What will happen when the ionization chamber in an AEC will be exposed to radiation?

III.A Ion Chambers

An ion chamber is a device in which two electrodes have been placed within an enclosed volume of gas to create an electric field. Ion pairs that are formed along the track of an energetic particle are caused to drift under the influence of this field. In its simplest form, the ion chamber is operated in current mode by measuring the average current corresponding to the drifting ions and electrons within the active volume of the chamber. If the electric field is sufficiently high to prevent loss of charge because of the recombination of the positive ions and electrons, a condition of ion saturation is reached. Then, the measured current is simply proportional to the rate of formation of charge within the ion chamber and is proportional to the intensity of the incident radiation. Ion chambers used in current mode are widely applied in radiation dosimetry and in the monitoring of high fluxes of radiation as encountered, for example, in nuclear reactor instrumentation systems.

Ion chambers can also be operated in pulse mode, but usually only for energetic, heavy charged particles. The amount of energy deposited by fast electrons over realistic dimensions in a gas is so small that the corresponding pulse amplitude is generally too small to be of use.

In a gas, approximately 35 eV of particle energy are expended per ion pair formed. The number of charge carriers that are produced per unit energy loss is therefore intermediate between that of semiconductor detectors and scintillation detectors. The corresponding energy resolution of pulse-type ion chambers therefore also lies between that of semiconductor detectors and scintillators, with values ranging from a few tenths to several percent.

Even for heavy charged particles, the pulse amplitude observed from ion chambers is very small. If we assume a deposited energy of 1 MeV, then about 30,000 ion pairs are formed in typical gases. Each ion pair carries one electronic charge, so Eq. (1) predicts an amplitude of about 50 μV across a collection capacitance of 100 pF. This amplitude is so small that great care must be taken to minimize electronic noise introduced in the preamplifier if the energy resolution is to be preserved. To help alleviate this problem, many gas-filled detectors are instead operated as proportional counters, as described next.

Electret Ion Chamber

EICs provide an integrated measurement of the average radon gas concentration during the exposure period. A positively charged electret is used in conjunction with an ionization chamber made of an electrically conductive plastic. Devices that are designed for short-term measurements use a short-term electret and a short-term chamber that incorporates a spring-loaded mechanism for exposing the electret to the full volume of the chamber at the time of placement. At the end of the exposure period, this mechanism is closed such that the electret is exposed to a tiny volume of air, thus effectively ‘turning off’ the device. Devices that are designed for long-term measurements use a less sensitive electret and a smaller chamber. All types of these devices have a filter in the opening of the chamber to preclude the passage of particulate radioactive materials, such as radon decay products, into the chamber. The electret, which is an electrically charged plastic disk, functions both as the source of an electric field and as a sensor in the device. As radon and its decay products undergo radioactive decay within the chamber, the emitted radiation ionizes the air. The positively charged electret collects negative ions that have been produced in the chamber air, resulting in a discharge of the electret that is related to the integrated ionization during the measurement period (Figure 4).

The electrical potential on the electret is measured before and after the exposure using an electret reader (an electrostatic voltmeter). Some long-term devices are read only by the manufacturers, whereas others can be read by another laboratory or by the end user. EICs that are read by the end user are analyzed by inputting the starting and ending potentials of the electret, the ambient background radiation in μR h− 1, the exposure duration in days, and the elevation at which the device was exposed into an algorithm supplied by the manufacturer. This algorithm calculates the calibration factor and the radon concentration based on calibration exposures performed by the manufacturer. The duration of the measurement period depends in part on the type of electret used (short- or long-term) and the size of the chamber. Short-term electrets and chambers are designed to measure radon for 2 days to 2 weeks. Alternatively, long-term devices can provide an integrated average radon concentration for measurement periods spanning several weeks to a year. EICs are not affected by temperature or humidity in the ranges normally encountered indoors, but condensing moisture should be avoided. The algorithm adjusts for effects due to elevation and ambient background radiation. Best results are obtained when the ambient background is measured or otherwise well known. Sources of gamma radiation in addition to ambient background, such as stone or ceramic ware containing natural radioactivity, can cause the device to give a falsely high reading. The chambers and electrets must be kept clean, because dust or particulates that come in contact with the electret may discharge it. These devices have been used in many countries and have the potential to provide accurate and precise radon gas measurements if standard operating procedures are carefully followed. A very useful function of the short-term device is that it can be opened and closed repeatedly during the exposure period, thus allowing radon measurements for specific periods of the day (e.g., school day hours and work hours) (Figure 5).

