Proportional Counters 

General

While proportional counters are most commonly used for quantifying alpha and beta activity, they are also used for neutron detection, and to some extent for x-ray spectroscopy.

The pulses produced by a proportional counter are larger than those produced by an ion chamber. This means that the proportional counter is more conveniently operated in the pulse mode (ion chambers usually operate in the current mode).

Unlike the situation in a GM detector, the pulse size reflects the energy deposited by the incident radiation in the detector gas. As such, it is possible to distinguish the larger pulses produced by alpha particles from the smaller pulses produced by betas or gamma rays.

General Types of Proportional Counters

                                  1. Gas Flow Proportional    -  with window (e.g., laboratory alpha-beta counters)

                                                                             -  windowless  (e.g., tritium measurements)

                                   2. Air Proportional (alpha counting only)

                                   3. Sealed proportional  (e.g., BF3, He-3 neutron detectors)

The Size of the Pulse 

The size of the pulse in a proportional counter depends on  two things:

1. Operating Voltage. The higher the operating voltage, the larger each avalanche becomes and the larger the pulse.

2. Energy Deposited in Detector Gas.  The greater the energy deposited in the detector gas by an incident particle of radiation, the larger the number of primary ion pairs, the larger the number of avalanches, and the larger the pulse.

The following diagram shows a charged particle traversing the detector gas. Four primary ion pairs (and four resulting avalanches) are produced. It is usually the case that many more ion pairs are produced by incident radiation than the four shown here. Keep in mind that the four avalanches contribute to a single pulse.

Townsend Avalanche

In a proportional counter, many electrons (10 - 10,000) reach the anode for each primary ion pair produced in the gas. The reason is that the electron of each primary ion pair creates further "secondary" ion pairs as it gets close to the anode. These secondary ion pairs are produced in what is called an avalanche.

 

Fill Gas

In general, the proportional gas should not contain electronegative components such as oxygen. Otherwise, electrons heading towards the anode will combine with the electronegative gas. If this happens, a negative ion goes to the anode rather than an electron, and unlike the electron, the negative ion will fail to produce an avalanche. The result is that the pulse is probably too small to exceed the threshold setting and be counted.

Despite the above, air is sometimes used as a proportional gas for alpha counting. Air could not serve as a proportional gas for beta detection because beta particles produce far fewer ion pairs in the gas than alphas. Since more electrons travel towards the anode following an alpha interaction in the gas, there is a greater chance that some of them will avoid interacting with oxygen and produce an avalanche. Also, if the detector is designed so that the electrons donít have far to travel to the anode, there is less chance that they will interact with the oxygen. Using air as the proportional gas allows the use of a thin window without the need for a gas flow system. However, it is essential that the air be dry. In high humidity conditions, air proportional counters are prone to generating spurious pulses. 

The fill gas in a proportional counter (and a GM detector) is usually a noble gas because noble gases are not electronegative and don't react chemically with the detector components. Of the noble gases, argon is the most widely used because of its low cost. Other noble gases with higher atomic numbers (e.g., krypton and xenon) might be used if increased sensitivity to x-rays or gamma rays is required.

Hydrocarbon gases (e.g., methane, propane and ethylene) can also serve as a fill gas, but they have the disadvantage of being flammable.

For certain applications in dosimetry, it is desirable that a detector have the same type of response as human tissue to radiation. To accomplish this a tissue equivalent gas mixture such as the following might be used: 64.4% methane, 32.4% carbon dioxide and 3.2% nitrogen.

He-3 and BF3 are the most commonly employed gases in neutron detectors. These gases serve a dual purpose. First, thermal neutrons undergo nuclear reactions with the He-3 or BF3 to produce charged particles. Second, the charged particles ionize the He-3 or BF3 to produce pulses. The interactions between the thermal neutrons and the gas are:

                                           n   +   He-3    ˇ    H-3   +   p+

                                           n   +   B-10    ˇ    Li-3   +   α+

Pure noble gases can be used for alpha counting at low voltages where the multiplication factor is below 100. As a rule however, a quench gas is added to prevent the proportional counter from acting like a geiger muller detector. During the formation of an avalanche, some gas molecules/atoms are excited rather than ionized. In other words, the energy absorbed by these proportional gas atoms/molecules promotes electrons to higher energy levels rather than frees them completely from the atoms/molecules. When the electrons deexcite and return to their original energy levels, they emit photons of visible light or UV. The problem with this is that these photons can interact with the proportional gas and cause the avalanche to spread along the anode. This can result in a non-linear relationship between the energy deposited in the detector gas and the size of the resulting pulse. These photons, particularly if they interact with the cathode wall, can also lead to the production of spurious pulses. The solution is to add a small amount of a polyatomic quench gas such as methane. The quench gas preferentially absorbs the photons, but unlike the fill gas (e.g., argon), it does so without becoming ionized.

P-10 gas, developed by John Simpson in the 1940s, is the most widely employed gas for gas flow proportional counters. It is a mix of 90% argon and 10% methane. 

Proportional Counters                   Museum Directory

Last updated: 07/25/07
Copyright 1999, Oak Ridge Associated Universities