Neutron cross section

An isotope (or nuclide) can be classified according to its neutron cross section and how it reacts to an incident neutron. Nuclides that tend to absorb a neutron and either decay or keep the neutron in its nucleus are neutron absorbers and will have a capture cross section for that reaction. Isotopes that undergo fission are fissionable fuels and have a corresponding fission cross section. The remaining isotopes will simply scatter the neutron, and have a scatter cross section. Some isotopes, like uranium-238, have nonzero cross sections of all three.

Isotopes which have a large scatter cross section and a low mass are good neutron moderators (see chart below). Nuclides which have a large absorption cross section are neutron poisons if they are neither fissile nor undergo decay. A poison that is purposely inserted into a nuclear reactor for controlling its reactivity in the long term and improve its shutdown margin is called a burnable poison.

Parameters of interest

The neutron cross section, and therefore the probability of a neutron–nucleus interaction, depends on:

• the target type (hydrogen, uranium...),
• the type of nuclear reaction (scattering, fission...).
• the incident particle energy, also called speed or temperature (thermal, fast...),

and, to a lesser extent, of:

• its relative angle between the incident neutron and the target nuclide,
• the target nuclide temperature.

Target type dependence

The neutron cross section is defined for a given type of target particle. For example, the capture cross section of deuterium ²H is much smaller than that of common hydrogen ¹H. This is the reason why some reactors use heavy water (in which most of the hydrogen is deuterium) instead of ordinary light water as moderator: fewer neutrons are lost by capture inside the medium, hence enabling the use of natural uranium instead of enriched uranium. This is the principle of a CANDU reactor.

Type of reaction dependence

The likelihood of interaction between an incident neutron and a target nuclide, independent of the type of reaction, is expressed with the help of the total cross section σ_T. However, it may be useful to know if the incoming particle bounces off the target (and therefore continue travelling after the interaction) or disappears after the reaction. For that reason, the scattering and absorption cross sections σ_S and σ_A are defined and the total cross section is simply the sum of the two partial cross sections:

${\displaystyle {\sigma }_{\text{T}}={\sigma }_{\text{S}}+{\sigma }_{\text{A}}}$

Absorption cross section

If the neutron is absorbed when approaching the nuclide, the atomic nucleus moves up on the table of isotopes by one position. For instance, ²³⁵U becomes ^236*U with the * indicating the nucleus is highly energized. This energy has to be released and the release can take place through any of several mechanisms.

1. The simplest way for the release to occur is for the neutron to be ejected by the nucleus. If the neutron is emitted immediately, it acts the same as in other scattering events.
2. The nucleus may emit gamma radiation.
3. The nucleus may β⁻ decay, where a neutron is converted into a proton, an electron and an electron-type antineutrino (the antiparticle of the neutrino)
4. About 81% of the ^236*U nuclei are so energized that they undergo fission, releasing the energy as kinetic motion of the fission fragments, also emitting between one and five free neutrons.

• Nuclei that undergo fission as their predominant decay method after neutron capture include ²³³U, ²³⁵U, ²³⁷U, ²³⁹Pu, ²⁴¹Pu.
• Nuclei that predominantly absorb neutrons and then emit beta particle radiation lead to these isotopes, e.g., ²³²Th absorbs a neutron and becomes ^233*Th, which beta decays to become ²³³Pa, which in turn beta decays to become ²³³U.
• Isotopes that undergo beta decay transmute from one element to another element. Those that undergo gamma or X-ray emission do not cause a change in element or isotope.

Scattering cross section

The scattering cross section can be further subdivided into coherent scattering and incoherent scattering, which is caused by the spin dependence of the scattering cross section and, for a natural sample, presence of different isotopes of the same element in the sample.

Because neutrons interact with the nuclear potential, the scattering cross section varies for different isotopes of the element in question. A very prominent example is hydrogen and its isotope deuterium. The total cross section for hydrogen is over 10 times that of deuterium, mostly due to the large incoherent scattering length of hydrogen. Some metals are rather transparent to neutrons, aluminum and zirconium being the two best examples of this.

Incident particle energy dependence

For a given target and reaction, the cross section is strongly dependent on the neutron speed. In the extreme case, the cross section can be, at low energies, either zero (the energy for which the cross section becomes significant is called threshold energy) or much larger than at high energies.

Therefore, a cross section should be defined either at a given energy or should be averaged in an energy range (or group).

As an example, the plot on the right shows that the fission cross section of uranium-235 is low at high neutron energies but becomes higher at low energies. Such physical constraints explain why most operational nuclear reactors use a neutron moderator to reduce the energy of the neutron and thus increase the probability of fission which is essential to produce energy and sustain the chain reaction.

A simple estimation of energy dependence of any kind of cross section is provided by the Ramsauer model, which is based on the idea that the effective size of a neutron is proportional to the breadth of the probability density function of where the neutron is likely to be, which itself is proportional to the neutron's thermal de Broglie wavelength.

${\displaystyle \lambda (E)=\frac{h}{\sqrt{2mE}}}$

Taking ${\displaystyle \lambda }$ as the effective radius of the neutron, we can estimate the area of the circle ${\displaystyle \sigma }$ in which neutrons hit the nuclei of effective radius ${\displaystyle R}$ as

${\displaystyle \sigma (E)\propto \pi (R+\lambda (E){)}^2}$

While the assumptions of this model are naive, it explains at least qualitatively the typical measured energy dependence of the neutron absorption cross section. For neutrons of wavelength much larger than typical radius of atomic nuclei (1–10 fm, E = 10–1000 keV) ${\displaystyle R}$ can be neglected. For these low energy neutrons (such as thermal neutrons) the cross section ${\displaystyle \sigma (E)}$ is inversely proportional to neutron velocity.

This explains the advantage of using a neutron moderator in fission nuclear reactors. On the other hand, for very high energy neutrons (over 1 MeV), ${\displaystyle \lambda }$ can be neglected, and the neutron cross section is approximately constant, determined just by the cross section of atomic nuclei.

However, this simple model does not take into account so called neutron resonances, which strongly modify the neutron cross section in the energy range of 1 eV–10 keV, nor the threshold energy of some nuclear reactions.

Target temperature dependence

Cross sections are usually measured at 20 °C. To account for the dependence with temperature of the medium (viz. the target), the following formula is used:

${\displaystyle \sigma ={\sigma }_0{\left(\frac{T_0}{T}\right)}^{\frac{1}{2}},}$

where σ is the cross section at temperature T, and σ₀ the cross section at temperature T₀ (T and T₀ in kelvins).

The energy is defined at the most likely energy and velocity of the neutron. The neutron population consists of a Maxwellian distribution, and hence the mean energy and velocity will be higher. Consequently, also a Maxwellian correction-term 1⁄2√π has to be included when calculating the cross section Equation 38.

Doppler broadening

The Doppler broadening of neutron resonances is a very important phenomenon and improves nuclear reactor stability. The prompt temperature coefficient of most thermal reactors is negative, owing to the nuclear Doppler effect. Nuclei are located in atoms which are themselves in continual motion owing to their thermal energy (temperature). As a result of these thermal motions, neutrons impinging on a target appears to the nuclei in the target to have a continuous spread in energy. This, in turn, has an effect on the observed shape of resonance. The resonance becomes shorter and wider than when the nuclei are at rest.

Although the shape of resonances changes with temperature, the total area under the resonance remains essentially constant. But this does not imply constant neutron absorption. Despite the constant area under resonance a resonance integral, which determines the absorption, increases with increasing target temperature. This, of course, decreases coefficient k (negative reactivity is inserted).