HETG Vacuum Storage Gratings, Version 1.0, June 1998

Busy-person Summary

A sample of HETG gratings has been aged in a high vacuum environment and retested periodically since late 1996 to determine if there are any changes in the performance parameters. The evaluations in early 1998 indicate that there are no trends that would change the calibration of the HETG instrument. Several changes in performance parameters have been detected but most seem to be caused by measurement or sample anomolies. Aging and evaluations of these gratings will continue through this year and updates will be made as testing is completed.

Contents

• Introduction
• Summary Results to Date
• The Vacuum Storage Gratings
• Why Do Measurements Change?
• LR Measurement Notes
• X-GEF Measurement Notes
• VSG Retest Procedure
• VSG Software

Introduction

Selected gratings were put into vacuum storage and their properties are being periodically measured. This vacuum storage grating (VSG) program will allow us to detect or set upper limits on changes in the grating properties with time. The properties being measured for each sample are the period and period variation ( "LR Measurements") and X-ray diffraction efficiency ( "X-GEF Measurements"). Familiarity with these quantities and test setups is assumed in the following.

The figure below shows a time line and events relevant to the HETG flight and VSG gratings. The flight gratings were X-GEF tested in the laboratory as they were produced between mid-1995 and September 1996 (date codes 950615 to 960914). Due to LR system upgrades and the relative ease of the test, all but a handful of the flight gratings were LR (re-)tested in the April to September 1996 time frame. These detailed, facet-by-facet laboratory data sets provide the basis of the instrument model and calibration.

The HETG was next tested as a full system at XRCF from December 1996 through April 1997. These system-level measurements represent an independent set of data to complement the laboratory measurements. The result of analyzing and synthesizing these two data sets is a final pre-flight calibration of the HETG and HETGS (HRMA-HETG-ACIS-S).

As the time line indicates, flight data will not be taken until almost two years after the XRCF calibration measurements were made! Thus, it is particularly important to estimate the degree to which the HETG properties might be changing with time - especially during the period from XRCF to flight first-light.

Figure vsg.1 : Time line of Flight and Vacuum Storage Gratings Activities

Summary Results to Date

As the diagram above indicates, a total of five (possibly six) vacuum storage retest cycles are planned to cover the XRCF to flight first-light time period. Currently three retests have been carried out, indicated by the vertical lines at 9611, 9707, and 9801 above.

A gross summary of Period changes at these times can be made by presenting the changes in the average period and the dp/p value for MEG and HEG grating ensembles:

         Vacuum Storage LR Results: MEG Ensemble
 
       Test Date :      9611xx     9707xx     9801xx 
           P_ave :     4000.36   [-68ppm]   [-11ppm]
           dp/p  :      192.07     [-22%]     [-18%]
 
 
         Vacuum Storage LR Results: HEG Ensemble
 
       Test Date :      9611xx     9707xx     9801xx 
           P_ave :     2001.95   [-70ppm]   [-37ppm]
           dp/p  :      229.48     [-14%]      [+5%]
 

A similar summary for the diffraction efficiency is not tabulated but may be inferred from Figure vsg.4 below.

The measurement results to date for the individual facets are presented in the following plots and tables:

• Facet Period Trends, Figure vsg.2
• Facet Period Variation (dp/p) Trends, Figure vsg.3
• Facet Period and dp/p Trends, Table vsg.1
• Facet First-order Efficiency Trends, Figure vsg.4 and Table vsg.2

Ideally these data would show unchanging horizontal lines; variations are, however, visible. The following items summarize significant variations seen in these data and their likely explanations. More details and an explanation of the measurement problem codes (e.g., LRmp1) are available in the sections that follow.

Period Plot Anomalies

• MA1019 at 9707 test: LRmp1 error due to test setup and MA1019 frame twists.
• HA2003 shows period trend: may be a real trend? Possibly a LRmp5 humidity effect (9611, 9707, 9801 had humidities: 31%, 38%, 1%). The dp/p map looks very similar from 9611 to 9801.
• HF2503 at 9707 test: LRmp1 error likely: the LR Camera Plots show the beams shifted 1.5 mm in X and reflected beam in Y.

