Answer to Question #534 Submitted to "Ask the Experts"

Q

I am Zainb from Jordan and I am studying physics in university. Can you please give me some information about alpha, beta, and gamma detectors, and can you please give me some information about the single- and multi-channel analyzers—how each of them works and the differences between them?

A

There are many good references in the literature that provide more detail than I can offer here. One of the most useful textbooks that deals with ionizing radiation detection is by Glenn F. Knoll, Radiation Detection and Measurement (3rd edition, John Wiley & Sons, Inc., New York, 2000). You asked for information about alpha, beta, and gamma detectors and also about the operation of multichannel analyzers and how they differ from single channel analyzers. I shall attempt to address each of these items. Since you have stated that you are a student of physics in college, I will assume that you have knowledge of physical principles and facts that might not be available to others. Most of the detectors used to measure ionizing radiation (such as alpha, beta, and gamma radiation) are based on the ability of the radiation to ionize materials or to excite atoms within materials. Most of the radiation detectors used in radiation measurements fall into three categories: gas-filled detectors, scintillation detectors, and semiconductor detectors. There are several other types as well, but these three categories are the most common, and I shall summarize some of them in relationship to the detection of alpha, beta, and gamma radiations. Many of the differences among the ways in which these radiations interact and what types of detectors are appropriate depend on the fundamental differences among the radiations themselves. Alpha particles are heavy (as radiations go), consisting of 2 neutrons and 2 protons, and have a charge of +2, and typical alpha particle energies are in the range from 3 to 10 MeV(million electron volts). Beta particles are energetic electrons, with a charge of –1, and produced by the transformation of a neutron in the nucleus to a proton, an electron (beta particle), and an antineutrino; typical beta particle energies range from a few thousand electron volts to several MeV. Gamma rays are electromagnetic radiations, having no mass and having energy given by the product of the frequency of the electromagnetic radiation and Planck's constant; energies range from roughly 20 thousand electron volts to 10 MeV. Gas-Filled Detectors There are three types of detectors most common among the gas-filled detectors. These are Geiger-Mueller (G-M) detectors, proportional gas detectors, and ionization chambers. In use the detectors are equipped with associated electronics necessary to process and record the events occurring in the detectors. These electronics may include a preamplifier, an amplifier, a discriminator (that allows rejection of pulses below a specified voltage level) and a device such as a scaler for recording events. All of these detectors use a defined gas cavity surrounded by a wall whose inner surface is electrically conducting and acts as one electrode and a second electrode (sometimes a central wire or rod) which penetrates the gas volume and is electrically insulated from the conducting wall. There is a voltage applied between the two electrodes. When ionizing radiation enters a gas-filled detector it may interact directly in the gas and produce ionization of the gas or it may interact in the wall surrounding the gas and knock charged particles, such as electrons, from the wall; these electrons may enter the gas and also cause ionization. When ionization of the gas occurs, the positive and negative ions are attracted towards the negative and positive electrodes, respectively. The three types of detectors mentioned above differ often in their physical sizes and shapes and the types of filling gases, but especially in terms of the high voltage applied between the electrodes. A common shape for gas detectors is cylindrical with a central conducting wire as the inner electrode. In such a shaped G-M detector the gas is almost always an inert gas, such as argon, and the voltage applied between the electrodes is sufficiently large, with the central wire being positive (referred to as the anode), so that when gas ionization occurs from radiation interactions, the electrons that are produced are accelerated sufficiently as they move toward the wire under the influence of the electric field so that if they collide with a neutral gas molecule they may cause another ionization event. This process can continue and multiply the number of ionization events (the ionization process propagates along the entire central wire anode) by a large factor and result in a very large electronic pulse when the ions are collected at the electrodes. G-M detectors can respond to alpha, beta, and gamma radiation. In order to detect alpha and beta radiation the detector must be equipped with a sufficiently thin window to allow these radiations to enter the gas. Alpha particles travel very short distances in materials (often less than 100 micrometers in solid