Custom Search
|
|
 
|
||
|
The ionization chamber is a detector that operates in the ionization region. EO 2.3 DESCRIBE the operation of an ionization chamber to include: a. Radiation detection b. Voltage variations c. Gamma sensitivity reduction Ionization chambers are electrical devices that detect radiation when the voltage is adjusted so that the conditions correspond to the ionization region (refer to Region II of Figure 6). The charge obtained is the result of collecting the ions produced by radiation. This charge will depend on the type of radiation being detected. Ionization chambers have two distinct disadvantages when compared to proportional counters: they are less sensitive, and they have a slower response time. There are two types of ionization chambers to be discussed: the pulse counting ionization chamber and the integrating ionization chamber. In the pulse counting ionization chamber, the pulses are detected due to particles traversing the chamber. In the integrating chamber, the pulses add, and the integrated total of the ionizations produced in a predetermined period of time is measured. The same type of ionization chamber may be used for either function. However, as a general rule, the integrating type ionization chamber is used. Flat plates or concentric cylinders may be utilized in the construction of an ionization chamber. The flat plate design is preferred because it has a well-defined active volume and ensures that ions will not collect on the insulators and cause a distortion of the electric field. The concentric cylinder design does not have a well-defined active volume because of the variation in the electric field as the insulator is approached. Ionization chamber construction differs from the proportional counter (flat plates or concentric cylinders vice a cylinder and central electrode) to allow for the integration of pulses produced by the incident radiation. The proportional counter would require such exact control of the electric field between the electrodes that it would not be practical. Figure 14 illustrates a simple ionization circuit consisting of two parallel plates of metal with an air space between them. The plates are connected to a battery which is connected in series with a highly sensitive ammeter.
Figure 14 Simple Ionization Circuit If a radioactive source that is an emitter of beta particles is placed near the detector, the beta particles will pass between the plates and strike atoms in the air. With sufficient energy, the beta particle causes an electron to be ejected from an atom in air. A single beta particle may eject 40 to 50 electrons for each centimeter of path length traveled. The electrons ejected by the beta particle often have enough energy to eject more electrons from other atoms in air. The total number of electrons produced is dependent on the energy of the beta particle and the gas between the plates of the ionization chamber. In general, a 1 MeV beta particle will eject approximately 50 electrons per centimeter of travel, while a 0.05 MeV beta particle will eject approximately 300 electrons. The lower energy beta ejects more electrons because it has more collisions. Each electron produced by the beta particle, while traveling through air, will produce thousands of electrons. A current of 1 micro-ampere consists of about 10" electrons per second. If 1 volt is applied to the plates of the ionization chamber shown in Figure 14, some of the free electrons will be attracted to the positive plate of the detector. This attraction is not strong because 1 volt does not create a strong electric field between the two plates. The free electrons will tend to drift toward the positive plate, causing a current to flow, which is indicated on the ammeter. Not all of the free electrons will make it to the positive plate because the positively charged atoms that resulted when an electron was ejected may recapture other free electrons. Therefore, the ammeter will register only a fraction of the number of free electrons between the plates. When the voltage is increased, the free electrons are more strongly attracted to the positive plate. They will move toward the positive plate more quickly and will have less opportunity to recombine with the positive ions. Figure 15 shows a plot of the number of electrons measured by the ammeter as a function of applied voltage.
Figure 15 Recombination and Ionization Regions At zero voltage, no attraction of electrons between the plates occurs. The electrons will eventually recombine, so there is no current flow. As the applied voltage is increased, the positive plate will begin to attract free electrons more strongly, and a higher percentage will reach the positive plate. As the voltage is increased further, a point will be reached in which all of the electrons produced in the chamber will reach the positive plate. Any further increase in voltage has no effect on the number of electrons collected. The simple ionization chamber shown in Figure 14 can also be utilized for the detection of gamma rays. Since the ammeter is sensitive only to electrons, gamma rays must interact with the atoms in air between the plates to release electrons. The gamma rays interact by compton scattering, photoelectric effect, or pair production. Each of these interactions causes some, or all, of the energy of the incident gamma rays to be converted into the kinetic energy of the ejected electrons. The ejected electrons move at very high speeds and cause other electrons to be ejected from their atoms. All of these electrons can be collected by the positively charged plate and measured by the ammeter. The plates in an ionization chamber are normally enclosed in metal, as shown by Figure 16.
