Abstract: An apparatus for measuring isotopic ratios provides four separate optical paths for separate measurement of each of two isotopes relative to a reference signal, using spectrally resolved infrared radiation. The design permits the measurements to be made accurately without significant time lags between measurements, and without interchanging of cells or filters.

Abstract: A system (20) for broadband electromagnetic absorption in an anechoic chamber (22) has walls (24) of the chamber (22) forming a sealed space. A plurality of ionization sources (30) are mounted on an inside surface of the walls (24), facing the sealed space (28), with a spacing to give one ionization source/m.sup.2 of chamber (22) surface. An electron beam source (32) is connected to the ionization sources (30). A source (36) of helium gas is connected to supply the helium to the space (28). An electronic unit (42) under test is inside the anechoic chamber, and is connected to a test system (44). The test system (44) is also connected to the electron beam source (32). The ionization sources (30) generate a helium plasma in the space (28) by pulsed operation of the electron beam source (32).