04/04/2026
RADIATION DETECTORS 101: Types of Ionizing Radiation Detectors โข๏ธ๐
In health physics, radiation detection and protection is considered the heart of the profession! Hence, health physicists always use ionizing radiation detectors, which are devices that detect and measure ionizing radiation.
Ionizing radiation detectors come in many forms, some small enough that you can comfortably hold them, while others are large detectors with large, sensitive volumes. They are generally classified according to their operating mechanism, and they are used depending on various applications. Some major classes of radiation detectors are as follows:
1. Gas-Filled Detectors ๐จ
MECHANISM: They operate through direct ionization produced by radiation as it interacts with a gas inside the cavity. A voltage is applied across the electrodes, which creates an electric field that transports the electrons to the anode and positive ions to the cathode. The collected electrons and ions create an electric signal (voltage pulse or current, depending on the mode) in the external circuit, which are then measured accordingly.
Depending on the applied voltage, these operate in three distinct regions:
โค Ionization chambers: They operate at lower voltages and are used for high-precision measurements.
โค Proportional counters: They operate at intermediate voltage regions and are used for spectroscopy, i.e. differentiating radiation types (e.g., alpha vs. beta).
โค Geiger-Mรผller counters: They operate at higher voltages and are used for radiation surveying.
2. Semiconductor Detectors ๐๏ธ
MECHANISM: In these solid-state detectors, radiation interacts with the crystal lattice, which excites electrons from the valence band across the bandgap into the conduction band. This creates electron-hole pairs within the semiconductor, which are then separated by an applied electric field (usually within a reverse-biased p-n junction). These free charges drift toward electrodes, then electrons to the anode and holes to the cathode, which generate electrical signals that are measured.
These detectors have high energy resolution and are therefore widely used in spectroscopy. They are compact in size and have a fast response time. Some examples include:
โค Silicon-based detectors
โค High-purity Germanium detectors
โค MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) detectors
โค Diamond detectors
3. Scintillation Detectors ๐ฆ
MECHANISM: These solid-state detectors rely on the excitation of electrons rather than direct charge collection. When radiation interacts with the scintillator material, it excites electrons to higher energy states, and as these electrons de-excite, they release energy in the form of visible or UV light photons. They are directed into a photomultiplier tube (PMT), which converts light back into electrons through the photoelectric effect, and a series of dynodes multiplies this electron signal into a measurable electrical signal.
These detectors are considered water-equivalent, with radiation absorption and scattering properties that closely match those of human soft tissue, which makes them ideal for radiation dose detection and medical imaging. Some scintillator materials, which emit light when struck by ionizing radiation, are as follows:
โค Organic plastic scintillators
โค Organic liquids
โค Organic crystals stilbene and anthracene
4. Luminescence Detectors ๐ก
MECHANISM: These solid-state detectors utilize the electron trapping process. When exposed to radiation, electrons are excited to the conduction band and then fall into metastable energy traps within the bandgap, which are created by crystal lattices containing deliberate impurities. If heated or subjected to light, the trapped electron returns to the valence band and emits light.
These detectors are typically used in the personal monitoring of radiation dose, and are usually wore by radiation workers (hence called personal dosimeters). There are two major types of luminescence detectors:
โค Thermoluminescent dosimeters (TLDs): Use heat to release stored energy as light.
โค Optically-stimulated luminescence dosimeters (OSLDs): Use light to release stored energy as light.
5. Chemical Detectors ๐งช
MECHANISM: As the name suggests, they rely on radiation, which induces a measurable, permanent chemical reaction in a medium. In general, the absorbed radiation energy breaks chemical bonds or alters oxidation state, and the amount of the new chemical product formed (or other quantities, such as optical density) is directly proportional to the absorbed dose.
There are multiple classes of chemical dosimeters:
โค Radiographic films
โค Radiochromic films
โค Fricke dosimeters
โค Alanine dosimeters
REFERENCES:
[1] Andreo, P., Burns, D.T., Nahum, A.E., Seuntjens, J., and Attix, F.H. Fundamentals of Ionizing Radiation Dosimetry. Wiley-VCH 2017.
[2] Turner, J.E. Atoms, Radiation, and Radiation Protection 3rd Edition. Wiley-VCH 2007.
[3] Lecture slides from Dr. John Paul O. Bustillo of DPSM
โ๏ธ & ๐จ: River Villanueva
