Detection efficiency
- Related Topics:
- radiation
- measurement
- nuclear physics
The intrinsic detection efficiency of any device operated in pulse mode is defined as the probability that a quantum of radiation incident on the detector will produce a recorded pulse. Especially for radiations of low intensity, a high detection efficiency is important to minimize the total time needed to record enough pulses for good statistical accuracy in the measurement. Detection efficiency is further subdivided into two types: total efficiency and peak efficiency. The total efficiency gives the probability that an incident quantum of radiation produces a pulse, regardless of size, from the detector. The peak efficiency is defined as the probability that the quantum will deposit all its initial energy in the detector. Since there are almost always ways in which the quantum may deposit only part of its energy and then escape from the detector, the total efficiency is generally larger than the peak efficiency.
For a given detector, efficiency values depend on the type and energy of the incident radiation. For incident charged particles such as alpha particles or beta particles, many detectors have a total efficiency that is close to 100 percent. Since these particles begin to deposit energy immediately upon entering the detector volume, a pulse of some amplitude is inevitably produced if the particle reaches the active volume of the device. Very often, any departure from 100 percent efficiency in these cases is due to absorption or scattering of the incident particle before it reaches the active volume. Furthermore, if the detector is thick compared with the range of the incident particle, most particles are fully stopped in the active volume and deposit all their energy. Under these circumstances, the peak efficiency also will be near 100 percent.
For incident gamma rays, the situation is quite different. Except for low-energy photons, it is quite possible for an incident gamma-ray photon to pass completely through the detector without interacting. In such cases, the total efficiency will then be substantially less than 100 percent. Furthermore, many of the gamma-rays may deposit only a fraction of their energy in the detector. These events do not contribute to the peak efficiency so, although they produce pulses, their amplitude does not indicate the initial energy of the incident gamma ray. Thus the peak efficiency values incorporate only those gamma-ray photons that interact one or more times in the detector and eventually deposit all their energy. The total efficiency for gamma rays may be enhanced by increasing the detector thickness in the direction of the incident gamma-ray flux. For a given thickness, the peak efficiency is enhanced by choosing a detector material with a high atomic number to increase the probability that all the energy of the original photon will eventually be photoelectrically absorbed. Full energy absorption could take place in a single photoelectric interaction but, more likely, it happens after the incident photon has Compton-scattered one or more times elsewhere in the detector. Alternatively, full absorption is also observed if pair production is followed by subsequent full absorption of both annihilation photons. Since these multiple interactions are enhanced in detectors of large volume, the peak efficiency for gamma-ray detectors improves significantly with increasing size.
Timing characteristics
One of the added benefits of pulse-mode operation is the fact that the arrival time of an individual quantum of radiation is closely related to the time of appearance of a pulse at the detector output. In many nuclear measurements, it is advantageous to be able to determine that two quanta are emitted in the same nuclear process and therefore may be sensed by two separate detectors in virtual time coincidence. Another example of the application of timing information is in the determination of the velocity or energy of a particle by measuring its flight time between its point of origin and a distant detector.
The timing information is carried by the leading edge or rising portion of the detector output pulse. The precision of timing measurements is enhanced in detectors that produce a prompt output pulse with a fast rise time. The time characteristics of the leading edge are related to the charge collection time from the detector, and the best timing performance is generally obtained from detectors in which the charges are collected most rapidly. For example, timing precision of less than one nanosecond can be obtained using organic scintillators for which the light (that is subsequently converted to charge in a photomultiplier tube) is emitted within a period of several nanoseconds following the deposition of the particle energy. On the other hand, timing measurements from gas-filled detectors may have an imprecision of up to one microsecond or more owing to the relatively long and sometimes variable charge-collection time of these devices.
Gas-filled detectors
The passage of a charged particle through a gas results in the transfer of energy from the particle to electrons that are part of the normal atomic structure of the gas. If the charged particle passes close enough to a given atom, the energy transfer may be sufficient to result in its excitation or ionization. In the excitation process, an electron is elevated from its original state to a less tightly bound state. Energy levels in typical gas atoms are only spaced a few electron volts apart, so that the energy needed for excitation is a small fraction of the kinetic energy of typical radiation quanta. The excited state exists for a specific lifetime before the atom decays back to the original ground energy state. Typical mean lifetimes for excited atomic states in gases are normally only a few nanoseconds. When the atom spontaneously returns to the ground state, the excitation energy is liberated, generally in the form of an electromagnetic photon. The wavelength of electromagnetic radiation for typical gases is in the ultraviolet region of the spectrum. Thus, for every excited gas atom that is formed, the observable result is the appearance of an ultraviolet photon. As a typical charged particle will create thousands of excited atoms along its track, a resulting flash of ultraviolet photons appears, originating along the track of the particle. Some detectors, based on directly sensing this ultraviolet light and known as gas scintillators, are described below (see below Scintillation and Cherenkov detectors). Similar ultraviolet photons also play an important part in the generation of a pulse from a Geiger-Müller tube.
