The HRC detection efficiency is determined by the product of the quantum efficiency of the CsI coated microchannel plates (MCPs) and the transmission of the UV/ion shield, a blocking filter in front of the MCPs. During the development of the flight instrument over the past two years, there have been material changes in the design of the UV/ion shield. The changes have come about because of measurements of the transmission properties of sample UV/ion shields outside the X-ray band. In this article I will discuss the trade-offs in the design of the HRC UV/ion shields, describe the measurements on the original design for the HRC-I UV/ion shield, describe the flight UV/ion shields, and finally present transmission data from calibrations performed in the HRC laboratory.
The UV/Ion shield, as its name implies, blocks ultraviolet light and ions as well as low energy electrons from interacting with the MCPs, mimicking X-ray events. There are actually four UV/Ion Shields in the HRC, one for the imaging detector and three for the spectroscopy detector. In order to maximize the low energy response of the HRC detectors it is necessary to keep the UV/ion shields thin to maximize their transmission. However, if they become too thin they will also transmit ultraviolet light. For the HRC-I the goal is for the UV/ion shield to reject UV so that the count rate due to a -1.5 V magnitude A star is less than 1 count/second. Aluminum coated thin plastic membranes can be made that provide adequate UV blocking while maintaining X-ray transmissions greater than a few percent down to energies of 0.15 keV. The original HRC-I UV/ion shield design was for a 6000 Åpolycarbonate membrane coated on each side with 350 Å of aluminum.
Figure 7: An example of a transmission curve for the original design of
the UV/Ion shield.
Figure 8: A schematic of the HRC focal plane geometry. The upper diamond is the
HRC-I detector and the lower rectangle is the HRC-S 3-segment array.
The nominal thicknesses of the UV/Ion shield material are shown. In
normal operation the spectrum will run along the lower part of the HRC-S
array where the aluminum is thinner in the outer segments to transmit
low energy photons.
The upper region provides a region of thicker aluminum that can be used
to reject low energy photons to assist in separating out the high
diffraction order contribution.
During subassembly calibrations measurements of this original design for the HRC-I UV/Ion shield a high transmission was found for the 2537 Åline from a Hg lamp by the University of Palermo Observatory. Further measurements of the transmission as a function of wavelength in the UV using smaller witness samples, performed at the University of Palermo Observatory and at SAO, featured resonance peaks in the transmission. Figure 7 is an example of one of these transmission measurements. Subsequent modeling of the HRC-I UV/Ion shield design showed that the high transmission could be understood as originating from Fabry-Perot interference effects caused by the Al/Polyimide/Al sandwich construction of the UV/Ion shield. This discovery led to a redesign of the flight UV/Ion shields for both the HRC-I and HRC-S. The basic fix was to replace the polycarbonate of the original design with polyimide, a material that is more absorbing in the UV. Additionally the aluminum was deposited on only one side to avoid Fabry-Perot effects.
Figure 9: The transmission of the HRC-I UV/Ion shield as a function of energy.
The x's are measured values. The line is the best fit model made up
of components listed in the upper part of the graph.
Energy is in eV.
The final design for the HRC UV/ion shields is shown in figure 8. The HRC-S UV/ion shields have a ``T" shaped pattern. The central portion of the center segment has X-ray and UV transmission properties similar to those in the HRC-I and provides a backup to the HRC-I. Since the HRC-S is the primary readout for the LETG, the areas outside the ``T" have thinner aluminum so that the low energy transmission is increased. The top of the ``T" with its thicker aluminum, especially in the outer segments, can be used to suppress the low energy X-ray contribution to a LETG spectrum by changing the aim-point of the telescope. This may particularly useful in trying to determine the contribution of higher order diffracted photons to a spectrum.
The transmission of witness samples from the manufacture of the flight HRC UV/ion shields have been measured in the laboratory. The X-ray transmission was measured at lines from alpha-particle fluoresced targets and at lines selected from an X-ray generator by a monochrometer. Figure 9 shows the measured transmission of the witness sample for the HRC-I UV/ion shield along with the best-fit model from Ralph Kraft of the HRC instrument team.
The best-fit model was found by allowing the areal mass density of each of the elemental constituents to vary and using the mass absorption coefficients tabulated by Henke et al. 1982. Using the nominal stoichiometry for the polyimide or the Henke et al. 1993 mass absorption cross sections resulted in much poorer fits. More information on the HRC UV/ion shields is available at:
http://hea-www.harvard.edu/HRC/calib/uvismodel.html
Michael Juda