A spherical array of twenty LiF 200 crystals was built to test the performances of a freestanding, self-focussing spherical crystal cosmic X-ray spectrometer. Measurements presently available show that the size of the image for a point source at infinite distance would be 3 mm (FWHM) along the focalisation axis and 2.1 mm (FWHM) along the dispersion axis. The mosaic spread on individual crystals is less than 0.1 degree. A slightly systematic deviation from the ideal bending (0.1 degree) is observed at the edges of most crystals and this appears to be the major limitation to spectrometer performance

Cantin, M.

Koch-Miramond, L.

Mougin, B.

Rocchia, R.

English

NASA Technical Reports Server

LABORATORY STUDIES ON A SPHERICALLY CURVED BRACC 
SPECTROMETER FOR COSMIC X-RAY SPECTROSCOPY 
M. Cantin, L. Koch-Miramond, 8. Mougin, R. Rocchia 
Cent re  d'Etudes Nucleaires d e  Saclay, France 
ABSTRACT 
A spherical array of twenty LiF 200 crystals has  been built to test t h e  performances of a 
freestanding, self -focussing spherical crystal  cosmic X-ray spectrometer.  Measurements 
presently available show tha t  t he  s ize  of the  image for  a point source at infinite distance 
would be 3mm(FWHM) along the  focalisation axis and 2.1 mm (FWHM) along the  dispersion 
axis. The mosaic spread on individual crystals is less than 0.1 degree. A slightly systematic 
deviation from the  ideal bending (0.1 degree) is observed at t h e  edges of most crystals and 
this appears to be the  major limitation to specirometer  performance. 
1. INTRODUCTION 
Precise piasma diagnostic on cosmic 
ray sources can  only be obtained from 
measurements of line strength ratios with 
Spherical Crystal  Imaging Spectrometer (SCIS e; Bragg spectrometers. A free-standin 
similar to the  one described by H. Schnopper 
and P. Taylor (1980) has been chosen for the  
E5A mission X 80 (L. Culhane, this Workshop;. 
A fre2-standing spectrometer employs a 
curveo array of crystals to simultaneously 
collect ,  focus and diffract  the  X rays emit ted 
by a distant source. The concave spherical 
shape provides the  minimum loss of spectral  
and spatial  resolution which may result from 
source extent,  spacecraft  pointing and 
alignment errors  (R. Criffith, this Workshop). 
W e  began studies on spherically bent 
lithium fluoride crystals in 1979 to 
demonstrate the feasibility of the  
spectrometer we proposed to NASA in collabo- 
ration with the  Smithsonian and Harvard 
Astrophysical Observatory (CFA). We describe 
hereaf ter  the  results obtained during this 
feasibility study. We will see tha t  these results 
a r e  directly applicable to the  Bragg 
spectrometer  of the  ESA mission X 80 whose 
main design goals a r e  : 
- spectral  resolution E / A E t  10 at 7 keV 
- size (full width at half maximum) of the 
image of a distant point source : 2.5 mm along 
the  dispersion axis and 4.5 mm along the 
focalisation axis for a radius of curvature of 
2.5 m. 
3 
11. TYPES OF CRYSTALS 
The  following results were obtained with 
LiF 200 crystals which were  proposed in the  
initial document (ref. ESA.SCL(79)3) for the  
obseivation of iron lines between 1.7 and 1.9A. 
These crystals can  b e  provided by cleavage or 
c u t  and polished. Cleavage along t h e  200 plane 
does not give very f la t  crystals : impurities 
produce cleavage s teps  of a few microns, 
even tens  of microns. Polishing gives surfaces 
with very good quality which allows optical  
uses. I t  is note-worthy tha t  polishing restilts in 
a bending of t h e  crystal  due to mechanical 
surface stress. For thr- tes ts  we  used square 
crystals  (5 c m  x 5 cm), 0.5 mm thick provided 
by Quartz  et Silice. Other  types of crystals 
(220 LiF, PET, TIAp) for fur ther  studies will be 
provided by the  same manufacturer.* 
111. TESTS AND SELECTION 
O F  FLAT CRYSTALS 
Planimetry 
The flatness af each crystal  is measured 
before bending. The required precision on the  
radius of curvature  AR/R A 5-10% imposes a 
severe selection: to understand that  we have 
to remind tha t  a 2.5 m curvature radius 
produces a sagi t ta  of only I25 microns on a 5 
c m  length. The surface of each  crystal  is 
scanned with a planimeter along several lines 
parallel to the  X and Y axis. The two sides of 
each  crystal  are explored with an  accuracy of 
I micron. Cleaved crystals with prohibitively 
https://ntrs.nasa.gov/search.jsp?R=19820018206 2020-03-21T07:36:05+00:00Z
524 ORIGINAL PAGE IS 
OF POOR QUALlW 
high cleavage steps were excluded. Polished 
crystals exhibit a strong uniformity and no 
selection is required for these. 
X-Ray tests 
They a r e  devoted to est imate  the effect 
of the  large scale mosaicity. The test bench 
consists of a point source, a crystal  holder and 
a radiographic film. Mosaicity means that, at a 
given point on the crystal, crystalline la t t ice  
makes an angle with the  upper surface. The  
different points on the  crystal, satisfying the  
Bragg relatio?, contribute to the  diffraction 
image on the  film which consists in a single 
regular line for a perfect  crystal  and several  
portions of bent lines for actual  crystals. The 
vertical  spread E of the  diffractioq pattern is 
proportional to the mosaicity W. Calibration 
gives €/W = 0.7 mm/arc.min. The two lines 
Km Kw2 of Copper a r e  clearly seen for each 
scaxning position. In most casesthe vertical  
spread is less than 3 to 4 mm corresponding to 
a mosaicity less than 0.1'. A few crystals only 
werr. rejected a f te r  this test. 
IV .  METHOD USED TO SPHERICALLY 
BEND THE LiF CRYSTALS 
The fiiF crystals a r e  0.5 mm thick and 50 
x 50 mm in atea. The nominal radius of 
curvature  is R = 2.50 m. Individual crystal  
holders 50 x 50 mm and 5 mm thick a r e  made 
in aluminium alloy. They can be  indi-iidually 
aligned by a classical optical  method using the  
rear face. The front face is covered by a 
spherical depo,it of STYCAST resin obtcined 
by molding, t he  surface being polished against 
a convex spherical shape to remove any 
imperfection. Then the  crystal  is glued on the  
holder during 72 hours or more pt room 
temperature  under pressure of ~lr 7.10 PascL;s. 
V. TESTS OF BENT CRYSTALS 
Me. wical measurements 
The same instrument as in sect.111 was 
used to est imate  the  crystal  sphericity. The 
nrofiles show slight cleavage steps. The set of 
tested crystals have radius clustered around 
the nominal vahe +5% with a maximum spread 
Of abG.Jt 2 8% 
X-ray tes ts  
The equipment is shown in fig.1. I t  
consists of an  X-ray generator wit') a copper 
anticathode.The s ize  of the  X-ray source is I x 
lmm2 ; after a tr ip  of 6.5 meters  in a tube 
filled with helium, X-rays reach the  LiF 
crystal. The par t  of the  radiation impinging 
t h e  crystal  with Bragg incidence is reflected 
and focussed, after another 6.5 m*:ters tr ip in 
helium, on t h e  sensor (film, PM, position 
sensitive proportional counter). For a perfect  
crystal ,  reflection should occur cn a vertical  
line. 
Test  bench in X rays for spherical 
crystals. 
Two tests were carried out : 
(i) for a given position of the  crystal  t he  image 
is scanned along a vertical  axis so as t3 
es t imate  focussing properties. The image 
brightness is not gaussian in most cases and 
may exhibit some spikes or bumps. On the  
ave iage  the  width with cleaved crystal  is 
smaller than wh!) polished ones, but the  shape 
is not so smooth. This is probably dine to small 
cleavage s teps  which give rite to different 
curvature  radii. 
(ii) The  crystal  is rotated around the  curvature 
center.  The detector  is located so as to 
measure r<p( copper line only. Then the  crystal  
is rotated abound a vertical axis going through 
the  theoretical  curvature  center. A perfectly 
bent crystal  would give a constant reflection 
for a rotation of 0 = 75 a r c  minutvs. Most of 
t h e  crystals show an  increased reflection at 
t h e  edges proving tha t  they a re  not properly 
curved at those place (R> It nominal). From 
t h e  width and shape of the  curves one can  
conclude tha t  local angular deviations from a 
perfectly spherical surface is less than 0 . 1 O .  
The reflectivity of polished crystals is about 
twice the  reflectivity of cleaved ones. 