low-atomic-number materials) so the window must be quite thin, usually about 1/3 the thickness of a standard sheet of paper or less. Beta particles are more penetrating, many of them able to travel a few mm in low-atomic-number solid materials. Gamma radiation is quite penetrating and, depending on its energy, can easily pass through rather large thicknesses of materials. If either an alpha particle or a beta particle enters the gas volume of a G-M detector it will most likely result in a detectable event. While gamma rays can also be detected by a G-M detector, the efficiency is much lower than that for alpha or beta particles that enter the detector. G-M detectors with relatively thick walls are often used for measuring the intensities of gamma radiation fields (these are usually measurements related to radiation dose). A G-M detector with a thin window can measure individual beta or alpha particles that pass through the window; however, if both types of particles enter the detector, it is not possible to distinguish the pulses by electronic means to tell which are from alpha and which are from beta particles. Gamma rays interact by setting free electrons which behave in the gas like beta particles, and it would also not be possible electronically to separate gamma radiation events from alpha and beta. By covering the thin window with a piece of paper it may be possible to eliminate response to alpha radiation and, by a difference technique, distinguish the alpha from beta and/or gamma. Because the pulses from a G-M detector are very large, the detector does not require as sophisticated electronics as do proportional detectors and ionization chambers. Proportional gas detectors for alpha and beta detection also are equipped with a thin window and also use a noble gas filling and operate similarly to G-M detectors except that they operate at a somewhat lower voltage (assuming the same detector configuration as the G-M) so that less multiplication of the ionization events takes place than in the G-M detector. The result is that electronic pulses produced by typical alpha particles are larger than those produced by typical beta interactions and, by properly selecting the applied voltage and the discriminator level, one can establish operating conditions so that alpha pulses can be distinguished from beta pulses. This is one of the most common applications of the gas proportional detector. Proportional detectors also respond with low efficiency to gamma radiation, if the voltage is set sufficiently high, but this is not a common application. Ionization chambers are used more for gamma radiation intensity measurements than for alpha and beta, although there are ionization chambers that have been designed for alpha or beta detection. The most commonly used ionization chambers for gamma measurements use air as the fill gas. They are operated at a low enough voltage so that no multiplication of the original ionization events occurs and only the ionization produced by those original events is collected. They are frequently used in a mode (called mean level mode) such that the signal from a large number of ionizing events is recorded or measured per unit time rather than attempting to resolve individual pulses. They are commonly used for gamma dose measurements in radiation protection. Scintillation Detectors Scintillation detectors respond to energy absorption from ionizing radiation by emitting light. The light is most often measured with a photomultiplier tube that converts the light to an electronic pulse. These detectors may be either inorganic crystals or organic compounds. All inorganic scintillators rely on the crystalline nature of the material for light production and most have impurity atoms, with ionization potentials less than atoms of the crystal, added as activators. Ionizing radiation may elevate electrons from the conduction band to the valence band of the crystal. The electrons can migrate in the conduction band and holes left in the valence band may also move and ionize a host atom that it encounters. The impurity ions introduce trapping levels in the energy gap between the valence and conduction bands. In brief, electrons may move into an excited level of the activator ion and then drop to the ground energy state with the emission of light. The most commonly used scintillation detector for alpha measurements is zinc sulfide activated with silver, ZnS(Ag), an inorganic scintillator. This material is not very transparent to light and is usually prepared as a large number of submillimeter-sized crystals attached with an adhesive to a flat piece of plastic or other material. The flat screen is optically coupled to a photomultiplier tube which is attached to associated electronics. The voltage and discriminator levels are selected so that the detector is sensitive to the rather large pulses from alpha interactions but insensitive to beta- or gamma-induced pulses. (Recall that alpha particles deposit all of their energies in small thicknesses of material compared to beta and gamma radiations.) Scintillation detectors for beta radiation are often organic scintillators. Organic scintillators