On design of FEL-based orbictron terahertz range radio spectrometer for application in art expertise technology of art works

Works of art expert examination is a type of activity intended to identify conformity of any work of art to data provided by examination ordering party. The experts examine and analyze work of art for its authenticity, i.e. its conformity to claimed creation time and authorship of a particular artist. Their work results in experts' report which confirms or rejects authenticity of a painting, icon, vase, jewelry, etc. The backbone of expert activity can be defined as identification and establishment of work of art status, as well as acquisition of primary data about it. Development and production of highly effective inspection toolkit designed to provide consistent results of art expertise is task of high current interest. Present-day study of art and art expertise involve wide range of different noncontact inspection methods, including infrared, ultraviolet and X-ray spectral analyses, acoustical surveillance methods, etc. Each of above methods have own limitations and disadvantages. Creation of femtosecond lasers boosted development of terahertz spectroscopy in time domain - difficult in its physical processes, complicate in its schematic solutions, labour-consuming in processing of measured results. Therefore search of new advanced methods and engineering solutions still remains a crucial task. Peculiarities and outlook of terahertz emission use in art expertise have been distinguished. We describe design and operating principle of radio spectrometer in terahertz range on basis of source of electromagnetic oscillations - laser with free electrons and energies of nonrelativistic particles - orbictron (acronym of open resonator, binary comb and electron terms). Orbictron is a vacuum generator with band electron stream, oscillative circuit as a double-mirror open resonator (OR) and periodic structure as a binary comb which acts as phased-array antenna emitting into intra-mirror volume. Object under examination is a part of intra-mirror volume of double-mirror high-Q (Q ≈ 7·10³) open resonator of orbictron. Narrow range of self-oscillations generated by orbictron (10-20 kHz) provides spectral resolution determined by Doppler broadening of lines only, without use of additional circuit for output signal frequency stabilization. Change of electromagnetic emission characteristics by object under examination is registered by a detector which is an electron collector for change of band electron stream.