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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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.
X-rays and radioactivity discharged a charged electroscope. Curie and Rutherford attributed the discharge to the ionization of air by these rays.
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
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
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.
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.
ShieldRange of Heavy Charged Particles in a Medium
Particles lose all their energy at a distance called range.
The range can be used to determine the energy of the particles and the radiation source.
Speed of 1 MeV a particle
1.6e-13 J = (½) m v2 = (½)(41.66e-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
m = mo / (1-(v/c)2)
This speed is a 80% of c, the speed of light.
ShieldScattering of Electrons in a Medium
Fast moving electrons are light charged particles.
They travel at higher speed., but scattered easily by electrons.
Range of b particles is not as well defined as heavy charged particles, but measured range is still a useful piece of information.
Bremsstrahlung (braking) radiation refers to photons emitted by moving electrons when they are influence by atoms.
Braking radiationInteraction of Beta particles with Matter
Beta particles interact with matter mainly via three modes:
Ionization (scattering by electrons)
Bremsstrahlung (braking) radiation
Annihilation with positrons
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:
When a photon transfers part of its energy to an electron, and the photon becomes less energetic is called Compton effect.
Gamma photons with energy greater than 1.02 MeV produce a electron-positron pair is called pair production.
Photoelectric effect Compton scattering pair production
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
Current (A) is proportional to charges collected on electrode in ionization chambers.
Gas multiplication due to secondary ion pairs when the ionization chambers operate at higher voltage.
Every ionizing particle causes a discharge in the detector of G-M counters.
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 - -
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.
Photons cause the emission of a short flash in the Na(Tl)I crystal.The flashes cause the photo-cathode to emit electrons.
-rayspectrum of 207mPb
Gamma ray spectrum of 207mPb (half-life 0.806 sec)
207mPb Decay Scheme
- Intensity (log scale)
-10 569 + 1063
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.
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.
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.
The Brookhaven 7-foot bubble chamberand the 80-inch 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 –
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 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.
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.