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. 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

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

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 substances Material Air Xe He NH3 Ge-crystal Average IE 35 22 43 39 2.9 Ionizing Radiation

Primary and Secondary Ion Pairs ooooooooooooooooo oooooooooooooooo oooooooo+-ooooooo oooooo-+ooooooooo oooo+-ooooooooooo oo-+ooo+-oooooooo +-ooooooooooooooo ooooooooooo+-oooo Primary ion pairs are caused directly by radiation. Secondary ion pairs are generated by high-energy primary electrons. Molecular density (molecules/mL) air = 2.7e19 water = 3.3e22 Ionizing Radiation

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

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

Range of Heavy Charged Particles in a Medium Particles lose all their energy at a distance called range.  source Shield Ionizing Radiation

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

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)2 v = 6.9e6 m/s Speed of 1 MeV (= 1.6e-13 J) electron 1.6e-13 J = (1/2) m v2 = (½) 9.1e-31 kg v2 Solving for v, v2 = 3.52e17 (m/s)2 or v = 5.9e8 m/s. exceeds c (=3e8 m/s), the speed limit Proper evaluation method shown next Still reasonable Ionizing Radiation

Proper Evaluation of Particle Speed The relativity mass m 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

Scattering of Electrons in a Medium Fast moving electrons are light charged particles. They travel at higher speed., but scattered easily by electrons.  source Shield Ionizing Radiation

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

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

Interaction of Beta particles with Matter Ionization Braking radiation Annihilation Beta particles interact with matter mainly via three modes: Ionization (scattering by electrons) Bremsstrahlung (braking) radiation Annihilation with positrons Ionizing Radiation

Interaction of Photons with Matter Photon Energies Visible red light 1.5 eV visible 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

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

The animated Compton effect Ionizing Radiation

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

Gamma-ray Three Modes of Interaction with Matter Photoelectric effect Compton scattering pair production Ionizing Radiation

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 x c: absorption constant x: thickness Ionizing Radiation

Ionizing Radiation Measurements Radiation Detectors - overall view electroscopes ionization chambers proportional counters Geiger-Muller counters solid-state detectors photographic films and photographic emulsion plates bubble chambers and cloud chambers scintillation counters and fluorescence screen Ionizing Radiation

Ionization Chambers Current (A) is proportional to charges collected on electrode in ionization chambers. Ionizing Radiation

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

Geiger-Muller Counters 1X00 V Every ionizing particle causes a discharge in the detector of G-M counters. Ionizing Radiation

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 - - Negative Positive electronic counter See: bo.iasf.cnr.it/ldavinci/programme/Presentazioni/Harrison_cryo.pdf Ionizing Radiation

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

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

Scintillation Detector and Photomultiplier tube Ionizing Radiation

Gamma ray spectrum of 207mPb (half-life 0.806 sec) 207mPb Decay Scheme 13/2+____________1633.4 keV - Intensity (log scale) 1063 -1e4 569 5/2-____________569.7 keV -1e3 1/2-____________0.0 stable -1e2 -10 569 + 1063 -1 Energy -ray spectrum of 207mPb Ionizing Radiation

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

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

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

Bubble Chambers The Brookhaven 7-foot bubble chamber and the 80-inch bubble chamber  Ionizing Radiation

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

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

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

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