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. How reactors made plutonium
When uranium-238 absorbs a neutron, it can become uranium-239. Two successive beta-minus decays convert it first to neptunium-239 and then to plutonium-239. In beta-minus decay a neutron becomes a proton, with an electron and an antineutrino emitted. Each decay advances the atomic number by one while leaving the mass number unchanged. At Hanford, chemical separation took advantage of plutonium’s different chemistry to recover it from irradiated material.
6. Why the research shifted to implosion
Reactor production yielded a mixture of plutonium isotopes. Plutonium-240 undergoes spontaneous fission more readily than plutonium-239 and contributes background neutrons. That made the original gun-type plutonium concept unreliable. The implosion approach used compression to change the density and neutron balance. Trinity was the first full-scale test of the general plutonium approach later used at Nagasaki.
7. Connect the physics to the human record
The rapidly growing chain reaction deposited energy in matter and the surrounding air. The airburst produced heat, blast, and ionizing radiation. The terrain at Nagasaki and the location of the airburst help explain the distribution of physical damage. The city’s museum and memorial hall preserve the accounts of how people experienced it and what survival cost.
At 11:02 a.m., the bomb detonated over Urakami. Teachers and children, postal workers, medical staff, and families in their homes were among the people below. The valley held homes and places of worship as well as wartime industry. Korean victims included people brought to Japan for wartime labor.
Sumiteru Taniguchi was delivering mail by bicycle when he was severely burned. Sachiko Yasui was six and later wrote about the losses her family lived through. Dr. Takashi Nagai helped organize relief despite his own injuries and his wife Midori’s death.
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 Nagasaki, the terrain influenced how the blast spread. Distance alone could not describe an individual’s exposure. Taniguchi’s account shows how the heat could cause lasting injury even where a person survived the initial blast.
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.
A postwar survey cited by Nagasaki estimated approximately 74,000 deaths by the end of 1945. Some people died after the day itself from burns, injuries, or acute radiation illness. Others lived for decades with the effects of injuries, illness, and bereavement.