operate at the molecular level, which means that the light emission occurs as a result of fluorescence as a molecule relaxes from an excited level following excitation by energy absorption from ionizing radiation. Molecules such as anthracene , trans-stilbene, para-terphenyl, and phenyl oxazole derivatives are among the many organic species that have useful scintillation properties. Such organic molecules dissolved in organic solvents are used as liquid scintillators. A classic application is in the measurement of low-energy beta radiation (e.g., from tritium, ¹⁴C, ³⁵S). In such cases the sample containing the radioactive beta emitter is dissolved in, or in some cases suspended in, the liquid scintillation solution. The emitted beta radiation transfers energy through the solvent to the scintillator molecule which then emits light that is detected by photomultiplier tubes. Liquid scintillators can also be used for alpha particle measurements and have had applications for low energy x-ray or gamma-ray measurements. Organic scintillator molecules can also be dissolved in an organic monomer which can then be polymerized to produce a plastic scintillator. Plastic scintillators can be fabricated in a wide variety of shapes and sizes. Very thin scintillators have been used for alpha detection, somewhat thicker scintillators for beta detection, and some large volume plastic scintillators have been used in gamma detection, particularly for dose-related measurements, but not for gamma energy spectral analysis. Other inorganic crystalline scintillators, especially sodium iodide activated with thallium, NaI(Tl), have been used for gamma-ray energy measurements. Such detectors can be grown as large single crystals that have a reasonably high efficiency for absorbing all of the energy from incident gamma rays. They are used most often with a photomultiplier tube to convert the light pulses to electronic pulses which are usually sorted, according to pulse height, using a multichannel analyzer. A pulse that represents full energy deposition by a gamma ray in the detector falls in a region of the distribution of pulses called the photopeak region and can be associated with a specific gamma ray energy. (See discussion later on multichannel analyzers). There exists a rather large number of inorganic scintillators; some examples of these include cesium iodide activated with thallium, CsI(Tl), bismuth germanate, Bi4Ge3O12, and barium fluoride, BaF2. Most have been used for gamma measurements but some have been prepared with thin windows and have been used for charged particle (e.g., alpha and beta) counting. Semiconductor Detectors Among the most popular semiconductor detectors for gamma radiation energy spectral analysis is germanium. Large volume cylindrical detectors frequently use what is called a coaxial geometry in which the outer surface serves as one electrode and a central conducting cylindrical core as the other electrode and a high voltage is applied between them. Gamma rays deposit their energy in the germanium and produce free electrons and holes (vacancies where the electrons were located in the crystalline germanium). The electrons and holes behave as negative and positive charges and are collected at the electrodes by the applied voltage. The amount of charge collected is correlated with the amount of energy deposited in the detector and therefore with the energy of the gamma ray that caused it. The detectors are used with multichannel analyzers to sort the pulses according to pulse height. It requires a lot less energy to create an electron-hole pair in semiconductor detectors than it does in scintillation detectors such as NaI(Tl) and, mostly because of this reason, the germanium detector has a much better energy resolution than the NaI(Tl) detector. This means that if multiple gamma-ray energies are being analyzed simultaneously, the germanium detector will do a better job of separating them cleanly. Because the likelihood of gamma-ray interactions depends strongly on the atomic number of the material the gamma rays are interacting with, the detection efficiency of germanium is considerably lower than that of NaI(Tl) because germanium has a lower effective atomic number. One practical disadvantage of germanium detectors is that they must be operated at liquid nitrogen temperatures to keep the thermal noise at an acceptably low level. Silicon is another semiconductor that has been used for some low-energy gamma-ray and x-ray energy analysis. It has a smaller atomic number than germanium and has lower efficiency and is less useful for high-energy gamma rays. Some specially designed silicon diode detectors have been made with the active volume (the depleted region) very close to the surface of the detector and have an extremely thin window so that alpha particles can enter the active volume and deposit all their energy there. The charge collected is a measure of the energy of the alpha particle, and these detectors are common for alpha particle spectroscopy. One of the most common types of such detectors is called a surface barrier detector. The detectors are usually used with multichannel