Thermal Neutron
Definition
A low-energy neutron (around 0.025 eV) that has been slowed by a moderator. The workhorse particle of NAA because slow neutrons are most efficiently captured by target nuclei.
- Energy
- About 0.025 eV
- Origin
- Fast neutron slowed by a moderator
- Role in NAA
- Efficiently captured by target nuclei
- Contrast
- Fast neutrons carry far higher energy
Common questions
Why are thermal neutrons preferred for activation analysis?+
Slow neutrons have a much higher capture cross-section for most target nuclides than fast neutrons, producing stronger induced radioactivity signals for trace-element identification.
What role does the moderator play in producing them?+
A material such as water or graphite slows fast neutrons from a reactor or source through repeated collisions until they reach thermal energies, without which far fewer neutrons would be captured by the sample.
Related terms
- BARC Trombay
- Bhabha Atomic Research Centre, Mumbai. Hosts the Dhruva and Apsara-U research reactors that have historically supplied neutron flux for Indian NAA work.
- Gamma Spectrometry
- Energy-resolving detection of gamma rays. Each radioisotope emits a characteristic gamma at a known energy, which is how NAA identifies elements.
- Half-Life
- The time required for half the radioactive atoms in a sample to decay. Set by the decay constant lambda through t1/2 =...
- HPGe Detector
- A high-purity germanium semiconductor crystal cooled with liquid nitrogen to about 77 kelvin. Resolution at the 1,332 keV gamma line of cobalt-60...
- INAA
- Instrumental Neutron Activation Analysis. The whole sample is irradiated, cooled, and counted directly. No wet chemistry. Non-destructive.
- Neutron Capture (N, Gamma)
- The reaction in which a stable nucleus absorbs a neutron and becomes a radioactive isotope of the same element, emitting a prompt...
- RNAA
- Radiochemical Neutron Activation Analysis. After irradiation, the analyst chemically separates the isotope of interest to remove interfering activity. More sensitive, destructive.