Radiation detectors are essential tools for measuring and analyzing ionizing radiation. From gas-filled Geiger-Müller counters to high-tech semiconductor devices, each type offers unique capabilities for different applications.

Understanding these detectors is crucial for radiation safety, environmental monitoring, and nuclear research. This section explores the principles behind various detector types, their strengths, and common uses in radiochemistry and related fields.

Gas-Filled Detectors

Geiger-Müller Counter

• Detects ionizing radiation using a gas-filled tube
• Consists of a cylindrical cathode and a thin wire anode running through the center
• Filled with an inert gas (helium, neon, or argon) and a small amount of a quenching gas (ethanol or halogen)
• When radiation enters the tube, it ionizes the gas molecules, creating ion pairs
• The strong electric field accelerates the ions towards the electrodes, causing further ionization and creating an avalanche effect
• The resulting electrical pulse is counted by the detector, giving a measure of the radiation intensity
• Operates in the Geiger-Müller region, where the applied voltage is high enough to cause complete ionization of the gas
• Cannot distinguish between different types or energies of radiation
• Commonly used for radiation safety monitoring and contamination checks

Proportional Counter

• Similar in construction to a Geiger-Müller counter but operates at a lower voltage
• The applied voltage is high enough to cause gas amplification but not complete ionization
• The magnitude of the output pulse is proportional to the energy of the incident radiation
• Can distinguish between different types and energies of radiation based on the pulse height
• Filled with a noble gas (argon or xenon) and a quenching gas (methane or carbon dioxide)
• Used for low-level radiation measurements and spectroscopy applications
• Provides better energy resolution than Geiger-Müller counters but lower sensitivity

Solid-State Detectors

Scintillation Detector

• Uses a scintillator material that emits light when exposed to ionizing radiation
• Common scintillators include sodium iodide (NaI), cesium iodide (CsI), and bismuth germanate (BGO)
• The scintillator is coupled to a photomultiplier tube (PMT) or a photodiode, which converts the light into an electrical signal
• The intensity of the light is proportional to the energy of the incident radiation
• Provides good energy resolution and high sensitivity
• Widely used for gamma-ray spectroscopy and radiation monitoring
• Can be made in various shapes and sizes to suit different applications (handheld devices, well detectors, or large-volume detectors)

Semiconductor Detector

• Uses a semiconductor material (silicon or germanium) as the active detection medium
• When radiation interacts with the semiconductor, it creates electron-hole pairs
• The number of electron-hole pairs is proportional to the energy of the incident radiation
• An applied electric field separates the electrons and holes, generating an electrical signal
• Provides excellent energy resolution, better than scintillation detectors
• Commonly used for high-resolution gamma-ray and X-ray spectroscopy
• Requires cooling to reduce thermal noise and maintain high performance (liquid nitrogen or electromechanical cooling)
• Examples include high-purity germanium (HPGe) detectors and silicon drift detectors (SDDs)

Specialized Detectors

Neutron Detectors

• Designed to detect neutrons, which are electrically neutral particles
• Often rely on neutron-induced nuclear reactions to generate detectable signals
• Common neutron detectors include:
  • Boron trifluoride (BF3) proportional counters: Utilize the 10B(n, α)7Li reaction
  • Helium-3 (3He) proportional counters: Utilize the 3He(n, p)3H reaction
  • Fission chambers: Contain a fissile material (235U) that undergoes fission when exposed to neutrons
• Used in nuclear reactors, neutron scattering experiments, and radiation safety monitoring

Gamma-Ray Spectrometer

• Designed to measure the energy spectrum of gamma radiation
• Commonly based on scintillation detectors (NaI or HPGe) coupled with multichannel analyzers (MCAs)
• The MCA sorts the detector pulses into different energy channels, creating a histogram of the gamma-ray energies
• Used for radionuclide identification, environmental monitoring, and nuclear physics research
• Can be portable (handheld) or laboratory-based systems

Alpha Spectrometer

• Designed to measure the energy spectrum of alpha particles
• Often uses semiconductor detectors (silicon) or ionization chambers
• Alpha particles have a short range in air, so the sample must be placed close to the detector in a vacuum chamber
• Provides high-resolution energy spectra for alpha-emitting radionuclides
• Used in environmental monitoring, nuclear forensics, and radiochemistry research

Liquid Scintillation Counter

• Used for measuring low-energy beta radiation and alpha particles in liquid samples
• The sample is mixed with a liquid scintillator cocktail, which contains a solvent and a scintillator
• When the radiation interacts with the scintillator, it produces light pulses that are detected by photomultiplier tubes
• Provides high counting efficiency for beta and alpha radiation
• Commonly used for radioimmunoassay, environmental monitoring, and radiochemistry applications
• Requires careful sample preparation and calibration to account for quenching effects (color, chemical, or optical)