5. Electret Detectors

Electret types of ion chambers make use of the drop of surface voltage on a plastic material. The plastic specimen is a dielectric material, usually Teflon, which is quasi-permanently charged. It is called an electret and usually has the shape of a disk about 1 mm thick and 10 mm in diameter. Electrets are prepared by being heated and simultaneously exposed to an electric field. Due to this process, many dipoles in the material become oriented in a preferred direction. After the heating, the material is “frozen” and is able to keep the position of its electric dipoles for a long period of time. A voltage gradient of several hundred volts can be maintained between the surfaces of the electret disk.

One surface of the electret is kept in contact with the wall of an ion chamber, which builds up an electric field in the chamber. Ionizing radiation causes a decrease of charge in that system, resulting in a partial neutralization of the charge at the electret. Measurement of the electret voltage difference before and after irradiation allows determination of the amount of ionization. The system has to be calibrated and can be used for determination of environmental radiation doses.

Amrani et al. (2000) used an electret ion chamber for the determination of the radon content of groundwater. They put an electret ion chamber together with a known amount of water in a leak proof container. The reading of the electret ion chamber provides the radon content in the air, and this value could be related to the concentration of radon in the water sample.

Measuring Ionizing Radiation

The particle counter, the ionization chamber, photographic film, and the thermoluminescent detector are four methods widely used to measure radiation dose, dose rate, and the quantity of radioactive material present.

Particle counters are designed to detect the movement of single particles through a defined volume. Gas-filled counters collect the ionization produced by the radiation as it passes through the gas and amplify it to produce an audible pulse or other signal. Counters are used to determine radioactivity by measuring the number of particles emitted by radioactive material in a given time.

Ionization chambers consist of a pair of charged electrodes that collect ions formed within their respective electric fields. Ionization chambers can measure dose or dose rate because they provide an indirect representation of the energy deposited in the chamber.

Photographic film darkens on exposure to ionizing radiation and is an indicator of the presence of radioactivity. Film is often used for determining personnel exposure and making other dose measurements for which a record of dose accumulated over a period of time is necessary, or for which a permanent record is required.

Thermoluminescent detectors (TLD) are crystals, such as NaI, that can be excited to high electronic energy levels by ionizing radiation. The excitation energy is then released as a short burst or flash of light, which can be detected by a photocell or photomultiplier. TLD systems are replacing photographic film because they are more sensitive and consistent. Liquid scintillators, organic phosphors operating on the same principle as TLDs, are used in biochemical applications.

IX.A Gas-Filled Detectors

Gas-filled detectors include ionization chambers, gas proportional counters, and Geiger–Müller (GM) counters. Basically, each of these typically consists of a cylindrical chamber with an axial central electrode, filled with a suitable gas. In principle, a given detector can be operated in each of these three detection modes by choosing the appropriate applied voltage. Operation as an ionization chamber involves use of an applied voltage that is large enough to collect all of the ion pairs (positive ion and removed electron) produced in the gas by a radioactive source, but not large enough to cause any gas amplification. In a given measurement, the ionization current can be most sensitively measured by an attached vibrating-reed electrometer operating in the rate-of-charge mode or, for larger ion currents, in the calibrated-resistor voltage drop mode. At higher applied voltages, gas amplification sets in, increasing the number of ion pairs collected per detected particle by a sizable amplification factor. This corresponds to operation as a gas proportional counter, capable of distinguishing between α particles and β particles, and operation at high counting rates with very little dead time loss of counts. In the proportional mode, the size of each output electrical pulse is directly proportional to the amount of energy dissipated in the gas of the detector. At still higher applied voltages (e.g., 1000–2000 V), the detector operates as a GM counter. Now each electrical output pulse is much larger than those in the proportional region and is no longer related to the energy of the detected α or β particle. Geiger–Müller counters, because of the detailed nature of the avalanche of ion pairs produced in a single counting event, have a dead time (usually in the range of a few hundred microseconds) following each interaction, which limits them to lower counting rates before dead time losses of counts become severe. For example, with a GM counter having a dead time of 300 μsec per pulse and a sample giving an observed counting rate of 20,000 counts per minute (cpm), 10% of the possible counts have already been lost because of the detector dead time period after each recorded count. With a gas proportional counter having a much shorter dead time period per pulse (e.g., 3 μsec), a 10% loss of counts is not reached until an observed counting rate of 2 × 106 cpm.

