3.5.5.1. Gas-flow and Sealed Detectors

Gas-flow and sealed proportional detectors have identical designs except that in the first case the detector gas is constantly changed and replenished while in the latter the gas is static. Both detectors consist of a cathode tube with a thin (20-100 mm thick) W-wire anode running through the center of it (Figure 3.5.5.1a). A voltage of 1 to 2 keV is applied between wire and tube. Sealed-gas detectors were first developed to a high degree of reliability by H. Geiger and W. Muller in 1928.

Figure 3.5.5.1a. A gas-flow proportional counter (after Goldstein et al. 1981).

The detector tube has a Mylar (polyethylene tetrapthalate) or polypropylene window through which X-rays from the analyzing crystal enter the detector. Whatever the thickness of the window, it still absorbs a significant portion of the X-rays from the analyzing crystal (Figure 3.5.5.1b). For example, a 5.5 mm Mylar window absorbs 50% Al-Ka, 70% Mg-Ka, 85% Na-Ka and 98% F-Ka X-rays. Polypropylene absorbs about 60% less than Mylar and is routinely used for light element detectors. NAU's  MBX probe uses a 1 mm polypropylene windows for the light element spectrometer and 6 mm Mylar windows for the other spectrometers.  All MBX detectors have thicker (25 mm) Be windows in their backs through which high-energy X-rays may exit to prevent them producing secondary radiation from the detector itself.

Figure 3.5.5.1b. X-ray absorption properties of various materials used as window membranes for gas proportional counters as a function of energy. Elements indicate wavelengths of Ka lines (after Potts 1987).

In a gas-flow detector, a mixture of 90% argon and 10% methane (P-10) continuously flows through the detector. The detector tube is not just sealed with P-10 gas inside, because some gas escapes through the thin windows and must be constantly replenished. Detector gas must be ultra-pure to avoid reaction with the wire and a loss of detector sensitivity. Electronegative impurities like O⁼ and CO^- are especially bad. The efficiency of a gas-flow detector can be improved by increasing the pressure, which is why some probes have high and low pressure detectors.

Sealed detectors are filled with Xe or Kr gas and are used for higher energy X-rays since these gases are ionized more efficiently than Ar by these wavelengths. Since they are used for more energetic X-rays, the windows can be thicker to prevent loss of the gas by diffusion. A Be or Al window about 25 mm thick is commonly used because the high-energy X-rays will gradually destroy Mylar or polypropylene windows.