How does one measure radioactivity?
Unlike other materials that we commonly have a need to measure, we cannot weigh radioactivity or collect it in a box, just as we cannot weigh or collect sunshine in a box. However, we can measure it indirectly by measuring the effects that it causes. Unlike that portion of sunshine that we can see, invisible nuclear radiation produces an electrical effect in materials through which it passes. If we measure the electrical effect, we can determine how much radiation passed through the materials. This is the basic operational principle for measuring radioactivity.
Are there instruments for measuring radioactivity?
The definitive method of testing for the presence of radioactivity is to make measurements with a suitable instrument, using suitable procedures. However, the operative word here is suitable. Some forms of radiation are extremely difficult to detect in typical field conditions. In addition, different instruments are sensitive to different types of radiation. There is no such thing as a “universal” instrument that will work in all circumstances. Furthermore, the contribution of naturally-occurring background must be accounted for when trying to determine whether radioactivity of significance exists. (Remember, everything is radioactive to some degree.)
What is the basis for detecting radiation using instruments?
The principles of “ionization” and “excitation” are the fundamental interaction processes that provide the basis for the operation of the radiation detection instruments.
What is “ionization”?
Ionization is the process whereby the radiation has sufficient energy to strip electrons away from atoms. The ionization process results in the formation of “free” electrons and a residual atom that is missing its full complement of orbital electrons. Radiations that are able to initiate the ionization process are known as “ionizing radiations”. Examples include particulate radiations (radiations with mass) such as alpha and beta particles, and photon (pure energy) radiations such as x-rays and gamma rays. Neutrons and protons are additional examples of ionizing radiations.
What is “excitation”?
Excitation refers to the process where the radiation does not impart sufficient energy to strip electrons from atoms, but rather “excites” or raises electrons to a higher energy state within the atom. The electrons are not physically removed from the atom. Once excited, the electrons will drop back to the “ground” or original state, typically emitting the energy associated with this transition in the form of x-rays.
Are there categories of radiation detection instruments?
Yes. Instruments used for the purpose of detecting the presence of ionizing radiation can be categorized in several ways. One way to distinguish them is whether they utilize a gas or a solid as the detecting medium.
What are gas-filled detectors?
Geiger counters, Proportional counters, and Ionization Chambers are examples of gas-filled detectors. Each of these three commonly utilized detectors contain a central wire known as the “anode” which initially carries a positive (+) charge with respect to the outer walls of the detector. This outer wall is known as the “cathode”. The volume between the wire and the outer walls is filled with gas, which may be air or a mixture of gases (e.g., argon and methane). Alpha and beta particle that enter the detector interact with atoms in the gas to strip electrons away from the atom, producing primary ion pairs. Ion pairs consist of a “free” electron and the positively charged atom which lost it. The electron component of the ion pair will be attracted to the anode (the wire). The positive member of the ion pair will head in the direction of the outer wall.
Gamma ray interactions will more than likely first occur with the outer cathode wall rather than the fill gas (This has to do with the fact that gamma rays have no charge, and are therefore more likely to interact/ionize atoms containing many electrons, as in a solid, rather than in a gas where the atoms are not as closely spaced.). These interactions will eject electrons from the wall; the electrons, as charged particles, will then ionize the gas as noted above.
What are solid detectors?
Sodium Iodide (NaI); Zinc Sulfide (ZnS); and Plastic scintillators are examples of instruments with solid detecting media. Sodium iodide and zinc sulfide detectors are inorganic crystalline solids which respond to gamma radiation and alpha radiation, respectively, by producing light flashes. (This is why they are called scintillators.) The term “inorganic” refers to the fact that no carbon atoms are present in the detector’s makeup. Plastic (organic) detectors, which also rely on the scintillation process, are principally used for the detection of beta radiation. Organic and inorganic detectors are often combined to detect multiple radiation types. For each of these three scintillators, the amount of light that the radiation creates in the detector is converted into an electronic signal and a corresponding “readout”on a ratemeter or scaler for recording purposes.
