A group of twelve optically polished flat samples were used to study the scattering of X-rays. The X-ray beam reflected from the twelve optical flat samples was analyzed by means of a long vacuum system of special design for these tests. The scattering measurements were made at 8.34A and 0.92 deg angle of incidence. The results for ten of the samples are comparable, the two exceptions being the fire polished samples

Froechtenigt, J. F.

English

NASA Technical Reports Server

Report MCR-73-143
DEVELOPMENT STUDY OF THE X-RAY SCATTERING PROPERTIES OF A
GROUP OF OPTICALLY POLISHED FLAT SAMPLES
Joseph F. Froechtenigt
Space Physics Laboratory
Martin Marietta Aerospace
P. 0. Box 179, Mail No. 1615
Denver, Colorado 80201
COPY
June 1973
Phase I Final Report for Contract NAS5-23198
Prepared for
GODDARD SPACE FLIGHT CENTER
Greenbelt, Maryland 20771
https://ntrs.nasa.gov/search.jsp?R=19740002525 2020-03-23T13:57:21+00:00Z
1. Report No.
MCR-73-143
2. Government Accession No. 3. Recipient's Catalog No.
4. Title and Subtitle
Development Study of the X-ray Scattering Properties
of a Group of Optically Polished Flat Samples
5. Report Dote
June 197-3
6. Performing Organisation Code
7. Author(s)
Joseph F. Froechteniet
8. Performing Organization Report No.
9. Performing Organization Nome and Address
Space Physics Laboratory
Martin Marietta Aerospace
P. 0. Box 179, Mail No. 1615
Denver, Colorado 80201
10. Work Unit No.
11. Controct or Cront No.
NAS5-23198
12. Sponsoring Agency Nome and Address
Howard Herzig
Goddard Space Flight Center
Greenbelt, Md. 20771
13. Type of Report and Period Covered
Phase I
Final Report
14. Sponsoring Agency Code
15. Supplementory Notes
16. Abstract
A group of twelve optically polished flat samples were used to study the scattering
of X-rays. The X-ray beam reflected from the twelve optical flat samples was
analyzed by means of a long vacuum system of special design for these tests. The
scattering measurements were made at 8.34A* and 0.92° angle of incidence. The results
for ten of the samples are comparable, the two exceptions being the fire polished
samples.
17. Key Word*
X-ray Optics, Scattering
18. Distribution Statement
19. Security Clastif. (of this report)
Unclassified
20. Security Class!f. (of this page)
Unclassified
21. No. of Pages 22. Price
PREFACE
The objective of Phase I of this program was to study the
X-ray scattering of 12 optically polished flat samples at 8.34A
and 0.92° angle of incidence. The tests indicate comparable
results for ten samples with the two fire polished samples
showing irregularities in the data.
-ii-
INTRODUCTION
It has been shown ~ that the Limiting factor in the per-
formance of X-ray telescopes is the scattering of soft X-rays by
the optical surfaces.
One important point learned in previous tests of soft X-ray
telescopes is that visible light tests do not predict the per-
formance in the soft X-ray spectral region. The manufacturing
techniques used were not sufficiently controlled to produce a
telescope giving predictable performance. For future X-ray astro-
nomy missions such as the HEAD, it is important to choose the
optimum materials and manufacturing techniques to provide higher
quality, lower cost telescopes. A previous study had made some
of the required measurements and answered some questions regard-
ing materials, but it also produced some questions which can only
be answered by continued research into the problem.
The first phase of this program involved measuring the pro-
perties of an X-ray beam reflected by optical flat samples at one
wavelength and one angle of incidence. In order to obtain data
which are suitable for application to grazing incidence mirror
construction, it is necessary to use a measurement system having
angular resolution of the order of telescope resolution - one arc
second or so. A total of one dozen optical flat samples were
tested in the course of this work.
-1-
EXPERIMENTAL PROGRAM
During the first phase of this program the scattering of
X-rays by polished optical flats was measured for 12 samples, at
one wavelength (8A), and at one angle of incidence (0.92°). The
products of the first phase of this program were the data re-
corded in the various tests. The following paragraphs in this
chapter describe these data.
Optical Flat Tests
The scattering of X-rays by polished optical flats was
measured by means of a special vacuum line which was approximate-
ly 36 feet long. The experimental configuration of the X-ray
scattering measurement equipment is shown in Figure 1. The vacu-
um system consisted of three stainless steel chambers, each
24 inches in diameter and 12 inches high. The three chambers
were interconnected by 6 inch I.D. stainless steel tubing. The
distance between the center line of each chamber was 17.2 feet.
The middle chamber housed the sample and secondary detector with
one end chamber enclosing the X-ray source and the other end
chamber enclosing the detector used for measuring the reflected
X-ray beam.
The X-ray source used in this phase of the program was
o
aluminum (8.34A), the anode voltage was 3.5 KV and the anode
current was 20 - 30 ma. The filament was essentially a line
source which was .005 inch diameter tungsten wire. The purpose
-2-
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of the slit in front of the source and the slits at the sample
