Radiation Detection Systems
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Solve questions (25) →Contents
- 5.1 Gas-Filled Detectors
- 5.1.1 General Principle of Operation
- 5.1.2 Voltage-Current Regions
- 5.1.3 Ionisation Chamber
- 5.1.4 Proportional Counter
- 5.1.5 Geiger-Müller (GM) Counter
- 5.1.6 Gas-Filled Detectors – Comparative Summary
- 5.2 Scintillation Detectors
- 5.2.1 General Principle of Operation
- 5.2.2 Inorganic Scintillation Crystals
- 5.2.3 Organic Scintillation Detectors (Liquid Scintillation Counter)
- 5.2.4 Photomultiplier Tube (PMT)
- 5.2.5 Silicon Photomultiplier (SiPM)
- 5.3 Semiconductor Detectors
- 5.3.1 General Principle of Operation
- 5.3.2 CZT (Cadmium Zinc Telluride – CdZnTe)
- 5.3.3 Other Semiconductor Detectors
- 5.4 Well Counter, Dose Calibrator, Probe Detectors
- 5.4.1 Well Counter
- 5.4.2 Dose Calibrator (Detailed)
- 5.4.3 Probe Detectors (Gamma Probe)
- 5.4.4 Thyroid Uptake Probe
- 5.5 Energy Spectrum and Resolution
- 5.5.1 The Energy Spectrum
- 5.5.2 Energy Resolution
- 5.5.3 Pulse Height Analyser (PHA)
- 5.5.4 Counting Statistics
- Key Points for the Board
Learning objectives and modules3 modules · about 2 h 35 min
Learning objectives
After this chapter you will be able to:
- Compare the operating regions, gas amplification and uses of the ionisation chamber, proportional counter and GM counter.
- Explain the signal chain of a scintillation detector (crystal, light, photocathode, PMT) and the properties of the main crystals.
- Explain the direct-conversion principle of semiconductor detectors (CZT, HPGe) and their advantage in energy resolution.
- State the principle, limits and appropriate use of the well counter, dose calibrator and probe detectors.
- Identify the peaks in an energy spectrum (photopeak, Compton edge, backscatter, escape, sum) and explain energy resolution and the PHA window.
Modules
The chapter is divided into study modules without changing the order of the text: read, watch the animations, then answer the questions linked to those headings.
- Module 15.1–5.2 Gas-Filled Detectors · Scintillation Detectors1 h 5 min · 3 animations · 25 questionsAnswer the questions →
- Module 25.3–5.4 Semiconductor Detectors · Well Counter, Dose Calibrator, Probe Detectors40 min · 1 animation · 17 questionsAnswer the questions →
- Module 35.5 Energy Spectrum and Resolution · Key Points for the Board50 min · 3 animations · 12 questionsAnswer the questions →
Times are estimates: 130 words per minute, 3 min per animation, 1.5 min per question.
MOST FREQUENTLY ASKED IN THIS CHAPTER
5.1 Gas-Filled Detectors
5.1.1 General Principle of Operation
Gas-filled detectors are devices that convert the ionisation produced by radiation in a gas into an electrical signal.
Basic components:
Mechanism of operation:
1. Radiation enters the gas volume → ionises gas atoms 2. Ion pairs are formed (positive ion + free electron) 3. Under the electric field, electrons move towards the anode and positive ions towards the cathode 4. The charges collected at the electrodes produce an electric current/pulse 5. This signal is measured to determine the amount of radiation
Formation and collection of ion pairs in a gas-filled detector, the six operating regions by voltage, and the discharge and dead time of a Geiger-Müller counter are shown.
Key point: an ion chamber collects the charge as formed, a proportional counter multiplies it but keeps energy information, and a GM counter gives the same large pulse for every event and cannot discriminate energy.