What is a Geiger-Mueller (GM) detector?
The GM detector is a gas detector operated at a relatively high voltage such that ionization in the detector region creates a large and easily detected electronic pulse. The pulse size is essentially the same regardless of the number of electrons initially ionized in the gas.
What is a GM detector used for?
G-M instruments are typically used as radiation detection instruments, meaning they are principally used to determine that radiation is present, rather than what the radionuclide’s identity is, or how much radioactivity is present. One particular earlier use of these detectors was in the prospecting for uranium ore. In more recent times, they have proven quite useful in performing contamination surveys for elevated radioactivity levels.
What are the most common types of GM detectors?
There are three commonly encountered GM detectors: The “end-window” detector, the “pancake” detector, and the “side-wall” detector.
What is an “end-window” GM?
The “end-window” detector uses a thin wall at its end (thinner than a piece of loose leaf paper) to allow most alpha and beta radiations to enter the detector without being stopped. This detector can also measure gamma/x-ray radiation.
What is a “pancake” GM?
The “pancake” detector, also known as a “frisker”, is similar to an end-window, with a wider diameter thin window to allow for the detector to cover more area as it is used, which results in faster equipment and personnel surveys.
What is a “side-wall” GM?
The “side-wall” probe detects beta and gamma/x-ray radiation using an aluminum or stainless steel outer wall which slides (or rotates) to expose the actual detector. In the “open” window configuration (that is, the sleeve is moved to expose the detector), beta and gamma/x-ray radiation above a certain energy can be detected; the “closed” window position (the sleeve covers the detector) detects only gamma/x-ray radiation.
What are some advantages of using GM detectors?
GM detectors are fairly reliable and rugged instruments useful for field use (primarily indoor) applications. They are easy to use and come in a wide variety of shapes and sizes. They are portable and lightweight, and they are relatively inexpensive. GMs respond quickly to radiation and are sensitive to relatively low levels of radiation (one ion pair can produce a “count” on the readout device). (They are more sensitive than an ionization chamber by a factor of 10.) GMs can detect a wide variety of radiations including alpha, beta, x-ray, and gamma rays. They are frequently used to survey for contamination on equipment and personnel and to perform source “leak” testing, and they can be modified to make exposure rate measurements (but only under proper conditions).
What are some limitations of using GM detectors?
GM detectors are principally detection, not measurement, devices. They are prone to breaking if the thin entrance window (found on pancake and end-window designs) is punctured. This can easily occur if the window comes in contact with a variety of objects (such as a blade of grass, paper clip, nail, paint flecks, etc.). In addition, they cannot discriminate on the basis of energy (spectroscopy is not possible), they can exhibit self-absorption in the counter wall and window for alpha and beta radiations, they have poor efficiency for gamma ray detection, and they are highly energy-dependent. (A GM tube can be calibrated to read the “true” exposure rate (milliroentgens per hour or “mR/hr”) for a specific source but will be valid only for other identical sources. The GM tube does not duplicate the conditions for which the “roentgen” (a unit of “exposure”) is defined.
GM detectors are plagued with what are known as resolving time (or “dead time”) losses when they are used in the presence of high radiation fields. (For example, at extremely high counting rates, the GM pulses are too small to be counted and the meter indicates a “zero” response, a condition known as saturation.) GM detectors have, in general, the worst resolving time losses of any gas-filled detector
What is a Proportional Counter?
The proportional counter is a gas detector operated at a voltage such that the electronic pulse produced is amplified and proportional to the amount of ionization created in the probe. This allows for discrimination against unwanted pulses. It also can be used to detect alpha radiation in the presence of beta and/or gamma radiation.
What is a proportional counter used for?
Proportional counters, like GM instruments, are typically used as radiation detection instruments. Because of their ability to detect alpha and/or beta radiations, proportional counters are often used for contamination surveys.
What are the most common types of proportional counters?
