Ionizing radiation detectors�Table of contents�

• Brief history
• The electromagnetic spectrum
• Generalities
• Ionizing Radiations
• Alpha Particles
• Beta Particles
• X Rays
• Neutrons
• Gamma Radiation
• Measuring and sensing ionizing radiations

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Dra. Rossana Madrid /Dr. Carmelo José Felice

Biomedical Transducers

Biomedical Engineering

FACET-UNT, Argentina

Updated 11/April/2021

Bibliography

• https://inis.iaea.org/collection/NCLCollectionStore/_Public/40/100/40100480.pdf?r=1&r=1
• Tanarro Sanz A. (1959). Junta de Energía Nuclear: Curso de introducción a la energía nuclear. Detectores de radiaciones nucleares.
• https://www.utoledo.edu/med/depts/radther/pdf/JC%20Chapter%209%20handout.pdf
• https://www.osti.gov/etdeweb/servlets/purl/21161069

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A bit of history

Electromagnetic spectrum

• When a charge accelerate or oscillate

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Electromagnetic radiation

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Magnetic & electric field

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• Spectrum radio to gamma waves

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• All waves are radiation

wave-particle duality

The Electromagnetic wave

Generating EM radio wave

Antenna is an open capacitor

(a dipole)

A dipole to produce and transmit EM waves

Dipole generates electric field (blue)

Current generates magnetic field (red)

Producing and transmitting �radio waves

ELECTROMAGNETIC SPECTRUM

MAIN CHARACTERISTICS OF ELECTROMAGNETIC WAVES

• Frequency f
• Wavelength λ
• Amplitude (Energy) E

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• When f wave energy

EXCITATION AND IONIZATION

< ----------------- Excitation ------------- >

<--Ionization-- >

PROPERTIES OF THREE PRIMARY RADIATION TYPE

amu: Atomic Mass Unit

IONIZING RADIATION

X RAYS

Discovered in 1885 by Roentgen

Frequency: 10¹⁷ to 10²¹ Hz

CORPUSCULAR RADIATIONS�Alpha Particles

• They are 2 p⁺ + 2 n⁰ (He nucleous) ejected by a radioactive atom

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• Alpha particles are very massive and ionizing power, but with very low penetring power

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• Velocity: 5% of light velocity

• They are emitted by nucleus of heavy elements located at the end of the PT (A > 100)
• These nuclei have many p+ and the electrical repulsion is very strong They emit α particles

CORPUSCULAR RADIATIONS�Alpha Particles

There are 2 types:

β^– Radiation

β⁺ Radiation

CORPUSCULAR RADIATIONS�Beta Particles

β-: it is an electron. Occurs when there is an unstable atomic nucleus with an excess of neutrons

β+: it is a positron. Occurs when there is an unstable atomic nuclei with an excess of protons

CORPUSCULAR RADIATIONS�Beta Particles

β^- decay

• A positron e⁺ (β⁺ particle) is emmited at v≈ c
• A proton is converted into a neutron, a positron, and an electron neutrino

Β⁺ decay

• An electron e^- (β^-particle) is emmited at v≈ c
• A neutron is converted into a proton, an electron, and an electron antineutrino

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NEUTRONS

• Particles from collisions between atoms
• Mass = ¼ of α particles
• Without charge great energy

very penetrating

• To stop them

Thick layers of concrete, lead or water.

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GAMMA RADIATION

• GR are very short wavelength electromagnetic waves, i.e. very high energy photons
• Originate from excited nuclei of radioactive atoms who spontaneously tend to less excited states
• No charge no mass, great penetration
• Unlike α and β radiation they produce indirect ionization
• Interaction with the electronic layers of the atoms

RADIATIONS

IONIZING RADIATION SENSORS

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1) Gas Ionization

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2) Luminescence excitation in solids

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3) Dissociation of matter

Radiation Detectors

Material of the detector depends on:

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• Type of radiation to be studied
• Information that you want to obtain:
• Energy of the radiation
• Time when the radiation was emitted
• Type of particle
• High rate of counts
• Very low rate of counts

Clasification

Detectors

Inmediats

By ionization

Gaseous

Semiconductors

By excitation

Scintillation

By ionization

Photographic Film

By excitation

Thermoluminescent

Radioluminescent

Retarded

EXCITATION OF LUMINESCENCE IN SOLIDS

Scintillation Detectors�Characteristics

• A small fraction of the Ec of the secondary particles is converted into light energy with high efficiency

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• The rest is transferred to the medium as heat or as vibrations of its crystal lattice.

Properties of the scintillation materials

• The conversion of Ec is linear over a wide range of energies

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• It is "transparent" to the λ that it emits by de-excitation

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• The decay time of the light pulses is short the signals generated are fast

Properties of the scintillation materials

• Refraction index very similar to that of glass efficient optical coupling

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• No material meets all the stated properties

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• Most used scintillators: inorganic or organic, plastic or liquid.

