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Channel electron multiplier: its quantum efficiency at soft x-ray and vacuum ultraviolet wavelengths John E. Mack, Francesco Paresce, and Stuart Bowyer This work was done at University of California, Space Sciences Laboratory, Berkeley, California 94720. J. E. Mack has now returned to State University College, De- partment of Geosciences, Buffalo, New York 14222. Received 24 November 1975. The channel electron multiplier (CEM) is a versatile low noise photon detector widely used both in the laboratory and in space. Its many advantages have been extensively discussed in the literature. 1 - 3 It is particularly useful in the 300-1050-Å band where its sensitivity exceeds that of any other presently available detector. 2 While its electrical characteristics are well known, its response to radiation in the 1-1500-Å region to which it is most sensitive has yet to be fully explored. Of particular importance for EUV as- tronomy 4 is the 50-300-A range for which practically no in- formation exists. An additional problem commonly en- countered with regard to the CEM quantum efficiency is that all available measurements refer to a wide variety of CEM models made by different manufacturers. These in- clude Johnson's 2 measurements of a Bendix S3029X in the 350-1700-Å range, Bowyer et al.'s 3 of a Bendix 4019 at 0.56 A, 2.1 A, and 304 Å, Weller and Young's 5 of a Bendix 4039C in the 304-1493-Å range, Parkes et α/.'s 6 of a Mullard B419BL in the 2-68-A range, Lapson and Timothy's 7 of a Bendix 4050-SX in the 461-1400-Å range, Timothy and Lapson's 8 of a Mullard B419BL in the 304-1350-Å range, and Paresce's 9 of a Bendix 4019 in the 1200-2500-Å range. There is no quantum efficiency measurement of a single type of CEM over its entire sensitive range. We have measured the quantum efficiency of a Galileo 10 CEM, model 3555899, over the 2-1600-Å wavelength range over which its sensitivity is higher than ~1%. We have also compared its efficiency to that of a quite different Bendix model 4019 in the same wavelength range to make sure the Galileo CEM response was typical of this type of detector. The Galileo CEM on which the measurements were made has a 28-mm diam extended cone connected to a helix channel electron multiplier section mounted as de- scribed by Hoshiko. 11 This particular CEM was selected from a number of this type available in our laboratory be- cause its electrical characteristics were typical. No special precautions were taken in handling or storage beyond those suggested by the manufacturer. The CEM cone was bi- ased at 800 V and the multiplier section at 3600 V insuring operation in the pulse saturated mode at pressures of sev- eral X 10 -6 Torr. The output pulses from the CEM were fed into a sensitive preamplifier, a shaper-discriminator network, and finally into a timer-sealer. The background counting rate of this detector was approximately 4 counts/ min. The CEM was placed in a vacuum chamber featuring a movable translation stage and a manipulator arm, both externally controlled. These were used to introduce the CEM, the primary standard, and necessary windows and filters in and out of the photon beam. At 2.1 A the source of photons was an Fe 55 radioactive source, and at 8.3 A an alpha-fluorescence source, exciting the L-K transition in an aluminum target, was used. Both sources were mechanically collimated to a beamwidth of 10°. The reference standard detector at these wavelengths was a gas-flow proportional counter with a polypropylene window of measured thickness of 70 X 10 -6 g cm -2 , filled with a gas mixture of 90% A and 10% CH 4 , 3 cm deep with a monitored pressure of 400 Torr. The pulse spectrum of the proportional counter signal was observed in each case, and a discriminator level set that assured virtually 100% pulse counting. The absolute efficiency of the counter was computed using the available elemental cross sections. 12 For the 44-170-Å region in x-ray source similar to the one described by Henke and Tester 13 was used to generate strong lines at 44 A, 68 A, 113 A, 136 A, and 170 A using C, B, and Be, Si and Al targets, respectively. The reference standard at these wavelengths was the same gas-flow pro- portional counter, filled with propane at a pressure of 200 Torr. The absorption efficiency of the gas was computed using available elemental cross sections. 12 The window transparency was measured by mounting it on an arm that could move it in and out of the beam, using a CEM as a monitor. A hollow cathode source of the type described by Paresce et al. 14 produced radiation in the 200-1600-Å range by using gases such as neon, argon, helium, and hydrogen yielding useful lines for calculation every 25-50 A through- out this region. The reference standard for the 200-1216- A range was a NBS calibrated windowless Al 2 O 3 photo- diode described by Canfield et al. 15 The 1216-1600-Å re- gion was covered by a NBS calibrated far uv photodiode having a MgF 2 window and a semitransparent rubidium telluride photocathode. Calibration errors for these diodes never exceeded 10% over the useful range of the devices. Radiation from the Henke and the hollow cathode source was fed into a 1.2-m grazing incidence monochromator with a 300-line/mm, Al overcoated grating set at an angle of in- cidence of 87°. The fixed exit port of the monochromator was connected to the vacuum chamber via a parabolic colli- mating mirror set at an angle of 81°. In this way the pho- ton beam entering the vacuum chamber was collimated to 15 min of arc and had a diameter of approximately 6 mm. A comparison of the output signals from the Galileo CEM and the standard detectors as they viewed the beam in turn yielded the quantum efficiency of the CEM at all the wave- lengths used. A beam intensity which was sufficiently in- tense to give a measurable diode signal was found to satu- rate the CEM. Hence, a filter was calibrated at each wave- length, then used to reduce the intensity incident of the CEM. Random efficiency variations of the order of 15% or less were observed across the funnel face at most wavelengths. The exceptions were at the shortest wavelengths, below 200 A, where the efficiency at the lip was about twice that at the center. This effect has been previously noted. 6 Re- sults of these measurements are shown in Fig. 1 where we have plotted the observed quantum efficiency in counts/ photon as a function of wavelength over the entire range where the response exceeds 1%. The results apply to the center of the cone, at 0° incidence angle and averaged over a beam about 6 mm in diameter. The error bars take into account both the standard calibration uncertainties and the measurement errors represented by the one sigma sta- April 1976 / Vol. 15, No. 4 / APPLIED OPTICS 861
