Nuclear reaction analysis
Fundamental principles of nuclear reaction analysis
Material 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. 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 since other methods fail in this field. Furthermore NRA is a highly sensitive tool with which concentrations of few 10 ppm can be verified.
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 one decays immediately to an alpha particle and an excited 12C isotope whereas the carbon emits a gamma quantum 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. (*)
In RNRA projectiles with energy equal to the resonance energy can only perform nuclear reactions at the sample surface. By increasing the acceleration energy of the ion beam one shifts the resonance reaction in a certain depth: The reactions take place not until the incomming ions are slowed down to ERes. If composition and density of the sample are known one can generate via energy loss (available by simulations, e.g. SRIM) a depth profile.
Drawing of of the MaRPel accelerator. Important instruments are colored in yellow, the beam path is marked in orange. (*)
- Tandem accelerator MaRPel supplies 15N2+ beam with energies up to 7.3 MeV
- "Amsel-steerer" enables to vary beam energy without adjusting the 90° magnet
- Low-level measurement unit: Vacuum chamber, NaI detector (10" x 10")
- Shielding: iron from a dutch warship which is free of radioactive contaminations; active anticoincidence shielding suppresses cosmic rays
- Energy spread for 6.385 MeV 15N: approx. Etot ~ 7 keV
15N2+ ion current at the target approx. 0.5 µA
Picture of the NRA measuring assembly. The iron box guarantees an excellent reduction of the background radiation. (*)
- G. Schatz, A. Weidinger: Nukleare Festkörperphysik; B. G. Teubner Verlag, 3. Auflage, Stuttgart (1997)
- P. Trocellier, C. Engelmann: Hydrogen depth profile measurement using resonant nuclear reaction: an overview, J. Radioanal. Nucl. Chem. 100 (1986) 117
- L.C. Feldman, J.W. Mayer: Fundamentals of Surface and Thin Film Analysis, North-Holland (1986)
- J.B. Marion, F.C. Young: Nuclear Reaction Analysis, Graphs and Tables, North-Holland (1968)
- M. Uhrmacher, M. Schwickert, H. Schebela, K.-P. Lieb: Miss MaRPel - a 3 MV pelletron accelerator for hydrogen depth profiling, J. Alloys Comp. 404-406 (2005) 307
- J. F. Ziegler, M. D. Ziegler, J. Biersack: The Stopping and Range of Ions in Solids; Version SRIM-2008.03, www.srim2008.org, Chester (2008)
- J. Bosman: Hoch sensitiver Nachweis von Wasserstoff mittels resonanter Kernreaktionsanalyse an einem Tandembeschleuniger, Diplomarbeit (2008), Georg-August-Universität Göttingen
We would like to thank Johannes Bosman kindly for provision of all pictures.