Why Neutrons Find Nuclei Easily on Atomic Level

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Discussion Overview

The discussion revolves around the behavior of neutrons in relation to atomic nuclei, particularly focusing on the conditions under which neutrons can effectively find and interact with nuclei, such as in the context of nuclear fission. The scope includes theoretical considerations, experimental implications, and conceptual clarifications regarding neutron interactions and critical mass scenarios.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants suggest that the abundance of atoms and the speed of thermal neutrons contribute to the likelihood of neutrons finding a nucleus, especially in a critical mass scenario.
  • Others point out that the DeBroglie wavelength of neutrons increases at lower energies, which leads to a larger capture cross-section and explains the use of moderators to slow down neutrons for better absorption.
  • One participant raises a question about the average distance a neutron travels before causing fission in a critical mass of U-235, noting that it may be on the order of centimeters and that many neutrons could escape without causing fission.
  • Another participant draws an analogy between neutron interactions and asteroids hitting planets, but acknowledges the need to consider quantum effects due to the subatomic scale of neutrons.
  • A participant expresses curiosity about the percentage distance from the equator of a nucleus within which a fast neutron could induce fission, suggesting a range of 5% to 20%.

Areas of Agreement / Disagreement

Participants express various viewpoints on the mechanisms by which neutrons interact with nuclei, with no consensus reached on specific distances or the effectiveness of different scenarios. The discussion remains unresolved regarding the exact parameters influencing neutron fission.

Contextual Notes

Some claims depend on assumptions about neutron behavior in critical mass scenarios, the effectiveness of moderators, and the quantum mechanical nature of subatomic particles, which are not fully explored or agreed upon in the discussion.

1907Quarter
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and nuclei are so small(on an atomic level!). is it that there are SO many atoms around and that thermal neutrons are still incredibly fast, that if there is a reflector surrounding(lets say we're talking about a critical mass of uranium and an uncontrolled chain reaction), that the neutron will inevitably find a nucleus?
 
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The other important point is that, since the DeBroglie wavelength of the neutron increases as the energy and momentum decreases, the capture cross-section gets much larger at low energies. This is why moderators are used to slow down the neutrons so that they are more likely to be absorbed.
 


so, assuming a critical mass of pure u235, and modifiers to slow down the neutron what is the AVERAGE distance a neutron will travel before it fissions an atom? (in picometers)
 


1907Quarter said:
so, assuming a critical mass of pure u235, and modifiers to slow down the neutron what is the AVERAGE distance a neutron will travel before it fissions an atom? (in picometers)
It's on the order of cm. In a critical mass of pure U-235, there would be no moderation or very little slowing down. Such systems are usually designed for prompt criticality, i.e., the fission is induced by fast (fission) neutrons. Many neutrons however would escape the mass.
 


is this not just like asking how come asteroids hit planets?
 


starting to put some numbers together, I will share some thoughts but I'm curious...within what percent off the equator of a nucleus will a fast neutron fission be possible? 5%? 10%? 20%?
 


Doofy said:
is this not just like asking how come asteroids hit planets?

No. A neutron is a tiny subatomic particle and we must account for quantum effects, which are very dominant at that scale.
 

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