Source: argon W = 26.4 eV per pair (ICRU); α range in argon 4.5 cm (NIST ASTAR, 1 atm), chamber cross-section 6 cm; β energy loss NIST ESTAR (1 MeV, 2.29 keV/cm); gas multiplication and GM dead time from the book. Time scales slowed and not to scale. Near the wire: Diethorn relation, for P-10 gas (90% Ar + 10% CH₄) K = 4.8×10⁴ V/cm·atm, ΔV = 23.6 V (Wolff 1974; Knoll); example wire radius 25 µm, cathode radius 3 cm.
How to use: a single click on the drawing pauses and resumes; ↑ ↓ switch the particle (α / β); in the voltage step drag on the right-hand chart to set the voltage; ← → change steps.
Figure 5.1: Principle of operation of a gas-filled detector: formation of ion pairs and their collection at the electrodes (schematic).
5.1.2 Voltage-Current Regions2 questions
The behaviour of gas-filled detectors varies with the applied voltage. As the voltage is increased, six regions are observed in turn:
1. Recombination region:
2. Ionisation region:
3. Proportional region:
4. Limited proportional region:
5. Geiger-Müller (GM) region:
6. Continuous discharge region:
→ The proportional counter (the ionisation chamber works in current mode and cannot discriminate energy). The GM counter CANNOT discriminate energy!
Formation and collection of ion pairs in a gas-filled detector, the six operating regions by voltage, and the discharge and dead time of a Geiger-Müller counter are shown.
Key point: an ion chamber collects the charge as formed, a proportional counter multiplies it but keeps energy information, and a GM counter gives the same large pulse for every event and cannot discriminate energy.
Source: argon W = 26.4 eV per pair (ICRU); α range in argon 4.5 cm (NIST ASTAR, 1 atm), chamber cross-section 6 cm; β energy loss NIST ESTAR (1 MeV, 2.29 keV/cm); gas multiplication and GM dead time from the book. Time scales slowed and not to scale. Near the wire: Diethorn relation, for P-10 gas (90% Ar + 10% CH₄) K = 4.8×10⁴ V/cm·atm, ΔV = 23.6 V (Wolff 1974; Knoll); example wire radius 25 µm, cathode radius 3 cm.
How to use: a single click on the drawing pauses and resumes; ↑ ↓ switch the particle (α / β); in the voltage step drag on the right-hand chart to set the voltage; ← → change steps.
Figure 5.2: The six operating regions of gas-filled detectors according to applied voltage and the corresponding detectors (schematic).
5.1.3 Ionisation Chamber2 questions
The detector operating in the ionisation region. There is no gas amplification; it measures only the primary ionisation.
Features:
Clinical applications:
Dose calibrator – detailed information:
→ The dose calibrator (well-type ionisation chamber).
→ The ionisation region.
→ NO. The dose calibrator works in current mode, not pulse mode → it cannot count single photons; it measures total activity.
5.1.4 Proportional Counter
The detector operating in the proportional region. There is gas amplification, and the collected charge is proportional to the primary ionisation.
Features:
Clinical applications:
5.1.5 Geiger-Müller (GM) Counter3 questions
The detector operating in the GM region. Even a single ionisation event produces a full avalanche.
Features:
Quenching:
Dead time:
Clinical applications:
→ The GM counter.
5.1.6 Gas-Filled Detectors – Comparative Summary
5.2 Scintillation Detectors
5.2.1 General Principle of Operation
Scintillation detectors are devices that convert the light (photon) signals produced by radiation in a scintillation material into an electrical signal.
Mechanism of operation:
1. Radiation interacts with the scintillation crystal/liquid 2. Atoms/molecules in the crystal are excited 3. Excited atoms emit light photons as they return to the ground state (scintillation) 4. The light photons reach the photocathode → photoelectrons are produced 5. The photoelectrons are multiplied by a photomultiplier tube (PMT) or silicon photomultiplier (SiPM) 6. An electrical signal is produced → analysed
Ideal properties of a scintillation material:
5.2.2 Inorganic Scintillation Crystals11 questions
Inorganic crystals produce light through transitions between the energy bands of activator dopants in the crystal lattice.
