What happens when a target nucleus captures a slow neutron?

What happens when a target nucleus captures a slow neutron?

• A. The nucleus captures a slow neutron, becoming unstable or radioactive.
• B. The nucleus captures a fast neutron, becoming unstable or radioactive.
• C. The nucleus emits a proton.
• D. The nucleus decays by alpha emission only.

Answer: (A)

When a target nucleus captures a slow neutron, it becomes a heavier isotope by one unit in mass number. Slow (thermal) neutrons have a high probability of being captured, so this process is common. The newly formed isotope is often in an excited state and may emit gamma radiation as it relaxes, but more importantly, many of these heavier isotopes are neutron-rich and unstable, so they decay radioactively toward stability. That’s why neutron capture by a slow neutron typically results in a nucleus that is unstable or radioactive.

When a target nucleus captures a slow neutron, it becomes a heavier isotope by one unit in mass number. Slow (thermal) neutrons have a high probability of being captured, so this process is common. The newly formed isotope is often in an excited state and may emit gamma radiation as it relaxes, but more importantly, many of these heavier isotopes are neutron-rich and unstable, so they decay radioactively toward stability. That’s why neutron capture by a slow neutron typically results in a nucleus that is unstable or radioactive.