In the recombination region (Region I), as voltage increases to V₁, the pulse height increases until it reaches a saturation value. At V₁, the field strength between the cathode and anode is sufficient for collection of all ions produced within the detector. At voltages less than V₁, ions move slowly toward the electrodes, and the ions tend to recombine to form neutral atoms or molecules. In this case, the pulse height is less than it would have been if all the ions originally formed reached the electrodes. Gas ionization instruments are, therefore, not operated in this region of response.

Ionization Region

As voltage is increased in the ionization region (Region II), there is no appreciable increase in the pulse height. The field strength is more than adequate to ensure collection of all ions produced; however, it is insufficient to cause any increase in ion pairs due to gas amplification. This region is called the ionization chamber region.

Proportional Region

As voltage increases to the proportional region (Region III), the pulse height increases smoothly. The voltage is sufficient to produce a large potential gradient near the anode, and it imparts a very high velocity to the electrons produced through ionization of the gas by charged radiation particles. The velocity of these electrons is sufficient to cause ionization of other atoms or molecules in the gas. This multiplication of electrons is called gas amplification and is referred to as Townsend avalanche. The gas amplification factor (A) varies from 10³ to 10⁴. This region is called the proportional region since the gas amplification factor (A) is proportional to applied voltage.

Limited Proportional Region

In the limited proportional region (Region IV), as voltage increases, additional processes occur leading to increased ionization. The strong field causes increased electron velocity, which results in excited states of higher energies capable of releasing more electrons from the cathode. These events cause the Townsend avalanche to spread along the anode. The positive ions remain near where they were originated and reduce the electric field to a point where further avalanches are impossible. For this reason, Region IV is called the limited proportional region, and it is not used for detector operation.

Geiger-Miiller Region

The pulse height in the Geiger-Miiller region (Region V) is independent of the type of radiation causing the initial ionizations. The pulse height obtained is on the order of several volts. The field strength is so great that the discharge, once ignited, continues to spread until amplification cannot occur, due to a dense positive ion sheath surrounding the central wire (anode). V₄ is termed the threshold voltage. This is where the number of ion pairs level off and remain relatively independent of the applied voltage. This leveling off is called the Geiger plateau which extends over a region of 200 to 300 volts. The threshold is normally about 1000 volts. In the G-M region, the gas amplification factor (A) depends on the specific ionization of the radiation to be detected.

Continuous Discharge Region

In the continuous discharge region (Region VI), a steady discharge current flows. The applied voltage is so high that, once ionization takes place in the gas, there is a continuous discharge of electricity, so that the detector cannot be used for radiation detection.

Radiation detectors are normally designed to respond to a certain type of radiation. Since the detector response can be sensitive to both energy and intensity of the radiation, each type of detector has defined operating limits based on the characteristics of the radiation to be measured. A large variety of detectors are in use in DOE facilities to detect alpha and beta particles, gamma rays, or neutrons. Some types of detectors are capable of distinguishing between the types of radiation; others are not. Some detectors only count the number of particles that enter the detector, while others are used to determine both the number and energy of the incident particles. Most detectors used in DOE facilities have one thing in common: they respond only to electrons produced in the detector. In order to detect the different types of incident particles, the particle's energy must be converted to electrons in the detector.

Gas-filled detectors are used, for the most part, to measure alpha and beta particles, neutrons, and gamma rays. The detectors operate in the ionization, proportional, and G-M regions with an arrangement most sensitive to the type of radiation being measured. Neutron detectors utilize ionization chambers or proportional counters of appropriate design. Compensated ion chambers, BF₃ counters, fission counters, and proton recoil counters are examples of neutron detectors.

Summary

The alpha curve is higher than the beta curve from Region I to part of Region IV due to the larger number of ion pairs produced by the initial reaction of the incident radiation. Detector voltage principles are summarized below.

Gas Amplification Region Summary

Recombination Region

The voltage is such a low value that recombination takes place before most of the negative ions are collected by the electrode.

Ionization Region

The voltage is sufficient to ensure all ion pairs produced by the incident radiation are collected.

No gas amplification takes place.

Proportional Region

The voltage is sufficient to ensure all ion pairs produced by the incident radiation are collected.

Amount of gas amplification is proportional to the applied voltage.

Limited Proportional Region

As voltage increases, additional processes occur leading to increased ionizations. Since positive ions remain near their point of origin, further avalanches are impossible.

Geiger-Muller Region

The ion pair production is independent of the radiation, causing the initial ionization. The field strength is so great that the discharge continues to spread until amplification cannot occur, due to a dense positive ion sheath surrounding the central wire.

Continuous Discharge Region

The applied voltage is so high that, once ionization takes place, there is a continuous discharge of electricity.