analyzers. Some silicon detectors have thicker active volumes and are used for beta particle energy analysis. There are a number of other less popular semiconductor detectors with poorer energy resolution than germanium and silicon, but that have been used in some applications in gamma-ray measurements. Two examples are cadmium telluride, CdTe, and mercuric iodide (HgI2), detectors with high efficiency associated with high atomic number constituents. Unlike germanium detectors, these detectors may be operated at room temperature. Single and Multichannel Analyzers These analyzing systems are typically used in association with some of the detectors noted above to make energy spectral measurements—e.g., to determine the energies of gamma rays or alpha particles emitted from a sample. As we noted earlier, all detectors are used in conjunction with an electronic discriminator, a device that rejects electronic pulses that fall below a specific voltage level fixed by the discriminator. A single-channel analyzer (SCA) uses two rather than one such discriminator. The discriminators are called upper and lower level discriminators. Pulses from the amplifier are fed to the analyzer, and if the pulse height falls between the lower and upper discriminators the usual logic is to allow such a pulse to be recorded (counted). The voltage levels of the two discriminators are adjustable so that the gap between them corresponds to a group of pulse heights within a fixed energy interval. For example, if one is interested in evaluating gamma rays of a discrete energy coming from a particular radionuclide, the discriminators are set to allow pulses that correspond to the gamma energy of interest to be recorded. Even though the gamma rays from a specific decay transition are of a discrete energy, there is a statistical spread of pulses coming from the detector and associated electronics so that the gap between the discriminators must be large enough to include most of such pulses. The voltage gap between the discriminators that corresponds to a given energy interval is frequently referred to as an energy window. By varying the position of the window (i.e., changing the voltage levels of each of the discriminators) it is possible to measure gamma rays of different energies, but it is a tedious process to attempt to measure many different energy gamma rays from a radioactive sample using a single-channel analyzer. For such cases a multichannel analyzer is a much more efficient system to use. The most important part of a multichannel analyzer is a device called an analog-to-digital converter (ADC). The ADC takes the analog voltage pulse from the detector and converts it to a digital signal which is a number proportional in magnitude to the height of the analog pulse. The analyzer system contains a number of equal width storage bins (called channels), usually up to a maximum of 8192. The width of the storage bins corresponds to a specific energy interval and can be set by the user, depending on the application. For example, in doing high resolution gamma spectroscopy with a germanium detector, one might set up each channel to represent a 0.5 keV (500 electron volts) width. Signals (the signal is basically a number) coming from the ADC are then sorted into the various channel locations, according to their sizes. For example, if the numerical value of a signal from the ADC corresponded to a 250 keV energy event, one count would be stored in channel location 500 (assuming the 0.5 keV/channel noted above); if a second signal from the ADC corresponded to a 1000 keV energy event a count would be stored in channel number 2000. After counting the sample of interest for some predetermined period of time, the output of the analyzer may be displayed on a CRT screen and/or printed out or plotted. The complete channel-by-channel output would be a listing of each channel from 1 to the maximum number of channels used along with the number of counts that had been accumulated in each respective channel. If the results are plotted, one sees the counts plotted versus channel number. Gamma-ray energies are identified by looking for Gaussian-shaped peaks (called photopeaks) that are centered at the channel corresponding to a specific energy. An ideal system has a linear response so that a peak centered at channel 500 in our above example would correspond to a 250 keV gamma ray, and a peak centered at channel 2000 would correspond to the 1000 keV gamma ray. Multichannel analyzers are commonly used for gamma spectroscopy as noted here using a germanium detector, but are also used with lower resolution NaI(Tl) detectors. They are also frequently used for energy measurements of charged particle, especially alpha particles where a detector such as a silicon surface barrier detector may be used. There are many more details and sophistications related to the entire area of radiation detection and I have provided only some summary information. I hope it is useful to you. If you have a stronger interest in the area I would encourage you to attempt to borrow or purchase a copy of the text by Knoll mentioned above. George Chabot, Jr., CHP, Ph.D.