Comparison of methods for determining the second-order detection efficiency of a VUV spectrometer.

Four independent procedures were developed and tested to measure the apparatus response function of a VUV spectrometer-detector system for unpolarized 46-nm radiation dispersed in second order. These measurements were made to allow the use of continuum synchrotron radiation for the calibration of the response of the spectrometer-detector system for dispersion of 92-nm radiation in first order with full correction for the effects of synchrotron radiation dispersed in second order. In the first method, synchrotron radiation was used in combination with a thin Al foil to block out synchrotron radiation at 92 nm while allowing 46-nm radiation to enter the spectrometer. In the second as well as the third method NeII 46-nm line radiation was used to measure the response function in first and second order. The line radiation was produced by (1) an electron beam exciting a Ne gas target for which the resulting VUV light illuminated the entire grating and (2) a duoplasmatron VUV light source operating with Ne gas producing a small spot of radiation that was scanned across the surface of the spectrometer grating. In the fourth method the difference in the spectral distributions of synchrotron radiation produced by electrons with different kinetic energies was employed to deduce the second-order detection efficiency. The ratio of the second- to firstorder response function for 46-nm radiation could be determined to a precision of 6% using the bandpass filter and electron-beam methods, 10% using the duoplasmatron method, and 250% using the multiple electron energy method.