Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://hea-www.harvard.edu/~bradw/cv/papers/xssSPIE.pdf Äàòà èçìåíåíèÿ: Thu Jan 2 23:02:33 2014 Äàòà èíäåêñèðîâàíèÿ: Fri Feb 28 12:19:02 2014 Êîäèðîâêà:

X-ray source system at the MSFC x-ray calibration facility
J. J. Kolodziejczak, R. A. Austin

Universities Space Research Association ES-84, Marshall Space Flight Center Huntsville, Alabama 35812
R.F. Eisner, M. K. Joy, M. Suikanen NASA ES-84, Marshall Space Flight Center Huntsville, Alabama 35812

E. M. Kellogg, B. J. Wargelin
Harvard-Smithsonian Center for Astrophysics 60 Garden Street, Cambridge, MA 02138

ABSTRACT
system has been assembled for use at the x-ray calibration facility at MSFC. The system consists of an electron impact point source with filters, a penning gas discharge source, and two monochromators fed by
rotating anode x-ray generator. The purpose and predicted performance characteristics of these elements are In preparation for calibrating the Advanced X-ray Astrophysics Facility, a multicomponent x-ray source

described as they apply to the AXAF calibration. The planned source characterizations, which will be
performed in the June, 1995 through June, 1996 time period, are also described.

1. INTRODUCTION
The X-ray source system (XSS) described in this paper is being developed for use at the X-Ray Calibration Facility (XRCF) at Marshall Space Flight Center. It is a part of NASA's Advanced X-Ray
Astrophysics Facility (AXAF) program. A key scientific goal for AXAF is to determine the observatory's x-ray response to a level near 1%. The XSS is an important component in the plan to implement this objective. A purpose of this paper is to describe the capabilities of the source system in terms of their relevance to AXAF calibration, therefore a brief description of the planned calibration process follows.

AXAF calibration is a three phase process. In their laboratories, instrument developers will perform the first phase by measuring the various characteristics of the flight instruments such as quantum efficiency,
spatial resolution and energy

the 4 parabolic and 4 hyperbolic shells, which combine to form AXAF's Wolter-I High-resolution mirror assembly (HRMA), is a phase-i calibration on the x-ray optics as are the synchrotron measurements made at Brookhaven3.
Once the various components are delivered, individual teams from the prime contractor, subcontractors, instrument development institutions, and MSFC will join forces for a 4-month-long 24-hour-per-day calibration marathon at the XRCF. A part of this time will be devoted exclusively to x-ray characterization of the HRMA using a set of instruments developed by personnel at the Smithsonian Astrophysical Observatory (SAO) called the HRMA X-ray Detection System (HXDS). The high count-rate capability of the HXDS relative to the flight instruments will permit a more precise HRMA calibration than would otherwise be possible, and will lead to a more balanced understanding of the telescope response relative to the instrument response. The instrument response will already be well understood as a result of phase-i calibration. The remainder of the time will be devoted to HRMA plus flight instrument calibration which includes the only ground x-ray calibration of the assembled flight gratings. Some of this grating work will also use the I-IXDS capabilities.

,2 The metrology performed by Hughes-Danbury Optical Systems on

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The third phase of calibration will happen on-orbit where a number of reasonably well-understood and stable astrophysical x-ray sources will be observed. These data will continue to accrue throughout the life of time. AXAF to maintain adequate understanding of those observatory characteristics which may change over

2. CALIBRATION AT THE XRCF
At a distance of 528 m, a 0.5 mm object subtends 0.2 arcsec. These respective dimensions are the sourceto-HRMA distance, nominal source size, and expected FWHM of the HRMA point-response function. Figure 1 shows the XRCF. At one end, Building 4718 houses the 22.9-m-long-by-7.3-m-diameter instrument chamber and associated control rooms. At the other end, building 600 houses the XSS. 38m from the XSS is building 500 which houses a portion of the beam monitoring equipment.

