Introduction to Radiation Detectors

For those who work with or around radiation, one of the most important factors is an awareness of the levels of radiation around them. This is primarily accomplished through the use of radiation detectors of varying types. A basic understanding of the different types of detectors out there and how they work can go a long way both to finding the best detector for the required task and also for maximizing the benefits of operating that detector.

A NOTE: “GEIGER COUNTERS”

Many people, thinking of radiation detection, tend to group them all together under the term “Geiger counters,” a misconception heartily encouraged by popular TV shows and movies. While one of the most common types of radiation detector is in fact called a “Geiger Mueller (G-M) tube,” the catchall phrase “Geiger Counter” isn’t always the most appropriate. It applies to a very specific type of detector, and generally to a specific application of that detector. Radiation detection devices are typically categorized by either the type of detector element employed, or by the application involved. People will refer to instruments as an Ion Chamber, or a Survey Meter, or a Contamination Meter, or a Frisker Probe. Popular culture has so thoroughly subverted the proper usage of “Geiger Counter” that using the phrase doesn’t generally provide enough information about the device in question.

FIRST RADIATION DETECTORS

Since the early days of radiation testing by Roentgen and Becquerel, scientists have sought ways to measure and observe the radiation given off by the materials they worked with. One of the earliest means of capturing any sort of data from radioactivity was a photographic plate. A photographic plate would be placed in the path/vicinity of a radioactive beam or material. When the plate was developed, it would have spots or be fogged from the exposure to the radiation. Henri Becquerel used a method similar to this to demonstrate the existence of radiation in 1896.

Another common early detector was the electroscope. These used a pair of gold leaves that would become charged by the ionization caused by radiation and repel each other. This provided a means of measuring radiation with a better level of sensitivity than was reliably possible using photographic plates. Depending on the arrangement of the device, they could be configured to measure alpha or beta particles, and were a valuable tool for early experiments involving radioactivity.

An interesting early device, borne out of a desire to measure the actual individual particles or rays being emitted by a radioactive substance, as opposed to a more gross measurement of a radioactive field, was the spinthariscope. Developed by William Crookes, who had also invented the Crookes Tube used by Wilhelm Roentgen to discover X-Rays, it used a zinc sulfide screen at the end of a tube, with a lens at the other end, with a small amount of a radioactive substance near the zinc sulfide screen. The zinc sulfide would react with the alpha particles emitted, and each interaction would result in a tiny flash of light. This was one of the first means of counting a rate of decay, albeit a very tedious one, as it meant scientists had to work in shifts watching and literally counting the flashes of light. The spinthariscope wasn’t very practical as a long term solution for radiation detection, though it did undergo a revival later in the 20^th century as an educational tool. This tendency of certain materials to give off light when exposed to radiation would also prove valuable in future radiation detection technologies.

These early devices, and many others, such as cloud chambers, were valuable in developing an understanding of the basic principles of radiation and conducting important experiments that set the stage for later developments. This included development of new types of radiation detectors, many of which are still in use today, such as G-M Tubes, Ion Chambers, and Scintillators.

WHERE/WHEN YOU’D NEED RADIATION DETECTORS

An important part of knowing what type of detector to use is to have an idea of how and where it will be used. Different applications and settings call for different types of detectors, as each detector type has various ways it can be specialized to fit a role. The applications for radiation detection instruments can be broadly categorized into a few different core tasks: measurement, protection, and search.

Radiation measurement tasks are for situations where there is a known presence of radioactive materials which need to be monitored. The goal with this type of detection is awareness. Awareness of the strength of an established radioactive field, the boundaries of a radioactive area, or simply of the spread of radioactive contamination. These are settings where the presence of radiation is expected, or at least considered likely. The requirements for detectors involved in these settings are unique, often with relatively higher measurement ranges or with modifications needed to specifically look for one type of radiation.