Period Variation (dp/p) Plot Anomalies

• MA1019 at 9801 test: LRmp2 - the 9801 and 9611 tests have the sample offset by ~ 2mm causing less of the -X bad edge region to be included in the 9801 measurement.
• HC2251 at 9611 test: LRmp2 - the facet has a very bad region in the -X, +Y quadrant. For the 9611 test this region did not fall into the LR active region, hence the 9611 dp/p is lower than the others.

First-order Efficiency Plot Anomalies

• HA2045 at 9707 test and 1 keV trend: Shows deviant efficiencies at 4 and 8 keV, due to X-GEFmp1 effects and a bad region identified on LR plot. HA2045 also shows a 1 keV trend to higher efficiencies but all of the change is due to the bad region - ignoring this region the 1 keV efficiency is stable to a few percent.
• HC2251 at 9707 and 9801 tests: X-GEFmp1: large changes in efficiencies mostly due to one region of the facet, other regions are stable.
• HF2503 shows 1 keV trend to higher efficiencies: may be a real trend? About 2/3 of the trend occured between the 960502 test and the later three tests. The facet does not appear to have any serious blemishes or erratic regions, regions 'p0 and 'p4 have the trend more so than 'm4. More investigation is required to identify a "cause".
• MD1367 shows drift to lower efficiency at 4 keV: shows up in all 5 regions of the grating, not a blemish effect.
• MEGs (e.g., ME1408, MB1165, MC1212) show a general drift to higher efficiency at 1 keV (of order +7 % in two years) : MB1165 shows the trend in all 5 test regions, not a blemish effect. Requires more investigation to identify a "cause".

The Vacuum Storage Gratings

General information on the 14 grating facets which make up the Vacuum Storage Gratings are given in the tables below. As Table vsg.4 indicates, the VSGs come from a subset of flight batches that together yielded 180 of the full 336 flight gratings. The Period and Diffraction properties of the flight gratings and VSGs are compared in Figures vsg.5 and vsg.6 - showing that the VSGs cover a wide range of flight grating types.

• Summary information on the VSGs Table vsg.3
• Fabrication Lots and the VSGs Table vsg.4
• Flight and VSG Period Properties Figure vsg.5
• Flight and VSG Diffraction Properties Figure vsg.6

Specific information and notes regarding each grating especially as it effects test results are given here, organized by the facet name.

MA1019

• Large frame twisting leads to LR beams covering full LR Camera range and hence sensitive to LRmp1 errors.
• Bad dp/p at -X edge enhances sensitivity to sample positioning, LRmp2.

HA2003

• May show consistent period drift to lower periods.

MA1040

MC1212

• Period is large indicating membrane stress; excluded from ensemble period analysis.
• Drift to higher efficiency at 1 keV?

HD2334

• Period is large indicating membrane stress; excluded from ensemble period analysis.

HA2045

• Period is large indicating membrane stress; excluded from ensemble period analysis.
• dp/p map shows bad region at X~0, +Y just outside of active region. Could lead to LRmp2 effects if mispositioned.
• Large change in HA2045 4, 8 keV efficiencies at 9707 tracked down to a strong variation of the efficiency with position on the facet. On the plus side ('p4) there is a blemish region (shows up in dp/p map) that is more transmitting to Mg K than normal - causing bizarre fits to the X-GEF data.
• The 1 keV efficiency trend for HA2045 is also dominated by this same blemish as the region-by-region percent changes of 1 keV efficienciy indicate:

           9511  9611  9707  9801
      'p4   0 %  20 %  33 %  42 %  <-- blemish region
      'p0   0 %  2.8%  0.0%  3.6%
      'm4   0 %  4.3% -0.2%  0.5%
  

HC2251

• dp/p blemish in the -X, +Y quadrant produces LRmp2.
• The X-GEF 'p4 region has large variations in X-ray efficiency (7of 9 points greater than 6%) as opposed to the 'p0 and 'm4 regions for which 17 of 18 points have less than 6% deviation from the 9512 measurement values. Need to check correlation of dp/p blemish with X-GEF bad region...

