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 13Nuclear 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 ReactionsThe 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 KinematicsConsider 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 fuel2) Moderator to slow down neutrons3) Control rods for safety and to control criticality of reactor4) Reflector to surround moderator and fuel in order to contain neutrons and thereby improve

efficiency5) Reactor vessel and radiation shield6) 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

(고등광기술연구원APRI)

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 products2) In nuclear reactors as fission fragments or decay products3) 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.

Over 150 new isotopes heavier than uranium have been discovered.