Neutron cross section

Neutron cross section is a measure of the likelihood of a neutron interacting with a specific nucleus. It is a key concept in nuclear physics and nuclear engineering, particularly in the design and operation of nuclear reactors and nuclear weapons. The neutron cross section is typically measured in barns (a unit of area, 1 barn = 10^-28 m^2).

Overview

The neutron cross section is not a physical cross section but a measure of the probability of interaction between a neutron and a nucleus. It is determined by the properties of the nucleus, including its size, shape, and nuclear structure, as well as the energy of the neutron. The neutron cross section varies greatly from nucleus to nucleus and is also strongly dependent on the neutron's energy.

Types of Neutron Interactions

There are several types of interactions that a neutron can have with a nucleus, each with its own cross section. These include elastic scattering, inelastic scattering, radiative capture, fission, and neutron-induced fission. The sum of the cross sections for all possible interactions is the total cross section.

Elastic Scattering

In elastic scattering, the neutron is deflected by the nucleus, but neither the neutron nor the nucleus is excited or transformed. The neutron and nucleus share kinetic energy, but their total kinetic energy is conserved.

Inelastic Scattering

In inelastic scattering, the neutron is absorbed by the nucleus, which is then excited to a higher energy state. The nucleus subsequently emits a gamma ray, returning to its ground state, and the neutron is re-emitted with lower energy.

Radiative Capture

In radiative capture, the neutron is absorbed by the nucleus, which then emits a gamma ray. The nucleus is left in a higher energy state, and the neutron is not re-emitted.

Fission

In fission, the neutron is absorbed by the nucleus, which then splits into two or more smaller nuclei, releasing a large amount of energy.

Neutron-Induced Fission

In neutron-induced fission, the neutron is absorbed by the nucleus, which then splits into two or more smaller nuclei, releasing a large amount of energy and additional neutrons.

Measurement of Neutron Cross Sections

Neutron cross sections are typically measured using neutron sources and neutron detectors. The neutron source emits neutrons, which are directed at a sample of the material whose cross section is to be measured. The neutron detector measures the number of neutrons that are scattered, absorbed, or cause fission in the sample.

Applications

Neutron cross sections are crucial in the design and operation of nuclear reactors and nuclear weapons. In a nuclear reactor, the neutron cross section of the fuel, the moderator, and the control materials must be known to predict and control the reactor's behavior. In a nuclear weapon, the neutron cross section of the fissile material must be known to predict the weapon's yield.