ME1408

• Period is large indicating membrane stress; excluded from ensemble period analysis.
• Drift to higher efficiency at 1 keV?

MB1165

• Drift to higher efficiency at 1 keV?

HB2121

HF2503

• Shows 1 keV trend to increasing efficiency with 2/3 of the increase between first and second X-GEF tests (first test is not on line). No obvious blemishes in X-GEF region and no erratic efficiency behavior in the test regions; regions 'p0 and 'p4 each have the 1 keV trend.

HE2423

• Bar parameters (0.70 width/period, 4000 A thick) are outside of the flight HEG range.

MD1367

• Drift to lower efficiency at 4 keV?

H2102

Why Do Measurements Change?

When a sample is re-measured there are several general categories of effects that can be responsible for a change in the measured quantity - these are discussed in general as well as with respect to the LR and X-GEF setups themselves.

measurement noise (mn)

• Definition: Measurement noise represents the random variations inherent in the measurement process due to true physical noise processes as well as low-level systematic effects (e.g., repositioning tolerance.)
• Trend Behavior: Produces a spread in the measurement values that is uncorrelated with sample and time.

sample variation (sv)

• Definition: Sample variation represents actual changes in the properties of the sample being measured.
• Trend Behavior: The simplest assumption for true sample variation is a monotonic variation with time that may continue to drift or possibly converge. Similar samples may show similar behavior, however exact tracking is not generally expected.

reference variation (rv)

• Definition: Reference variation represents an actual change in the reference components of the measuring system.
• Trend Behavior: The resulting trend would be similar to the sample variation - however, because it occurs in the measuring system it will show up as a correlated and similar variation among all samples.

measurement problem (mp)

• Definition: Measurement problem represents gross changes in the measurement system which effect the resulting measured value. Examples are operator setup errors, equipment malfunctions, contamination effects, etc.
• Trend Behavior: Produces large (compared to noise), non-monotonic-in-time variations which may or may not be corelated sample-to-sample.
• What can be done?: By studying data sets with obvious measurement problems the source of the measurement problem can often be identified. Once identified, data sets already taken can be checked for the same (perhaps at a lower level) problem and procedures can be modified to detect or prevent the problems.

LR Measurement Notes

A variety of measurement problems can be caught by reviewing the LR data sheets:

• LRmp1 : Check "LR Camera Plots" to verify that the measurement points are largely within the camera's range and and in a similar, centered range to previous measurements.
• LRmp2 : Check that all X,Y contour plots to verify that the patterns show features at the same X,Y locations to within ~ 1 mm (e.g., MA1019 9611 and 9801 are offset by ~ 2 mm inX and 0.5 mm in Y.) Gratings with large gradients and/or edge problems will convert these measurement region shifts into period and dp/p changes.
• LRmp3 : Check the "Roll Angle" for substantial (> 0.4 arc minute) change from previous measurements. This roll difference causes slightly different regions on the sample to be measured.
• LRmp4 : Check that the before and after reference points ("R"s on "LR Camera Plots") are the same (overlap to form a single "R").
• LRmp5 : Check the temp and humidity for extremes
• LRmp6 : TBD

LR reference variation, that is a change in the period of the LR reference samples built into the LR system , is unlikely. However illumination of different regions of the samples may create apparent changes - this could be investigated further. The general stability of the LR system can be monitored with the Reference Stability data and plots - however, changes in these plots (not shown) do not immediately translate into sample period changes.

LR sample variations may show up as clear changes in the dp/p pattern or clear changes in the frame flatness (LR surface plot.)

Other ideas and actions for improved LR testing are recorded here:

• Create a checklist of known measurement problems and mark a copy of it for each measurement.
• Look at past wafer measurements, especially HEG in range 970317 to 970710 to explain the 9707 results?
• Include periodic wafer measurements to complement the reference samples. Select wafers for bright beams and a period close to the samples.
• On retests, perform LR measurement before and/or after X-GEF measurement?
• On past data look for effects of HeCd laser intensity varying over time.
• Measure the built-in LR reference samples' periods at various locations.

