How Nuclear Fission Was Discovered: Evidence and Explanation

A sample of uranium is bombarded with neutrons. When the radioactive products are analysed, one of them behaves chemically like barium. There is a serious problem with that result: uranium has 92 protons, while barium has just 56. The usual model of neutron capture cannot explain how uranium suddenly becomes an element with 36 fewer protons.

What would you conclude? That the experiment was contaminated, that the chemistry was wrong, or that something unexpected had happened inside the nucleus?

In December 1938, Otto Hahn and Fritz Strassmann faced exactly this problem. Their chemical evidence pointed towards an extraordinary conclusion: uranium nuclei were breaking into smaller nuclei. Lise Meitner and Otto Frisch then developed the physical explanation, showing how a heavy nucleus could split and release an enormous amount of energy.

The discovery of nuclear fission came from two different kinds of scientific reasoning: careful chemical identification and a new physical model of the nucleus.

01What did scientists expect neutrons to do?

To understand why the discovery was so surprising, we need to go back a few years.

In 1932, James Chadwick identified the neutron. Unlike a proton, a neutron has no electric charge. This makes it particularly useful for investigating atomic nuclei.

A positively charged proton is repelled by the positively charged nucleus it approaches. A neutron does not experience this electrostatic repulsion, so it can penetrate the nucleus more easily.

Enrico Fermi recognised the potential of neutrons and began bombarding different elements with them in 1934. His group investigated the radioactive products that formed after neutron irradiation.

When they investigated uranium, they believed they had produced elements heavier than uranium. These hypothetical new elements were called transuranium elements.

The reasoning was straightforward.

A uranium-238 nucleus could capture a neutron to form uranium-239:

\[ {}^{238}_{92}\mathrm{U}+{}^{1}_{0}\mathrm{n} \rightarrow {}^{239}_{92}\mathrm{U} \]

The resulting uranium-239 nucleus could undergo beta-minus decay. A neutron changes into a proton, increasing the atomic number by one:

\[ {}^{239}_{92}\mathrm{U} \rightarrow {}^{239}_{93}\mathrm{Np} + {}^{0}_{-1}\mathrm{e} + \bar{\nu}_e \]

The new element is neptunium, with atomic number 93.

This seemed like a reasonable explanation for the radioactive substances being produced.

The important prediction was that neutron capture and subsequent radioactive decay would produce elements close to uranium in the periodic table, or even heavier elements.

Why wasn’t fission considered?

Scientists already knew that nuclei could undergo radioactive decay and release small particles, particularly alpha particles.

However, there was no accepted explanation for a heavy nucleus splitting into two substantial fragments.

Consider the difference between uranium and barium:

A uranium nucleus producing barium would require a major redistribution of its nucleons. This was far beyond the nuclear transformations scientists were accustomed to explaining.

There was an interesting earlier suggestion. In 1934, chemist Ida Noddack had proposed that uranium might break into lighter elements during neutron bombardment. However, her suggestion did not establish an experimental mechanism, and it attracted little attention.

The prevailing assumption remained that uranium would capture neutrons or undergo relatively small nuclear transformations.

That assumption would soon be challenged by the chemistry.

02Hahn and Strassmann: the chemical evidence

Otto Hahn and Fritz Strassmann were working at the Kaiser Wilhelm Institute for Chemistry in Berlin.

They had been investigating the radioactive products formed when uranium was exposed to neutrons. Their aim was to identify the elements produced and understand their radioactive decay.

Lise Meitner had been a central collaborator in their earlier research. However, she was forced to leave Germany in July 1938 because of Nazi persecution and continued her work from Sweden.

Hahn and Strassmann continued the experiments without her.

The experimental arrangement

The experiment began with a radium-beryllium neutron source.

This source produced neutrons that were directed towards a uranium sample. After irradiation, the uranium was dissolved, allowing the resulting radioactive substances to be separated using chemical techniques.

The important point is that they weren’t simply measuring how much radiation the sample produced.

They wanted to identify the chemical elements responsible for that radiation.

Their procedure involved several stages:

1. Uranium was exposed to neutrons from the radium-beryllium source.
2. The irradiated material was dissolved.
3. Chemical carriers were added to help isolate tiny quantities of radioactive products.
4. Different chemical groups were separated using precipitation and fractional crystallisation.
5. The radioactivity of the resulting fractions was monitored.

