1. A nucleus has quantum states
Protons and neutrons are quantum particles. The strong nuclear interaction binds them at short distances, while positively charged protons repel one another electrically. The nucleus has a lowest-energy state and can occupy excited states. A useful picture treats the nucleus as a deformable drop, while a quantum description accounts for effects such as nucleon pairing and shell structure.
2. Follow an incoming neutron
A neutron has no electric charge, so it can approach a nucleus without crossing the electrostatic barrier that repels a proton. Scattering, capture, and fission have probabilities that vary with neutron energy and isotope. A cross section expresses the likelihood of a specified reaction in units of area. Peaks called resonances can occur when the incoming energy matches an accessible nuclear state.
3. Follow the nucleus through fission
Neutron absorption can form an excited compound nucleus. As it deforms, the energy needed to stretch its surface competes with the electric repulsion between protons. The fission barrier is the energy hurdle along that deformation. Quantum tunneling helps explain how spontaneous fission can occur even when a nucleus lacks the classical energy to cross the barrier. In induced fission, neutron absorption adds excitation and can make crossing the barrier much more probable.
- Neutron absorption
- Excited compound nucleus
- Deformation and splitting
- Fragments, neutrons and radiation
4. Account for the energy
The products of fission are more tightly bound overall. The difference in rest mass appears as released energy, E = Δmc². Most of the immediate energy is kinetic energy of the fragments. Prompt neutrons and gamma rays carry a share, and unstable fragments later decay. A single fission, a self-sustaining chain reaction, and the varied exposure of people below an airburst are physically distinct stages.
5. Why the project separated uranium isotopes
Uranium-235 and uranium-238 have the same number of protons and nearly identical chemistry. Their different neutron counts change nuclear behavior and create a small mass difference. Oak Ridge’s wartime plants exploited physical properties related to mass. Uranium enrichment changed the makeup of an existing element. It did not create a new element.
6. How the material shaped the engineering
The uranium route supported a gun-type concept, in which bringing separate components together changed the neutron balance. The project had to coordinate isotope production, metallurgical research, mechanical testing and integration with the aircraft. Tests with nonnuclear articles could examine handling and flight behavior. The full nuclear test at Trinity addressed the project’s other design, the plutonium route.
7. From one nucleus to the people below
Neutrons released by fission can trigger more fissions. Some instead escape or are absorbed without fission. A growing population releases energy rapidly, creating the heat and pressure wave that spread through Hiroshima, alongside ionizing radiation. The city’s records and survivors’ accounts show how location, shelter, fire, and access to treatment shaped what happened to each person.
At 8:15 a.m., students clearing buildings for firebreaks were already out on the streets. Other people were at home, working, or passing through the city. Those exposed included Koreans, Taiwanese, Chinese, and American prisoners of war as well as Japanese residents. Where a person stood, whether a wall shielded them, and who could reach them afterward all mattered.
Setsuko Thurlow was thirteen and had been mobilized for wartime work. She recalled escaping a collapsed building. Sadako Sasaki, who was two at the time, survived the bombing and died of leukemia ten years later. Their experiences span two very different timescales of the disaster.
8. How heat, blast and radiation injured people
Thermal pulse and fires
The fireball radiated energy outward as light and heat. Surfaces and exposed skin absorbed part of that energy, causing burns. Walls and other obstructions could interrupt the direct line of sight. Fires that followed created separate hazards, including smoke and entrapment in damaged buildings.
Blast pressure and kinetic injury
The rapid expansion of hot air produced a shock wave and strong winds. The pressure could injure the body directly. Moving air and debris carried momentum into people and structures, while collapsing buildings crushed or trapped occupants. In Hiroshima, fires and collapsed buildings could cut off routes to help. Setsuko Thurlow’s account gives a person’s perspective on what a pressure wave and structural collapse meant inside an occupied building.
Ionizing radiation and delayed illness
Prompt gamma rays and neutrons deposited energy in tissue. Ionization can damage DNA and other molecules. At high doses, damage to rapidly dividing cells in the bone marrow and gut can lead to bleeding, infection and acute illness over days or weeks. Exposure could also occur after the blast through residual radioactivity.
Long-term studies of survivors found an excess of leukemia and several solid cancers. Those findings describe how risk changed across groups. The injuries, exposures and later diagnoses of individual survivors must be understood through their own medical and personal histories.
Hiroshima estimates about 140,000 people died by the end of December 1945. This includes people who died in the weeks and months after the attack. Surviving the first day could mean years of treatment, family loss, or the later emergence of radiation-related illness.