X-GEF Measurement Notes

Identified X-GEF measurement problems are listed here:

• X-GEFmp1 : check for similarity of 'm4 and 'p4 regions to catch gratings that have local blemishes or gross non-uniformities (e.g., HA2045). Small changes in repositioning can create large efficiency changes.

X-GEF reference variation is more likely an effect than LR reference variation. The two reference gratings mounted in the X-GEF, MX078 and HX220, have been there continously since before 950615. They are maintained in vacuum, subject to any low-level contaminants in the system, and exposed to atmosphere whenever samples are being changed. The absolute efficiency of these gratings can be directly measured by looking at the ratio of the zero-order beam with the gratings in place to the direct X-ray beam. This is most accurately carried out with the SSD because of its superior energy resolution. Plots of these SSD-measured zero-order efficiencies as a function of time are available:

• MX078 SSD-measured zero-order efficiencies Figure vsg.7
• HX220 SSD-measured zero-order efficiencies Figure vsg.8

Other ideas and actions for improved X-GEF testing are recorded here:

• Source spectral composition changes? Check SSD/FPC spectra before doing X-GEF retests. Compile past source picture books.
• Get holder drawings, etc. to definitely relate LR map to X-GEF test locations.
• Analysis s/w may have evolved since the earliest tests - reanalyze all VSG data sets uniformly. Need disk space for data.
• Reference gratings can have variation of efficiency with position and either system drifts or test adjustments may cause a different region to be used offsetting all measured results.

VSG Retest Procedure

The following is a high-level list of tasks that need to be carried out in order to perform a VSG retest. The retest period generally refers to the period during which X-GEF measurements are made - for the 14 VSGs this is about two weeks. Note, however, that there are preliminary and follow up activities as well.

• Order Refurbished HeCd Laser

  2 months prior to the retest period the LR HeCd laser should be sent out for refurbishment unless its intensity is outstanding.

• Refurbish X-GEF setup as needed

  The Manson source filament is changed if TBD condition obtains. The PSPC anode is swapped if it has been used for more than 25 X-GEF tests. Send spent anode plane to Ordella for refurbishment.

• Qualify X-GEF setup

  An X-GEF test on a pair of HEGs is carried out. The resulting data are analyzed and inspected for source spectral quality, SMD operation, SSD operation, PSPC operation, reference grating stability, etc. The Manson source filament is changed if TBD condition obtains.

• Install and Align HeCd Laser

  If refurbished, the HeCd laser must be installed and aligned.

• Qualify LR setup

  Measurements of a grating and holography monitor wafer are made for HEG and MEG samples. The results are reviewed by test scientist or engineer.

• Retrieve the VSGs from the vacuum storage facility

• Perform LR tests on the VSGs

  The analysis results are reviewed for signs of LR measurement problems.

• Perform X-GEF Tests

  Two gratings are tested in each overnight X-GEF run. Runs are not made on weekend nights (Sat.-Sun. and Sun.-Mon.). Runs are not started during "heat-wave" conditions when overnight temperatures may rise sharply due to lack of air conditioning. Analysis results are reviewed such that test N+2 is not started until acceptable results are obtained from test N (e.g., The Wednesday overnight run is not started without confirmation that the Monday overnight data were OK.) All X-GEF tests should be noted in the file ~tgs/XGEF/X-GEF_log.txt as well as in the X-GEF notebook.

• Review test results

• Return the VSGs to the vacuum storage facility

VSG Software

The analysis and presentation of the VSG measurement results is carried out by the IDL routine vsg_info.pro. This routine has multiple sections starting with "if 0 EQ 1 then begin" which may be changed to "if 1 EQ 1 then begin" to produce that section's product. In this way a large variety of VSG-related analyses are assembled in one location.

This web page is: http://space.mit.edu/HETG/vsg9806/vsg.html.
Please send any comments and updates to: dd@space.mit.edu.