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Transmutations
• The changing of one element to another is called transmutation
• This occurs whenever there is an alpha decay or a beta decay
• Remember, that for a gamma decay, the nucleus just changes internal energy levels, but doesn’t change the identity of nucleons
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Transmutations
• Consider an alpha decay
• 238U -> 234Th + alpha (4He)
• Have to count the number of protons and neutrons
• Protons [U-92, Th-90, He-2] Balance!!
• Neutrons [U-146, Th-144, He-2] Balance!!
• Mass No. [U-238, Th-234, He-4] Balance!!
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Transmutations
• Consider a beta decay
• 234Th -> 234Pa + beta
• This means a neutron changed into a proton
• So mass number stays the same
• Proton number increases by one and the thorium changes into protactinium
• Actually an anti-neutrino appears also
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Transmutations
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Artificial Transmutation
• First example was Rutherford bombarding nitrogen with alpha particles
• 4He + 14N -> 17O + 1H
• He viewed the reaction with a cloud chamber
• The particles left a vapor trail in the mist
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H H H
http://education.jlab.org/glossary/isotope.html
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Isotopes of Carbon
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Emission of alpha particles :
• helium nuclei • two protons and two neutrons • charge +2e • can travel a few inches through air• can be stopped by a sheet of paper,
clothing.
Alpha Decay
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Alpha Decay
Uranium Thorium
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Beta Decay
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Beta Decay
Thorium Protactinium
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Types of Radioactive Decay Positron Emission
Loss of a positron (a particle that has the same mass as but opposite charge than an electron)
e01
C116
B115 + e0
1
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Types of Radioactive Decay Electron Capture (K-Capture)
Addition of an electron to a proton in the nucleus– As a result, a proton is transformed into a neutron.
p11 + e0
−1 n1
0
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• Gamma radiation electromagnetic energy that is released.
• Gamma rays are electromagnetic waves.
• They have no mass.• Gamma radiation has no charge.
– Most Penetrating, can be stopped by 1m thick concrete or a several cm thick sheet of lead.
Gamma Decay
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Which is more penetrating? Why?
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Nuclear Stability
• Depends on the neutron to proton ratio.
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Neutron-Proton Ratios• Any element with more than
one proton (i.e., anything but hydrogen) will have repulsions between the protons in the nucleus.
• A strong nuclear force helps keep the nucleus from flying apart.
• Neutrons play a key role stabilizing the nucleus.
• Therefore, the ratio of neutrons to protons is an important factor.
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Neutron-Proton Ratios
For smaller nuclei (Z 20) stable nuclei have a neutron-to-proton ratio close to 1:1.
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Neutron-Proton Ratios
For smaller nuclei (Z 20) stable nuclei have a neutron-to-proton ratio close to 1:1.
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Neutron-Proton Ratios
For smaller nuclei (Z 20) stable nuclei have a neutron-to-proton ratio close to 1:1.
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Stable Nuclei
• Nuclei below the belt have too many protons.
• They tend to become more stable by positron emission or electron capture.
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Some Trends
Nuclei with 2, 8, 20, 28, 50, or 82 protons or 2, 8, 20, 28, 50, 82, or 126 neutrons tend to
be more stable than nuclides with a different number of nucleons.
Nuclei with an even number of protons and neutrons tend to be more stable than nuclides that have odd numbers of these nucleons.
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Measuring Radioactivity
• One can use a device like this Geiger counter to measure the amount of activity present in a radioactive sample.
• The ionizing radiation creates ions, which conduct a current that is detected by the instrument.
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Objectives
• Explain the relationship between number of nucleons and stability of nuclei.
• Describe the different types of radioactive decay and their effects o the nucleus.
• Define nuclear fission, chain reaction, and nuclear fusion, and distinguish between them.
• Discuss the possible benefits and the current difficulty of controlling fusion reations.
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Energy in Nuclear Reactions• There is a tremendous amount of energy
stored in nuclei.
• Einstein’s famous equation, E = mc2, relates directly to the calculation of this energy.
• In chemical reactions the amount of mass converted to energy is minimal.
• However, these energies are many thousands of times greater in nuclear reactions.
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Einstein• "The intuitive mind
is a sacred gift and the rational mind is a faithful servant. We have created a society that honors the servant and has forgotten the gift."
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Man-Made Radioactive Decay on Earth
• Fission
• Fusion– Occurs naturally in space– Powers the sun– Supernovas allow atoms to fuse into heavier
elements, this is how the other elements came into existence
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• "Any intelligent fool can make things bigger, more complex, and more violent. It takes a touch of genius -- and a lot of courage -- to move in the opposite direction."
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Fission
• Nuclear fission occurs when scientists bombard a large isotope with a neutron.
• This collision causes the larger isotope to break apart into two or more elements.
• These reactions release a lot of energy.
• You can calculate the amount of energy produced during a nuclear reaction using an equation developed by Einstein: E=mc2
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Nuclear Fission
1n + 235U -> 91Kr + 142Ba + 31n
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Nuclear Fission• How does one tap all that energy?• Nuclear fission is the type of reaction carried out
in nuclear reactors.
