Doppler Effect of Nuclear Resonance Level

Abstract

EXPERIMENTAL and theoretical work has shown the existence of remarkably narrow resonance levels for the absorption of slow neutrons in certain nuclei^1–5. In the cases of rhodium, silver (group A) and gold, the natural half-value width can be estimated as 0·06 volt, 0·05 volt and 0·12 volt, respectively. These values, however, may include errors of the order of 100 per cent. It has been pointed out by Frisch⁵ that these widths are comparable with the Doppler width which such resonance must show in consequence of the thermal agitation of the capturing nuclei. According to Bethe and Placzek³, the Doppler width is E_r being the resonance energy and M the mass of the nucleus. This leads to the following widths at room temperature: 0·07 volt for rhodium, 0·1 volt for silver, 0·12 volt for gold. In the case of silver, the Doppler width seems to be even greater than the natural width. In this case, heating a silver absorber should influence considerably the absorption in the middle of the resonance line. In consequence of the Doppler broadening, the absorption in the middle of the line should decrease with increasing temperature of the absorber. Therefore this effect should be observed with a thin detector, which is sensitive principally to the middle of the line.