A LOW-LEVEL DETECTION SYSTEM FOR HYDROGEN ANALYSIS WITH THE REACTION 1H (15N,ALPHA-GAMMA)12C

A spectrometer has been constructed for measurements of low hydrogen concentrations by detecting 4.43 MeV gamma-rays from the reaction H-1 (N-15,alphagamma) C-12 at the 6.39 MeV resonance. It consists of an annular large-volume (2600 cm3) BGO detector, an annular plastic-detector anticoincidence shield around it, and Pb and Cd shields. The spectrometer is placed in the target hall of the laboratory located 20 m deep inside the granite bedrock. The background counting rate arising mainly from impurity of the BGO materials is 0.023 Hz in the energy range 3.9-4.9 MeV. The detection sensitivity of 10 at.ppm H in silicon substrates was determined by measurements of concentration distributions of 2x10(13)-1x10(17) 40 KeV protons cm-2 in single-crystal silicon substrates. The range profiles of 40 keV protons simulated by molecular dynamics calculations resulted in the correction factor of 1.21 for the commonly used electronic stopping power of silicon given by Ziegler et al. [The Stopping Power and Ranges of Ions in Matter (Pergamon, New York, 1985) vol. 1].