Ultrasonically tunable filter for ψ-rays

A new type of an ultra-narrow frequency transmission filter for ψ-rays is presented. The principle of its operation is based on the Mössbauer-sideband-spectroscopy 1–3]: A resonant absorber onto a thin piezo-electric polymer foil vibrates piston like at an ultrasonic frequency ω. At certain definite amplitudes the zero-order-sideband, i.e. the transition energy ω₀, is fully transmitted without any resonant absorption, whereas simultanously sidebands displaced by energies ±hhΩ from ω₀ are resonantly absorbed. Using several absorbers of this type in series having different frequencies a broadband filter for ψ-rays with a well defined transmission-profile can be designed. Symmetrically, on both sides of the central transmission window there exist resonant absorption bands. The energy width of these different bands is only limited by the possible ultrasonic frequencies and can be adjusted from 10^?o to 10^?5 eV. First experimental results using such an assembly of absorbers are presented. The limit of the efficiency of the filter, caused by non-resonant scattering is discussed.