Nuclear fission 1938 — from the lab bench to E = mc² (3D)
About this app
Hahn and Strassmann's work bench as a reconstruction: irradiate, separate chemically, measure. In the nucleus uranium-235 splits according to the liquid-drop model, a mass-defect calculator shows where the energy comes from, plus chain reaction with moderator and Lise Meitner's interpretation.
With explain mode: an AI tutor walks you through the app, points out what matters in the scene and answers your questions.
What was discovered in 1938
In December 1938 Otto Hahn and Fritz Strassmann irradiated uranium with neutrons in Berlin and then found barium by chemical means, an element with nuclei only just over half as heavy. Early in 1939 Lise Meitner and Otto Frisch explained what had happened: the uranium nucleus splits into two medium-weight nuclei and releases two to three neutrons. Together the fragments weigh slightly less than before. According to E = mc² this missing mass becomes energy, about 200 MeV per fission.
What the app shows
The app shows its own reconstruction of the lab bench with neutron source, paraffin block, uranium sample, glassware, counter tubes, amplifier and counter. The experiment runs in three steps: irradiate, separate chemically, measure. A switch shows that the instruments were spread over three rooms in 1938. In the nucleus view the uranium nucleus splits in the liquid-drop model, phase by phase. The mass panel computes the mass defect, and a schematic chain reaction shows the difference between subcritical, critical and supercritical.
What you can try
Play the experiment and watch barium precipitate while the counter tallies the decays. In the nucleus, scrub the sequence back and forth: approach, capture, oscillation, necking, fission. Compare three example reactions and read off in the magnifier how much mass is missing. Change the chain reaction and switch the moderator off. The history panel sets out the roles of Hahn, Strassmann, Meitner and Frisch.
Frequently asked questions
- Where does the energy in nuclear fission come from?
- From missing mass. Uranium-235 and a neutron together weigh 236.0526 u, the fragments barium-141, krypton-92 and three neutrons only 235.8666 u. The difference of 0.186 u gives about 173 MeV by E = Δm · c², and about 200 MeV per fission including the later decays of the fragments.
- What role did Lise Meitner play in the discovery of nuclear fission?
- Lise Meitner started the uranium experiments with Otto Hahn in 1934 and led the physics side until she had to flee in 1938. Together with Otto Frisch she provided the interpretation as nuclear fission, including the energy balance. Only Hahn received the 1944 Nobel Prize in Chemistry.
- Did Otto Hahn discover the atomic bomb?
- No. Hahn and Strassmann discovered a nuclear reaction in very few atoms, detectable only through their radiation. Enrico Fermi achieved the first chain reaction in Chicago in 1942, and the bomb was built in the US Manhattan Project. Hahn took no part in it.
- Is the lab bench in the app a photo of the museum exhibit?
- No. The app shows its own schematic reconstruction based on published descriptions. Shapes, sizes and layout are estimated and marked as a reconstruction.
Subject: Physics experiments (Deutsches Museum)
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