There are several commonly encountered proportional counters, including an “air proportional counter” and a “gas flow detector”. An “air” proportional detector which is designed to count only alpha radiation using air as the counting gas. In contrast, a gas-flow detector is designed to detect alpha and/or beta radiations depending on the radiation(s) of interest and the way the instrument is set up. A variety of different counting gases are used (all heavier than air) to provide a slow flow of gas into the probe. Gas-flow units are available either as hand-held devices or as bigger units designed with a much larger detection areas, which are useful in accelerating surveys of open floor areas.
What are the advantages of using a proportional counter?
Proportional counters have several advantages. They are versatile in that they can be used for a variety of different applications. They are also available in a variety of shapes and sizes. As with Geiger counters, these counters are sensitive to the formation of one ion pair. The size of the electronic pulse is proportional to the initial number of ion pairs. Proportional counters can detect a variety of radiations (including alpha, beta, gamma, x-ray, and neutrons). They can also distinguish among radiation types based on the shape of the pulses they create (spectroscopy is possible). They have a low background for alpha particles (as low as “0” counts per minute). They exhibit little or no dead time (unlike certain GM detectors) which allows the counting of higher activity sources. For certain types, they are easily portable and, for air proportional counters, an external gas tank is not necessary.
What are the disadvantages of using a proportional counter?
Proportional counters require a stable high voltage supply due to the nature of gas amplification, and external amplification (preamplifiers, amplifiers) to produce pulses of sufficient size for detection. In general, these detectors are more expensive than GM counters. They demand greater expertise on the part of the user since they have a “finicky” nature (i.e., more attention to maintenance is typically required than with other instruments). They also have an adverse sensitivity to environmental conditions such as heat, humidity (conditions which negatively impacts on counter performance). Proportional counters have poor efficiencies for higher energy x- and gamma rays. In addition, there is a potential fire hazard associated with their use of fill gases (e.g., propane). Before use, some proportional detectors must be purged with its counting gas before it can be used. Finally, some designs are bulkier than others, and there are transportation issues associated with the counting gases.
What is an Ionization Chamber?
An ionization chamber (or detector) is a gas detector operated at a voltage such that all charge produced by ionizing radiation will be collected but without any further amplification of the signal. (Recall that GM detectors and proportional counters do amplify the signal). Because of this, the signal produced in the chamber is small and requires a lot of external amplification. The instrument will give a true measure of charge produced in air, hence it is the ideal instrument to measure the health physics quantity “exposure” measured in roentgens.
What is an ionization chamber used for?
Ionization chambers are conventionally used for measuring the exposure rate from gamma and X-ray radiation. These devices are calibrated to read out in typical units such as roentgens per hour (R/hr), milliroentgens per hour (mR/hr), and microroentgens per hour (µR/hr).
Unlike GM and proportional counters, ionization chambers are measurement, rather than detection devices. In other words, beyond simply detecting the presence of radiation (typical units for this application as described previously are “cpm”), ionization chambers can accurately measure/quantify higher levels of radiation. The readout units (R/hr, etc.) reflect this advancement.
Ionization chambers are extremely useful for measuring a broad range of exposure rates, from the low levels found in environmental applications to much higher radiation levels. The latter includes applications such as surveys in nuclear facilities where much higher activity sources can be found. The use of a conventionally-designed GM or proportional detector would be inappropriate for these situations.
What are some common types of ionization chambers?
The typical ionization chamber is a fairly lightweight, portable survey meters (and therefore hand-held instrument) used for measuring exposure rates. The typical range is 1 mR/hr to 5 R/hr, but some instruments are designed for much higher rates (up to 1000 R/hr). In the last several years, ionization chambers designed to measure lower-level exposure rates associated with the environment (µR/hr) have been developed as well.
In addition, smaller devices known as pocket ionization chambers are also available. These small instruments are designed to be carried as a dosimeter used to provide an immediate readout of the dose received. They are generally not considered as reliable and accurate as other types of personnel monitoring devices such as thermoluminescent dosimeters (TLD’s) or film badges.
