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Los Alamos National Laboratory Research Quarterly, Fall 2002
A Modular Neutron Detector
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Gas-Filled Radiation Detectors

A gas-filled radiation detector is usually a glass tube that contains two concentric electrodes and a gas such as argon. The outer electrode is a metal tube; the inner electrode is a wire stretched between the ends of the tube along its axis. Energetic charged particles, x-rays, or gamma rays entering the detector strip electrons from the gas atoms to produce positively charged ions and negatively charged electrons. An electric field created by several hundred volts or more across the electrodes draws the ions to the negative electrode and the electrons to the positive electrode. The electron flow produces a current pulse, which is the detection signal. The charge produced by a 1-million-electronvolt charged particle coming to rest in the gas is about 5 femtocoulombs.

The magnitude of the electric field determines the detector's mode of operation. In order of increasing electric field, the detector operates as an ionization chamber, a proportional counter, or a Geiger-Müller tube. The modular neutron detector operates as an ionization chamber.

The field in an ionization chamber is high enough to prevent the electron-ion pairs produced by the radiation from recombining but too low for the electrons to produce additional electron-ion pairs in collisions with gas atoms. Because the current pulse produced by each radiation packet is proportional to the energy the packet ultimately deposits in the detection gas, the detector can measure the distribution of the deposited energies. This feature allows a neutron detector to discriminate between neutrons and background gamma rays. (Most of the background radiation comes from gamma-ray-emitting radioactive isotopes in the environment, such as potassium-40, uranium-238, and thorium.) However, the current pulse is so weak that it must be amplified electronically.

In a proportional counter, the field is high enough for the electrons to produce additional electron-ion pairs—an amplifying process called gas multiplication. A proportional counter requires only minimal electrical amplification. However, the field is still small enough to preserve the proportionality between the current pulse's amplitude and the energy deposited by the radiation packet. Thus, a proportional counter can also discriminate between neutrons and background gamma rays. The amplification factor of a proportional counter increases as the field increases.

At an even higher field, however, a gas-filled detector's proportionality is destroyed, and the amplitude of the detection signal is constant no matter what the deposited energy. A radiation detector operating in this mode is a Geiger-Müller tube, which needs little or no electrical amplification but cannot be used to measure the deposited energies. This tube is the heart of the Geiger counter.

 

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