PHASE SHIFT INDUCED TRANSPARENCY OF GAMMA RADIATION

摘要

The "gamma-echo" effect has been observed experimentally. The effect is observed by modifying the time-differential Moessbauer spectroscopy experimental technique. A gamma-echo is produced when the source of recoil-free radiation is moved abruptly during its radiative lifetime. The original interpretation used the semi-classical optical model. Recently a one-dimensional quantum mechanical model has been developed. The advantage of the new model is that it gives a clear physical picture of the resonant gamma ray forward scattering process. The gamma-echo effect is analyzed here using the new model. The analysis shows that the effect is due to the constructive interference between the source radiation, after the abrupt source movement, and the radiation corning from the absorber. Here the source modulation is taken to be such that the source is moved a finite distance instantaneously. Thus there is an instantaneous change of phase. Numerical results, using the familiar ~(57) Fe recoil-free resonant transition, show that the largest gamma-echo effect is produced when the phase shift is π. In fact it is possible to recover, in the forward direction, almost all the radiation incident on the absorber. Thus it is seen that the effect is not a gamma echo but a π phase shift induced transparency.