During AXAF X-ray calibration a variety of x-ray source configurations will be used to illuminate the HRMA. Detectors near the HRMA and in building 500 will continuously monitor the output. A flow
proportional counter (FPC) mounted on a moving stage and a solid state spectroscopic detector (SSD) will monitor the flux in building 500, while an array of 4 flow proportional counters surrounds the HRMA entrance aperture. One of these is also on a remotely controlled stage. These detectors will also be used for on-axis characterization of the XSS. Off-axis tests will use a High Resolution microchannel-plate Imager (HRI)4. The specific planned characterizations are described in later sections.

MSFC Bldg. 4718 houses: 7.3 m dia. x 22.9 m L. instrument chamber and control rooms

Figure 1. NASA's X-ray Calibration Facility at MSFC.

3. CAPABILITIES THE XSS NEEDS TO PROVIDE
The design of the XSS was driven by the performance capabilities of AXAF, in terms of spatial and energy resolution, efficiency and count rate limits. The result was a collection of x-ray generators, filters and

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monochromators which satisfy specific needs as well as more general requirements. The sensitivity of AXAF model parameters to energy, spectral purity and source size was also considered in selecting suitable source configurations.

3.1 Source Size
The half power diameter of the HRMA point response function is expected be <1 arcsec over much of its energy range so a 0.5 mm maximum source size is chosen to minimize source contributions to the image while

still permitting a reasonable flux without damaging a target. The image of the 0.2 arcsec source would contribute <8% to the measured 1 arcsec encircle energy. High resolution pinhole images of the sources using the HRI during characterization will provide the data necessary to account for the finite source size to
the necessary accuracy4.

3.2 Spectral Purity
The lack of energy resolution in the two microchannel plate detectors requires a beam which is free of both lines at widely differing energies and as much continuum as possible. Both the high-resolution camera (HRC) flight instrument, and high-speed imager(HSI) HXDS instrument will provide data which is more directly related to models if high spectral purity is maintained. We will use an electron impact source with continuum removing filters to provide clean lines. At interesting energies where such lines do not exist, monochromators may be needed.

3.3 Intensity
Both the counting rate limits and the surface brightness limits of the detectors define the source intensity range. Table 1 lists approximate values for these parameters:

Instrument
HRC ACIS HXDS-SSD HXDS-FPC HXDS-HSI
Table 1 .

count rate limit

brightness limit

sec4
200 200 20000 20000 2000

cm2 sec

2x105
1011 1011

iÜ

5 x 106

Approximate count rate and detected brightness limits for the various instruments to be used at the XRCF.

The FPC and SSD numbers are derived from the smallest apertures available on those detectors (5 micron

diameter). A large dynamic range in source intensity is clearly desirable to take full advantage of this
assortment of detectors. The brightness limit for the AXAF CCD Imaging Spectrometer (ACTS) is based on a limit of 1 photon per 32-64 pixels per frame. The ACIS value accounts for a factor of 0. 1 to limit the Poisson probability of getting 2 or more photons per frame to 1 % for in-focus on-axis measurements where most of the

counts are contained within a few pixels. As an example, from these numbers we can derive a maximum acceptable source intensity as low as 3 x i06 photons sr1 sec for ACTS point spread function measurements and as high as io12 photons sr' sec for core measurements with the SSD and 5 micron pinhole. Both of
these cases are for 1.5 keY photons.

3.4 Line widths
Measurements need to be made of the resolution and line spread functions of the low energy transmission grating (LETG) and high energy transmission grating (HETG). The later consists of medium energy gratings (MEG) and high energy gratings(HEG). All three of these should have resolving power greater than 1000 near

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the low-energy ends of their spectral ranges and therefore require narrow lines for calibration. The low-energy end of the HEG is 1 keV and the Mg-K line at 1 .25 keV is naturally narrow with E/dE>3000. The energy range of the LETG extends to below 0.1 keV and the 0.095 eV Al-IY & V line produced by a continuous discharge Penning source5 with E/dE>l0000 should be acceptable. The energy range of the MEG extends down to O.2 keV. For this the 0.277 keV C-K line narrowed to E/dE>2000 using a grating monochromator is planned, however, recent work on the Penning source6 has shown that a bright Ar-I line at 0.22 eV is attainable and may be narrow enough. Preliminary measurements indicate E/dE>1000 for this line.