What are the advantages of using an ionization chamber?
Advantages of these instruments include: Their capability for measuring exposure rates over a wide range; a flat energy response above about 100 keV; and their ability to detect alpha and beta radiation (when warranted) with a proper design and calibration.
Are there disadvantages to the use of ionization chambers?
Yes. They have slow response times which requires careful observation to allow the instrument to reach a maximum reading. They are generally insensitive to low levels of radiation (which precludes their use in searching for low-activity lost sources or contamination). In addition, they are sensitive to the effects of temperature, pressure, and humidity changes
What is a scintillation detector?
A scintillation detector, sometimes called a “scintillator” is a device that emits light when ionizing radiation interacts with the detector. The light is converted into an electrical signal and recorded on a readout device. The amount of light is proportional to the amount of energy deposited, allowing energy discrimination if desired.
What are scintillation detectors used for?
Scintillation detectors are used for a variety of applications, including gamma-ray spectroscopy, environmental applications (conducting environmental walkover land surveys, aerial radiological surveys for the presence of gamma- emitting radionuclides, etc.), nuclear medicine, whole body counting, geological surveys, detecting/locating low-activity sources, and performing contamination surveys. The military, for example, has used them to detect the presence of depleted uranium projectiles both at their training facilities and in combat situations.
What are some common types?
There are many different types of scintillators. The most commonly used scintillators include zinc sulfide, sodium iodide, and plastic scintillators.
What is a zinc sulfide detector?
Zinc sulfide detectors which are designed to detect alphas in the presence of other types of radiation by energy discrimination. A thin coating of zinc sulfide (a phosphor) is placed behind the thin entrance window. Alpha particles penetrate the window and interact with the phosphor, producing light flashes. Beta particles and gamma rays travel too quickly to interact with the phosphor and hence are not detected.
What is a sodium iodide detector?
Sodium iodide (NaI) scintillation detectors are designed to detect low levels of gamma/x-ray radiation. These detectors typically read out in units of cpm, but with proper calibration and within proper energy limits, they may be used as a microroentgen (“micro-R”) meter to measure low exposure rates. A sodium iodide detector is a solid chunk of material with an outer casing. The thickness of the casing prohibits detection of alpha and beta radiation.
What is a plastic scintillator?
Plastic scintillators are increasing in popularity and have been used in a variety of settings, including military applications. These detectors are made of organic scintillation material that is dissolved in a solvent and subsequently polymerized. Because of the ease with which they can be shaped and fabricated, they have become an extremely useful form of organic scintillator. Depending upon the material and way in which it is packaged, plastic scintillators can be used for alpha, beta, gamma or neutron detection.
What are the advantages of using a scintillation detector?
There are a number of advantages of using a scintillation detector. For example, because they can detect a variety of radiations, including alphas, betas, gammas, x-rays, protons, and neutrons, they have a wide variety of applications. They are generally portable, have a low background for alpha detection (near 0), have a fast response time and they have a low dead time.
What are the disadvantages of a scintillation detector?
Disadvantages include the fact that these detectors are sensitive to light leaks through the mylar window (in the case of alpha scintillators). They have a high background (up to several thousand cpm) in the case of sodium iodide and plastic scintillators. They are energy dependent devices (unless the detector is specifically calibrated to the energy of interest, the detector is limited to its conventional role of detecting, not measuring, the presence of radiation). NaI crystals are prone to damage (impact, temperature changes, sensitivity to water damage). Zinc sulfide scintillators are prone to window punctures. All scintillators can be affected by magnetic fields, adversely affecting the instrument response. They also contain photomultiplier tubes (which convert the scintillator’s light flashes into an electronic signal) which are fragile, require a well-regulated power supply, represent a shock hazard, and operationally degrade over time.
Is there any final advice on measuring radioactivity?
To avoid a false sense of security from measurement results, individuals who use radiation detection instruments or perform radiation surveys appropriately will always do the following:
If any one of these steps is overlooked or omitted, the measurement results should not be assumed to be reliable.