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Chain Reaction
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Nuclear Fission
• Bombardment of the radioactive nuclide with a neutron starts the process.
• Neutrons released in the transmutation strike other nuclei, causing their decay and the production of more neutrons.
• This process continues in what we call a nuclear chain reaction.
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Nuclear Fission
• If there are not enough radioactive nuclides in the path of the ejected neutrons, the chain reaction will die out.
• Therefore, there must be a certain minimum amount of fissionable material present for the chain reaction to be sustained: Critical Mass.
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Critical Mass
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Atomic Bombs• Because of the tremendous amount of energy released in
a fission chain reaction, the military implications of nuclear reactions were immediately realized.– The first atomic bomb was dropped on Hiroshima, Japan, on
August 6, 1945.
• In an atomic bomb, two pieces of a fissionable isotope are kept apart. Each piece by itself is subcritical.
• When it’s time for the bomb to explode, conventional explosives force the two pieces together to cause a critical mass.
• The chain reaction is uncontrolled, releasing a tremendous amount of energy almost instantaneously.
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Mushroom Cloud
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Atom Bomb
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Nuclear Power Plants• If the neutrons can be controlled, then the
energy can be released in a controlled way. Nuclear power plants produce heat through controlled nuclear fission chain reactions.
• The fissionable isotope is contained in fuel rods in the reactor core. All the fuel rods together comprise the critical mass.
• Control rods, commonly made of boron and cadmium, are in the core, and they act like neutron sponges to control the rate of radioactive decay.
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Nuclear Power Plants (cont)• In the U.S., there are approximately 100 nuclear
reactors, producing a little more than 20% of the country’s electricity.
• Advantages– No fossil fuels are burned.
– No combustion products (CO2, SO2, etc) to pollute the air and water.
• Disadvantages– Cost - expensive to build and operate.
– Limited supply of fissionable Uranium-235.
– Accidents (Three Mile Island & Chernobyl)
– Disposal of nuclear wastes
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Nuclear ReactorsIn nuclear reactors the heat generated by the reaction is used to produce steam that turns a turbine connected to a generator.
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Nuclear Reactors
• The reaction is kept in check by the use of control rods.
• These block the paths of some neutrons, keeping the system from reaching a dangerous supercritical mass.
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Three Mile Island
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Chernobyl
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Nuclear Fusion
• Fusion is when lighter nuclei are fused into a heavier nucleus.
• Fusion powers the sun. Four isotopes of hydrogen-1 are fused into a helium-4 with the release of a tremendous amount of energy.
• On Earth, H-2 (deuterium) & H-3 (tritium) are used.
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Fusion Reactions
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Nuclear Fusion
• Fusion would be a superior
method of generating power.– The good news is that the
products of the reaction are
not radioactive.
– The bad news is that in order to achieve fusion, the material must be in the plasma state at several million kelvins.
– Tokamak apparati like the one shown at the right show promise for carrying out these reactions.
– They use magnetic fields to heat the material.
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Positron emission tomography, also called PET imaging or a PET scan, is a diagnostic examination that involves the acquisition of physiologic images based on the detection of radiation from the emission of positrons. Positrons are tiny particles emitted from a radioactive substance administered to the patient.
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Isotopic Dating
• Cosmic rays (mostly high energy protons) strike upper atmosphere and cause transmutations that result in many protons and neutrons being sprayed out
• Protons tend to grab electrons from other atoms and become simple hydrogen
• Neutrons keep going and smash into other atoms
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Isotopic Dating
• When a neutron strikes nitrogen
• 1n + 14N -> 14C + 1H
• In the atmosphere, 14C is about 1 part in 1011
• Reacts with plants just like 12C
• 14C decays via beta emission
• 14C -> 14N + beta half life of 5730 years
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Half-Life
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Half-Life
• There are a large range of half-lives seen in nature
• Half-lives are unaffected by the nuclei’s surroundings and only depend on what goes on inside the nucleus
• 238U has a half-life of 4.5 billion years
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Half-Life
• To summarize, one-half of the sample will decay in one half-fife
• One-half of that one-half will decay in the next half life
• One-half of that one-fourth will decay in the next half life
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Carbon Dating
• When a plant dies, it stops the intake of carbon
• Since the 14C decays, after 5730 years, half of it will be gone
• We can just weigh a piece of dead wood, calculate how much 14C it originally had and measure to how much it has now to get the age
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Uranium Dating
• We know the half -life of 238U and 235U• They have series that end in 206Pb and 207Pb• Compare how much U vs. special lead and
calculate the age of the rock!!• Carbon dating only good for about 50,000
years• Uranium rock dating good for millions of
years
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ExampleYou have 100 g of radioactive C-14. The half-
life of C-14 is 5730 years.
• How many grams are left after one half-life? Answer:50 g
• How many grams are left after two half-lives?
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Problem
A sample of 3x107 Radon atoms are trapped
in a basement that is sealed. The half-life of
Radon is 3.83 days. How many radon atoms
are left after 31 days?
answer:1.2x105 atoms