Abstract
Since the lower excited states of nuclei have very small widths (« 1eV.), resonant scattering of gamma-rays requires precise matching of the energy available from the gamma-ray with the energy necessary to excite the scattering nucleus.
Resonant scattering should be observable if (1) the emitting and scattering nuclei are of identical type, (2) the gamma-transition goes to the ground state, and (3) the source and scatterer are given such a relative velocity that Doppler effect restores the energy lost by the gamma-ray to nuclear recoils Thermal velocities of the emitting and scattering nuclei broaden and correspondingly weaken the resonant scattering peak, and the cross section at the optimum speed of 32E/A cm/sec. is 3.6 × 10-3(IΓ/E3)(A/T)1/2 cm2, where E and Γ are the energy and intrinsic width of the excited state in electron volts, I the isotopic abundance of the resonantly scattering isotope, A its atomic weight and T the absolute temperature.
Preliminary experiments have been made with the 0.411 MeV. radiation from the nucleus 198Hg, the source being carried by a high-speed rotor up to a speed of about 7 × 104 cm/sec. and the scatterer being liquid mercury (10% 198Hg) A small but apparently significant increase of scattering was found, corresponding to a width Γ of the order of 10-5 eV
No such increase was observed with 181Ta gamma-rays scattered from tantalum carbide.
The negative result for 181Ta and the positive result for 198Hg are consistent with the latest information about the life-times of the excited states concerned, viz. 1.1 × 10-8 sec. for 181Ta and less than 2 × 10-10 sec. for 198Hg.