3.4 Spontaneous fission and neutron emission

Spontaneous fission is a rare decay mode where heavy nuclei split without external triggers. It's most common in actinides like uranium and plutonium, producing lighter nuclei and neutrons. This process is crucial for nuclear energy and weapons.

Neutron emission during fission can lead to chain reactions if enough fissile material is present. Understanding criticality is key for controlling nuclear reactors and preventing accidents. Delayed neutrons play a vital role in reactor safety.

Spontaneous Fission Process

Mechanism and Products

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• Spontaneous fission occurs when a heavy nucleus splits into two smaller nuclei without any external stimulation
• Fission fragments are the lighter nuclei produced by the splitting of a heavier nucleus during spontaneous fission
  • Typically have atomic masses ranging from 70 to 160 amu (atomic mass units)
  • Fission fragments are usually neutron-rich and undergo beta decay to reach stability
• Fission yield refers to the relative abundance of specific fission fragments produced during spontaneous fission
  • Varies depending on the parent nucleus and its excitation energy
  • Typically represented by a double-humped curve, with peaks around atomic masses 95 and 140

Occurrence in Actinides

• Actinides, elements with atomic numbers 89 to 103, are prone to spontaneous fission due to their large atomic nuclei
  • Examples include uranium (U), plutonium (Pu), and curium (Cm)
• The probability of spontaneous fission increases with increasing atomic number and neutron-to-proton ratio
  • Heavier actinides like californium-252 ($^{252}$Cf) and fermium-257 ($^{257}$Fm) have relatively short half-lives due to high spontaneous fission rates

Neutron Emission and Criticality

Neutron Emission in Fission

• Neutron emission occurs during spontaneous fission, as the fission fragments are typically neutron-rich
  • On average, 2-3 neutrons are released per fission event
• Delayed neutrons are emitted by some fission fragments after a delay, typically seconds to minutes after the initial fission event
  • Delayed neutrons are crucial for controlling nuclear reactors, as they provide a longer response time for adjusting reactor conditions

Criticality and Chain Reactions

• Critical mass is the minimum amount of fissile material required to sustain a nuclear chain reaction
  • Depends on factors such as the type of fissile material, its purity, and the geometry of the system
• When the number of neutrons produced by fission equals the number of neutrons lost through absorption and leakage, the system is said to be critical
  • Supercritical systems have increasing fission rates, while subcritical systems have decreasing fission rates
• Nuclear reactors are designed to maintain a controlled, self-sustaining chain reaction by carefully balancing neutron production and loss
  • Control rods, made of neutron-absorbing materials like boron or cadmium, are used to regulate the reactor's criticality