IT-NMT 105: SCINTILLATION DETECTORS (PT 1)

Scintillation

A general term referring to the process of giving off light.

Scintillation

Any material that can release a photon in the UV or visible-light range, when an excited electron in the scintillator returns to its ground state.

Scintillation Detectors

Operates on the principle of converting ionizing radiation into visible light.

Scintillation Detectors

When high-energy photons or charged particles interact with the scintillation material, they excite the atoms of the crystal, leading to the emission of light.

Sodium Iodide

most common scintillation material

Scintillation Detectors

They enable the detection and analysis of gamma rays emitted from radiopharmaceuticals within the body

Sir William Crookes in 1903

• invented the first inorganic scintillator detector,

• a Zinc sulfide screen which produced weak scintillations when struck by α particles

Curran and Baker in 1944

Replaced the naked eye with the photomultiplier tube and revived the use of scintillators

Robert Hofstadter in 1948

developed thallium-activated sodium iodide or NaI(Tl)

thallium-activated sodium iodide or NaI(Tl)

• The most common inorganic scintillator employed in nuclear medicine

• developed for use in radiation detection (to detect lower energy gamma rays ex: emitted by Tc-99m)

1. Bismuth germanate

2. Lutetium oxyorthosilicate

3. Gadolinium oxyorthosilicate

SCINTILLATORS WITH A HIGHER ATOMIC NUMBER AND DENSITY:

Scintillation Process

When radiation strikes the scintillating material, it excites the atoms, causing them to emit light.

Light to electrical signal

The emitted light is converted into an electrical signal by a photomultiplier tube (PMT)

Signal amplification

The signal is amplified and processed to provide radiation detection and measurement.

Scintillation Crystals

made to exacting tolerances and require exceptional care in the manufacturing process.

Scintillation Crystals

must be optically transparent, without cracks or boundaries that could cause scintillation photons to be reflected.

Scintillation crystals

quite fragile, and can fracture under conditions of mechanical stress or rapid temperature change (>5°C or 9°F per hr)

excellent scintillation light yield

• number of scintillation photons emitted per eV of radiation energy absorbed

• the most desirable characteristic of sodium iodide

excellent scintillation light yield

the greater number of scintillation photons leads to greater precision in measuring the energy of the absorbed gamma-ray.

scintillation photons produced in sodium iodide

• range from 325 to 550 nm

• primarily in the ultraviolet spectrum but just touching the high end of the visible-light spectrum.

bialkali photomultiplier tube

the most common type of PMT used in radiation detection applications.

1. High atomic number

2. efficient at stopping gamma rays.

properties of sodium iodide crystals

Doping with Thallium (NaI(Tl))

Increases light yield

function of sodium iodide crystal

Emits light when struck by radiation.

Photomultiplier Tubes

crucial for detecting the light produced by the scintillation process.

Photomultiplier Tubes

amplify the small number of photons emitted by the crystal into a detectable electrical pulse

Photomultiplier Tubes

Converts light into an electrical signal.

PMT Functionality

Multiplies the number of electrons via dynodes to amplify the signal.

Importance of PMT

Essential for detecting weak light signals from scintillation events.

Photomultiplier Tubes

Its basic structure is an evacuated cylinder enclosed in glass, with a photocathode on one end, an anode at the opposite end, and small curved dynodes in between.

The electrical potential to the dynodes

causes multiplication of the electrical signal created at the photocathode.

Photomultiplier Tubes

The passage of an electron through the focusing grid, its interaction with the first dynode, and its multiplication at the second dynode is shown, using a multiplication factor of 3

Photocathode

• Located at the front of the PMT

• converts incoming photons into photoelectrons.

Focusing Grid

This component helps to direct the photoelectrons toward the dynodes, enhancing the efficiency of the signal amplification.

Focusing Grid

Ensures electrons are directed onto the dynodes.

Dynode Cascade

Each dynode multiplies the number of electrons.

Amplification

A few initial electrons result in a large number of electrons at the output.

Dynodes

• A series of dynodes within the PMT

• multiply the number of electrons through successive stages, leading to significant amplification of the initial signal

PRE-AMPLIFIER

This component serves to enhance the signal before it is sent to the main amplifier, helping to minimize noise.

AMPLIFIER

Adjusting the gain allows the detector to be calibrated for different levels of radiation intensity, optimizing performance.

PULSE HEIGHT ANALYZERS

• This is a crucial component for energy discrimination.

• It categorizes the pulses based on their heights, allowing for detailed analysis of the energy of the incoming radiation

single-channel analyzer

• focuses on a specific energy window,

• Isolates pulses of a specific height (energy level).

multichannel analyzer

• captures a spectrum of energies, providing a comprehensive view of the radiation detected.

• Measures and records a range of pulse heights across an energy spectrum.

creation of electron pulses, high-voltage power, amplification

Requirements of signal detection

CREATION OF ELECTRON PULSES

Scintillation photons strike the photocathode, liberating electrons

HIGH-VOLTAGE POWER

High voltage is necessary to maintain the electron multiplication across dynodes.

Stability Requirements:

Voltage needs to be stable to ensure consistent amplification.

AMPLIFICATION

Following the initial pulse creation, additional amplification may occur through further electronic components

photopeak, compton edge, backscatter peak, escape peak

4 parts of energy spectrum

Photopeak

main peak corresponding to full-energy absorption

Compton Edge

Result of partial energy absorption in the compton scattering process

Backscatter peak

lower energy peak caused by radiation that scatters from the detector environment

Escape peaks

result of photons escaping from the detector after partial interaction

140keV photon

scintillation crystal: information carriers for Tc-99m

662 keV photon

scintillation crystal: information carrier for Cs-137

PERCENT ENERGY WINDOW

A window width is specified as a percent of the photopeak energy.

narrow window (2-5%)

% energy window: used for calibration

wider window (15-20%)

% energy window: for imaging and other measurements

ENERGY RESOLUTION

Evaluates how well a given detector distinguishes between gamma rays of closely spaced energies.

FULL-WIDTH AT HALF MAXIMUM

measures the width of the photopeak relative to the photopeak energy.

Peak broadening

primarily due to the statistical variations in the number of electrons created at the photocathode

higher-energy gamma rays

create more electrons at the photocathode than do lower-energy gamma rays.

Sodium iodide scintillation detectors

• have energy resolution in the range of 6–8% FWHM for Cs-137

• 10-12% for Tc-99m

Cs-137

always used to compare energy resolution between detectors

constancy reading

at least 5% deviation from true value

COUNTERS, SCALERS, TIMERS

These devices are used to quantify the detected events over time, essential for applications in nuclear medicine, radiation safety, and environmental monitoring

Counters

Count the number of radiation events

Timers

Measure the time duration for counting events

Rate Meters

Measure the rate of radiation events over time.