Neutron Reactions for NDP
Nuclear Properties of Elements (Isotopes) for NDP Analysis
The table below presents the most common reactions of use for NDP analysis. Particle and mass energies are calculated using the most recent mass values for nuclides and particles (IUPAC 2020). Neutron capture cross sections are for thermal neutrons (0.0253 keV). The values are the average of ENDF/B-VIII.0, JENDL-4.0u, TENDL-2019, JEFF-3.3, BROND-3.1, CENDL-3.2, EAF-2010, and IRDFF-II, where available within each evaluation for the nuclide reaction and energy. Variation of the average cross section reported (Standard Deviation) is generally less than 0.15%, except for some of the radioactive nuclide reactions, where the uncertainties are large.
Principle reactions available for NDP applications:
NDP determinations of the abundances for nuclides are typically determined by taking a ratio of the respective reaction cross sections to that of boron. This is because boron-10 mass standards exist that are traceable to SI values. Reliable mass standards for B-10 atom content and isotopic standards for boron are SRM 2137 and SRM 951a respectively.
Variations in natural isotopic ratios for the lighter elements of boron and lithium in real-world materials represent the largest source of uncertainty in the determination of elemental abundance for samples.
Other nuclides may be impractical to determine by NDP due to their low cross section, low abundance, high specific activity, or some combination of these properties. In an extreme example, Au-197 has a 4E-21b cross section for the (n,α) reaction. O-17, given in the above table, is difficult due to its low abundance, small cross section, and interference from ubiquitous boron contamination. However, enriching the O-17 nuclide in the sample to atomic percent level makes it possible to profile. The disadvantage of its low abundance is also an advantage as the low natural abundance means atmospheric contamination not an issue.
One nuclide missing from the table above is 65Zn (t1/2 = 243.66(9) days). It has an approximately 2 barn cross section for the (n,α) reaction. It also has a much smaller (n,p) cross section.
A few nuclides have fission neutron cross sections up to 0.2 barn cross sections for (n,p) and (n,α) reactions (with the reaction threshold being approximately an MeV and higher) for example 23Na, 24Mg, 27Al, 28Si, 31P, 37Cl, 54Fe... Unlike reactions using thermal neutrons and lower energies, the laboratory center-of-mass changes for high energy neutron reactions and therefore influences how the depth scale is calculated.
The 10B reaction is one frequently utilized reaction in NDP analysis. It is also one of the more interesting reactions. In 6% of the reactions the reaction leads to a ground state 7Li particle. However about 94% of the events go first through an excited state 7Li that promptly decays (while in recoil motion) with the release of a (477 keV) gamma ray. In the figures below are a depiction of the charged particle spectrum obtained from a surface layer of boron with its various reaction particles labeled and an energy spectrum from a sample containing a thin boron layer.
Another reaction of much interest for NDP measurements is the Lithium reaction for the study of lithium ion batteries. The reaction is simple with only two monoenergetic particles. The particles are more energetic and penetrating than those of the boron reaction revealing concentration profiles over a deeper region of the sample.
If you wish to explore the reactions above and a few additional neutron induced reactions having charged particle emissions with cross sections in excess of ≈1 barn (some listed are slightly <1 b), then the reactions listed below are available on the JANIS website. There you will find plots of reaction probabilities for these reactions as a function of neutron energy. A number of these reaction involve radioactive nuclides with short half-lives or minor abundance stable isotopes. There are several good research projects for NDP implied among these reactions.