8.2 Scintillation and semiconductor detectors

Scintillation and semiconductor detectors are crucial tools in nuclear physics. They convert radiation into measurable signals, allowing scientists to study radioactive decay and nuclear reactions. These detectors differ in their materials and operating principles, offering unique advantages for various applications.

Scintillators use light-emitting materials and photomultiplier tubes, while semiconductors rely on electron-hole pair creation in crystals. Both types have strengths: scintillators are versatile and cost-effective, while semiconductors offer superior energy resolution. Understanding their properties is key to choosing the right detector for specific experiments.

Scintillation Detectors

Scintillation Process and Photomultiplier Tube

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• Scintillation process converts ionizing radiation into visible light
• Incident radiation excites atoms or molecules in scintillator material
• Excited atoms quickly de-excite, emitting photons in visible spectrum
• Scintillator materials include organic crystals (anthracene), inorganic crystals (NaI(Tl)), and organic liquids (liquid xenon)
• Photomultiplier tube (PMT) amplifies weak light signals from scintillator
• PMT consists of photocathode, dynodes, and anode
• Photocathode converts light photons into electrons through photoelectric effect
• Dynodes multiply electrons through secondary emission
• Typical PMT gain ranges from 10^5 to 10^7
• Anode collects amplified electron signal for further processing

Sodium Iodide Detector and Energy Resolution

• Sodium iodide (NaI) detector widely used for gamma-ray spectroscopy
• NaI crystal typically doped with thallium (Tl) to enhance light output
• NaI(Tl) offers high detection efficiency for gamma rays due to high atomic number of iodine
• Light output of NaI(Tl) proportional to energy deposited by incident radiation
• Energy resolution of NaI(Tl) detector typically ranges from 6% to 10% at 662 keV
• Factors affecting energy resolution include statistical fluctuations in scintillation process, light collection efficiency, and PMT performance
• NaI(Tl) detectors operate at room temperature, making them convenient for field applications
• Limitations include hygroscopic nature (sensitive to moisture) and relatively poor energy resolution compared to semiconductor detectors

Semiconductor Detectors

Germanium Detector Characteristics

• Germanium detectors offer superior energy resolution for gamma-ray spectroscopy
• High-purity germanium (HPGe) crystals used to minimize impurities
• Germanium has lower band gap (0.67 eV) compared to silicon (1.12 eV)
• Cooling to liquid nitrogen temperature (77 K) required to reduce thermal noise
• Depletion region created by applying reverse bias voltage
• Incident radiation creates electron-hole pairs in depletion region
• Number of electron-hole pairs proportional to energy deposited
• Charge carriers collected by electrodes, producing electrical signal
• Energy resolution of HPGe detectors typically 0.1% to 0.3% at 662 keV
• Excellent for identifying closely spaced gamma-ray energies in complex spectra

Silicon Detector Applications

• Silicon detectors widely used for charged particle detection
• Operate at room temperature due to larger band gap compared to germanium
• Silicon surface barrier detectors measure alpha particles and heavy ions
• Lithium-drifted silicon (Si(Li)) detectors used for X-ray spectroscopy
• Silicon drift detectors (SDD) offer improved energy resolution and count rate capability
• Silicon photomultipliers (SiPM) combine advantages of solid-state detectors and PMTs
• SiPMs consist of arrays of avalanche photodiodes operating in Geiger mode
• Applications include particle physics experiments, medical imaging (PET scanners), and radiation monitoring

Energy Resolution Comparison

• Energy resolution quantifies detector's ability to distinguish between closely spaced energies
• Expressed as full width at half maximum (FWHM) divided by peak centroid energy
• Semiconductor detectors offer superior energy resolution compared to scintillators
• HPGe detectors achieve ~0.2% resolution at 662 keV
• Si(Li) detectors typically offer 150-200 eV resolution for X-rays
• Factors affecting energy resolution include charge carrier statistics, electronic noise, and charge collection efficiency
• Trade-offs between energy resolution, detection efficiency, and operational requirements (cooling, cost) influence detector choice for specific applications

Interaction Mechanisms

Photoelectric Effect

• Dominant interaction mechanism for low-energy gamma rays (< 100 keV)
• Incident photon transfers all its energy to a bound electron
• Electron ejected from atom with kinetic energy equal to photon energy minus binding energy
• Cross-section proportional to Z^4-5 of absorber material
• Results in full-energy peak in gamma-ray spectrum
• Auger electrons or characteristic X-rays emitted as atom de-excites
• Photoelectric effect crucial for X-ray spectroscopy and medical imaging (CT scans)

Compton Scattering

• Elastic scattering of photon by free or loosely bound electron
• Incident photon transfers part of its energy to electron
• Scattered photon emerges with reduced energy at an angle θ
• Energy of scattered photon given by Compton formula: $E' = \frac{E}{1 + \frac{E}{m_ec^2}(1-\cos\theta)}$
• Produces continuous distribution of energies in spectrum (Compton continuum)
• Compton edge corresponds to maximum energy transfer (180° scattering)
• Dominant interaction mechanism for intermediate gamma-ray energies (0.1-10 MeV) in low-Z materials
• Applications include Compton cameras for medical imaging and homeland security

Pair Production

• Occurs when photon energy exceeds 1.022 MeV (twice electron rest mass)
• Photon converts into electron-positron pair in presence of nuclear Coulomb field
• Threshold energy: 2m_ec^2 = 1.022 MeV
• Excess energy above threshold shared as kinetic energy between electron and positron
• Cross-section increases with photon energy and atomic number of absorber
• Positron quickly annihilates with electron, producing two 511 keV photons
• Spectrum features include full-energy peak, single escape peak, and double escape peak
• Dominant interaction mechanism for high-energy gamma rays (> 10 MeV)
• Exploited in positron emission tomography (PET) for medical imaging