and detector were to yield a theoretical FWHM value of the scat-
tering curves which, when compared to the experimental FWHM
value, would give a value of the amount of scattering produced
by the surface imperfections of the sample.
In the middle chamber, 17.2 feet away from the X-ray source,
the sample was mounted in the sample holder which could be ro-
tated in either direction in one arc minute steps about an axis
parallel to the sample surface and perpendicular to the incoming
beam. A secondary detector was placed adjacent to the sample
holder to monitor the direct beam from the X-ray source so that
any variations that occurred at the source could be taken into
account in the final analysis of the scattering data. A beam
stop with a one inch diameter hole was placed directly in front
of the sample between the sample and the detector end of the
system. The beam stop helped reduce the amount of X-rays scat-
tered from the chamber walls or X-rays from the direct beam from
reaching the primary detector. In the end chamber 17.2 feet from
the sample the primary detector was placed on a mount that could
be translated across the scattered X-ray beam in 0.1 arc second
intervals. The detector was a flow type proportional counter
with a 1/4 mil aluminum window. A .004 inch wide slit was placed
directly in front of the window of the detector.
-4-
The system was aligned by means of a laser so that the
angle of incidence between the sample and the X-ray source was
0.92°. After the sample and the primary detector were put in
position, the chamber was closed and the pressure reduced to
approximately 1 x 10 Torr. The high voltage for the X-ray
source was then turned on and the scattered beam was located.
After the peak of the scattered beam was located, the primary
detector was positioned +30 arc seconds from the peak. A typical
run consisted in translating the detector and slit across the
scattered X-ray beam in one arc second intervals and counting
for 100 seconds at each point from +30 arc seconds to -30 arc
seconds.
Two types of data were collected for each sample. The first
was a scan of intensity values as a function of angle as the de-
tector was scanned across the reflected beam. Readings were
. J
taken every arc second so that a detailed shape of the scatter-
ing curve could be determined. These curves were expected to be
symmetrical, but the results indicated that they were not always
regular in shape. These curves were used to find the position
of the peak reflected intensity, and the width of this peak was
interpreted as a characteristic of the sample. The second type
of data consisted of readings taken at nine selected points (0,
+5, +10, +20, +30 arc seconds) about the peak. These points
were used in making calculations of the percentage of radiation
scattered away from the main reflected beam.
-5-
Data Format
The experimental configuration used in this phase of the
program had the source slit set at .002 inch, the sample slit
set at .002 inch (equivalent to .004 inch), and the detector slit
set at .004 inch.
From geometrical considerations, assuming no scattering, the
X-ray beam reflected from the sample should have a trapezoidal
intensity profile that has an eight arc second FWHM. The 17.2 ft
working distance was chosen so that one arc second was equal to
.001 inch of linear motion. The value Sw was defined as the dif-
ference between the observed curve width (FWHM) and the calcu-
lated width (FWHM) based on geometrical considerations. Negative
values of Sw are thought to be due to error in setting the slits
or in the motion of the system during the course of a run. The
error in S is about +2 arc seconds.
As shown in the graphs in Figures 2-13, the negative
angles are in a direction away from the surface of the sample.
The failure of the nine points data to be exactly symmetrical
should be interpreted as an uncertainty in the position of the
detector. This positional uncertainty in locating the peak was
+3 arc seconds, but the relative locations of the nine.points
were uncertain by less than +1 arc second.
Scattering Curves
Figures 2 through 13 show the relative intensity curves ob-
tained from the 61 point raw data. Each individual graph contains
-6-
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Figure 6 - Scattering Curve
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Figure 11 - Scattering Curve O _' ^
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20
-13-
an identification number in the upper right hand corner. The
first number is the sample number, the second number is the wave-
length, and the last number is the angle of incidence at which
the sample was positioned. Each graph also contains a value re-
presentative of the scattering width in arc seconds, designated
as Sw- This value was defined as the difference between the
observed curve width (FWHM) and the calculated width (FWHM).
The following summarized observations are made for samples
which indicate obvious irregularities.
Sample No. G6 showed a double peak and wide FWHM with a
slight bump occurring at +30 arc seconds. Sample No. G12
showed a slight bump near the peak and had a wide FWHM
with slight irregularities near +30 arc seconds.
Nine Points Data