3.5 Photon energies
A part of the phase- 1 calibration of AXAF components includes synchrotron measurements to map details

in the spectral response3'7'8. Time constraints prevent complete duplication of this effort at the XRCF,
however spot checks in critical energy ranges and coarse spectral scans to search for unexpected features are

needed for validation of combined models. A grating monochromator for low energies and a crystal
monochromator for high energies, both supplied by high-output sources, were selected to fulfill this need. E/dE in the range 30-100 is sufficient for these measurements.

3.6 Other
Constraints on beam spatial uniformity, source mechanical stability, power supply electrical stability, change out time, and other elements of the system have arisen from the 1% AXAF calibration goal as well as test time constraints.

4. X-RAY SOURCE SYSTEM
Figure 2 shows a block diagram of the x-ray source system. All the components are mounted on four carts which ride on rails. The rails are attached to a 3 m x 3 m optical table. The table is mounted on a vibration isolated concrete pier. A stationary filter chamber mates with the end of the guide tube and provides a common interface for the equipment on each of the 4 carts. One cart supports the electron impact point source. Another supports the grating monochromator (HIREFS®) and rotating anode source. A third supports the crystal monochromator (DCM) and a second rotating anode source. Finally, the fourth supports either the Penning source or an unencumbered rotating anode source. Each of these components is discussed below.

4.1 Penning Source
In the penning source5, gas discharge current ionizes gas atoms which then impact cathode targets and sputter ionized metal atoms into the gas. Further collisions generate numerous bright EUV and soft x-ray lines. We currently plan to use Argon gas and Al cathodes to produce the needed Al-IV and Ar-I lines for LETG and MEG calibration, however we will also investigate alternative gases like PlO to try to excite the C-K line. The water-cooled Penning source heads for the XSS were built by Smithsonian Astrophysical Observatory.

Gas pressure is set to 25 mTorr to start the Penning source, then a current limit is set -5OO mA and
voltage is adjusted until ignition. Pressure is reduced as low as possible to maximize flux and either current or voltage are set to adjust the flux up to the maximum current. The current limits the system since the source

does not run at maximum voltage when the current is maximum. Table 2 summarizes the performance
characteristics:
Maximum current 1000 mA Source size ).5 mm wide slit in the LETG dispersive direction Expected flux >i010 sr sec per line E/dE >15000 @ 0.095 keV9; >1000 @ 0.22 keV6 Cathode lifetime 4 hr Target material Aluminum Gas Argon Table 2. Penning source performance characteristics.

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Figure 2. The XRCF X-ray Source System

0.035

0
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0.03
0 025

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I

0.005

0

_iiili iiliiult iiliiil iiili ii
wavelength (Angstroms)

56 56.02 56.04 56.06 56.08 56.1 56.12 56.14
Figure 3. Spectrum from the penning source using HIREFS® monochromator with 10 pm entrance slit and 5 xm exit slit with channeltron detector, 1 kV, 600 mA, 6 mTorr, Argon gas, Aluminum cathodes. The

theoretical HIREFS® resolution is 0.028 Angstroms, and the measured FWHM is 0.04 Angstroms, so the resulting FWHM of the line is 0.028, which gives an E/dE of 2000.

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Before using the Penning source for AXAF calibration, we will need to determine the following: 1)
dependence of output flux upon current, and gas pressure in the manifold, 2) variation of source flux over time

(temporal stability), and 3) characterize the Ar I, 0.22 keV and the Al IV, 0.095 keV lines. The emission region of the Penning source has a diameter of a few millimeters, so the source will be operated with a slit aperture which will define the source size, and pinhole camera images of the source will be unnecessary. Measurements 1 and 2 will be performed in the on-axis configuration with the Bldg. 500 FPC serving as the detector. These important measurements will answer practical questions such as: what current and pressure settings give optimum performance and how long for a particular current and pressure can we operate the source before change-out is necessary. Spectral characterization will be made off-axis with the HIREFS®
configured as a spectrometer and the HRI serving as the detector. Resolving powers of >2000 will be adequate for measuring the positions, widths (at least upper limit determinations), and relative intensities of the 0.22 keY and 0.095 keY calibration lines and nearby lines which might complicate calibration of the LETG and HETG spectrometers. Preliminary measurements with HIREFS® in a monochromator configuration and a Channeltron detector6 suggest these two lines will adequately serve our calibration needs. Figure 3 is a plot of the Penning source data from this preliminary experiment.

