Fast neutrons vs thermal neutrons

In summary, fast neutrons and thermal neutrons are two types of neutrons found in nuclear reactions. They differ in energy levels and speeds, with fast neutrons having higher energy and moving at higher speeds, while thermal neutrons have lower energy and move at slower speeds. Both types of neutrons are used in different types of nuclear reactions, with fast neutrons being more effective in fission reactions and thermal neutrons being more effective in fusion reactions. Through a process called moderation, fast neutrons and thermal neutrons can be converted into each other, which is important in controlling nuclear reactions and producing energy.
  • #1
nuke21
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If an nuclide can be fissioned by thermal neutrons does that always mean it can be fissioned by fast neutrons. If not how can one tell this by looking at the neutron cross section?
 
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  • #2
Nuclides that readily fission by thermal neutron absorption are termed 'fissile' as opposed to those which can only be fissioned by fast neutrons which are termed 'fertile'. Fertile really means that the nuclide can absorb a neutron and decay to a fissile nucleus.

Fissile radionuclides (e.g., U-233, U-235, Pu-239, . . . ) to fission by fast neutrons, but the fission cross-section is lower.
 
  • #3


Fast neutrons and thermal neutrons are both types of neutrons with different energies. Fast neutrons have higher energies (around 1 MeV) while thermal neutrons have lower energies (around 0.025 eV). The energy of a neutron plays a crucial role in nuclear reactions, including fission.

In terms of fission, fast neutrons are more effective at inducing fission in certain nuclides compared to thermal neutrons. This is because fast neutrons have more kinetic energy, which allows them to penetrate deeper into the nucleus and interact with the nucleons more effectively. On the other hand, thermal neutrons have less kinetic energy and are more likely to be absorbed or scattered by the nucleus, reducing their effectiveness in inducing fission.

However, just because a nuclide can be fissioned by thermal neutrons does not necessarily mean it can also be fissioned by fast neutrons. This is because the fissionability of a nuclide depends on many factors, such as the size and shape of the nucleus, its binding energy, and the energy of the neutron. Therefore, it is possible for a nuclide to be fissionable by thermal neutrons but not fast neutrons.

One way to determine the fissionability of a nuclide by different types of neutrons is by looking at its neutron cross section. The neutron cross section is a measure of the probability of a neutron interacting with a nucleus. A higher cross section indicates a higher probability of interaction and therefore a higher chance of fission.

By comparing the neutron cross section of a nuclide for thermal neutrons and fast neutrons, one can determine if it is more likely to undergo fission with one type of neutron over the other. If the cross section is significantly higher for fast neutrons, then the nuclide is more fissionable with fast neutrons. However, if the cross section is similar for both types of neutrons, then the nuclide is fissionable by both thermal and fast neutrons.

In summary, while thermal neutrons are generally less effective at inducing fission compared to fast neutrons, the fissionability of a nuclide depends on various factors and cannot be determined solely based on its ability to undergo fission with thermal neutrons. The neutron cross section can provide information on the fissionability of a nuclide with different types of neutrons.
 

What are fast neutrons and thermal neutrons?

Fast neutrons and thermal neutrons are two types of neutrons that can be found in nuclear reactions. Fast neutrons have high energy and move at high speeds, while thermal neutrons have lower energy and move at slower speeds.

What is the main difference between fast neutrons and thermal neutrons?

The main difference between fast neutrons and thermal neutrons is their energy levels and speeds. Fast neutrons have higher energy and move at higher speeds, while thermal neutrons have lower energy and move at slower speeds.

How are fast neutrons and thermal neutrons used in nuclear reactions?

Fast neutrons and thermal neutrons are both used in different types of nuclear reactions. Fast neutrons are used in nuclear fission reactions, while thermal neutrons are used in nuclear fusion reactions.

Which type of neutron is more effective for nuclear reactions?

The effectiveness of fast neutrons and thermal neutrons in nuclear reactions depends on the type of reaction. Fast neutrons are more effective in nuclear fission reactions, while thermal neutrons are more effective in nuclear fusion reactions.

Can fast neutrons and thermal neutrons be converted into each other?

Yes, fast neutrons and thermal neutrons can be converted into each other through a process called moderation. In this process, fast neutrons are slowed down and converted into thermal neutrons, while thermal neutrons can be accelerated to become fast neutrons. This conversion is important in controlling nuclear reactions and producing energy.

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