13.1 Nuclear Reactions
13.2 Reaction Kinematics
13.3 Reaction Mechanisms
13.4 Fission
13.5 Fission Reactors
13.6 Fusion
13.7 Special Applications
CHAPTER 13
Nuclear Interactions and Applications
The invention of particle accelerators in the 1930s
Opens a new era in nuclear and particle physics that continues today.
Nuclear reactions and mechanisms.
Nuclear power generation by fission and fusion reactors.
Applications of nuclear particle techniques The search for the discovery of new elements
13.1: Nuclear Reactions
The first man-made nuclear reaction by Rutherford in 1919.
A nitrogen target bombarded with 7.7 MeV alpha particles, which emitted protons.
The first electrostatic accelerator, by Van de Graaff in 1931.
The first cyclotron by E. O. Lawrence and M. S. Livingston in 1932.
13.2: Reaction Kinematics
Consider the reaction: x + X → y + Y
For a target X at rest, conservation of energy Rearranging
The difference between the final and initial kinetic energies is precisely the difference between the initial and final mass energies.
EXAMPLE 13.3 Calculate the ground state Q value for the reaction in which Rutherford first observed a nuclear reaction. The kinetic energy of the a particles was 7.7 MeV.
What was the sum of the kinetic energies of the exit channel?
The final reaction products share 6.5 MeV of energy.
When Q < 0, kinetic energy is converted to mass energy Endoergic reaction (흡열반응).
When Q > 0, mass energy is converted to kinetic energy Exoergic reaction (발열반응).
Threshold Reaction Energy
Consider the reaction of x + X → y + Y in the center-of-mass system
Laboratory system Center-of-mass system
Minimum kinetic energy of the incident particle needed to initiate a nuclear reaction
The speeds of x and X in the center-of-mass system
At threshold,
In the center-of-mass system, total linear momentum is zero The conservation of energy in the center-of-mass system
The threshold energy defined by
This reaction will not take place if the particle’s KE is less than 1.533 MeV.
Therefore, we need an accelerator.
13.4: Fission
In fission a nucleus separates into two fission fragments.
One fragment is typically somewhat larger than the other.
Fission occurs for heavy nuclei because of the increased Coulomb forces between the protons.
We can understand fission by using the semi-empirical mass formula based on the liquid drop model.
A careful examination of the semi-empirical mass formula reveals that spontaneous fission occurs for nuclei with
Fission may also be induced by a nuclear reaction Induced fission
A neutron absorbed by a heavy nucleus forms a highly excited compound nucleus that may quickly fission.
An example of induced fission is
Most probable fission is asymmetric (one mass larger than the other).
Thermal Neutron Fission
This nucleus, in a highly excited and unstable state, is agitated and becomes deformed.
Finally, it becomes so deformed that the Coulomb force overcomes the nuclear force (which acts only over very short distances), and the
nucleus separates—much like a liquid drop.
Prompt neutrons are emitted simultaneously with the fissioning process.
Even after prompt neutrons are released, the fission fragments undergo beta decay, releasing more energy.
Most of the ~200 MeV released in fission goes to the kinetic energy of the fission products, but the neutrons, beta particles, neutrinos, and gamma rays typically carry away 30~40 MeV of the kinetic energy.
Because several neutrons are produced in fission,
these neutrons may subsequently produce other fissions.
This is the basis of the self-sustaining chain reaction.
A sufficient amount of mass is required for a neutron to be absorbed, called the critical mass.
Chain Reactions
A self-sustaining controlled fission process requires that not all the neutrons are prompt.
Some of the neutrons are delayed by several seconds and are emitted by daughter nuclides
These delayed neutrons allow the control of the nuclear reactor.
13.5: Fission Reactors
Several components are important for a controlled nuclear reactor:
1) Fissionable fuel
2) Moderator to slow down neutrons
3) Control rods for safety and to control criticality of reactor
4) Reflector to surround moderator and fuel in order to contain neutrons and thereby improve efficiency
5) Reactor vessel and radiation shield
6) Energy transfer systems if commercial power is desired
In boiling water reactors (BWRs)
the moderating water turns into steam, which drives a turbine producing electricity.
In pressurized water reactors (PWRs)
the moderating water is under high pressure
and circulates from the reactor to an external heat exchanger where it produces steam,
which drives a turbine.
Boiling water reactors are inherently simpler than pressurized water reactors.
However, the possibility that the steam driving the turbine may become radioactive is greater with the BWR.
The two-step process of the PWR helps to isolate the power generation system from possible radioactive contamination.