MATTER DISSOCIATION

• The IR gives up energy, breaking the chemical bonds of matter (dissociation)

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Photographic plate effect it measures the intensity of the blackening of the plate

X-ray film (Radiographic plate)

• Emulsion:
• Sensitive to light – Little sensitive to x-rays
• Darker at more radiation
• When it is exposed to x-Rays
• It ionizes
• Start the conversion to metallic Ag

Analog transduction of the X-Rays beam�1) Fluoroscopic screen

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Luminosity proportional to incident X-radiation

Analog Transduction of the resulting beam

DOSIMETERS

• There are two types:

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• Ambient Dosimeter

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• Personal Dosimeter

They analyze dose or amount and type of radiation

Types of personal dosimeters

1) Based on gas ionization:

• Feather dosimeter
• Advantages
• Disadvantages

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2) Photographic films

• Advantages
• Disadvantages

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3) Thermoluminiscence

• Radiation generates charge carriers that accumulate in material defects
• Heating the material enables the trapped states to interact with phonons, i.e. lattice vibrations, to rapidly decay into lower-energy states, causing the emission of photons in the process. It emits visible light.
• Advantages

Types of personal dosimeters

GASEOUS DETECTORS

Voltage dependent regions

To identify different radiations use Filters

Ionization chamber

• Continuous radiation
• Pulsed radiation

Ionization chamber�continuous radiaton

Ionization chamber�output voltage

Continuous radiation ionization chamber

• Chamber contains sample or radiation passes through the chamber giving up all its energy
• All electrons and ions produced are collected by chamber capacitance
• Sample emission is continuous, therefore current is also continuous
• Current peak is proportional to sample activity
• Disadvantages: very small currents and isolated pulses cannot be measured

Ionization chamber�pulsed radiation

Pulsed Ionization chamber

• Radiation passes through the chamber producing ions and electrons
• Charges are attracted by electrodes (wall chamber and central wire)
• First arrive electrons (μs) then ions (ms)
• Pulse peaks are proportional to incident energy
• Capacitor discharge depends on R and chamber capacitance
• Disadvantage: very small currents must be amplified

Output pulse shape from pulsed ionization chamber

Time to collect electrons (t^-), ions (t⁺) and discharge (blue lines)

If RC> time to collect all charges, amplitude of the pulse measures the original charge

Ionization Chambers�Continuous and Pulsed

• Continuous ionization chamber are used as activity monitor
• If we do not connect the continuous chamber, charges are collected and stored for use as personal dosimeter.
• Pulsed ionization chamber is used as pulse monitor, but only for high energy radiation because current are very small
• Solution: proportional counter, where pulses are amplified by secondary ionization without electronic amplifier

Proportional Counter

Proportional Counter:characteristics

• Largest electric field allows e^- to generate secondary ionizations.

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• Accelerated secondary electrons produce new ionizations

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• Secondary ionizations (~1000-100000)

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• The chamber always works proportionally to primary events number

Output pulse shape from proportional counter

Ions travel more and more slowly when go away from the central wire

Proportional Counter

• The proportional counter does not have general applicability in clinical nuclear medicine

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• It is used in research to detect alpha and beta particles.

Geiger-Müller Counter

Geiger-Müller Counter

The Geiger-Müller and Proportional detectors use wires thinner than the ionization chamber, to achieve higher voltages.

Filling Gases

• Ionization Chamber: air
• Proportional Counter
• Methane
• CO₂
• mixture of Ar and methane
• Geiger Counters
• mixture of gases: Ar or Ne and an organic compound (ethanol) or a halogen gas (Cl or Br)

Dead Time

Solid State Detectors

Energy levels

Atoms of a solid crystal

Si

Operating Principle similar to the ionization chamber

Solid state Detectors

Solid state Detectors

Nuclear spectrums and detectors

Fundamental State of ¹³⁷Cs

β^- Decay

(It emits an e^-)

Excited Level of ¹³⁷Ba

Fundamental Level of ¹³⁷Ba

How to make an energy spectrum

1. Measure the current pulses
2. Clasify them by current amplitude
3. Group them in Channels
4. Calibrate Channel vs Energy
5. Graph Counts per channel vs Energy or,
6. Graph Counts per channel vs #channel

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Measure the current pulses

Clasify them by current amplitude

Group them by Channel

Calibrate Channel vs Energy

Nuclear Spectrums and Detectors

¹³⁷Cs

Photomultiplier Tubes

Photomultiplier Tubes

Photomultiplier Tubes

# dynodes depends on multiplication factor required (10⁵-10⁶)

BIBLIOGRAPHY

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Fraden, J. Handbook of Modern Sensors, Physics, Design and Applications. 3rd Ed. AIP Press, 2004.

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Pallás-Areny, R. and Webster, J. Sensors and Signal Conditioning. 2nd Ed. John Wiley and Sons, Inc. 2001.

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Principios de detección de radiación. Protección radiológica. Instituto Balseiro.

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