by the head mechanic Jenkinson, of a well-engineered spectrometer. The set-up is shown diagrammatically in figure 1.

The Braggs described the instrument as a spectrometer and measured variations in scattering angle arising either as a result of variations in wavelength or as a result of variations in interplanar spacings in crystals. In the latter sense we would now describe the instrument as a diffractometer.

The X-ray tube or 'bulb' is operated from an induction coil which provides an intermittent high voltage. The X-ray source (the anti-cathode or target) is a block of heavy metal (osmium, rhodium, palladium, platinum) which is 'bombarded' with the ions of residual gas in the tube. Maintaining the residual gas at an appropriate level (not too 'hard' not too 'soft') was a continual headache and remained so until the invention of the Coolidge tube in which the cathode was replaced by a heated filament providing a source of electrons.

The X-rays pass from the target through a hole in a lead screen and strike the crystal mounted on a graduated turntable. Reflected X-rays pass through a slit of variable width into an ionisation chamber which records their intensity, and which is also independently rotatable (there was no 'θ-2θ' coupling as there is in diffractometers today).