4.2 Electron Impact Point Source and Filters

The electron impact point source is a modification of the one used for the VETA-I test4. The
modifications include a new bias cup design with independent bias control and a new power supply. The anodes or targets are oil-cooled and quickly replaceable so that a variety of materials can be selected to generate the required lines. The filter wheel assembly houses two independently controllable filter wheels which will include open positions for unfiltered output. The filter wheel assembly is a stationary element so each source subsystem interfaces to it and the x-ray beam always passes through it. Table 3 summarizes the performance characteristics of these items.
Voltage Range Current Range Source size Take-off angle Expected count rates Target materials Filter Wheels Filter positions per wheel Filter Materials
1-30 keV 0.050-5.0 mA (70 W max.) 0.5 mm x 0.5 mm
300

see figs. 4 and 5 see table 4 2 32 2,4, and 6 mfp selectable, see table 4

Table 3. Electron impact point source performance characteristics.

Filters are selected for each target in 2 and 4 mean-free-path thicknesses and since there are two wheels, 2, 4, or 6 mean free path filter choices are currently planned. The list of targets and filters includes those in table 4. The final two columns in this table list the voltage and filter thickness combination which optimizes the line-to-continuum ratio. Figures 4 and 5 show the predicted HRC and ACIS count rates per mA for most of these targets. Where the modeled materials differ from the currently planned materials, the modeled materials appear in parentheses in table 4.

The predictions given in Figures 4 and 5 are base on a physical radiation transport

and are

convolved with current HRMA and instrument models which are shown in ref. 1 1 . Accuracy of a factor of 2-3 is

assumed. The fluxes are expected to be adjustable, up a factor of 5 and down a factor of 20, using the current adjustment. Figure 5 shows that the intensity range does not extend low enough for in-focus measurements with ACTS for any of the lines above 1 keY, since we will need count rates below 1 sec and the values in this range are almost all more than 50 ec1 . Two options for dealing with this problem may be considered:

(1) thicker filters, or (2) lower voltage. Although thicker filters would continue to improve the line-tocontinuum ratio for most cases, there is a problem in that a small percentage variation in the thickness can lead to substantial beam nonuniformity. The preferred option would be to lower the voltage, for example, the

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worst case is Cr with over 600 counts sec mA at 5 times the line energy and 6 mfp, but at 2 times the line
energy and 6 mfp (case not shown) the rate is only 70. At 1 .2- 1.3 times the line energy an acceptable intensity is likely. This lowering of the operating voltage will be at the expense of line-to-total-continuum ratio, but the energy resolution of ACIS will probably still permit a good measurement of counts in the line using spectral fitting algorithms12.

A similar problem exists with the HRC but to a smaller extent and lowering the voltage is still likely to be the preferred option. For the same example, the simulation indicates that reducing the voltage on the Cr target

from 5 to 2 times the line energy reduces the rate from 1 18 to 13 sec mA while the line fraction only
changes from 0.85 to 0.75. Obviously, the uncertainties in the simulation will require us to measure the source intensity and repeat this analysis to assure that acceptable counting rates can be achieved.