Ionization chambers, gas proportional counters, and GM counters can all be used to measure the radioactivity of radioactive gases (e.g., CO2 containing some 14CO2) by placing the gaseous sample inside the detector or flowing it steadily through the detector. In the two that operate as pulse counters (rather than ionization current measurement), the gaseous sample usually must be mixed with a suitable counting gas. When solid samples of α or β emitters are to be counted, the sample is usually placed close to one end of the cylindrical counter. If the counter is a windowless flow counter, there is no window separating the sample from the counter interior, and a counting gas is flowed steadily through the counter. Such a counter is often used for α emitters or radionuclides that emit very low energy β particles (e.g., the 0.019-MeV Emax, β− particles of tritium), since such particles are readily absorbed by all but the thinnest of counter windows. For most β counting of solid samples by gas proportional or GM counters, the sample is separated by a thin window from the counter gas that is sealed inside the counter. Such thin windows, often made of mica or Mylar, absorb the lowest-energy β particles, but allow the more energetic ones to pass into the interior of the detector. Once inside the counter, α and β particles are counted with essentially 100% efficiency. Both α and β particles lose an average of about 35 eV of kinetic energy per ion pair produced in a gas, the exact value depending on the particular gas.

Besides their use in research and radiotracer studies, all three of these kinds of gas-filled detectors are used as safety monitoring instruments (discussed later). For such uses, a battery-operated portable instrument is usually employed. The corresponding research instruments, however, are usually ac-operated, not readily portable, have a lead shield around the detector (to reduce the counting rate due to background gamma radiation), and have more complicated electronics (e.g., a stable high-voltage supply, a pulse amplifier, and either a scaler or ratemeter).

3.4.9.2 Working Procedure

PID introduces a gas sample into an ionization chamber at one end of a drift tube. UV light from a PI source ionizes the ionizable molecules contained in the gas sample. The PI source includes either multiple UV lamps (each having a specific energy level for discriminating between potential constituents of the gas sample) or one multiple energy level UV lamp with different light bandwidth window zones and a zone selector. A shutter grid separates the ionization chamber from the drift tube. When the shutter grid opens, an electric field in the drift tube attracts ions, that travel against the flow of a drift gas until it is being collected by an electrode at the end of the drift tube. A time required for the ions to travel the length of the drift tube is characteristic of this type of ion. Thin mesh electrodes in the drift tube sustain a uniform electric field so that groups of ions traveling down the drift tube create well-defined current pulses at the collector electrode (Yang and Hsi, 2003).

Electret ionization chambers

A more recent application of the primitive total ionization chambers (such as the electroscopes used, for example, by Rutherford in the early 1900s), is based on the use of an electret, which holds a charge over a long period and is discharged by exposure to radiation. The loss of a charge is then measured by an electrostatic voltmeter and related to radon exposure through a calibration process. The chamber also responds to γ-radiation and the total signal must be corrected for this response. At average radon concentrations and γ-levels, the two signals are approximately equal. When used as a screening technique for short-term measurements, the assessment of the γ-exposure can be estimated with any available instruments, such as Geiger counter.

However, for the long-term measurements required for the assessment of human exposure, paired chambers must be used in order to ensure the accurate discrimination of the radon signal from that of the γ-background.

IV.A.5.b Active ionization detectors

In an active (or current-type) ionization chamber, electrons collected at the anode compose a direct current that can be amplified in an electrometer tube and measured with a microammeter. For situations requiring high accuracy, the small current may be measured with a vibrating reed electrometer that transforms a small direct current into pulses that can be amplified. Essentially all ionization chambers are evaluted in one of these ways.