NaI(Tl) – Thallium-doped Sodium Iodide:
Properties of NaI(Tl):
→ NaI(Tl).
→ It is hygroscopic (absorbs moisture → the crystal deteriorates).
→ It gives the optimal balance between sensitivity and resolution for the 140 keV photons of Tc-99m. Thick crystal → sensitivity increases but spatial resolution decreases. Thin crystal → resolution increases but sensitivity decreases.
BGO (Bismuth Germanate – Bi₄Ge₃O₁₂):
LSO (Lutetium Oxyorthosilicate – Lu₂SiO₅:Ce):
LYSO (Lutetium Yttrium Oxyorthosilicate – Lu₁.₈Y₀.₂SiO₅:Ce):
GSO (Gadolinium Oxyorthosilicate – Gd₂SiO₅:Ce):
Properties sought in an ideal scintillator:
Difference in scintillator design between SPECT and PET:
→ LSO or LYSO (short decay time).
→ Its decay time is too long (300 ns).
5.2.3 Organic Scintillation Detectors (Liquid Scintillation Counter)2 questions
Organic scintillation materials are compounds that produce fluorescent light at the molecular level.
Liquid Scintillation Counter (LSC):
Applications:
Quenching – an important problem in LSC:
→ The liquid scintillation counter (LSC). The beta energy of tritium is very low (18.6 keV) → it cannot be measured with gas-filled detectors or a NaI crystal. In LSC the sample is mixed into the liquid → 4π geometry → high efficiency.
→ Usually an LSC or a well counter.
5.2.4 Photomultiplier Tube (PMT)
The PMT is a vacuum tube that converts the weak light signals from the scintillation crystal into strong electrical signals.
Components and principle of operation:
1. Photocathode: Converts light photons into photoelectrons (photoelectric effect)
2. Dynodes: 10-12 successive electrodes → the number of electrons increases 3-6-fold at each dynode
3. Anode: Collects the electrons from the last dynode → an electrical signal is produced
PMT properties:
A 140 keV γ photon converted to light in a NaI(Tl) crystal, to electrons at the photocathode and multiplied at the dynodes; carrier numbers and photopeak width in NaI and CZT are shown.
Key point: in a scintillation detector the energy passes through several conversions and carriers are lost at each; at 140 keV ~900 photoelectrons give ~10% resolution, CZT's ~30,000 carriers ~5–6%.
Source: book (NaI ~38 photons/keV, ~415 nm, ~230 ns; quantum efficiency 20–30%; dynode ×3–6, 10–12 dynodes; CZT 3–5 eV per carrier). Assumptions: ~70% of light reaches the photocathode, QE 25%, ×4; CZT W ≈ 4.6 eV (published value). In CZT other effects such as charge trapping also limit resolution. Micro scale: I K shell 33.17 keV, I Kα 28.6 keV (Bearden 1967); photoelectron path from the integral of the NIST ESTAR stopping power (NaI); mean free path at 28.6 keV in NaI 0.33 mm (NIST XCOM). Magnetic field: r = m·v/(e·B), ≈ 11 µm for a 100 eV electron at 3 T (~100 V between dynodes assumed); orbits are drawn enlarged.
How to use: a single click on the drawing pauses and resumes; ← → change steps; in the photocathode and dynodes step ↑ ↓ or the buttons below switch the MR field on and off.
Figure 5.3: Conversion of light into an electrical signal in the scintillation crystal and photomultiplier tube (schematic).
5.2.5 Silicon Photomultiplier (SiPM)5 questions
The SiPM is the semiconductor alternative to the PMT. It is a matrix of avalanche photodiodes (APDs).
SiPM properties:
Analogue and digital SiPM:
→ The PMT is sensitive to magnetic fields → it cannot work in the MR magnetic field. SiPMs (or APDs) are used in PET/MR.
→ Compact, insensitive to magnetic fields, low voltage.