The unexpected identification of radium

During their investigations, Hahn and Strassmann identified radioactive products that appeared to behave like radium.

This initially seemed reasonable.

Radium and barium are both Group 2 elements. They have similar chemical properties, meaning they can behave similarly during chemical separation.

The researchers therefore suspected that the radioactive products were isotopes of radium.

However, there was a problem.

They repeatedly struggled to separate the radioactive substances from barium.

If the substances really were radium, they should be separable from barium using appropriate chemical methods.

Hahn and Strassmann needed a way to test this possibility properly.

The crucial fractional crystallisation experiment

Fractional crystallisation is a technique used to separate substances based on differences in their solubility.

When a solution is cooled or concentrated, different compounds crystallise under different conditions. Their distribution between the crystals and the remaining solution can therefore be used to distinguish them.

Hahn and Strassmann used this technique to investigate whether their radioactive products were actually radium.

They added barium compounds to the sample and attempted to separate the suspected radium from the barium.

They also performed comparison experiments using known radium isotopes.

This comparison was important. If their method could separate genuine radium from barium, they could use it to test the identity of the unknown radioactive material.

The result was striking:

• Genuine radium could be separated from barium.
• The unknown radioactive products could not.
• The unknown radioactivity continued to follow the barium through repeated chemical separations.

The radioactive substances were therefore behaving like barium itself, rather than radium.

This was much stronger evidence than simply finding radioactivity in a barium-containing precipitate.

There was another important confirmation.

If the radioactive parent substance was barium, its beta-minus decay would produce lanthanum.

If it were radium, the proposed decay pathway would instead involve actinium.

Hahn and Strassmann found that the daughter product behaved chemically like lanthanum rather than actinium.

The evidence was becoming difficult to dismiss.

Why the chemical evidence mattered

Chemical behaviour is determined primarily by an element’s atomic number.

The number of protons determines the nuclear charge, which determines the arrangement of electrons in a neutral atom and therefore its chemical properties.

This means chemical analysis can identify an element even when only tiny quantities of a radioactive isotope are present.

Hahn and Strassmann had identified radioactive barium produced from uranium.

They had not yet established the complete physical mechanism, but their result had a remarkable implication.

A nucleus with atomic number 92 had produced a radioactive nucleus with atomic number 56.

The original uranium nucleus must have undergone a much more dramatic transformation than scientists had expected.

The dates of the discovery

The historical sequence is worth getting right.

• 19 December 1938: Hahn and Strassmann reached the crucial conclusion that their radioactive products were barium isotopes.
• 22 December 1938: They submitted their research paper for publication.
• 6 January 1939: Their paper appeared in Naturwissenschaften.
• Christmas 1938: Meitner and Frisch developed the physical interpretation of the results.
• 11 February 1939: Meitner and Frisch published their explanation in Nature.

The dates are documented by the Max Planck Institute for Chemistry, which preserves the historical record of Hahn’s work and his laboratory diary.

Notice the distinction: Hahn and Strassmann established the chemical evidence. They did not provide the complete physical explanation of nuclear fission.

That was the next problem.

03Meitner and Frisch: how could a nucleus split?

Hahn sent Meitner a letter describing the unexpected chemical results.

Meitner was living in Sweden, and her nephew Otto Frisch was visiting her over Christmas in 1938. Frisch was a physicist working in Copenhagen at Niels Bohr’s Institute for Theoretical Physics.

They discussed the strange finding.

Initially, the idea seemed almost impossible. How could a single neutron cause a uranium nucleus to produce something as light as barium?

Meitner and Frisch realised that the neutron was not acting like a tiny chisel, chipping a large piece out of the uranium nucleus.

Instead, they considered a different model.

The liquid-drop model of the nucleus

The liquid-drop model treats the nucleus as a collective system of protons and neutrons that can change shape.

Imagine a small water droplet.

A droplet tends to maintain a roughly spherical shape because surface tension pulls it together. If it is disturbed strongly enough, it can become elongated, develop a narrow neck, and eventually separate into two droplets.

Meitner and Frisch applied a similar idea to the uranium nucleus.

There were two important forces involved.