Figure 16 Ionization Chamber This metal chamber serves to shield the plates from outside electric fields and to contain the air or other gas. Gamma rays have very little trouble in penetrating the metal walls of the chamber. Beta particles and alpha particles, however, are stopped by the metal wall. For alpha and beta particles to be detected, some means must be provided for a thin wall or "window." This window must be thin enough for the alpha and beta particles to penetrate. However, a window of almost any thickness will prevent an alpha particle from entering the chamber. Neutrons can also be detected by an ionization chamber. As we already know, neutrons are uncharged; therefore, they cause no ionizations themselves. If the inner surface of the ionization chamber is coated with a thin coat of boron, the following reaction can take place.
A neutron is capturved by a boron atom, and an energetic alpha particle is emitted. The alpha particle causes ionization within the chamber, and ejected electrons cause further secondary ionizations. Another method for detecting neutrons using an ionization chamber is to use the gas boron trifluoride (BF3) instead of air in the chamber. The incoming neutrons produce alpha particles when they react with the boron atoms in the detector gas. Either method may be used to detect neutrons in nuclear reactor neutron detectors. When using an ionization chamber for detecting neutrons, beta particles can be prevented from entering the chamber by walls thick enough to shield out all of the beta particles. Gamma rays cannot be shielded from the detector; therefore, they always contribute to the total current read by the ammeter. This effect is not desired because the detector responds not only to neutrons, but also to gamma rays. Several ways are available to minimize this problem. Discrimination is possible because the ionizations produced by the alpha particles differ in energy levels from those produced by gamma rays. A 1 MeV alpha particle moving through the gas loses all of its energy in a few centimeters. Therefore, all of the secondary electrons are produced along a path of only a few centimeters. A 1 MeV gamma ray produces a 1 MeV electron, and this electron has a long range and loses its energy over the entire length of its range. If we make the sensitive volume of the chamber smaller without reducing the area of the coated boron, the sensitivity to gamma rays is reduced. Figure 17 illustrates how the chamber may be modified to accomplish this reduction.
Figure 17 Minimizing Gamma Influence by Size and Volume In Figure 17(b) there is half as much gas in the sensitive volume as in the chamber in Figure 17(a). As a result, gamma rays have only half as much gas to interact with; therefore, half the number of electrons are produced. The area which is boron-coated has not changed, and both chambers produce the same number of neutron-induced alpha particles. Also, the gamma rayinduced electrons produce fewer ionizations because the range of these electrons is longer than the dimensions of the sensitive volume. The range of neutron-induced alpha particles is short, and all of the energy will be dissipated within the sensitive volume, even when the volume is smaller. Gamma interference can also be minimized by reducing the pressure of the gas inside the chamber. The reduction in pressure reduces the number of atoms within the sensitive volume and has the same effect as reducing the volume. Ionization chamber sensitivity to gamma rays can also be reduced by increasing chamber sensitivity to neutrons. This is accomplished by increasing the boron-coated area, as shown in Figure 18. Both ionization chambers shown in Figure 18 have the same sensitive volume.
Figure 18 Minimizing Gamma Influence with Boron Coating Area The ionization chamber in Figure 18(b) has twice the boron area as the ionization chamber in Figure 18(a). The result is that more neutron-induced alpha particles are produced, and neutron sensitivity is increased. Ionization chambers supplied commercially are designed to minimize gamma sensitivity by both of the techniques described previously. Gamma sensitivity can be minimized but not eliminated. For reactors operating near peak power, neutrons are the dominant radiation, and almost all of the current is due to neutrons. These chambers are used at high reactor powers and are referred to as uncompensated ion chambers. The uncompensated ion chamber is not suitable for use at intermediate or low power levels because the gamma response at these power levels can be significant compared to the neutron response. Summary Ionization chambers are summarized below. Ionization Chamber Summary When radiation enters an ionization chamber, the detector gas at the point of incident radiation becomes ionized. Some of the electrons have sufficient energy to cause additional ionizations. The electrons are attracted to the electrode by the voltage potential set up on the detector. If the voltage is set high enough, all of the electrons will reach the electrode before recombination takes place. Gamma sensitivity reduction is accomplished by either reducing the amount of chamber gas or increasing the boron coated surface area.
|
 
|
|
|
||