Probably the simplest of the active ionization chambers, yet the type least familiar in terms of widespread use, is the free-air ionization chambers. This detector is used as a primary standard in national standardization laboratories throughout the world. The chamber is a parallel-plate design that satisfies the operational definition of exposure in units of roentgens. The photon beam is collimated as it enters the chamber and interacts in a volume of air defined by the collimator aperture and the electric field between the collecting electrodes. The chamber features a guard ring and guard wires to maintain straight lines of force between the two electrodes. The entire device is enclosed, usually with a lead-lined material. Ions produced in the chamber volume due to photon interactions are collected at the plates. The current flow is measured by an external circuit, and from it the number of ions produced in the volume and, ultimately, the exposure can be calculated.

For this measurement to be valid, electronic equilibrium must exist in the detector. In other words, all the energy of primary electrons produced in the sensitive volume of the chamber must be dissipated in the chamber. Obviously, many electrons produced in the detector volume by photon interactions will leave the sensitive volume. Electronic equilibrium is maintained by making the entire chamber larger than the maximum range of the primary electrons in air. In this situation, primary electrons produced in the sensitive volume that leave the volume are replaced by primary electrons that were produced outside the sensitive volume but enter it. Thus, electronic equilibrium is obtained as an electron of equal energy enters into the sensitive volume for every electron that leaves.

The thickness of air between the entrance port and the collecting volume needed to provide electronic equilibrium increases with increasing photon energy. For example, 9 cm of air is required for highly filtered, 250-kV X-rays, whereas for 500-kV X-rays, the air thickness is 40 cm. This fundamental requirement limits the use of free-air chambers since the size of the chamber for higher photon energies is extremely large. For example, the NIST has three free-air ionization chambers. The chambers are intended to cover the X-ray generating potentials of 10–60 kV, 20–100 KV and 60–250 kV. These chambers were manufactured at the NIST, but similar chambers are commercially available with a useful range up to ∼300 keV. At ranges greater than this, photon energy, operational difficulties, chamber size, and so forth limit this detector's usefulness even in a standards laboratory.

The surface dose due to beta-emitters may be determined by use of the extrapolation chamber. This special ionization chamber is a parallel-plate detector similar in design to the free-air chamber described above. However, in this design the distance between the plates can be varied. Usually, one plate that acts as a thin window is placed as close as possible to the source to be measured. A series of measurements is obtained while the spacing between the plates is decreased. The results of these measurements are plotted and extrapolated to zero spacing. This gives the dose at the surface of the beta source and eliminates secondary gas or wall effects.

The use of an extrapolation chamber is a good example of the application of the Bragg-Gray principle to the measurement of absorbed dose (discussed earlier in this article). The chamber, introduced by Failla, is quite useful because it recognizes the fundamental requirement that the detector cavity be small compared with the electron ranges. The chamber has also been used for measurements in areas where no electronic equilibrium exists, for example, at interfaces between two dissimilar materials. Currently, there is increased interest in the use of extrapolation chambers for measurements in beta-radiation fields found in nuclear utilities. However, this is a special application of this detector system since the chamber is not sufficiently rugged to survive in routine use in these environments. Tissue-equivalent extrapolation chambers have also been designed and used in a number of dosimetry research activities. However, there has been no widespread application of this system to routine dosimetry.

Ionization chambers have found wide use in surveys for radiation protection purposes. The ionization chamber is the only gas-filled detector that allows the direct determination of the absorbed dose. This is because the measured current is directly proportional to the ionization produced in the sensitive volume and that in turn is directly proportional to the energy deposited in the detector.

A number of active detectors have been designed with characteristics similar to the condenser R chambers discussed in the previous section. One such system is a precision instrument designed specifically for the measurement of ionizing radiation used in medical diagnostic and therapeutic procedures. The individual chambers have walls constructed of air-equivalent materials, and the sensitive volume is filled with air. A preamplifier located close to the detector allows a reasonably long run of cable between the detector and the readout. The readout system functions either as a rate-meter or as an integrating device. In addition, a high-voltage supply for the chamber is an integral part of readout. The entire system is very stable and accurate, and state-of-the-art solid-state electronics makes it easy to operate. As with the condenser R chambers, the detectors may be purchased with a calibration traceable directly to the NIST. These systems have found wide use in instrument calibration facilities in many utilities. The systems provide immediate and accurate indications of the exposure rates, which lends confidence to portable calibration procedures. In addition, the systems can be used in the integrate mode to monitor standard exposures of pocket chambers or TLD badges.

