Back to " HEALTH PHYSICS INSTRUMENTS "

Gas-Filled Detectors

A gas-filled detector is basically a metal chamber filled with gas and containing a positively biased anode wire. A photon passing through the gas produces free electrons and positive ions. The electrons are attracted to the anode wire and collected to produce an electric pulse.

At low anode voltages, the electrons may recombine with the ions. Recombination may also occur for a high density of ions. At a sufficiently high voltage nearly all electrons are collected, and the detector is known as an ionization chamber. At higher voltages the electrons are accelerated toward the anode at energies high enough to ionize other atoms, thus creating a larger number of electrons. This detector is known as a proportional counter. At higher voltages the electron multiplication is even greater, and the number of electrons collected is independent of the initial ionization. This detector is the Geiger-Müller counter, in which the large output pulse is the same for all photons. At still higher voltages continuous discharge occurs.

 

The different voltage regions are indicated schematically in Figure 1.3. The actual voltages can vary widely from one detector to the next, depending upon the detector geometry and the gas type and pressure.         Figure 1.3: Gas Detector Output vs. Anode Voltage

 

Ionization Chamber

The very low signal output for the ionization chamber makes this detector difficult to use for detecting individual gamma rays. It finds use in high radiation fluxes in which the total current produced can be very large. Many radiation monitoring instruments use ionization chambers. Absolute ionization measurements can be made, using an electrometer for recording the output.¹

¹A.C. Melissinos, Experiments in Modern Physics, Academic Press, New York (1966), p. 178.]

Proportional Counter

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Proportional counters can be purchased in different sizes and shapes, ranging from cylindrical with end or side windows to "pancake" flat cylinders. They may be sealed detectors or operate with gas flow, and may have thin beryllium windows or be windowless. A detector is typically specified in terms of its physical size, effective window size and gas path length, operating voltage range and resolution for the 5.9 keV x ray from a ⁵⁵Fe source (Mn x ray). Typical resolutions are about 16 to 20% full-width at half maximum (FWHM).

 

Operating voltages depend upon the fill gas as well as the geometry. For x rays, noble gases are often used, with xenon, krypton, neon and argon common choices. Xenon and krypton are selected for higher energy x rays or to get higher efficiencies, while Neon is selected when it is desired to detect low energy x rays in the presence of unwanted higher energy x rays. Sometimes gas mixtures are used, such as P-10 gas, which is a mixture of 90% argon and 10% methane.

Geiger-Müller Counter

The Geiger-Müller counter produces a large voltage pulse that is easily counted without further amplification. No energy measurements are possible since the output pulse height is independent of initial ionization. Geiger-Müller counters are available in a wide variety of sizes, generally with a thin mica window. The operating voltage is in the plateau region (see Figure 1.3), which can be relatively flat over a range of bias voltage. The plateau is determined by measuring the counting rate as a function of the anode voltage.

The discharge produced by an ionization must be quenched in order for the detector to be returned to a neutral ionization state for the next pulse. This is accomplished by using a fill gas that contains a small amount of halogen in addition to a noble gas. The voltage drop across a large resistor between the anode and bias supply will also serve to quench the discharge since the operating voltage will be reduced below the plateau.

The Geiger-Müller counter is inactive or "dead" after each pulse until the quenching is complete. This dead time can be hundreds of microseconds long, which limits the counter to low count rate applications.