AG Hofsäss - II- Institute of Physics
Fundamental principles of nuclear reaction analysis
Very efficient, very sensitive and very element specific materials characterization can be performed by means of Nuclear Reaction Analysis (NRA). This method uses accelerated particles which initiate a nuclear reaction with specific target atoms in the sample. Often the nuclear reaction exhibits a rather sharp energy resonance. The emitted radiation is characteristic for this reaction and can be detected. From the intensity of the radiation one is able to determine the concentration of the particular atomic sort.
Analyses via nuclear reactions are well suited for the detection of light elements like H, Li and B, since most other methods fail in this field. Furthermore NRA is a highly sensitive tool with which concentrations of few 10 ppm can be verified.
11B detection using a 400-500 keV proton ion beam
The cross section for the reaction 11B(1H,2α)α is close to 1 barn and allows easy and background-free detection of 11B . The emitted alpha particles have energies up to about 6 MeV. Moreover, the decay channel to an excited state of 8Be with an extremely short half life leads to a broad energy distribution of the alpha particles. The width of this distribution of 1 MeV is deterimend by the energy-time uncertainty and corresponds to a life time of the 8Be of about 10-22 seconds. The method allows detection of B below 1014 atoms/cm2. We use this technique to analyse the B dopant concentration in graphene films.
This nuclear reaction experiment is offered as advanced physics lab course to work with nuclear energy level digrams, to carry out the quantitative determination of reaction cross sections and to evaluate the energy-time uncertainty.
Hydrogen detection and depth profiling with the 15N-method
Desired or undesired, hydrogen exists in many materials and has an decisive effect on the behavior of the substances. Examples are penetration and inclusion of hydrogen in metals which leads to material embrittlement and stability reduction. In polymers (e.g. a-C:H) the change of the hydrogen fraction results in altering of electrical, optical, thermal and tribological properties which topic is still an active field of research - with NRA as a powerful tool for such studies.
An important point of view for these investigations is the concentration profile measurement as a function of depth whereas two different possibilities are available: the Resonant Nuclear Reaction Analysis (RNRA) or the non-resonant Nuclear Reaction Analysis (NRA).
A standard method for hydrogen detection with RNRA is based on the reaction 1H(15N,αγ)12C, visualized in the figure below.
Illustration of the nuclear reaction for hydrogen detection with 15N ions. (*)
A 15N ion beam is focussed on a hydrogen containing sample. Is the energy of the nitrogen ions equal to the resonance energy ERes both nuclei fuse to the 16O compound nucleus. This decays immediately to an alpha particle and an excited 12C isotope whereas the carbon emits a gamma photon with a well-defined energy of Eγ = 4.43 MeV to reach its ground state. By measuring this photon one gets an unambiguous proof for this nuclear reaction and thus a confirmation for hydrogen identification.
Principle of hydrogen profiling with with the 15N method. (*)