Introduction to Radiation Detection
Ensuring safe, accurate, and precise delivery of highly complex radiation fields benefits greatly from radiation detectors. Detectors help safeguard against excessive radiation exposure for patients, staff, and visitors in cancer centers. Different applications and settings call for different types of detectors. In cancer centers, applications include quality assurance testing, environmental monitoring, and personnel monitoring. In this blog, the common radiation detection instrumentation used in cancer centers will be discussed. There is a wide variety of radiation detection instrumentation available; however, this blog will only include Geiger-Muller Counters, Ionization chambers, Radiochromic films, and personal radiation badges.
Geiger- Muller Counters
Geiger- Muller (GM) counters are a hand-held survey type meter that is made up of a GM tube. The tube is filled with inert gas, such as helium or argon, at low pressure, to which a high voltage is applied. The tube conducts electrical charge when a particle or photon of incident radiation creates an ionization in the gas molecules. Ionization is the process by which an atom acquires a negative or positive charge by gaining or losing an electron. In this case, ionization results from the loss of an electron after collision with the incident particle or photon. The ionization is amplified within the tube by the Townsend Discharge effect. Townsend Discharge is a gas ionization process in which free electrons are accelerated by an electric field into more gas molecules. The interaction of the electrons with the gas molecules causes more free electrons to be created and the process of ionization continues to accelerate. The amplification of ionization leads to the development of a pulse that is fed to processing and display electronics. To stop the Townsend Discharge, a halogen gas or organic material is added to the gas mixture in a process called quenching.
GM counters can display dose rates in real-time and are sensitive to even low levels of radiation, this makes them important to quality assurance for Brachytherapy because they can ensure that the radioactive seed has properly been retracted from the patient. Although the real-time function of GM counters is useful, there are some limitations to this detection device. This includes the inability to differentiate between different radiation types and the dead time experienced by GM tubes at high count rates. Dead time is when the tube indicates lower rates than actual due to an insensitive period after each ionization of the gas during which any further incident radiation will not result in a count.
Ionization Chambers
Ionization chambers are similar to GM counters in the aspect that they are both filled with gas. The gas-filled chamber has two electrodes; anode and cathode, in which a voltage is applied to maintain an electrical field. The anode is positively charged with respect to the cathode. When the gas between the electrodes is ionized by incident ionizing radiation, it creates positive ions and electrons under the influence of the electric field. These particles move to the electrodes of the opposite polarity causing an ionization current which is a measurement of the total ionizing dose entering the chamber. The output signal of an ion chamber is a continuous current, unlike the GM counter that produces a pulse output. Additionally, ion chambers do not experience dead time. These properties of ion chambers make them useful for quality assurance and beam characterization research.
Radiochromic Film
Radiochromic films (RCFs) consist of a layer of radiation-sensitive material on a thin polyester base with a coating. Ionizing radiation produces a polymerization process in the radiation-sensitive active layer. Polymerization is the process of a chemical change due to the effect of the material under irradiation. The change in the material property modifies the optical properties of the film; thus, causing a color change. RCFs are sensitive to almost any type of ionizing radiation and at the same time almost totally insensitive to room light. To obtain the actual results the films need to be scanned and converted to a dose via software; thus, the real-time readout is not available. However, the wide range of doses that RCFs can detect allows them to be used in various applications in medical physics. For example, RCFs are used to ensure the precision of brachytherapy machinery.
Personal Radiation Badges
Individuals who could be exposed to radiation during their employment over 10% of the appropriate permissible occupational limit or individuals who enter “High Radiation” and “Very High Radiation” Areas are required to wear personal monitoring devices in the form of a radiation dosimeter. A dosimeter can calculate the effective dose of radiation that an individual receives from their working environment and ensures that they are not receiving enough radiation to damage their bodies. Specifically, a dosimeter is the accumulated dose that is delivered by ionizing radiation over a period of time. A dosimeter can either be a whole-body or extremity badge. Typically, extremity badges are only used when personnel is handling radioactive materials.
There are two main categories of dosimeters; Thermoluminescent (TLD) and Optically Stimulated Luminescence (OSL). A TLD dosimeter measures exposure from a crystal embedded within the detector. This crystal emits visible light when it is heated by radiation, the intensity of this visible light is used to calculate the ionizing radiation exposure that the individual is surrounded by. TLDs are reusable and easy to use; however, they can only be read once. OSL dosimeters are composed of crystalline solids that are used to determine the effective dosage of radiation that an individual is exposed to. OSL dosimeters are typically used in situations where real-time information of radiation levels is not required. Instead, they provide a detailed record of the accumulated dose of radiation over time. An OSL dosimeter uses aluminum oxide to absorb X-ray energy, release it, and measure the precise dose of ionizing radiation that is received. The essential difference between a TLD dosimeter and an OSL dosimeter is that a TLD dosimeter requires heat to function, whereas an OSL dosimeter only requires optical stimulation.
Additionally, OSL dosimeters can be read out more than once and are more sensitive than TLDs; thus, making them more suitable for low radiation environments.
Conclusion and Further Resources
For those who work with or are exposed to radiation, it is vital to understand the radiation environment around them. Detectors are used to measure and determine the different types and severity of this radiation. With these measurements, physical structures and procedures can be established to further reduce harmful radiation exposure to patients, staff, and visitors. In next month’s blog, radiation shielding in cancer centers will be discussed. In the meantime, check out the resources below for more information.
https://med-pro.net/tld-dosimeter-vs-osl-dosimeter/
https://www.nature.com/articles/s41598-019-41705-0
https://www.radiation-therapy-review.com/Ionization_Chamber.html
https://www.radiation-therapy-review.com/Geiger-Muller_Counter.html
https://www.mirion.com/learning-center/radiation-detector-types/introduction-to-radiation-detectors