1. The strong nuclear force

The strong nuclear force holds protons and neutrons together inside the nucleus.

It is attractive over very short distances and is responsible for the stability of ordinary atomic nuclei.

2. Electrostatic repulsion

Every proton has a positive electric charge.

Like charges repel, so the protons inside a heavy nucleus push against one another through electrostatic repulsion.

For a heavy nucleus such as uranium, this repulsion is substantial because it contains 92 protons.

The nucleus is held together by the strong nuclear interaction, but the electrostatic repulsion makes it increasingly difficult to maintain stability as the nucleus becomes heavier.

How neutron capture triggers fission

When a neutron is captured, it transfers energy to the nucleus.

The resulting compound nucleus is excited and can undergo collective oscillations.

These oscillations can deform the nucleus from its approximately spherical shape.

As it stretches, the electrostatic repulsion between its protons becomes increasingly important. If the deformation becomes sufficiently large, the nucleus can pass into a configuration where it separates into two fragments.

The two positively charged fragments then repel one another and accelerate away.

This is nuclear fission.

The liquid-drop analogy is useful, but it has limits. A nucleus is not made of liquid molecules, and its behaviour is governed by quantum physics and nuclear forces. The model is a way of understanding collective nuclear deformation, not a literal description of a liquid inside an atom.

What did the fragments look like?

The products of fission are called fission fragments.

They are generally medium-mass nuclei, rather than two identical halves of the original nucleus.

The exact fragments can vary between fission events.

One important feature is that the fragments are often neutron-rich. Heavy nuclei such as uranium contain a greater proportion of neutrons than many stable medium-mass nuclei.

After fission, the fragments can therefore undergo radioactive decay, particularly beta-minus decay, to move towards more stable nuclear compositions.

This explains why Hahn and Strassmann observed several different radioactive products rather than just one.

Naming the process

Meitner and Frisch published their physical explanation in Nature on 11 February 1939.

Their paper was titled Disintegration of Uranium by Neutrons: A New Type of Nuclear Reaction.

Frisch suggested the name nuclear fission, borrowing the term from biological cell division, where one cell divides into two.

The name captured the essential idea: a large nucleus had divided into smaller nuclei.

You can read their original paper here:

Meitner and Frisch, Nature, 11 February 1939.

04Where did the enormous energy come from?

Explaining how uranium could split was only part of the problem.

Meitner and Frisch also needed to explain why the process released so much energy.

Their reasoning relied on the relationship between nuclear mass, binding energy, and Einstein’s mass-energy equivalence.

Nuclear binding energy

Nuclear binding energy is the energy required to separate a nucleus completely into its individual protons and neutrons.

A strongly bound nucleus has a high binding energy.

One useful way to compare different nuclei is to consider their binding energy per nucleon.

A nucleon is either a proton or a neutron.

For light nuclei, binding energy per nucleon generally increases as the nuclei become heavier. It reaches a maximum around iron and nickel, then gradually decreases for very heavy nuclei.

This gives us an important prediction.

A very heavy nucleus such as uranium can release energy by splitting into medium-mass nuclei because the resulting fragments can be more tightly bound.

The mass defect

If the fission fragments are more tightly bound, their combined rest mass is lower than the original system’s rest mass.

The difference is called the mass defect.

This does not mean that mass-energy has disappeared. The reduction in rest mass is converted into other forms of energy, mainly the kinetic energy of the fission fragments, along with energy carried by neutrons and radiation.

Einstein’s equation describes the relationship:

\[ E=\Delta mc^2 \]

Where:

• \(E\) is the energy released, measured in joules.
• \(\Delta m\) is the mass defect, measured in kilograms.
• \(c\) is the speed of light, approximately \(3.00\times10^8\ \mathrm{m\,s^{-1}}\).

Meitner estimated that the mass difference in a typical fission event was approximately one-fifth of the mass of a proton.

That is a very small amount of mass.

However, the speed of light is squared in the equation. Even a tiny mass difference therefore corresponds to a substantial amount of energy.

A second way to check the energy estimate

There was another useful piece of reasoning.

After the nucleus splits, the two fragments are both positively charged. They repel each other.

Meitner and Frisch estimated the energy associated with this electrostatic repulsion and found that it was also of the order of 200 MeV.

This was an important cross-check.