Radiation protection is similar to radiation measurement applications in the sense that it is usually in a setting where radiation is expected to be found. However, the goals are different. With radiation measurement settings, the goal is to monitor the radioactivity itself, to be aware of fluctuations, boundaries, etc. With radiation protection, the goal is monitoring people. Radiation dosimetry is the most common example of this, with radiation badges being worn by medical personnel, nuclear industry workers, and many other occupationally exposed workers all over the world. The importance of this is that it provides protection from the most harmful effects of radiation exposure through awareness, in that a wearer can keep informed of how much radiation they’ve been exposed to, and how that corresponds to potential health effects, and alter their behavior or position or schedule accordingly.

Radiation search differs from the other two basic categories of radiation detection applications in that it is predicated both on the fact that radiation is not expected in the area, and the desire to keep things that way. Primarily the goal of radiation security personnel, first responders, or groups such as customs & border inspectors, radiation search has a different set of requirements to mirror the significantly different circumstances in which it takes place. Detectors need to be highly sensitive, with the concern being more about smaller, concealed radioactive sources or materials. Spectroscopy is often very helpful as well, since it is typically a small subset of radioactive isotopes that are of concern, and being able to filter those out that are present due to legitimate reasons such as medical treatment or just an accumulation of a naturally occurring radioactive substance is important.

These three categories, and the varying tasks that fit inside them, help determine what the best type of instrument or detector is best suited for the task.

TYPES

When talking about radiation detection instruments, there are three types of detectors that are most commonly used, depending on the specific needs of the device. These are: Gas-Filled Detectors, Scintillators, and Solid State detectors. Each has various strengths and weaknesses that recommend them to their own specific roles.

GAS FILLED

The first type of radiation detector, gas-filled detectors, are amongst the most commonly used. There are several types of gas-filled detector, and while they have various differences in how they work, they all are based on similar principles. When the gas in the detector comes in contact with radiation, it reacts, with the gas becoming ionized and the resulting electronic charge being measured by a meter.

The different types of gas-filled detectors are: ionization chambers, proportional counters, and Geiger-Mueller (G-M) tubes. The major differentiating factor between these different types is the applied voltage across the detector, which determines the type of response that the detector will register from an ionization event.

ION CHAMBER

At the lower end of the voltage scale for gas-filled detectors are Ionization Chambers, or Ion Chambers. They operate at a low voltage, meaning that the detector only registers a measurement from the “primary” ions (in actuality pair of ions created: a positively charged ion and a free electron) caused by an interaction with a radioactive photon in the reaction chamber. Thus the measurement that the detector records is directly proportional to the number of ion pairs created. This is particularly useful as a measure of absorbed dose over time. They are also valuable for the measurement of high-energy gamma rays, as they don’t have any of the issues with dead time that other detector types can have.

However, ion chambers are unable to discriminate between different types of radiation, meaning they cannot be used for spectroscopy. They can also tend towards being more expensive than other solution. Despite this, they are valuable detectors for survey meters. They are also widely used in laboratories to establish reference standards for calibrations.

PROPORTIONAL

The next step up on the voltage scale for gas-filled detectors is the proportional (or gas-proportional) counter. They are generally devised so that for much of the area inside the chamber, they perform similarly to an ion chamber, in that interactions with radiation create ion pairs. However, they have a strong enough voltage that the ions “drift” towards the detector anode. As the ions approach the detector anode, the voltage increases, until they reach a point where a “gas amplification” effect occurs.

Gas amplification means that the original ions created by the reaction with a photon of radiation causes further ionization reactions, which multiply the strength of the output pulse measured across the detector. The resulting pulse is proportional to the number of original ion pairs formed, which correlates to the energy of the radioactive field that it is interacting with.

The makes proportional counters very useful for some spectroscopy applications, since they react differently to different energies, and thus are able to tell the difference between different types of radiation that they come into contact with. They are also highly sensitive, which coupled with their effectiveness at alpha and beta detection and discrimination, makes this type of detector very valuable as a contamination screening detector.