Material
Al
B

Energy (keV) Filter
1.486

mfp (jim)
9.19
1.27

Al
B

Best V (x line energy)
3

Best t
(x mfp) 6 6 6 6 6

Be C
Cr

0.183 0.1085 0.277
5.41

4
4 4 5 5 5 3 3 5 3

Be
C8H8 V Ni Mn Mg Ti Co Cr Ti

0.903
5.16

22
24.1

Cu

Fe
Mg N from TiN Ni

8.03 6.4
1.254

22.6
11.8 0.5

6 6 6
4 6 6 2 6

0 from Si02
Ti

Zn
Ag Au Co Cu

Cr Fe
Mo Ni Sn Ti

w
Zn Zr

0.3924 7.47 0.5249 4.51 8.62 2.98 9.71 0.7762 0.9297 0.5728 0.705 2.293 0.852 3.444 0.4522 8.398 1.012 2.04

23.2
0.432

Cu
Rh (Ag) Ta (Au) Co Cu Cr

20.3 27.3 2.5 (1.87)

5
3 5 3 3 3 3 3

6.2 (6.4) 0.58 0.72
0.51 0.59

Fe
Nb (Mo)

2 6 6 6 6 6
6 6 2 6

Ni
Cd (Sn)

Ti
Cu Zn
Zr

2.5 (1.62) 0.624 2.5 (3.17) . 0.663
25.2 0.94
1.98

2
3 5 3 3 5 2

6 6 6

Table 4. Point Source Targets and Filters. Best V and best t are the combination of voltage (in the range from 2 to 5 times the line energy) and thickness (in the range from 0 to 6 mfp) that gives the highest simulated line-to-total-continuum ratio.

Driven by the scientific requirements for AXAF calibration, we have determined that the following
parameters must be verified to fall within certain limits or be characterized to sufficient accuracy: source size, source concentration, temporal stability, positional stability, operating range, output flux. To verify that the source diameter is no greater than the required 0.5 mm, pinhole camera images will be taken of the source,

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off-axis, for each of the 25 or so anode materials over a range of bias voltages and anode voltages and currents. Pinhole sizes (diameters 25(m and 70(m) and source-pinhole, pinhole-HRI distances will be chosen to assure

that better than the budgeted 0. 1 mm image resolution is maintained. Pinhole images will also provide
information about source concentration (i.e., verify that intensity from the source falls below 10-4 of its peak value at distances greater that 0.25 mm from the center of the spot), and the range of voltages and currents over which a compact source can be maintained. Output flux will be measured on-axis with the BND-500 FPC and SSD. For a given anode, these measurements will allow us to choose the necessary anode voltage and current settings to obtained a desired output flux. Temporal stability will be characterized during beam mapper scans which will be made to determine x-ray filter uniformity (see below).

To adequately characterize the x-ray filters, measurements of absolute transmission, scattering and
vignetting will need to be made. For each anode/filter combination, absolute transmission will be measured to an accuracy of 5% or better. To ensure that no filters having pinholes will be used for the calibration, the FPC beam mapper will make measurements at 36 points over the region of the filter which intercepts the beam reaching the HRMA. Filters having greater-than-normal nonuniformity will be discarded. By mapping the beam, we will also be able to determine if beam is being vignetted by the filter wheel assembly. To determine if the filter wheel assembly is producing any scattering of the beam, an occulting disk mounted on the Bldg.

500 beam mapper will be placed in the beam. If there is no scattering, no x-rays will be detected by the
detectors at the HRMA entrance.
4.3 High-Output Source

To obtain a usable count rate through the monochromators, and especially in the case of the high resolution line at C (0.277 keV), a more powerful source than the point source described in section 4.2 is necessary. To satisfy this need we selected a rotating anode x-ray generator system manufactured by Rigaku Corp. In this water-cooled system, the anode is a beveled disk rotating at 6000 rpm. By selecting a low takeoff angle, a line shaped electron beam can be projected into a point-like x-ray source. Figure 6 illustrates this.
Table 5 is a summary of the rotating anode source performance characteristics.

anode materials anode take-off angles

W or C
14° for HIREFS®
70 for DCM and unencumbered

cathode sizes
Voltage range Current Range Power

Cl: 0.2mm x 2 mm/ 5-25 kV C2: 0.2mm x 2 mm/ 20-40 kV C3: 0.5mm x lOmm/ 5-25 kV 5-40 kV in 1 kV steps 10-450 mA in 3 mA steps Cl or C2 with W: 2500 Watts Cl or C2 with Carbon: 300 Watts C3 with Carbon: 2000 Watts C3 with W: 15000 Watts

Intensity

>iO14 -

1

r1 in C-K line

Source sizes

see fig. 7 for W for HIREFS®: C3: 0.5 mm wide x 2.5 mm high C1/C2: 0.5 mm wide x 0.2 mm high for DCM and unencumbered: C1/C2: 0.25 mm wide x 0.2 mm high C3: 0.5 wide x 1.2 mm high

Table 5. Rotating anode source performance characteristics.

