8.6.1.2. Fission Fragments

Table of Contents

What the fragments are

In nuclear fission, a heavy nucleus splits into two main smaller nuclei. These two nuclei are called fission fragments. They are the primary products of the split and they carry away most of the mass and much of the released energy.

A simple example is the fission of uranium 235 after it absorbs a neutron. The split can happen in many different ways, so there is not just one pair of fragments. One possible reaction is

$$
{}^{235}_{92}\mathrm{U} + {}^{1}_{0}\mathrm{n}
\rightarrow
{}^{141}_{56}\mathrm{Ba} + {}^{92}_{36}\mathrm{Kr} + 3\,{}^{1}_{0}\mathrm{n}
+ \text{energy}
$$

Here barium 141 and krypton 92 are the fission fragments. In another event, a different pair may be produced.

Why many different fragments are possible

A fissile nucleus does not always split into equal halves. Instead, it can break into many possible combinations, as long as the basic conservation laws are satisfied. The mass numbers and atomic numbers of the fragments, together with emitted neutrons, must add up correctly.

Because of this, fission produces a distribution of fragment masses rather than one fixed result. Some fragment pairs occur more often than others.

In fission, there is no single unique fragment pair. A heavy nucleus can split into many different fragment combinations.

Typical properties of fission fragments

Freshly produced fission fragments have some common features. They are usually medium mass nuclei, they move very fast, and they are often unstable. They also tend to contain too many neutrons compared with stable nuclei of similar size.

This neutron richness is important because the fragments often undergo radioactive decay later, especially beta decay, to move toward greater stability.

The table below summarizes the main features.

Property	Typical behavior of fission fragments
Size	Medium mass nuclei
Charge	Positive, because they are atomic nuclei
Stability	Often radioactive
Neutron content	Usually neutron rich
Speed	Very high immediately after fission
Energy	Carry a large part of fission energy

Unequal splitting

A striking fact about many fission processes is that the split is often asymmetric. Instead of producing two equal fragments, the nucleus more commonly forms one heavier fragment and one lighter fragment.

For many common fission events, one fragment has a mass number around 90 to 100, and the other around 130 to 145. This pattern is much more common than a perfectly equal split.

This can be shown schematically.

Rendering scientific sketch…

Asymmetric fission into two main fragments

Fragment motion and kinetic energy

The two fragments repel each other strongly because both are positively charged. This electric repulsion pushes them apart at high speed immediately after the split. As a result, the fragments carry a large amount of kinetic energy.

This kinetic energy is one of the main forms of energy released in fission. When the fragments move through surrounding material, they lose this energy by colliding with atoms and ionizing matter. That deposited energy eventually appears as heat.

Most of the energy released in nuclear fission appears first as kinetic energy of the fission fragments.

Excited fragments

The fragments are often not produced in their lowest energy states. They may be formed in excited nuclear states. An excited fragment can release extra energy by emitting gamma rays, and later many fragments also decay by beta emission.

So the immediate products of fission are not always the final stable nuclei. The original fission fragments are the first large nuclei formed in the split, and these may transform afterward.

A simplified sequence is

$$
\text{heavy nucleus} \rightarrow \text{fission fragments} + \text{neutrons}
$$

followed by possible later changes such as

$$
\text{fragment} \rightarrow \text{new nucleus} + \beta + \bar{\nu}
$$

and sometimes gamma emission:

$$
\text{fragment}^* \rightarrow \text{fragment} + \gamma
$$

Fragment mass distribution

If we count how often different fragment masses are produced, we obtain a fragment mass distribution. For many heavy nuclei, this distribution shows two peaks, one for lighter fragments and one for heavier fragments. This reflects the fact that asymmetric fission is common.

A schematic mass yield curve looks like this.

Rendering scientific sketch…

Typical two-peaked fission fragment mass distribution

This graph is only qualitative, but it shows the important idea that some fragment masses are favored.

Fragments and emitted neutrons

The two main fragments are not the only products of fission. Additional neutrons are usually emitted as well. These neutrons are not called fission fragments. The term fission fragments refers to the two main daughter nuclei formed in the split.

It is useful to keep this distinction clear.

Product of fission	Meaning
Fission fragments	The two main nuclei produced by the split
Prompt neutrons	Neutrons emitted immediately after fission
Gamma rays	Radiation from excited fragments
Later beta decay products	Nuclei formed as unstable fragments decay

Fission fragments are the main daughter nuclei, not the emitted neutrons.

Why fission fragments matter

Fission fragments are important because they determine many practical effects of fission. Their kinetic energy is a major source of heating in a reactor. Their radioactivity contributes to decay heat after fission has occurred. Their chemical identities also matter in reactor fuel behavior and radioactive waste.

So, although fission is often introduced as the splitting of a nucleus, the actual fragments produced are central to understanding what happens next.

Summary

Fission fragments are the two main nuclei formed when a heavy nucleus splits. They are usually unequal in size, neutron rich, highly energetic, and often radioactive. Many different fragment pairs are possible, and the distribution of fragment masses is typically asymmetric with two favored mass regions. Their motion carries most of the immediate fission energy, and their later radioactive decays remain important after the fission event itself.

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