→ Dark current (noise), temperature sensitivity.
5.3 Semiconductor Detectors
5.3.1 General Principle of Operation
Semiconductor detectors are solid-state detectors that convert the electron-hole pairs produced by radiation in a semiconductor material into an electrical signal.
Mechanism of operation:
1. Radiation enters the semiconductor material 2. An electron passes from the valence band to the conduction band → an electron-hole pair is formed 3. Under the applied electric field, electrons and holes move towards opposite electrodes 4. The collected charge forms an electrical signal
Advantages of semiconductor detectors:
5.3.2 CZT (Cadmium Zinc Telluride – CdZnTe)5 questions
CZT is a semiconductor detector material with increasing use in nuclear medicine.
CZT properties:
Advantages of CZT over NaI(Tl):
A 140 keV γ photon converted to light in a NaI(Tl) crystal, to electrons at the photocathode and multiplied at the dynodes; carrier numbers and photopeak width in NaI and CZT are shown.
Key point: in a scintillation detector the energy passes through several conversions and carriers are lost at each; at 140 keV ~900 photoelectrons give ~10% resolution, CZT's ~30,000 carriers ~5–6%.
Source: book (NaI ~38 photons/keV, ~415 nm, ~230 ns; quantum efficiency 20–30%; dynode ×3–6, 10–12 dynodes; CZT 3–5 eV per carrier). Assumptions: ~70% of light reaches the photocathode, QE 25%, ×4; CZT W ≈ 4.6 eV (published value). In CZT other effects such as charge trapping also limit resolution. Micro scale: I K shell 33.17 keV, I Kα 28.6 keV (Bearden 1967); photoelectron path from the integral of the NIST ESTAR stopping power (NaI); mean free path at 28.6 keV in NaI 0.33 mm (NIST XCOM). Magnetic field: r = m·v/(e·B), ≈ 11 µm for a 100 eV electron at 3 T (~100 V between dynodes assumed); orbits are drawn enlarged.
How to use: a single click on the drawing pauses and resumes; ← → change steps; in the photocathode and dynodes step ↑ ↓ or the buttons below switch the MR field on and off.
Figure 5.4: Indirect conversion chain in scintillation detectors, direct conversion in CZT, and the effect on energy resolution (schematic).
Disadvantages of CZT:
Clinical applications:
Dedicated cardiac CZT SPECT designs:
→ Semiconductor detectors (CZT).
→ Superior energy resolution.
→ Thanks to its wide band gap, thermal noise is low at room temperature. Germanium (Ge) detectors, by contrast, require cooling with liquid nitrogen → their clinical use is limited.
5.3.3 Other Semiconductor Detectors
Silicon (Si) detectors:
Germanium (Ge) detectors:
5.4 Well Counter, Dose Calibrator, Probe Detectors
5.4.1 Well Counter4 questions
The well counter is a highly sensitive scintillation detector used to measure the activity of small-volume radioactive samples (blood, urine, tissue samples).
Structure:
Features:
Clinical applications:
Well counter vs dose calibrator – important differences:
→ The well counter (NaI(Tl) well counter).
→ Well counter: scintillation detector, pulse mode, low activity. Dose calibrator: ionisation chamber, current mode, high activity. They CANNOT be used interchangeably!
Figure 5.5: Comparison of the structure and operating principle of the well counter and the dose calibrator (schematic).
5.4.2 Dose Calibrator (Detailed)5 questions
The dose calibrator is the basic device used to measure radiopharmaceutical activity in nuclear medicine. Its structure and operating principle are explained in 5.1.3; details of clinical use are given here.
Measurement principle:
Radionuclide selection (dial/pot setting):
Geometry effects:
Quality control tests (see Chapter 27 – detailed):
5.4.3 Probe Detectors (Gamma Probe)3 questions
The gamma probe is a small hand-held detector used to localise radioactive tissue/lymph nodes during surgery.
Structure:
Clinical applications:
Use of the gamma probe in sentinel lymph node criteria:
→ The gamma probe.