Figure 7 shows the predicted source flux from the W anode. The low-energy intensity decreases with increased voltage, in part, because of the low take-off angle which requires a longer path for a generated
photon to escape from the material and therefore more absorption for the more deeply penetating high voltage

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electron. As a result, we will always want to run the rotating anode source at the lowest possible voltage when using the grating monochromator. We will need to tune the voltage to the appropriate value for a given energy when using the crystal monochromator.

The RAS, being much like the EIPS, will be characterized in the same way. In this case there will be only two anode materials, W and C.

cathode

wide e

aperture

narrow e beam defines source width

projected source height

Figure 6. Diagram of the rotating anode source.
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10 20 Energy (keV) Figure 7. Source Intensity for Tungsten at 3 energies ant 14° take-off angle.
1

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4.4 Grating Monochromator
A HIREFS®SXR1.75 grating monochromator13 from Hettrick Scientific Inc. (HSI) was selected for low energy applications below -'1.5 keY since it has fixed entrance and exit slit locations and a sufficient range of resolution settings to satisfy both our need for narrow lines to examine the grating line response, and the requirement to perform low resolution efficiency scans. HIREFS® stands for High Resolution Erect Field Spectrometer. The addition of an exit slit converts it to a monochromator.

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The path of photons through the HIREFS® begins as they pass through an adjustable entrance slit and reflect off a gold-coated cylindrical mirror which focuses the beam in the non-dispersive direction. This mirror

is adjustable to permit beam focusing directly onto the exit slit for high spatial resolution or up to 1 .9 m outside the exit slit to provide a factor of --'3 increase in throughput. A second gold-coated mirror, this one spherical, focuses the beam in the dispersive direction over the distance to the exit slit. Along the way the beam is intercepted by a reflection grating which disperses only the selected wavelength to the adjustablewidth exit slit. The beam is always focused by the spherical mirror in the dispersive direction at the exit slit, however the output of the slit would be astigmatic were it not for the initial cylindrical mirror which corrects
for the astigmatism if adjusted to focus on the exit slit. Number of selectable gratings Energy Range

4 (SX,SA,SB,SC)
SA: SB: SC: SX: 0.77-1.6 keY 0.25-0.83 keY 0.08-0.28 keY 0.04-0.12 keV

Energy resolution Output beam intensity Entrance slits

See Fig. 8 See Fig. 9

small slits: 5, 10, 20, 50, 100, 200 j.Lm medium slits: 75,100,150,200,300,400 pm Exit slits 5, 10, 20, 50, 100, 200 pm Table 6. Performance characteristics of the HLREFS monochromator.

Energy resolution is adjustable by changing the entrance and exit slit widths. The resulting resolving
power depends on wavelength as shown in Figure 8. The points plotted in Figure 8 are measured values of the energy resolution performed by HSI using a laser plasma source prior to delivery of the monochromator.

The beam intensity will naturally be a function of both wavelength and resolving power. Figure 9 is plots the predicted output intensity based on a simple scalar diffraction grating model which includes reflections from the 2 mirrors and the grating. Plots of the first 3 order are shown, and indicate that higher orders may be a significant contributor to the output up to about 0.8 keY. We intend to investigate the use of filters to reduce higher orders, but their effects must be considered when planning AXAF calibration measurements.
1 E.'-

E 1E+8
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i\

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Figure 9. Predicted intensity out of HIREFS® monochromator with 200/100 .tm entrance/exit slits.

