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Ionizing Radiation radioactivity measurements. High energy particles and photons that ionise atoms and molecules along their tracks in a medium are called ionizing radiation . For example, a, b, g, cosmic rays and X-rays are ionizing radiation.

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Ionizing Radiationradioactivity measurements

High energy particles and photons that ionise atoms and molecules along their tracks in a medium are called ionizing radiation. For example, a, b, g, cosmic rays and X-rays are ionizing radiation.

Most radioactive measurement are based on their ionizing effect.

Ionizing radiation causes illness such as cancer and death. Radiation effect is a health and safety concern.

Ionizing radiation can also be used in industry for various purposes.

Light and microwaves that do not ionize atoms and molecules are called non-ionizing radiation.

Ionizing Radiation

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Discovery of Ionization by Radiation

X-rays and radioactivity discharged a charged electroscope. Curie and Rutherford attributed the discharge to the ionization of air by these rays.

Ionizing Radiation

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Ionization Energy of Gases

The minimum energy required to remove an outer electron from atoms or molecules is called ionization potential. Ionizing radiation also remove electrons in atomic inner shell, and the average energy per ion pair is considered ionization energy

He + 25 eV  He+ + e-

He+ + 54 eV  He2+ + e-

Ionization energy (IE eV) per ion pair of some substancesMaterial Air Xe He NH3 Ge-crystalAverage IE 35 22 43 39 2.9

Ionizing Radiation

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Primary and Secondary Ion Pairs

Primary and secondary ion Pairs










Primary ion pairs are caused directly by radiation. Secondary ion pairs are generated by high-energy primary electrons.

Molecular density (molecules/mL)air = 2.7e19water = 3.3e22

Ionizing Radiation

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Interaction of Heavy Charged Particles with Matter

Fast moving protons, 4He, and other nuclei are heavy charged particles.

Coulomb force dominates charge interaction.

They ionize and excite(give energy to) molecules on their path.

Ionizing Radiation

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Energy Loss of Heavy Charged Particles in Matter

Stopping power is the rate of energy loss per unit length along the path.

Stopping power is proportional to the mass number A, and to the square of atomic number, Z2, of a medium, but inversely proportional to the energy of the particle E.

The surges of ion density before they stop give the Bragg peaks.

Ionizing Radiation

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Range of Heavy Charged Particles in a Medium

Particles lose all their energy at a distance called range.

Ionizing Radiation

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Range of Heavy Charged Particles in a Medium

The range can be used to determine the energy of the particles and the radiation source.

Ionizing Radiation

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Speed of Particles

Speed of 1 MeV a particle

1.6e-13 J = (½) m v2 = (½)(41.66e-27 kg) v2

Solving for v v2 = 4.82e13 (m/s)2v = 6.9e6m/s

Speed of 1 MeV (= 1.6e-13 J) electron

1.6e-13 J = (1/2) m v2 = (½) 9.1e-31 kg v2Solving for v, v2 = 3.52e17 (m/s)2or v = 5.9e8 m/s.

exceeds c (=3e8 m/s), the speed limitProper evaluation method shown next

Still reasonable

Ionizing Radiation

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Proper Evaluation of Particle Speed

  • The relativity massm of a particle of kinetic energy Ek is the sum of the rest mass and its kinetic energy

    • m = Ek MeV + 0.51 MeV (rest mass of electron)

  • For an electron with Ek = 1.0 MeV, m = 1.51 MeV

  • The speed of an electron with a kinetic energy 1.0 MeV is evaluated by applying the Einstein’s equation:

m = mo / (1-(v/c)2)

This speed is a 80% of c, the speed of light.

Ionizing Radiation

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Scattering of Electrons in a Medium

Fast moving electrons are light charged particles.

They travel at higher speed., but scattered easily by electrons.

Ionizing Radiation

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Range of Light Charged Particles in a Medium

Range of b particles is not as well defined as heavy charged particles, but measured range is still a useful piece of information.

Ionizing Radiation

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Braking Radiation of b particles Influenced by Atom

Bremsstrahlung (braking) radiation refers to photons emitted by moving electrons when they are influence by atoms.

Ionizing Radiation

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Braking radiation

Interaction of Beta particles with Matter

Beta particles interact with matter mainly via three modes:

Ionization (scattering by electrons)

Bremsstrahlung (braking) radiation

Annihilation with positrons

Ionizing Radiation

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Interaction of Photons with Matter

Photon Energies

Visible red light 1.5 eVvisible blue light 3.0 eV

UV few eV-hundreds eV

X-rays 1 to 60 keV

Gamma rays keV - some MeV

Interactions of gamma rays with matter:

photoelectric effect

Compton effect

Pair productions

Ionizing Radiation

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Compton Effect of Gamma Rays

When a photon transfers part of its energy to an electron, and the photon becomes less energetic is called Compton effect.

Ionizing Radiation

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The animated Compton effect

Ionizing Radiation

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Pair Production of Gamma Rays

Gamma photons with energy greater than 1.02 MeV produce a electron-positron pair is called pair production.

Ionizing Radiation

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Gamma-ray Three Modes of Interaction with Matter

Photoelectric effect Compton scattering pair production

Ionizing Radiation

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Attenuation of Gamma Rays by Matter

Gamma-ray intensity decreases exponentially as the thickness of the absorber increases.

