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Radiation. Electromagnetic Radiation. X-rays. William Roentgen German Physicist - 1895. Was working with cathode ray tubes when he noticed that a phosphorescent material in his lab was glowing several meters away.
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X-rays William Roentgen German Physicist - 1895 • Was working with cathode ray tubes when he noticed that a phosphorescent material in his lab was glowing several meters away. • He made a phosphorescent screen and was shocked when he put his hand in front of the screen – he could see the outline of his bones!! • He did not know what kind of rays were responsible for this phenomenon so he called them X-rays Roentgen and famous picture of his wife’s hand
Electromagnetic Spectrum (of waves) • (x-rays were part of EM spectrum) • Light, heat (infra-red), microwaves, ultra-violet, x-rays, gamma rays and radio waves are all just energy waves of different frequencies. • The sun emits radiation across the EM spectrum
Frequency and Wavelength • Frequency (f) - the number of waves that go by each second (Hertz – Hz) • Wavelength (λ) - the length of one complete wave. (eg. Radio waves from station 95.3 MHz, 95 300 000 waves go by each second and a wave length of about 3m)
Electromagnetic Spectrum Low Frequency Low Energy High Frequency High Energy
Summary of EM radiation • All EM radiation travels at the speed of light (3.0X108 m/s) • Frequency is proportional to Energy • Frequency is inversely proportional to wavelength • EM radiation has NO MASS and can travel without particles
Discovery of Radioactivity • Becquerel put different elements in the sun and then placed them on photographic plates in dark drawers to study phosphorescence. • One day in 1896 there was no sun and he put Uranium on a photographic plate in a dark drawer. The next day the plate was cloudy! Energy was coming from Uranium itself! Henri Becquerel French Physicist
Discovery of Radium and Polonium • Marie and Pierre Curie spent years purifying radioactive elements. • They discovered new radioactive elements Radium and Polonium in1898. Marie and Pierre Curie of Poland and France
Three types particle radiation • Rutherford found three different types of particles were emitted. • He called them alpha(), beta() and gamma(γ) particles Ernest Rutherford New Zealand
Penetrating ability of particles • alpha particles stopped by paper • beta particles stopped by 5 mm of aluminum • gamma rays stopped by 30 cm of dense lead
Charge on three types of radiation In a magnetic field • Alpha particles deflected one way, • Beta particles deflected the other way • Gamma rays not deflected at all. γ(0) (-) (+)
Ionizing Radiation alpha particle beta particle Where does all of this radiation come from? Radioactive Atom X-ray gamma ray
Radiation Types Clarified + - • Particle radiation consists of particles that have mass. ie. alpha or beta particles. • Electromagnetic Radiation consists of massless waves/photons of higher and lower energies. ie. X-rays, gamma rays, light, radio waves, UV, IR, microwaves +
Structure of the Atom (review) • Atoms all contain a dense nucleus with protons and neutrons. • Elements are arranged on the periodic table ONLY by the number of protons in the nucleus • Electrons travel around the nucleus and attempt to balance the charge from the protons.
Isotopes • Isotopes are elements with the same number of protons and a different number of neutrons eg. Carbon-12 and Carbon-14 (this is also the charge on the nucleus)
Stable Isotopes • Elements can only accept certain numbers of neutrons. Too many or too less, and elements become unstable and will decay (orange) or not form at all (white)
Radioactive Decay • Radioactive decay is the spontaneous decay of atoms by emitting alpha, beta or gamma particles. • New elements are always formed during alpha and beta decay • (This is the sort of thing that happens with radioactive waste)
Beta Decay • Beta decay involved the ejection of a beta particle (could either be an electron or a positron) • Electrons come from a neutron and change it to a proton!!
Gamma Decay • The “*” denotes high energy • Gamma rays are emitted when a particle has too much energy. No new elements are formed.
Sources of Gamma Rays • Supernova explosions, neutron stars, and black holes are all sources of celestial gamma-rays
Balancing Nuclear Equations • Both MASS and CHARGE must be conserved in any nuclear reaction • This means that the sum of masses and atomic numbers on the right and left sides of the equations must be equal! eg. The decay of Uranium-238:
Half Life • The half-life is the amount of time it takes for half of the unstable atoms in a sample to decay. • The half-life for a given isotope is always the same no matter how many particles you have or what happened in the past. • For example, if an element has a half life of 4 days and starts off with 16g of unstable particles, then after the first four days, 8g will remain. After the next four days, 4g will remain. After four more days, 2g will remain etc.
Decay applet http://www.lon-capa.org/~mmp/applist/decay/decay.htm
Carbon-14 Dating • Cosmic rays change Nitrogen-14 in to Carbon-14 in the atmosphere. This radioactive form of Carbon-14 is absorbed into organisms through carbon dioxide. • Once the organism dies, the carbon slowly decays to nitrogen-14 • The half-life of carbon-14 is about 5730 years.
Uses of Radiation • Pet Scans – Uses positrons to get moving 3d image by reacting with radioactive injections • Cat Scans – 3D X-ray image • X Rays (do not include Cat Scans) • Food irradiation • Tracers, leakage, and wear in industry, density and thickness measurements • Cancer Treatment • Activation analysis, crime solving – composition determining using spectrometry • Smoke detectors • Microwave ovens • Cell/mobile phones
The Dangers of Radiation • Both high frequency electromagnetic radiation and particle radiation can ionize atoms (give them a charge). • Ionized atoms can change the DNA leading to the reproduction of cancerous cells • Types of ionizing radiation include alpha, beta (particle), gamma, x-rays and neutrons (EM).
How do we measure radiation? Geiger Muller Tube (Geiger Counter) • Has a large cylindrical cathode (positive electrode) and a tungsten anode (negative electrode) with a voltage across them. • When alpha, beta or gamma radiation ionizes a gas particle, the positive ion moves to the cathode and the electron moves to the anode, thus sending a signal to the counter.