→ NaI(Tl) or CZT.
→ The 10% rule: all nodes giving ≥10% of the (ex vivo) count of the hottest node are removed (frequently used node/background thresholds for a "hot" node are also ex vivo ≥10:1, in vivo ≥3:1). Using the gamma probe together with blue dye (patent blue/isosulfan blue) increases the success rate.
5.4.4 Thyroid Uptake Probe
The thyroid uptake probe is a special scintillation detector used to measure radioactive iodine uptake by the thyroid gland.
Structure:
Principle of operation:
Normal values:
Clinical use:
5.5 Energy Spectrum and Resolution
5.5.1 The Energy Spectrum3 questions
The energy spectrum is a graph showing the energy distribution of the events recorded by the detector. X axis: energy (keV); Y axis: counts (number of events).
Components of the gamma-ray energy spectrum:
1. Photopeak (photoelectric peak / full-energy peak):
2. Compton edge and Compton plateau:
3. Backscatter peak:
4. Lead X-ray peak:
5. Iodine escape peak:
6. Sum peak (pile-up peak):
→ The iodine escape peak (140 - 28 = 112 keV).
→ The point corresponding to the maximum energy transfer in Compton scattering (180° scattering).
→ At high count rates (pile-up).
→ Photons scattered through 180° from materials outside the crystal (collimator, shielding).
The energy spectrum of photons reaching the detector builds up event by event (photopeak, Compton, backscatter, lead X-ray, escape and sum peaks); FWHM measurement and the pulse-height analyser window are shown.
Key point: the photopeak is full energy absorption; Compton, backscatter, escape and X-ray peaks are partial or secondary events. The energy window rejects photons scattered in the patient; a narrower window means less scatter but also fewer counts.
Source: Compton edge and backscatter from formula (Tc-99m 49.8 / 90.7, Cs-137 477.6 / 184.3 keV); Pb Kα₁ 74.97, Kβ₁ 84.94 keV; I K X-ray 28.6 keV; Compton angles from the Klein–Nishina distribution. NaI FWHM 10% at 140 keV (∝ 1/√E), CZT 5.5%. Event-type fractions are representative. HPGe FWHM: 0.5–1.0 keV at 122 keV (Mirion and ORTEC data sheets, 2025); 0.84 keV here. Escape peaks follow the detector's own K X-ray: I 28.6; Cd 23.2 / Te 27.5; Ge 9.9 keV (Bearden 1967).
How to use: a single click on the drawing pauses and resumes; the buttons below the drawing pick the source, the detector or the window width; ← → change steps and selections.
Figure 5.6: Components of the Tc-99m and Cs-137 gamma energy spectra in a NaI(Tl) detector (schematic).
5.5.2 Energy Resolution3 questions
Energy resolution is the ability of the detector to distinguish photons of different energies from one another.
Definition and formula:
Typical energy resolution values:
Factors affecting energy resolution:
1. Statistical fluctuations: Statistical variation in the number of light photons/ion pairs produced → the most important factor
2. Detector material: The lower the energy needed to create one signal unit → more signal → better statistics → better resolution
3. PMT gain variation: Fluctuations in the electron multiplication process in the PMT 4. Electronic noise: Noise in the amplifier and other electronic components 5. Crystal homogeneity: Structural defects in the crystal affect the light output
Clinical importance of energy resolution:
The energy spectrum of photons reaching the detector builds up event by event (photopeak, Compton, backscatter, lead X-ray, escape and sum peaks); FWHM measurement and the pulse-height analyser window are shown.
Key point: the photopeak is full energy absorption; Compton, backscatter, escape and X-ray peaks are partial or secondary events. The energy window rejects photons scattered in the patient; a narrower window means less scatter but also fewer counts.