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The two monochromators will pose the most formidable characterization challenge. The following must be determined: 1) absolute energy scale in the range 0. 1-2 keV, 2) energy resolution, 3) throughput, 4) source size, 5) beam angular size, 6) intensity uniformity, 7) temporal stability, 8) spectral purity, and 9) higher order diffraction. Measurements of 1 and 2 will be performed off-line by scanning through narrow lines provided by the Penning and electron impact point sources; the HRI will serve as the detector. By using lines with well known energies, we can pin down the energy scale of the HIREFS® . By making wavelength scans of intrinsically narrow lines it will be possible to determine the energy resolution of the RIREFS® or at least determine if the energy resolution is adequate at a particular energy. Pinhole camera measurements made offaxis will be used to measure the source size in the non-dispersion direction. The remaining measurements, 3, 5, 6, 7, 8, and 9 will be made on-axis with the Bldg. 500 FPC and SSD. With the rotating anode source connected to the HIREFS® , throughput measurements will be made for a variety wavelength settings and source voltage/current settings. For this same configuration, beam angular size, intensity uniformity, and temporal stability will characterized during beam maps over the region of the beam which will intercept the HRMA. The degree of spectral purity and higher order diffraction will be determined by examining energy spectra obtained with the FPC and SSD.

4.5 Crystal Monochromator
The crystal monochromator for the XSS was designed and built by the National Institute of Standards and Technology (NIST)14. This double crystal design has two 5-position turrets which hold the various crystals. Photons enter through an entrance slit, and those with the selected energy are reflected at the Bragg angle off the two crystals into a parallel, but offset, beam. The Crystals selected are Silicon , Germanium, Potassium Dihydrogen Phosphate (KDP), and Thallium Acid Phthalate (TAP). Number of selectable crystal
MinimumfMaximum Bragg Angle Energy Range

5 positions available, 4 used (Si, Ge, KDP,TAP)
19.880/81.600

Si-400: 4.62-13.42 keV (2d=2.71551 Ang.)

Ge-ill: 1.92-5.58 eV (2d=6.53287 Ang.)
KDP-01 1: 1.23-3.572 keY (2d=10.20773 Ang.) TAP-OOi: 0.49-1.41 keY (2d=25.7568 Ang.)

See Fig. 10 Energy resolution See Comments in Text Output beam intensity Entrance slits 0.3 mm Exit slits None Table 7. Performance characteristics of the double crystal monochromator.

Figure 10 shows the energy resolution of the various crystals. For the purposes of estimating count rates for AXAF calibration planning we will assume the throughput to be 1% (nominally 10% efficiency from each crystal) of the flux from the W-anode rotating anode source as shown in figure 7. To minimize contributions

from higher orders in the DCM we may need to detune the angles of the crystals, and the effect of this
detuning on the count rate has not yet been predicted. Characterization of the DCM will following nearly an identical procedure as above and the following must be determined: 1) absolute energy scale in the range 1-10 keY, 2) energy resolution, 3) throughput, 4) source size, 5) beam angular size, 6) intensity uniformity, 7) temporal stability, 8) spectral purity, and 9) higher order diffraction.

5. SYSTEM OPERATION
During AXAF calibration, a test conductor has the job of managing the overall test procedure. Requests from the test conductor for changes to the source configuration will come via a computerized procedure script. Since the 4 month measurement time will be tightly scheduled, the XSS has been designed to make every type of change as quickly as possible. The 4 carts which support the different configurations slide across the rails into position and are then lowered onto a set of precision hard points. These hard points are precisely

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Si
6000
5500 5000 4500

Ge

>4000
Ui

0
Ui

00

w

10

15

20

25

2

3

4

5

6

7

8

9

10

11

X-rayenergy

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X-ray energy

(key)

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0.175

0.150

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Cl

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20

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7

8

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0.5

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1.5

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2.5

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X-ray energy

( keV)

X-rayenergy

(keV)

Figure 10. NIST Double crystal monochromator energy resolution vs. energy. Curves are from ref. 14.