For close encounters between an incident charged particle and a gas atom, enough energy may be transferred to totally remove an electron. This is the process of ionization, and it results in the creation of an ion pair. Because the ionized atom is electron-deficient, it carries a net positive electric charge and is called a positive ion. The other member of the ion pair is the electron that is no longer bound to a specific atom and is known as a free electron. Most free electrons are formed with low kinetic energy, and they simply diffuse through the gas, taking part in the random thermal motion of all the atoms. Some free electrons are formed with enough kinetic energy to cause additional excitation and ionization. These are called delta rays, and their motion follows short branches away from the primary ionization and excitation that is created directly along the track of the incident charged particle.
The ionization potential, or the minimum energy required to remove an electron, is about 10 eV for the gases typically used in radiation detectors. Approximately 30 eV of energy loss by the incident charged particle is needed on average to create one ion pair. The remainder of the energy is expended in various excitation processes. For a 1-MeV charged particle that transfers all its energy to the gas, about 30,000 ion pairs will be formed along its track. Both the positive ions and the free electrons can be made to drift in a preferred direction by applying an external electric field. It is the movement of these charges that serves as the basis for the electrical signal produced by the important category of gas-filled detectors that includes ion chambers, proportional counters, and Geiger-Müller detectors.
Ion chambers
An ion chamber is a device in which two electrodes are arranged on opposite sides of a gas-filled volume. By applying a voltage difference between the two electrodes, an electric field is created within the gas. The ion pairs formed by incident radiation experience a force due to this electric field, with the positive ions drifting toward the cathode and the electrons toward the anode. The motion of these charges constitutes an electric current that can be measured in an external circuit.
Ion chambers are frequently operated as current-mode devices. The current-voltage characteristics of a typical ion chamber under constant irradiation conditions are shown in . At low applied voltages, there is some tendency for the positive and negative charges to collide and recombine, thereby neutralizing them and preventing their contribution to the measured current. As the voltage is raised, the stronger electric field separates the charges more quickly, and recombination is eventually made negligible at a sufficient applied voltage. This point marks the onset of the ion-saturation region, where the current no longer depends on applied voltage; this is the region of operation normally chosen for ion chambers. Under these conditions the current measured in the external circuit is simply equal to the rate of formation of charges in the gas by the incident radiation.
Air-filled ion chambers operated in current mode are a common type of portable survey meter used to monitor potential personnel exposure to gamma rays. One reason is that the historical unit of gamma-ray exposure, the roentgen (R), is defined in terms of the amount of ionization charge created per unit mass of air. Because of the close connection of the signal produced in an ion chamber with this definition, a measurement of the ion current under proper conditions can give an accurate measure of gamma-ray exposure rate over a wide range of incident gamma-ray energies.
The magnitude of the current observed from a typical ion chamber for a modest gamma-ray exposure rate is quite small. For example, at a gamma-ray exposure rate of 10−3 roentgen per hour (a small but significant level for personnel monitoring purposes), the expected ion current from a one-litre ion chamber at atmospheric pressure is about 0.1 picoampere (pA). These low currents require the use of sensitive electrometers for their accurate measurement.
Ion chambers are sometimes operated in a manner similar to passive detectors in integration mode. In this case, the ion chamber is first connected to a constant voltage source V0. The chamber has an inherent capacitance C, and this initial charging step has the effect of storing an electrical charge on it equal to CV0. The chamber is then disconnected from the voltage source and exposed to the radiation. During the exposure period, ion pairs are formed in the gas and are swept to their corresponding electrodes by the electric field created by the voltage on the chamber. At the end of the exposure period, the voltage on the chamber will have dropped, as the ionization charge that is collected serves to partially discharge the stored charge CV0. The chamber is then read out by recording the voltage drop ΔV that has occurred. If there are no other losses (such as leakage current across insulators), the amount of ionization charge created during the exposure is simply given by CΔV. Small pocket chambers of this type are frequently used to monitor exposure of personnel at radiation-producing facilities.
Ion chambers are rarely operated in pulse mode, and this mode of operation is only considered for high-dE/dx particles that can deposit large amounts of energy in the gas. The main problem is the small size of the voltage pulse that is produced by the interaction of a single quantum of radiation. The deposition of 1 MeV of energy in an ion chamber with a typical capacitance of 100 picofarads (pF) results in a voltage pulse with amplitude of only about 50 microvolts (μV). While it is possible to work with signals of such low level using careful techniques, it is much more common to use gas-filled detectors in pulse mode in the form of proportional or Geiger-Müller counters.
