Section 7.1.4.1. Scintillation counters

The most frequently used detector is the scintillation counter (Parrish & Kohler, 1956). It has two elements: a fluorescent crystal and a photomultiplier tube, Fig. 7.1.2.1(c). For X-ray diffraction, a cleaved single-crystal plate of optically clear NaI activated with about 1% Tl in solid solution is used. The crystal is hygroscopic and is hermetically sealed in a holder with thin Be entrance window and glass back to transmit the visible-light scintillations. The size and shape of the crystal can be selected, but is usually a 2 cm diameter disc or a rectangle 20 × 4 × 1 mm thick. A small thin crystal has been used to reduce the background from radioactive samples (Kohler & Parrish, 1955). A viscous mounting fluid with about the same refractive index as the glass is used to reduce light reflection and to attach it to the end of the photomultiplier tube. The crystal and photomultiplier are mounted in a light-tight cylinder surrounded by an antimagnetic foil. The high X-ray absorption of the crystal provides a high quantum-counting efficiency.

A Cu Kα quantum produces about 500 visible photons of average wavelength 4100 Å in the scintillation crystal (which matches the maximum spectral sensitivity of the photomultiplier), but only about 25 will be effective in the photomultiplier operation. High-speed versions with special pulse-height analysers have recently become available; they are linear to about 1% at 10⁵ counts s⁻¹ and can be used at rates approaching 10⁶ counts s⁻¹ (see Rigaku Corporation, 1990).

The detector system is as described in Subsection 7.1.3.1.