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sized pins attached to thick steel pillars which are in turn, rigidly mounted to the optical table. They are used

to insure repeatable repositioning of each source without extensive realignment after each co'ifit,ratinn
change.

Data describing which configuration is active, and its electrical and mechanical parameters will be stored each time something changes and at frequent regular intervals throughout the test. Whenever possible, critical parameters like voltage, current, and wavelength settings will be automatically entered. Other parameters, like which anode is being used, and alignment parameters, will be prompted by computer and manually entered.

6. REFERENCES
1. D. Dewey, D.N. Humphries, G.Y. McLean, D.A. Moshella, "Laboratory Calibration of X-Ray Transmission Diffraction Gratings", Proc. SPIE, Vol. 2280, pp. 257-271, 1994.

2. K.A. Flanagan, M. Barbera, S. Murray, M. Zombeck, "Calibration program for the AXAF High Resolution Camera", Proc. SPIE, Vol. 2280, pp. 243-256, 1994. 3. D.E. Graessle, Ri. Brissenden, J. Cobuzzi, J.P. Hughes, E.M. Kellogg, F.E. Mootz, D.A. Schwartz, P.O. Slane, M.V. Zombeck, R.L. Blake, J. Davis, Fsibility study of the use of synchrotron radiation in the calibration of AXAF: initial reflectivity results", Proc. SPIE, Vol. 1546, pp. 13-25, 1991. 4. P. Zhao, E.M. Kellogg, D.A. Schwartz, Y. Shao, M.A. Fulton, "Intensity Distribution of the X-ray Source for the AXAF VETA-I Mirror Test", Proc. SPIE, Vol. 1742, pp. 26-39, 1992.

5. D.S. Finley, S. Bowyer, F. Paresce, and Roger F. Malina, "Continuous discharge Penning source with emission lines between 50A and 300A", Applied Optics, Vol. 18, No. 5, pp. 649-654, 1992.

6. E.M. Kellogg, BJ. Wargelin, TJ. Norton, JJ. Kolodziejczak, "Penning Source for calibration of x-ray and EUV optics and spectrometers at wavelengths as short as 50A", Proc. SPIE, this volume, 1995.
7. C.S. Nelson, T.H. Markert, Y.S. Song, M.L.Schattenburg, D.E. Graessle, K.A. Flanagan, R.L. Blake, J. Bauer, E.M. Gullikson, "Efficiency measurements and modeling of AXAF high energy transmission gratings", Proc. SPIE, Vol. 2280, pp. 191-203, 1994.

8. M. Barbera, D. Breslau, K. Flanagan, D. Graessle, M. Zombeck, "Synchrotron x-ray transmission measurements in the calibration program for the UV/lon Shields of the AXAF HRC", Proc. SPIE, Vol. 2280, pp. 229-242, 1994. 9. M. C. Hettrick, J.H. Underwood, PJ. Batson, MJ. Eckart, "Resolving Power of 35000 (5 mA) in the extreme ultraviolet employing a grazing incidence spectrometer", Applied Optics, Vol. 27, No. 2, pp. 200-202,
1988.

10. M.E. Sulkanen, J.J. Kolodziejczak, G. Chartas, "Numerical simulation of electron impact x-ray sources", Proc. SPIE, this volume, 1995.
1 1. M.C. Weisskopf, S.L. O'Dell, R.F. Eisner, L.P. VanSpeybroeck, "AXAF-- an overview", Proc. SPIE, this volume, 1995.

12. G. Chartas, K.A. Flanagan, J.P. Hughes, E.M. Kellogg, D. Nguyen, M.V. Zombeck, M. Joy, J.J. Kolodziejczak, "Correcting x-ray spectra obtained from the AXAF VETA-I mirror calibration for pile-up, continuum, background and deadtime," Proc. SPIE, Vol. 1742, pp. 65-74, 1992.

13. M. C. Hettrick, Instruction Manual for the HIREFS®SXR1.75 Base Monochromator, July, 1993.
14. J.-L. Staudenmann, R.D. Deslattes, Design Principles of the AXAF-I monochromator, July, 1994.

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