|1.13.04||Identify the parameters that affect the number of ion pairs collected in a gas-filled detector.|
GAS FILLED DETECTORS
Figure 2. Basic Gas-filled Detector
Any contained gas volume that has a pair of electrodes can serve as a gas filled ionization detector. The detector can be almost any shape or size but is usually cylindrical. The cylinder walls are usually used as one electrode and an axial wire mounted in the center is used as the other electrode. Insulators support the axial electrode. It should be noted that the size, shape, and configuration is a function of the desired detector characteristics. (See Figure 2)
The gas used in the detector can be almost any gaseous mixture that will ionize, including air. Some ionization detectors, particularly ionization chambers use only air, while other detectors use gas mixtures that ionize more readily to obtain the desired detector response.
A gaseous mixture in a normal undisturbed state has positive and negative charges which are balanced such that no net charge is observed. When a particle or ray interacts with the gas atoms or molecules (and in some gases, the detector materials), energy is added to the gas and one or more electrons may be split off of the parent atom or molecule. The most common process results in a single negatively charged electron, leaving behind a positively charged atom. Together the negative electron and positive atom (minus one electron) are called an ion-pair.
If left undisturbed, the negative ions can be collected by a positive ion and return to a neutral state.
If a voltage potential is established across the two electrodes, electric fields are set up in the gas volume between the electrons. In most detectors, the center electrode is positively charged, and the shell of the detector is negatively charged. If an ion pair is created between the electrodes, the electron will be attracted to the center electrode, while the positively charged ion will be attracted to the detector shell. When either ion reaches the electrode, electric currents are set up. Because of mass differences, the electron reaches the electrode first. It takes up to 1,000 times longer for the positive ion to reach the side.
The amount of current flow is representative of the energy and number of radiation events that caused ionization. The readout circuitry analyzes this current and provides an indication of the amount of radiation that has been detected.
Ion Pair Production
For a gas filled ionization detector to be of value for radiological control purposes, the manner in which the response varies as a function of the energy, quantity, and type of radiation must be known. Factors such as the size and shape of the detector, the pressure and composition of the gas, the size of the voltage potential across the electrodes, the material of construction, the type of radiation, the quantity of radiation, and the energy of the radiation can all affect the response of the detector. Detectors for a special purpose are designed to incorporate the optimum characteristics necessary to obtain the desired response.
Type of Radiation
Each type of radiation has a specific probability of interaction with the detector media. This probability varies with the energy of the incident radiation and the characteristics of the detector gas. The probability of interaction is expressed in terms of specific ionization with units of ion pairs per centimeter. A radiation with a high specific ionization, such as alpha, will produce more ion pairs in each centimeter that it travels than will a radiation with a low specific ionization such as gamma. In Table 1, note the magnitude of the difference between the specific ionization for the three types of radiation.
Energy of the Radiation
Review of the data in Table 1 will reveal that, generally, the probability of interaction between the incident particle radiation and the detector gas (and therefore the production of ions) decreases with increasing radiation energy. In photon interactions, the overall probability of interaction increases because of the increasing contribution of the pair production reactions. As the energy of the particle radiation decreases, the probability of interaction increases, not only in the gas, but also in the materials of construction. Low energy radiations may be attenuated by the walls of the detector and not reach the gas volume. Obviously, this must be accounted for in the design of the detector.
Table 1. Specific Ionization In Air at STP.
Quantity of Radiation
As the number of radiation events striking a detector increases, the overall probability of an interaction occurring with the formation of an ion pair increases. In addition, the number of ion pairs created increases and therefore detector response increases.
The probability of an interaction occurring between the incident radiation and a gas atom increases as the number of atoms present increases. A larger detector volume offers more "targets" for the incident radiation, resulting in a larger number of ion pairs. Since, each radiation has a specific ionization in terms of ion pairs per centimeter, increasing the detector size also increases the length of the path that the radiation traverses through the detector. The longer the path, the larger the number of ion pairs.
Type of Detector Gas
The amount of energy expended in the creation of an ion pair is a function of the type of radiation, the energy of the radiation, and the characteristics of the absorber (in this case, the gas). This energy is referred to as the ionization potential, or W-Value, and is expressed in units of electron volts per ion pair. Typical gases have W-Values of 25-50 eV, with an average of about 34 eV per ion pair.
Detector Gas Pressure
In the section on detector size, it was shown the probability of interaction increases with detector size. In many cases, there is a practical limit to detector size. Instead of increasing detector size to increase the number of "target" atoms, increasing the pressure of the gas will accomplish the same goal. Gas under pressure has a higher density (more atoms per cm3) than a gas not under pressure, and therefore offers more targets, a higher probability of interaction, and greater ion pair production. For example, increasing the pressure of a typical gas to 100 psig increases the density by about 7 times.
Voltage Potential Across the Electrodes
Once the ion pair is created, it must be collected in order to produce an output pulse or current flow from the detector. If left undisturbed, the ion pairs will recombine, and not be collected. If a voltage potential is applied across the electrodes, a field is created in the detectors, and the ion pairs will be accelerated towards the electrodes.
The stronger the field, the stronger the acceleration. As the velocity of the electron increases, the electron may cause one or more ionizations on its own. This process is known as secondary ionization. The secondary ion pairs are accelerated towards the electrode and collected, resulting in a stronger pulse than would have been created by the ions from primary ionization.
Effect of Voltage Potential on the Detector Process
If the applied voltage potential is varied from 0 to a high value, and the pulse size recorded, a response curve will be observed. For the purposes of discussion, this curve is broken into six regions. The ion chamber region, the proportional region, and the Geiger-Mueller region are useful for detector designs used in radiological control. Other regions are not useful. In the recombination region, the applied voltage is insufficient to collect all of the ion pairs before some of them recombine. In the limited proportional region, neither the output current nor the number of output pulses are proportional to the radiation level. Calibration is impossible. In the continuous discharge region, the voltage is sufficient to cause arcing and breakdown of the detector gas.
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