Their mass-energy calculation predicted enough energy to explain the motion of the fragments. The electrostatic calculation independently suggested a similar amount of energy.

The two explanations were consistent.

Frisch later tested the physical prediction using an ionisation chamber. The energetic fragments produced unusually large electrical pulses as they ionised the gas inside the chamber.

This provided direct physical evidence that heavy, highly charged fragments were being produced.

His report, Physical Evidence for the Division of Heavy Nuclei under Neutron Bombardment, appeared in Nature on 18 February 1939.

The chemical evidence had identified the products. The liquid-drop model explained how the nucleus could split. Mass-energy calculations explained why the process released so much energy. Independent physical measurements supported the interpretation.

These were separate pieces of evidence pointing towards the same conclusion.

05Nuclear fission was not the same as a chain reaction

There is one more important distinction.

Discovering that a uranium nucleus can split is not the same as demonstrating that one fission event can trigger a continuing sequence of further fission events.

During fission, neutrons can be released alongside the two main fragments.

Those neutrons may interact with other nuclei. If they cause further fission events, the process can repeat.

This is called a nuclear chain reaction.

The distinction is important:

In 1939, researchers including Frédéric Joliot, Hans von Halban, and Lew Kowarski investigated the neutrons released during fission.

Their experiments provided evidence that fission could produce secondary neutrons, establishing an important condition for a possible chain reaction.

However, demonstrating neutron emission was not enough to create a sustained reaction. Scientists still needed to understand neutron losses, absorption, and the conditions required to maintain the process.

The first controlled, self-sustaining nuclear chain reaction was achieved on 2 December 1942 by Enrico Fermi and his team at the University of Chicago, using Chicago Pile-1.

That was a separate scientific and engineering achievement, almost four years after the original chemical discovery.

The US Department of Energy’s historical account of the first reactor provides further detail.

06Who actually discovered nuclear fission?

It is easy to compress the history into one sentence: Otto Hahn discovered nuclear fission.

That statement misses some important scientific contributions.

Different researchers solved different parts of the problem.

Meitner’s contribution deserves particular attention.

She had worked with Hahn for many years and was central to the research programme that led to the discovery. Although she was no longer in Berlin during the final chemical experiments, Hahn shared the results with her, and she worked with Frisch to explain them.

Their joint interpretation was published separately from Hahn and Strassmann’s chemical paper.

There is also an important detail about scientific recognition.

Hahn alone received the 1944 Nobel Prize in Chemistry for the discovery of fission of heavy nuclei. Strassmann, Meitner, and Frisch did not share that award.

The Nobel record is available here: Nobel Prize in Chemistry 1944.

A historically accurate account therefore distinguishes the experimental discovery from the physical interpretation rather than assigning the entire achievement to one person.

07Questions and solutions

Question 1

Uranium has an atomic number of 92. During neutron bombardment, Hahn and Strassmann identified a radioactive product with the chemical properties of barium, which has an atomic number of 56.

Explain why this observation challenged the accepted model of neutron-induced nuclear reactions.

Solution 1

The identification of barium showed that uranium could produce a much lighter element, something the accepted neutron-capture model could not explain.

The expected reaction involved neutron capture, followed by radioactive decay that generally increased the atomic number:

\[ {}^{238}_{92}\mathrm{U} +{}^{1}_{0}\mathrm{n} \rightarrow{}^{239}_{92}\mathrm{U} \]

Subsequent beta-minus decay would increase the atomic number from 92 to 93.

However, barium has an atomic number of 56.

The difference is:

\[ 92-56=36 \]

This is not a small change involving the capture of a neutron or the emission of an alpha particle.

It suggests that the uranium nucleus has undergone a major transformation, producing a much lighter nuclear fragment.

The observation challenged the assumption that heavy nuclei could only undergo relatively small nuclear changes.

Question 2

Hahn and Strassmann initially suspected that the radioactive products were radium isotopes because radium and barium have similar chemical properties.

They performed fractional crystallisation experiments using barium compounds.

Their results showed that:

• Known radium isotopes could be separated from barium.
• The unknown radioactive products remained with the barium fractions.

Explain why the second observation was significant and why the comparison with known radium isotopes strengthened their conclusion.