A multitude of special detectors have been designed and used for dosimetry in mixed-radiation fields. These systems use paired chambers, one of which is sensitive essentially to only one components of the field and the other detector which is sensitive to both components of the radiation field. If the detectors have been properly calibrated, the exposure rate (or dose rate) of the radiation field for each component can be obtained by subtraction. One such system uses a chamber constructed of tissue-equivalent material through which a tissue-equivalent gas is flowing. This detector is sensitive to both gamma and neutron radiation. The other detector is constructed of graphite, and the filling-gas is carbon dioxide. The graphite is sensitive only to gamma radiation, whereas the tissue-equivalent chamber is sensitive to both neutron and gamma radiation. The neutron component of the radiation field can be obtained by substracting the dose rate indicated by the graphite detector from that indicated by the tissue-equivalent detector.

IV.A.3 The Electron-Capture Detector

This detector is a device based on certain gas-phase ionization phenomena within the ionization chamber. Its schematic diagram is given inFig. 11. The carrier gas molecules, flowing through the ionization chamber, are bombarded by the radioactive rays from the source of radiation (usually a foil containing 63Ni or 3H) incorporated into the detector body. In a rather complicated process, radicals, positive ions, and low-energy electrons are created. Application of electric potential between the electrodes permits the easily collected electrons to be continuously monitored as the so-called standing current (typically, around 10−9  A). This steady current provides a baseline value for the measurement of substances with a strong affinity to such low-energy electrons. When an electron-capturing solute enters the detector, it decreases the population of electrons by an electron attachment process. A decrease of standing current thus occurs during the passage of a solute band, resulting in a negative chromatographic peak.

The decrease of standing current due to the electron-capture process is proportional to the solute concentration in a process reminiscent and formally similar to Beer's law of optical absorption, except that thermal-energy electrons rather than photons are involved:

(12)E=E0exp−Kxc,

where E is the number of electrons reaching the anode per second, E0 is the initial number of electrons, K is the electron-capture coefficient (a function of molecular parameters), x is the detector geometrical constant, and c is the solute concentration.

The electron-capture detector is a selective measurement device since only certain compounds exhibit appreciable affinities toward the low-energy electrons. Among the structures exhibiting strong electron affinities are various halogenated compounds, nitrated aromatics, highly conjugated systems, and metal chelates. The detector is extremely sensitive (amounts between 10−12 and 10−15  g can be detected) to various pesticides, herbicides, dioxins, freons, and other substances of great environmental concern. To achieve this extremely high sensitivity for normally noncapturing types of molecules (e.g., hormones and drug metabolites), various electron-capturing moieties can be introduced via chemical derivatization (a controlled sample alteration).

I.B History of the Field

Although the cosmic radiation was first detected in 1911 by V. Hess from balloon flights with ionization chambers, its exact nature was not known for several decades. By the late 1930s, it was known that the cosmic rays are atomic nuclei moving at high energies. In the late 1940s, W. Libby and co-workers established the use of cosmogenic radiocarbon, 14C, to date terrestrial samples. The activities of 14C measured in objects of known ages agreed well with predicted values, showing that the intensities of the cosmic rays had been fairly constant over the past several thousand years. Also in the 1940s, helium was measured in a number of iron meteorites, but it was assumed that all of the helium was made by the radioactive decay of uranium and thorium. C. Bauer argued in a short note published in 1947 that most of the helium was produced by cosmic radiation. Soon F. Paneth and others measured that about 20% of the helium in iron meteorites was 3He, thus confirming the origin of the helium as the product of nuclear reactions between the energetic particles in the cosmic rays and the iron. At this time, accelerators were starting to produce protons with energies of a few GeV, and nuclear scientists were systematically studying spallation reactions similar to those that make helium in iron meteorites.

