Nuclear Physics and Radiation Richard Lasky Summer 2010 Nuclear Structure The nucleus is composed of particles called nucleons Electrically charged are called protons No electrical charge are called neutrons Mass of nucleons are 2000 times the mass of electrons So the mass of an atom is almost equal to the
mass of the nucleus Nuclear Structure Atomic Number and Mass Number Isotopes Nuclei that contain the same number of protons but different numbers of neutrons are called isotopes An example of an Atom with many isotopes is
Carbon Isotopes are Rare 98.9% of naturally occurring Carbon is C12 Mass of the Nucleus The mass of a nucleus is specified in unified atomic mass units(u) 1 u = 1.66 X 10 kg Or 931.5 Mev/c
The total mass of a stable nucleus is always less than the sum of the masses of its protons and neutrons The difference is referred to as the total binding energy Total Binding Energy The total binding energy represents the amount of energy that must be put into a nucleus in order to break it apart into separate protons and neutrons To be stable, the mass of the nucleus must
require that extra energy to break it up Calculation of binding energy
Calculation of Binding Energy For a particle 4He: Calculated mass of 2 protons and 2 neutrons: proton = 1.007276470 gm/mole. neutron = 1.008665012 gm/mole .Mass calculated 2 proton 2 neutron mass calculated = 4.032 gm/mole
Calculate binding energy: E = m c c =2.99792458 x 10 m/sec . 1 mass defect E = m c E = 2.73 X 10 joule/ mole Average binding energy per nucleon The greater the binding energy per nucleon of an atom, the greater it's stability
The average energy/nucleon increases as A increases up to plateau of 8.7 Mev After that it decreases slightly Stability of the nucleus Since protons are positively charged !!! Why dont the protons repel each other causing the nucleus to come apart? The strong nuclear force holds all nucleons
(protons and neutrons) together This force is: stronger than the electrical force It is short range This is compared to electrical and gravitational forces that are long range Forces in nature There is another nuclear force that shows up with some radioactivity called the weak
nuclear force Four known forces in nature 1. 2. 3. 4. Strong nuclear Weak nuclear Electromagnetism
Gravity Radioactivity First discovered by Henri Becqurerel in 1896 Found that uranium would darken a photographic plate through a protective wrapping Next Marie Curie and Pierre Curie isolated two unknown elements that were radioactive Polonium and Radium
They discovered that the source of radioactivity is deep within the nucleus and is the result of the disintegration or decay of an unstable nucleus Radioactive isotopes occur in nature or can be produced in the laboratory http://www.youtube.com/watch?v=lFXUfK_C8j Y Radioactivity The three types of radiation are named after the first three letters of the Greek alphabet
Alpha Positively charged Nuclei of helium atoms Barely penetrate a piece of paper Beta Negatively charged Electrons Pass through 3mm of aluminum Gamma
Neutral High energy photons Extremely penetrating - pass through several centimeters of lead http://www.neok12.com/php/watch.php?v=zX557e4374675e794c186673&t=R adioactivity Alpha Decay When a nucleus emits an -particle it will be very different because it loses two protons and two neutrons The spontaneous emission of an alpha particle occurs in elements of mass number greater than about 150, such as uranium, thorium, and plutonium.
The reason alpha decay occurs is because the nucleus has too many protons which cause excessive repulsion. In an attempt to reduce the repulsion, a Helium nucleus is emitted. Beta Decay Beta decay occurs when the neutron to proton ratio is too great in the nucleus and causes instability In basic beta decay, a neutron is turned into a proton and an electron
Beta Decay/Positron emission There is also positron emission when the neutron to proton ratio is too small A proton turns into a neutron and a positron and the positron is emitted A positron is basically a positively charged electron Gamma Decay Gamma decay occurs because the nucleus is at too high an energy The nucleus falls down to a lower energy state
and, in the process, emits a high energy photon known as a gamma particle http://video.google.com/videoplay?docid=423 3777797536713826# Radiation Effects on Humans The unit used to measure radiation dosage is the rem stands for roentgen equivalent in man
represents the amount of radiation needed to produce a particular amount of damage to living tissue the total dose of rems determines how much harm a person suffers A Geiger counter is used to measure radiation A dosimeter is used to measure radiation on a person Effects of Radiation Exposure on Human Health
dose of just 25 rems causes some detectable changes in blood doses to near 100 rems usually have no immediate harmful effects doses above 100 rems cause the first signs of radiation sickness including:
nausea vomiting headache some loss of white blood cells Effects of Radiation Exposure on Human Health Doses of 300 rems or more cause temporary hair loss, but also more significant internal harm, including damage to nerve cells and the cells that line the digestive tract. Severe loss of white blood cells, which are the body's main
defense against infection, makes radiation victims highly vulnerable to disease. Radiation also reduces production of blood platelets, which aid blood clotting, so victims of radiation sickness are also vulnerable to hemorrhaging Half of all people exposed to 450 rems die doses of 800 rems or more are always fatal http://www.youtube.com/watch?v=l93naDmF gzE&feature=related Radioactive Decay The nuclei of radioactive isotopes do not all
decay at the right time Like any atomic process this is random We cant predict when a given nucleus will decay But we can determine, on probabilistic basis, approximately how many will decay over a certain time period Half Life The number of decays N that occur in a short interval t is proportional to the total number
N of radioactive material present N = - N tN t N t = decay constant Radioactive Decay Law http://www.youtube.com/watch?v=515BlcHo mw8&feature=related Half Life The half-life of an isotope is defined as the
time it takes for half the original amount of isotope to decay The half life is related to the decay constant by this formula: T() = .693/N t http://www.lon-capa.org/~mmp/applist/decay /decay.htm
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