GM TUBE

The last major class of gas-filled detectors is the Geiger-Mueller tube, the origin of the name “Geiger Counter.” Operating at a much higher voltage than other detector types, they differ from other detector types in that each ionization reaction, regardless of whether it is a single particle interaction or a stronger field, causes a gas-amplification effect across the entire length of the detector anode. Thus they can only really function as simple counting devices, used to measure count rates or, with the correct algorithms applied, dose rates.

After each pulse, a G-M has to be “reset” to its original state. This is accomplished by quenching. This can be accomplished electronically by temporarily lowering the anode voltage on the detector after each pulse, which allows the ions to recombine back to their inert state. This can also be accomplished chemically with a quenching gas such as halogen which absorbs the additional photons created by an ionization avalanche without becoming ionized itself.

Due to the extensive reaction G-M tubes experience with each pulse of radiation, they can experience something called “dead time” at higher exposure rates, meaning that there is a lag between the pulse cascade and when the gas is able to revert to its original state and be ready to detect another pulse. This can be accommodated for with calibration, or with algorithms in the detection instruments themselves to “calculate” what the additional pulses would be based on the existing measurement data.

SCINTILLATORS

The second major type of detectors utilized in radiation detection instruments are Scintillation Detectors. Scintillation is the act of giving off light, and for radiation detection it is the ability of some material to scintillate when exposed to radiation that makes them useful as detectors. Each photon of radiation that interacts with the scintillator material will result in a distinct flash of light, meaning that in addition to being highly sensitive, scintillation detectors are able to capture specific spectroscopic profiles for the measured radioactive materials.

Scintillation detectors work through the connection of a scintillator material with a photomultiplier (PM) tube. The PM tube uses a photocathode material to convert each pulse of light into an electron, and then amplifies that signal significantly in order to generate a voltage pulse that can then be read and interpreted. The number of these pulses that are measured over time indicated the strength of the radioactive source being measured, whereas the information on the specific energy of the radiation, as indicated by the number of photons of light being captured in each pulse, gives information on the type of radioactive material present.

Due to their high sensitivity and their potential ability to “identify” radioactive sources, scintillation detectors are particularly useful for radiation security applications. These can take many forms, from handheld devices used to screen containers for hidden or shielded radioactive material, to monitors set up to screen large areas or populations, able to differentiate between natural or medical sources of radiation and sources of more immediate concern, such as Special Nuclear Material (SNM).

SOLID STATE

The last major detector technology used in radiation detection instruments are solid state detectors. Generally using a semiconductor material such as silicon, they operate much like an ion chamber, simply at a much smaller scale, and at a much lower voltage. Semiconductors are materials that have a high resistance to electronic current, but not as high a resistance as an insulator. They are composed of a lattice of atoms that contain “charge carriers,” these being either electrons available to attach to another atom, or electron “holes,” or atoms with an empty place where an electron would/could be.

Silicon solid state detectors are composed of two layers of silicon semiconductor material, one “n-type,” which means it contains a greater number of electrons compared to holes, and one “p-type,” meaning it has a greater number of holes than electrons. Electrons from the n-type migrate across the junction between the two layers to fill the holes in the p-type, creating what’s called a depletion zone.

This depletion zone acts like the detection area of an ion chamber. Radiation interacting with the atoms inside the depletion zone causes them to re-ionize, and create an electronic pulse which can be measured. The small scale of the detector and of the depletion zone itself means that the ion pairs can be collected quickly, meaning that the instruments utilizing this type of detector can have a particularly quick response time. This, when coupled with their small size, makes this type of solid state detector very useful for electronic dosimetry applications. They are also able to withstand a much higher amount of radiation over their lifetime than other detectors types such as G-M Tubes, meaning that they are also useful for instruments operating in areas with particularly strong radiation fields.