Source: Compton edge and backscatter from formula (Tc-99m 49.8 / 90.7, Cs-137 477.6 / 184.3 keV); Pb Kα₁ 74.97, Kβ₁ 84.94 keV; I K X-ray 28.6 keV; Compton angles from the Klein–Nishina distribution. NaI FWHM 10% at 140 keV (∝ 1/√E), CZT 5.5%. Event-type fractions are representative. HPGe FWHM: 0.5–1.0 keV at 122 keV (Mirion and ORTEC data sheets, 2025); 0.84 keV here. Escape peaks follow the detector's own K X-ray: I 28.6; Cd 23.2 / Te 27.5; Ge 9.9 keV (Bearden 1967).
How to use: a single click on the drawing pauses and resumes; the buttons below the drawing pick the source, the detector or the window width; ← → change steps and selections.
Figure 5.7: Definition of energy resolution by FWHM and comparison of NaI(Tl) and CZT photopeaks (schematic).
5.5.3 Pulse Height Analyser (PHA)3 questions
The PHA is the electronic circuit that classifies and selects the electrical pulses from the detector according to their energy values.
Principle of operation:
Types of PHA:
Energy window setting:
→ 140 keV ± 10% = 20% window (126-154 keV).
→ Scatter decreases and contrast increases, BUT sensitivity (counts) decreases.
→ Sensitivity increases, BUT scatter increases and contrast decreases. Setting the energy window is a balance between sensitivity and contrast.
The energy spectrum of photons reaching the detector builds up event by event (photopeak, Compton, backscatter, lead X-ray, escape and sum peaks); FWHM measurement and the pulse-height analyser window are shown.
Key point: the photopeak is full energy absorption; Compton, backscatter, escape and X-ray peaks are partial or secondary events. The energy window rejects photons scattered in the patient; a narrower window means less scatter but also fewer counts.
Source: Compton edge and backscatter from formula (Tc-99m 49.8 / 90.7, Cs-137 477.6 / 184.3 keV); Pb Kα₁ 74.97, Kβ₁ 84.94 keV; I K X-ray 28.6 keV; Compton angles from the Klein–Nishina distribution. NaI FWHM 10% at 140 keV (∝ 1/√E), CZT 5.5%. Event-type fractions are representative. HPGe FWHM: 0.5–1.0 keV at 122 keV (Mirion and ORTEC data sheets, 2025); 0.84 keV here. Escape peaks follow the detector's own K X-ray: I 28.6; Cd 23.2 / Te 27.5; Ge 9.9 keV (Bearden 1967).
How to use: a single click on the drawing pauses and resumes; the buttons below the drawing pick the source, the detector or the window width; ← → change steps and selections.
Figure 5.8: Pulse selection with the energy window in the pulse height analyser and the effect of window width (schematic).
5.5.4 Counting Statistics4 questions
Radioactive decay is a random process and follows the Poisson distribution.
Poisson distribution:
Clinical importance:
Minimum detectable activity (MDA):
Dead time and paralysis:
→ √1,000,000 = 1000.
→ 1000/1,000,000 = 0.1%.
→ Increase the counts (a longer time or more activity).
→ The count paradoxically falls (paralysis) → a dangerous situation: a high radiation field may appear as a low count!
Figure 5.9: Relative standard deviation as a function of the number of counts and dead-time losses in paralysable/non-paralysable systems.
Key Points for the Board
Gas-Filled Detectors – Quick Comparison
Order of Voltage Regions (Memorise!):
Scintillation Crystals – Quick Comparison
PMT vs SiPM Comparison
Device-Use Matching (Memorise!):
Components of the Energy Spectrum (Memorise!)
Energy Window Rules
Counting Statistics Formulas (Memorise!)
Dead Time and Paralysis
Scintillator and Photodetector Technology – Additional Points
Frequently Asked Board Questions – Quick Answer Cards
Detector Materials – Table of Numerical Values (Memorise!)
The following numerical values are often asked in board exams:
Energy Needed to Create One Ion Pair/Signal Unit (Memorise!)
Critical Comparison: Sensitivity vs Energy Resolution vs Dead Time
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