Solution 2

The results supported the conclusion that the unknown radioactive products were barium isotopes rather than radium isotopes.

Simply finding radioactivity in a barium precipitate would not have been sufficient.

Radium and barium are chemically similar, so they can initially be carried through the same precipitation process. The radioactive material might therefore have been radium mixed with barium.

The fractional crystallisation experiment provided a more specific test.

If the unknown substance were radium, it should behave like the known radium isotopes and separate from barium under the same chemical conditions.

The comparison experiment demonstrated that the separation method actually worked for genuine radium.

However, the unknown radioactive material continued to follow the barium fractions.

This meant its chemical behaviour was consistent with barium, not radium.

The control experiment was crucial because it ruled out the possibility that the failure to separate the unknown material was simply caused by an ineffective chemical technique.

Further evidence came from the radioactive daughter product. It behaved like lanthanum rather than actinium, supporting the identification of the parent substance as barium.

Together, these results provided strong chemical evidence for radioactive barium being produced from neutron-irradiated uranium.

Question 3

Meitner and Frisch estimated that the mass defect associated with a typical fission event was approximately \(0.18\ \mathrm{u}\).

Calculate the energy released in:

(a) MeV.

(b) Joules.

Use:

\[ 1\ \mathrm{u}\,c^2=931.5\ \mathrm{MeV} \]

\[ 1\ \mathrm{MeV}=1.602\times10^{-13}\ \mathrm{J} \]

Explain what the calculated energy represents physically.

Solution 3

(a) The energy released is approximately \(168\ \mathrm{MeV}\).

Start with mass-energy equivalence:

\[ E=\Delta mc^2 \]

Using the supplied conversion:

\[ E=0.18\times931.5 \]

\[ E=167.67\ \mathrm{MeV} \]

Therefore:

\[ \boxed{E\approx168\ \mathrm{MeV}} \]

(b) The energy released is approximately \(2.69\times10^{-11}\ \mathrm{J}\).

Convert MeV into joules:

\[ E=167.67\times1.602\times10^{-13} \]

\[ E=2.686\times10^{-11}\ \mathrm{J} \]

Therefore:

\[ \boxed{E\approx2.69\times10^{-11}\ \mathrm{J}} \]

Physically, this energy comes from the difference between the initial total rest mass and the final total rest mass of the nuclear products.

The fission fragments are more tightly bound than the original heavy nucleus.

The reduction in rest mass is converted into energy, mainly the kinetic energy of the fragments.

The result is consistent with the approximate energy release of 200 MeV per fission event.

Question 4

A uranium-238 nucleus captures a neutron and forms an excited uranium-239 nucleus.

The excited nucleus subsequently undergoes fission, producing two neutron-rich fragments.

One fragment is initially identified as radioactive barium. It later undergoes beta-minus decay to form lanthanum.

Explain:

(a) Why neutron capture can trigger nuclear fission.

(b) What happens to the atomic number and mass number of the barium fragment during beta-minus decay.

(c) Why identifying lanthanum as a daughter product does not contradict the original identification of barium.

Solution 4

(a) Neutron capture can excite and deform the uranium nucleus until it splits into two smaller fragments.

The captured neutron transfers energy to the uranium nucleus, producing an excited compound nucleus.

According to the liquid-drop model, the nucleus can undergo collective oscillations and become elongated.

The strong nuclear force holds the nucleons together, while electrostatic repulsion between the protons opposes this attraction.

For a sufficiently excited heavy nucleus, the deformation can progress until the nucleus separates into two fragments.

Once separated, the positively charged fragments repel one another and move apart.

The neutron triggers the process, but the electrostatic repulsion helps drive the fragments apart.

(b) The atomic number increases by one, while the mass number remains unchanged.

During beta-minus decay, a neutron changes into a proton:

\[ {}^{1}_{0}\mathrm{n} \rightarrow {}^{1}_{1}\mathrm{p} + {}^{0}_{-1}\mathrm{e} + \bar{\nu}_e \]

The total number of nucleons remains unchanged.

However, the number of protons increases by one.

For barium:

\[ Z=56 \]

After beta-minus decay:

\[ Z=57 \]

The element therefore becomes lanthanum.

(c) Lanthanum is the daughter product, not the original fission fragment.

