Radioactivity Chapter 25
Nuclear Radiation • Nuclear chemistry • study of the structure of atomic nuclei • changes they undergo.
The Discovery of Radioactivity • Wilhelm Roentgen (1845–1923) • 1895-invisible rays were emitted when electrons bombarded the surface of certain materials. • caused photographic plates to darken. • named the invisible high-energy emissions X rays.
The Discovery of Radioactivity • Henri Becquerel (1852–1908) was studying phosphorescence • minerals that emit light after being exposed to sunlight • phosphorescent uranium salts produced spontaneous emissions that darkened photographic plates.
The Discovery of Radioactivity • Marie Curie (1867–1934) and her husband Pierre (1859–1906) took Becquerel’s mineral sample (called pitchblende) and isolated the components emitting the rays. • darkening of the photographic plates was due to rays emitted specifically from the uranium atoms present in the mineral sample.
The Discovery of Radioactivity • Marie Curie named the process by which materials give off such rays radioactivity • the rays and particles emitted by a radioactive source are called radiation.
Types of Radiation • isotopes are atoms of the same element that have different numbers of neutrons. • Isotopes of atoms with unstable nuclei are called radioisotopes • emit radiation to attain more stable atomic configurations in a process called radioactive decay • lose energy by emitting one of several types of radiation.
Why do some atoms decay? • The nucleus contains tightly packed protons and neutrons (nucleons) • The strong nuclear force keeps the nucleons packed together even though protons want to push each other away • Stable atoms have a neutron to proton ratio of about 1:1
As atomic number increases, more neutrons are required to have enough of a strong force to keep the protons pushed together • The neutron to proton ratio for stable atoms increases to 1.5:1
Band of Stability • When the number of protons and neutrons are plotted, the stable nuclei are found within the “band of stability” • Radioactive isotopes are outside the band of stability • They will undergo nuclear reactions to become more stable • All elements higher than atomic# 83 are radioactive
Basic Assessment Questions Calculate the neutron-to-proton ratio for . Topic 26 Question 3
Types of Nuclear Radiation • Alpha • Beta • Gamma
Alpha Radiation • Release of 2 protons and 2 neutrons • Equivalent to a He nucleus • Charge of 2+ • Mass = 4 amu • Largest and slowest • Least penetrating can be stopped by paper • Changes to a different element with a lower atomic mass and lower atomic number • Example: Polonium-212 (atomic# 84) is converted to Lead-208 (atomic# 82)
Beta Radiation • Decay of a neutron into a proton and electron • Electron is emitted, proton stays • Forms a new element b/c of addition of proton • Decay of the proton into a neutron and positron (like a positive electron) • The positron is emitted as a beta particle • Faster than alpha particles can be stopped by aluminum foil
Gamma Radiation • Not a particle • Electromagnetic wave with short wavelength and high frequency & energy • No mass, no charge • Very fast speed of light • Stronger than X-ray • Stopped by several centimeters of lead
Transmutation: changing one element into another through radioactive decay • Adding or removing a proton changes the atomic number, resulting in a different element • Half-Life: amount of time for half of a sample of a radioactive element to decay into something else • Can range from a fraction of a second to billions of years • Amount remaining=initial amount(1/2)t/T • t=total time • T=half-life
Half-life mf:final mass mi:initial mass n:# of half-lives
Half-life • Fluorine-21 has a half-life of 5.0 seconds. If you start with 25 g of fluorine-21, how many grams would remain after 60.0 s?
parent nuclide alpha particle daughter nuclide Nuclear Decay • Alpha Emission Numbers must balance!!
electron positron Nuclear Decay • Beta Emission • Positron Emission
electron Nuclear Decay • Electron Capture • Gamma Emission • Usually follows other types of decay.
Types of Transmutation • Induced transmutation • Nucleus of an unstable isotope (radionuclide) is struck with a high velocity charged particle • Particle accelerator • Need lots of energy and unstable nucleus • Elements atomic 93 and higher (transuranium elements) • Natural transmutation • Occurs naturally as a radioisotope decays to become more stable
Basic Assessment Questions What element is formed when polonium-214 ( ) radioisotope undergoes alpha decay? Give the atomic number and mass number of the element. Topic 26 Question 1
Basic Assessment Questions What element is formed when undergoes beta decay? Give the atomic number and mass number of the element. Topic 26 Question 2
Basic Assessment Questions Topic 26 Question 4 Write a balanced nuclear equation for the beta decay of the following radioisotope.
Nuclear Fission • Fission = divide • Neutron hits an unstable atom • Nucleus splits into two fragments of about the same mass • Some single neutrons are released (energy) • These neutrons can smash into other atoms • Causes a chain reaction
Nuclear reactors • Nuclear power plants use the process of nuclear fission to produce heat in nuclear reactors. • The heat is used to generate steam, which is then used to drive turbines that produce electricity.
Little Boy: $2billion in research; made of Uranium-235; equal to 20,000 tons of TNT; 140,000 people died; 2/3 of the city destroyed • Fat Man: Plutonium-239; 70,000 people died; 40% of the city destroyed
Hydrogen Bomb • 1000 times more powerful than atomic bomb • March 1, 1954; Bikini Atoll in Pacific • Never in war • Fission reaction triggers fusion of Hydrogen isotopes
Nuclear Fusion • Opposite of fission • Two nuclei fuse together to form one nucleus with a larger mass • Not simple sum of masses • Some mass lost as energy • Requires high temperature: Thermonuclear reaction • Occurs in the sun and stars • 4 H combine to form one He, 2e- and energy
Radiation Detectors • Cloud Chamber • supersaturated water or ethanol • radioactive particle flows through and knocks e- off • vapor condenses showing path • alpha: short/thick trails; beta: long/thin • Bubble Chamber • superheated liquid • e- knocked off again • bubbles are formed
Measuring Radiation • Geiger Counter • produces electric current when near radiation • Results in clicks or a digital reading
Using Nuclear Reactions in Medicine • Tracers: monitor body processes • Iodine-131 • emits beta particles • used to detect tumors in thyroid gland • also used: Carbon-11 and Sodium -24
Cancer Treatment • damage cancer cells • Gold -198 or Iridium -192 -- implanted in or near tumor • Cobalt-60 • outside body • emits gamma rays
Positron Emission Tomography (PET) • Fluorine-18 attached to molecules that go to brain • positrons are emitted and collide with electrons forming 2 gamma rays • the gamma rays are detected and indicate brain activity
http://www.hpwt.de/Kern2e.htm • http://www.colorado.edu/physics/2000/isotopes/radioactive_decay3.html • http://www.msd.k12.or.us/schools/mhs/projects/Fission/frames.html • http://www.cnn.com/SPECIALS/cold.war/experience/the.bomb/history.science/