I = Io e–c x

I: Intensity at distance xc: absorption constantx: thickness

Ionizing Radiation

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Ionizing Radiation Measurements

  • Radiation Detectors - overall view

    • electroscopesionization chambersproportional countersGeiger-Muller counters

    • solid-state detectors

    • photographic films and photographic emulsion plates

    • bubble chambers and cloud chambers

    • scintillation counters and fluorescence screen

Ionizing Radiation

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Ionization Chambers

Current (A) is proportional to charges collected on electrode in ionization chambers.

Ionizing Radiation

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Proportional Counters

Gas Multiplication





X00 V

Proportional counters

Gas multiplication due to secondary ion pairs when the ionization chambers operate at higher voltage.

Ionizing Radiation

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Geiger-Muller Counters

1X00 V

Every ionizing particle causes a discharge in the detector of G-M counters.

Ionizing Radiation

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Solid-state Detectors

A P-N junction of semiconductors placed under reverse bias has no current flows. Ionizing radiation enters the depleted zone excites electrons causing a temporary conduction. The electronic counter register a pulse corresponding to the energy entering the solid-state detector.

+ + depleted - -

P + - N

+ + zone - -



electronic counter


Ionizing Radiation

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A simple view of solid-state detectors

Solid-state detectors are usually made from germanium or cadmium-zinc-telluride (CdZnTe, or CZT) semiconducting material. An incoming gamma ray causes photoelectric ionization of the material, so an electric current will be formed if a voltage is applied to the material.

Digirad has developed and made commercially available the world's first solid-state, digital gamma camera for the nuclear medicine imaging market. Our proprietary, solid-state imaging technology is based on a patented, silicon photodiode technology that replaces the vacuum photomultipier tubes (PMTs) used in all other gamma cameras. These photodiodes are coupled to individual scintillation crystals to create a unique detection element for each addressable spatial location of the camera's head. We call this Digital Position Sensing™ technology. It provides images with excellent contrast and spatial resolution.

Ionizing Radiation

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Scintillation Counters

Photons cause the emission of a short flash in the Na(Tl)I crystal.The flashes cause the photo-cathode to emit electrons.

Ionizing Radiation

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Scintillation Detector and Photomultiplier tube

Ionizing Radiation

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-rayspectrum of 207mPb

Gamma ray spectrum of 207mPb (half-life 0.806 sec)

207mPb Decay Scheme

13/2+____________1633.4 keV

- Intensity (log scale)




5/2-____________569.7 keV




1/2-____________0.0 stable


-10 569 + 1063

-1 Energy

Ionizing Radiation

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Fluorescence Screens

Fluorescence materials absorb invisible energy and emit visible light.

J.J. Thomson used fluorescence screens to see electron tracks in cathode ray tubes. Electrons strike fluorescence screens on computer monitors and TV sets give dots of visible light.

Röntgen saw the shadow of his skeleton on fluorescence screens.

Rutherford observed alpha particle on scintillation material zinc sulfide.

Fluorescence screens are used to photograph X-ray images using films sensitive visible light.

Ionizing Radiation

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Cloud and Bubble Chambers

The ion pairs on the tracks of ionizing radiation form seeds of gas bubbles and droplets. Formations of droplets and bubbles provide visual appearance of their tracks, 3-D detectors.

C.T.R. Wilson shared the Nobel prize with Compton for his perfection of cloud chambers.

Ionizing Radiation

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Image Recorded in Bubble Chambers

Charge exchange of antiproton produced neutron-antineutron pair.

p + p  n + n (no tracks)

Annihilation of neutron-antineutron pair produced 5 pions.

n +n  3p+ + 2p- + ?

Only these tracks are sketched.

Ionizing Radiation

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Bubble Chambers

The Brookhaven 7-foot bubble chamberand the 80-inch bubble chamber 

Ionizing Radiation

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Image from bubble chamber

This image shows a historical event: one of the eight beam particles (K- at 4.2 GeV/c) which are seen entering the chamber, interacts with a proton, giving rise to the reactions

K– p  – K+ K0

K0  + –

–  0 K–

K+  + 0

0  p –

Ionizing Radiation

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Photographic Emulsions and Films

Sensitized silver bromide grains of emulsion develope into blackened grains. Plates and films are 2-D detectors.

Roentegen used photographic plates to record X-ray image.

Photographic plates helped Beckerel to discover radioactivity.

Films are routinely used to record X-ray images in medicine but lately digital images are replacing films.

Stacks of films record 3-dimensional tracks of particles.

Photographic plates and films are routinely used to record images made by electrons.

Ionizing Radiation

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Overall View

Ionizing radiation interacts with matter in various ways: ionization (photoelectric effect), excitation, braking radiation, Compton effect, pair production, annihilation etc.

Mechanisms of interaction are utilized for the detection of ionizing radiation.

Function and principles of electroscope, ionization chambers, proportional chambers, Geiger-Muller counters, solid-state detectors, and scintillation counters, bubble chambers, and cloud chambers have been describe.

Ionizing Radiation

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The Sudbury Neutrino Observatory

SNO will contain 1000 tonnes of heavy water, held in a 12-m diameter spherical acrylic vessel. It has the ability to detect all three types of neutrinos.

Ionizing Radiation