The original radioactive fragment can correctly be identified as barium even though it later becomes lanthanum.

The chemical identity changes because beta-minus decay changes the number of protons in the nucleus.

This distinction is important when interpreting radioactive decay chains.

The element identified immediately after fission is not necessarily the element that will be present after subsequent radioactive decays.

Question 5

A researcher is given the following experimental observations from neutron-irradiated uranium:

• An unknown radioactive product remains with barium during repeated fractional crystallisation.
• A known radium tracer separates from barium under the same experimental conditions.
• The daughter product of the unknown radioactive material behaves chemically like lanthanum.
• A known actinium tracer separates from lanthanum during the corresponding chemical tests.

The researcher proposes two competing explanations:

Model A: Uranium produces radium, which is simply carried through the chemical separation with barium.

Model B: Uranium undergoes nuclear fission, producing radioactive barium as one of its fragments.

Answer the following:

(a) Which model is better supported by the experimental evidence? Explain why the comparison experiments matter.

(b) Assume that the uranium nucleus undergoes binary fission into two main fragments, with no other charged particles emitted. If one fragment has atomic number 56, determine the atomic number of the complementary fragment.

(c) Does identifying radioactive barium establish the exact mass numbers of both fragments? Explain.

(d) Does the evidence establish that a self-sustaining nuclear chain reaction has occurred? Justify your answer.

Solution 5

(a) Model B is strongly supported: the unknown radioactive product behaves chemically like barium, not radium.

The first observation alone would not be conclusive.

Radium and barium have similar chemical properties, so they can initially follow the same chemical separation pathway.

However, the known radium tracer provides a control.

It demonstrates that the fractional crystallisation procedure can separate radium from barium under the experimental conditions.

The unknown radioactive material does not behave like that tracer. It remains with barium through repeated separations.

The daughter-product experiment provides further evidence.

If the parent is barium, beta-minus decay increases its atomic number from 56 to 57, producing lanthanum.

The daughter product follows lanthanum rather than actinium, while the known actinium tracer can be separated from lanthanum.

This is consistent with the proposed barium-to-lanthanum decay pathway.

The important distinction is that the researchers did not simply fail to separate two chemically similar substances. Their control experiments showed that genuine radium and actinium behaved differently from the unknown radioactive products.

The combined evidence therefore strongly supports Model B.

(b) The complementary fragment must have atomic number 36, corresponding to krypton.

Conservation of electric charge requires the total proton number to remain unchanged.

The original uranium nucleus has:

\[ Z=92 \]

One fragment has:

\[ Z_1=56 \]

Let the complementary fragment have atomic number \(Z_2\).

Then:

\[ Z_1+Z_2=92 \]

Substituting:

\[ 56+Z_2=92 \]

\[ Z_2=92-56 \]

\[ \boxed{Z_2=36} \]

Element 36 is krypton.

Therefore, under the stated binary-fission assumption, the complementary fragment must be krypton.

This is a useful example of how conservation laws allow us to infer information about an unobserved product.

(c) No. Chemical identification establishes the element, but not its exact isotope mass number.

Chemical behaviour tells us the atomic number because it depends on the number of protons.

However, different isotopes of the same element have different numbers of neutrons while retaining the same chemical identity.

For example, radioactive barium isotopes can have different mass numbers but all behave chemically as barium.

To determine the exact mass numbers of the fission fragments, additional measurements or nuclear analysis would be needed.

Mass-number conservation would also have to account for any neutrons emitted during fission.

The chemical evidence therefore establishes an important part of the reaction without identifying every nuclear product.

(d) No. The experiment establishes evidence for fission, not a self-sustaining chain reaction.

The chemical experiments identify radioactive barium and support the interpretation that the uranium nucleus has split.

However, they do not establish whether the fission events release enough neutrons to trigger further fissions.

A chain reaction requires neutrons produced in one fission event to cause additional fission events.

Some neutrons may escape or be absorbed without producing another fission.

The chemical separation experiments do not measure this neutron balance.

Further experiments would be required to establish neutron emission and determine whether the reaction could sustain itself.

This is the central distinction between discovering nuclear fission and engineering a nuclear chain reaction.

The next useful concept is neutron balance: understanding how the neutrons produced in individual fission events determine whether the process stops, continues, or becomes self-sustaining.