Maria Wagner, Gerhard Winkler, Herbert Vonach

Institut für Radiumforschung und Kernphysik
der Universität Wien, A-1090 Vienna, Austria

Horst Liskien

Commission of the European Communities
Joint Research Centre, Geel Establishment
Central Bureau for Nuclear Measurements, B-2440 Geel, Belgium

The cross section for the reaction 93Nb(n,n')93mNb was measured by foil activation to be (265.9 ± 9.8) mbarn at a neutron energy of (7.91 ± 0.02) MeV relative to the 238U(n,f) cross section employing a small ionization chamber as neutron fluence monitor. The mass of the fissile reference deposit was assessed relative to the mass of Al foils by means of a separate irradiation experiment with 14.8 MeV neutrons, thus relying on the 238U(n,f)/27Al(n,α) cross section ratio at this energy. The induced 93mNb activity was measured detecting the Nb K-shell x rays from the 30.7 keV isomeric transition by means of a Si(Li) x-ray detector. Sample size effects due to self-attenuation of the Nb x rays, extension of the sample, inhomogeneous foil activation and K-shell x-ray fluorescence induced by background radiation were experimentally determined. The result indicates, together with other recently available experimental data, that the cross section information included in the International Reactor Dosimetry File should be revised.

KEYWORDS: 93Nb(n,n')93mNb, activation cross section, En= 7.9 MeV, reactor dosimetry, fluence monitor