The nine points data was processed so that tables showing
relative intensities could be prepared. The intensity values at
the nine points were defined as:
I_/ = Intensity at -30 arc seconds
I_o • Intensity at -20 arc seconds
I_2 = Intensity at -10 arc seconds
I_l = Intensity at -5 arc seconds
I = Intensity at peak value
!]_ = Intensity at +5 arc seconds
12 = Intensity at +10 arc seconds
-19-
Io = Intensity at +20 arc seconds
I- = Intensity at +30 arc seconds
f
The relative intensity RI was then defined as:
RIJ - V'o
This definition produced a set of relative numbers which
would then be compared with one another and these values are
listed in Table 1.
The percent scattered, S, was defined as :
= . .
This then is another number which can be used as an index of
sample performance in comparisons. These values of percent scat-
tered are tabulated in Table 2. The percent scattered- values
represent an intensity that must come only from scattered radi-
ation. They provide a calculation method which is objective and
does not depend on experimental uncertainties in slit width. The
method of adding the area under the curve is subject to large
uncertainties because of possible slit width errors.
Reflecting Efficiency Measurements
The "efficiency factor", see Table 2, is not an attempt at
measuring reflecting efficiency, but, rather is used to normalize
the data against fluctuations in source intensity. The secondary
detector records an intensity (not flux) simultaneously with that
obtained in the primary detector. The values recorded will vary
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Sample
1
2
3
4
5
6
7
8
9
10
11
12
Scattering
Width
(sw)
0.3
-0.6
0.4
-0.4
0.3
13.4
-0.8
-1.0
0-9
0.4
0.6
8.7 '
Percent
Scattered
(S)
1.30
0.69
0.75
0.76
1.33
12.77
1.00
1.10
0.91
0.83
0.64
11.77
Efficiency
Factor
(E)
0.01077
0.01230
0.01811
0.01507
0.01207
0.00842
0.01938
0.00884
0.01274
0.01067
0.01144
0.01903
TABLE 2 - Phase I Test Results 8A - 0.92°
-22-
as the source output varies, but their ratio will not vary
(neglecting counting statistics). Thus, the shape of the curves
is not influenced by fluctuations in the source intensity, but
the absolute reflecting efficiency is not measured. The effici-
ency factor used in this report is defined as the ratio of se-
condary detector intensity to that recorded by the primary detec-
tor at the peak of the reflected intensity curve. This number can
be used, however, to give some insight into the location of cri-
tical angles in reflecting efficiency.
' i
Compilations of scattering widths, percent scattered, and
the efficiency factor are given in Table 2.
STATISTICAL CONSIDERATIONS
There was no statistical requirement with respect to inten-
sity in the case of the 61 point scattering curves. The time
interval was fixed at 100 seconds, and the number of counts col-
lected in this period was used.
The nine points data were taken so that the number of counts
at the peak was greater than 10,000 for the one angle of incidence.
These times ranged from 100 seconds to 500 seconds.
Twelve government furnished samples were tested in this phase
of the program. These were numbered as follows:
Sample No. Material Polishing Process
Gl CER-VIT Bowl Feed
G2 CER-VIT Fresh Feed
-23-
Sample No. Material
G3 CER-VIT
G4 CER-VIT
G5 ULE
ULE
ULE
G6
G7
G8 CER-VIT with Vacuum
Deposited Gold Film
G9 CER-VIT with Vacuum
Deposited Aluminum Film
G10 Aluminum with Polished
Polishing Process
Ionic Bombardment
Ionic Bombardment
Fresh Feed
Fire Polish
Ionic Bombardment
Bowl Feed
Bowl Feed
Fresh Feed
Electroless Nickel Coating
Gil Optical Grade Quartz Bowl Feed
G12 Optical Grade Quartz Fire Polish
The only irregularity observed on almost all of the samples
was a thin ring left by the teflon spacers between the samples.
All samples appeared to have a good polished surface.
Throughout this phase of the program, all samples were
placed in the sample holder at a definite orientation. This ori-
entation was defined as 0° (angle with respect to X-ray beam) and
determined for the sample by an arrow on the side of the sample.
CONCLUSIONS
The samples showed a low percent scatter for 8A and for 0.92°
angle of incidence. There were two exceptions, samples No. G6
and No. G12, both of which show irregularities in the scattering
-24-
curve and a high percent scatter. Both of these samples were
fire polished. The remaining ten samples show comparable results.
REFERENCES
1. R. S. Wriston, Technical Report No. ED-2002-907-1, NASA
Contract NAS 8-24000, September 1969, "S-056 Quartz Mirror
Test, Flight Optics B".
2. R. S. Wriston, Technical Report No. ED-2002-907-2, NASA
Contract NAS 8-24000, October 1969, "S-056 Quartz Mirror
Test, MSFC Mirror".
3. R. S. Wriston, Technical Report No. ED-2002-907-3, NASA
Contract NAS 8-24000, February 1970, "S-056 Quartz Mirror
Test, Flight Optics A".
4. J. F. Froechtenigt, Technical Report No. ED-2002-1593, NASA
Contract NAS 8-24000, February 1973, "ATM Experiment S-056
X-ray Reflectivity and Scattering Trends".
5. R. S. Wriston and J. F. Froechtenigt, Report No. MCR-72-31,
NASA Contract NAS 5-20287, February 1972, "X-ray Scattering
Study".
-25-


Development study of the X-ray scattering properties of a group of optically polished flat samples

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