Testing an ionization chamber with gaseous samples and measurements of the(n, alpha) reaction cross sections

A new ionization chamber with gaseous samples (GIC) has been designed and t ested on the thermal and resonance neutron beams of FLNP's neutron sources. The exposed gas volume serves as a target for neutrons. The obtained therm al cross sections for the O-17(n, alpha)C-14, Ne-21(n, alpha)O-18 and Ar-36 (n, alpha)S-33 reactions are (233 +/- 12) mb, (0.18 +/- 0.09) mb and (5.43 +/- 0.27) mb, respectively. These measurements have been performed on a pur e beam of thermal neutrons from the high flux reactor IBR-2; and they demon strated high efficiency and reliability of the method. Compared to samples on substrates, the application of gaseous samples makes the beam background essentially lower, and what is more important, the background component is totally absent due to the absence of Li and B microimpurities in gaseous s amples while they do present in the samples on substrates. The method is al so applicable to measurements with resonance neutrons. The recovery capabil ity of the GIC after the reactor power pulse has also been tested using the protons from the He-3(n, p)H-3 reaction, alpha-particles from a U-source a nd a pulsed precision generator. (C) 1999 Elsevier Science B.V. All rights reserved.