Energy Transfer
Breeder Reactors
A more advanced kind of reactor is the breeder reactor, which produces more fissionable fuel than it consumes.
The chain reaction is:
Fast breeder reactors have been built that convert 238U to 239Pu.
Breeder reactors hold the promise of providing an almost unlimited supply of fissionable material.
Plutonium is highly toxic, and there is concern about its use in unauthorized weapons production.
13.6: Fusion
Similar to the energy emitted by stars, if two light nuclei fuse together, they also form a nucleus with a larger binding energy per nucleon and energy is released. This reaction is called nuclear fusion.
Energy is released if two isotopes of hydrogen fuse together in the reaction.
Proton-Proton chain reaction
Another cycle due to carbon is also able to produce
4He.
The series of reactions responsible for the carbon or CNO cycle are
Proton-proton and CNO cycles are the only nuclear reactions that can supply the energy in stars.
C-N-O cycle reaction
Nuclear Fusion on Earth
Some scientists believe that controlled nuclear fusion ultimately represents our best source of terrestrial energy.
Among the several possible fusion reactions, three of the simplest involve the three isotopes of hydrogen.
Three main conditions are necessary for controlled nuclear fusion:
1) The temperature must be hot enough to allow the ions, for example,
deuterium and tritium, to overcome the Coulomb barrier and fuse their nuclei together. This requires a temperature of 100~200 million K.
2) The ions have to be confined together in close proximity to allow the ions to fuse. A suitable ion density is 2~3 × 1020 ions/m3.
3) The ions must be held together in close proximity at high temperature long enough to avoid plasma cooling. A suitable time is 1~2 s.
Because of the large amount of energy produced and the relatively small Coulomb barrier,
D-T fusion reaction will most likely be preferred.
Controlled Thermonuclear Reaction: Tokamak
D + T reaction: The tritium will be derived from two possible reactions:
The two major schemes to control thermonuclear reactions:
Magnetic confinement fusion (MCF), Inertial confinement fusion (ICF).
Magnetic confinement of plasma in MCF is done in a tokomak, first developed in the former USSR in the 1960s
The concept of inertial confinement fusion is to use an intense high-powered beam of heavy ions or light (laser) to implode a pea-sized target (a few mm in diameter) composed of D + T to a density and temperature high enough to cause fusion ignition.
The National Ignition Facility at Livermore uses 192 lasers to create a thermonuclear burn for research purposes.
Sandia National Laboratories has used a device called a Z-pinch that uses a huge jolt of current to create a powerful magnetic field that squeezes ions into implosion and heats the
plasma.
France is building a device named Laser Megajoule with similar objectives as the NIF.
Controlled Thermonuclear Reaction: Inertial confinement
(고등광기술연구원 APRI)
13.7: Special Applications
A specific isotope of a radioactive element is called a radioisotope.
Radioisotopes are produced for useful purposes by different methods:
1) By particle accelerators as reaction products
2) In nuclear reactors as fission fragments or decay products 3) In nuclear reactors using neutron activation
An important area of applications is the search for a very small concentration of a particular element, called a trace element.
Trace elements are used in detecting minute quantities of trace
elements for forensic science and environmental purposes.
Medicine
Over 1100 radioisotopes are available for clinical use.
Radioisotopes are used in tomography, a technique for displaying images of practically any part of the body
Single-photon emission computed tomography (SPECT),
Positron emission tomography (PET),
Magnetic resonance imaging (MRI).
Mining and Oil
Radioactive sources to search for oil and gas.
A source and detector are inserted down an exploratory drill hole to examine the material at different depths.
Neutron sources called PuBe (plutonium-beryllium), AmBe (americium-beryllium) are particularly useful.
The neutrons activate nuclei in the material surrounding the borehole, and these nuclei produce gamma decays characteristic of the particular element.
Materials
Neutrons are particularly useful because they have no charge and do not ionize the material, as do charged particles and photons.
They penetrate matter easily and introduce uniform lattice distortions or impurities.
Because they have a magnetic dipole moment, neutrons can probe bulk magnetization and spin phenomena.
Small Power Systems
Alpha-emitting radioactive sources have been used as power sources in heart pacemakers.
Smoke detectors use
241Am sources of alpha particles as current generators. The scattering of the alpha particles by the smoke
particles reduces the current flowing to a sensitive solid-state device, which results in an alarm.
Spacecrafts have been powered by radioisotope generators (RTGs)
since the early 1960s.
New Elements
No transuranic elements those with atomic number greater than
Z = 92 (uranium), are found in nature because of their short half-lives.
Reactors and especially accelerators have been able to produce 24 of these new elements up to Z = 118.