In the decade after Paneth's initial measurements of cosmogenic helium in iron meteorites, the study of cosmogenic nuclides in meteorites advanced rapidly. Cosmogenic noble-gas isotopes other than helium were measured in several stony and iron meteorites. Then a number of cosmogenic radionuclides were observed in meteorites, starting with the detection of 12.3-year 3H by low-level gas counting techniques. Shortly thereafter, other cosmogenic radionuclides, such as 26Al, 10Be, and 60Co, were observed in meteorites, and the cosmogenic pair of 3H/3He was used to determine the exposure ages of meteorites.

Around 1960, a number of systematic studies of cosmogenic nuclides in meteorites were done. Noble-gas isotopes were measured in many pieces from slabs of several iron meteorites, such as Carbon and Grant. Many radionuclides were measured in several freshly fallen meteorites, such as the iron Yardymly (then called Aroos) and the stone Bruderheim. Several research groups also measured noble-gas isotopes in a suite of meteorites. New techniques to measure cosmogenic radionuclides, such as nondestructive γ-ray spectroscopy, were developed.

In parallel with the rapid growth in cosmogenic-nuclide measurements, a number of theoretical models were developed. Simple models in which the primary cosmic-ray particles are exponentially attenuated and secondary particles are produced and removed were used by several investigators, such as P. Signer and A. Nier, who applied such a model to their noble-gas data for the Grant iron meteorite. Numerous experiments at accelerators in which thin or thick targets were bombarded with high-energy protons established production ratios for many cosmogenic nuclides. J. Arnold, M. Honda, and D. Lal estimated the energy spectrum of primary and secondary cosmic-ray particles in iron meteorites and calculated production rates of cosmogenic nuclei using cross sections for many reactions.

These measurements and models for cosmogenic nuclides in meteorites were applied to a number of studies, such as the constancy of cosmic rays over time. The ratio of the measured radionuclide activities to the calculated values showed no systematic trends for half-lives less than a million years. H. Voshage and H. Hintenberger found that the 40K/41K ratios measured in iron meteorites were inconsistent with the ratios for other radioactive/stable pairs of cosmogenic nuclides, implying that the fluxes of cosmic rays have been higher over the past 106 years than over the past 109 years. E. Fireman, R. Davis, O. Schaeffer, and co-workers used the measured 37Ar/39Ar ratios in studies of the spatial variation of cosmic rays between 1 and about 3 astronomical units (AU) from the sun (the region of space in which most meteorites probably traveled). A variety of pairs of radioactive and stable nuclides, such as 3H/3He and 39Ar/38Ar, were used to determine the lengths of times that meteorites were exposed to cosmic rays.

During the 1960s, additional measurements were made and the ideas used to interpret the observations were refined. Measurement techniques for the tracks produced in certain minerals by heavy nuclei were developed and applied to meteorites. Some meteorites, those with high concentrations of trapped gases and tracks, were realized to have been exposed to energetic solar particles on the surface of some parent object. The orbits of three stony meteorites, Pribram, Lost City, and Innisfree, were accurately determined by several photographic networks, and all had aphelia in the asteroid belt and perihelia near 1 AU. Bombardments were done at accelerators to simulate the cosmic irradiation of meteorites and used to predict the profiles for the production of nuclides in meteorites.

In the early 1970s, the studies of cosmogenic nuclides in meteorites declined as most investigators were studying lunar samples. Techniques for studying nuclear tracks were developed and used to study the irradiation history of meteorites. Some new methods for measuring cosmogenic nuclides were perfected using lunar samples. The lunar-sample studies confirmed meteoritic results about the galactic cosmic rays and gave us our first detailed knowledge of cosmogenic nuclides produced by the solar cosmic rays. In the late 1970s, interest in the studies of meteorites increased, especially for stones. Measurements of the distributions of the cosmic rays in the solar system by various satellites helped in interpreting the meteoritic cosmic-ray record.

In the 1980s, it was recognized that some meteorites have been ejected from planetary objects, the Moon and Mars. Many meteorites were being recovered from ice fields in Antarctica and from several deserts and arid regions around the world, which greatly expanded the numbers of meteorites available for study. Thermoluminescence was now being routinely used for many studies. Cosmogenic nuclides were being measured in samples much smaller than previously possible because of the use of improved or new measurement techniques (such as accelerator mass spectrometry). By the end of the 20th century, the study of the cosmic-ray records of meteorites was a mature field with gradual but steady advances in all of its aspects.