KAHIBARO
Table of Contents
Neutrons Released in Fission
In nuclear fission, a heavy nucleus splits into two medium sized fragments and releases a large amount of energy. One of the most important products of this process is the neutron. These emitted neutrons are the key to understanding why fission can continue from one nucleus to another.
When a fissile nucleus such as uranium 235 absorbs a neutron, it becomes unstable and may split. During the splitting process, the nucleus usually emits additional neutrons. A typical fission event releases about 2 to 3 neutrons on average, although the exact number varies from one event to another.
A single fission event usually produces more than one neutron. This is the essential reason a chain reaction is possible.
Why Neutrons Are Emitted
Inside a heavy nucleus, there are many protons and neutrons packed closely together. After the nucleus deforms and divides, the two fission fragments are often left with more neutrons than they can stably hold. As a result, some neutrons are expelled during or shortly after the split.
These neutrons are not all emitted in exactly the same way. Most are released almost immediately as part of the fission event itself. These are called prompt neutrons. A much smaller number appear later, after some fission fragments undergo radioactive decay. These are called delayed neutrons.
Prompt and Delayed Neutrons
Prompt neutrons are emitted extremely quickly, almost at the moment of fission. They make up the great majority of fission neutrons. Delayed neutrons are released later, because certain fission fragments are unstable and decay in ways that emit neutrons.
The delayed neutrons are only a small fraction of the total, but they are very important in reactor control. Even though they are rare, their later emission gives more time for the neutron population in a reactor to change gradually rather than instantly.
| Type of neutron | When emitted | Relative amount | Importance |
|---|---|---|---|
| Prompt neutron | Immediately after fission | Very large majority | Sustains the main chain reaction |
| Delayed neutron | After decay of some fission products | Small fraction | Crucial for reactor control |
Most fission neutrons are prompt neutrons. Delayed neutrons are few, but they are essential for practical control of reactors.
Average Number of Neutrons
The average number of neutrons produced per fission is often written as $\nu$. Its value depends on the nucleus and on the conditions of the fission process. For uranium 235, a common approximate value is
$$
\nu \approx 2.4
$$
This means that one fission event produces, on average, about 2.4 neutrons. Some events may produce 2 neutrons, some 3, and some more or less. The value $\nu$ is an average over many fission events.
This average matters because not every emitted neutron will go on to cause another fission. Some neutrons escape from the material, and some are absorbed without causing fission.
Energy of Emitted Neutrons
The neutrons released in fission usually leave with significant kinetic energy. These newly emitted neutrons are commonly called fast neutrons. Their energies are often around a few mega electron volts, typically near $2 \, \text{MeV}$.
Fast neutrons can cause further fission in some nuclei, but in many reactor systems they are slowed down by collisions with other atoms before they are more likely to trigger additional fission events. The details of slowing down and moderation belong to reactor behavior, but the main point here is that fission does not produce neutrons at rest. It produces energetic neutrons.
Fission neutrons are usually born as fast neutrons, with kinetic energies of the order of a few $\text{MeV}$.
Role in the Chain Reaction
The production of neutrons is what allows fission to spread through a sample of fissile material. If one neutron causes one fission, and that fission releases several more neutrons, then those neutrons may cause further fissions.
A simplified picture is:
$$
1 \text{ neutron} \to 1 \text{ fission} \to \nu \text{ neutrons}
$$
If enough of these neutrons produce new fissions, the process becomes self sustaining.
A chain reaction does not depend only on how many neutrons are produced, but neutron production is the starting point. Without emission of multiple neutrons per fission, there would be no continuing fission sequence.
Neutron Balance Idea
Although a fission event may produce about 2 to 3 neutrons, not all of them remain available for future fission. In real materials, neutrons can follow several paths. A neutron may cause another fission, be captured without fission, or escape from the material.
This makes neutron production a balance problem. The number produced must be large enough that at least one effective neutron per fission continues the process.
A simple conceptual relation is
$$
\text{neutrons produced} = \text{neutrons causing fission} + \text{neutrons lost}
$$
where “lost” includes both absorption without fission and leakage out of the system.
Statistical Nature of Neutron Production
Neutron production is not identical in every fission event. It is a statistical process. One nucleus may emit 2 neutrons, another 3, another 4. Because of this, physicists usually speak about average neutron yield rather than an exact fixed number.
This statistical behavior becomes important when analyzing reactors and other multiplying systems. Over many fission events, the average behavior becomes predictable even though individual events vary.
Visualizing Neutron Emission
The diagram below shows a simplified fission event in which an incoming neutron is absorbed, the heavy nucleus splits, and several neutrons are emitted.
Typical Example
A useful example is the fission of uranium 235 after absorbing a neutron:
$$
{}^{235}_{92}\mathrm{U} + {}^{1}_{0}\mathrm{n}
\to
\text{fission fragments} + \text{neutrons} + \text{energy}
$$
The exact fragments can differ from one event to another, so there is not just one single fission equation. However, in many cases the number of emitted neutrons is around 2 or 3.
Key Idea to Remember
Neutron production is the central feature that makes fission unique as a multiplying nuclear process. The emitted neutrons carry energy, appear mostly promptly, and can trigger further fissions.
Key facts about neutron production in fission:
$$
\text{average neutrons per fission} \approx 2 \text{ to } 3
$$
Most are prompt neutrons, a small fraction are delayed neutrons, and these neutrons make a self sustaining chain reaction possible.