526 
ORIGINAL PAGE IS 
OF POOR QUALln 
Fina!Iy the whole panel is rotated around 
the  center of curvature tc test the  response in 
the  dispersion plane. Tb *;K.B ShOM t he  
received intensity versus q&! of ro"dt.u.n for 
the  whole panel. The upper c'rrve cor- csponds 
to an aperture of 5 x 30 mn. :n front of t he  
detector, t he  lower curve to an  aperture  of 5 
mm diameter. The P;iS 1 diffracted intensity 
corresponds as expected to 4 times the  mean 
intensity given by individual crystals. The 
imperfections of the Curvature at the edges of 
the crystals explain the irregular shape of the 
curves. From the  ratio of diffracted 
intensities between the  upper and lower curves 
in fig.4 one deduces the size of the image in 
the  dispersion plane : 4.2 mm (FWHM).  During 
these tests we have monitored the focussing 
properties of the  m a y  which shewed 
acceptable variations. 
. .  . .  ! .  . . ................ . . . .  . . . - . . . -  
FiR.4 Diffracted X-ray intensity distribution 
along the  dispersion direction versus rotation 
angle of the crystals around a vertical axis 
going through the theoretical curvature 
center. 
VI. CONCLUSIONS 
The results gathered with a limited size 
sample of twenty 200 LiF crystals (16 polished 
and 4 cleaved ones) show that  : 
- the  focussing and dispersion curvatures are 
slightly bet ter  with cleaved crystals than with 
polished ones but reflection coefficient is 
increased by a factor two by polishing. 
- a slightly systematic deviation from the ideal 
bending of less than 0.1' observed at the edges 
of most of the  crystals is the major limitation 
of the  spectrometer performance. 
i Quartz et Silice : 8, rue d'Anjou 
75008 PARIS. 
This deviation results in : 
(i) A slight degradation of the energy 
resolution and focussing properties (non 
awsian distribution, existence of wings). 
ii) A non-uniform energy range coverage. 
The presently available measurements 
show tha t  the size of t h e  image for a point 
source at infinite distance (flight 
configuration) along the focussing direction 
would be 3 mm (FWHM) or 6 mm ( F W  at 1/8 
max.). These figures are better than the 
estimated ones based on a mosaicity of W = 
0.1' (5.4 mm F W H M  or 7.8 mm at 1/8 max.). 
This proves tha t  the mosaicity induced 
degradation has been, on the  average of 5 
crystals, overestimated. The effect of badly 
curved individual crystals does not show when 
considered in a set of several crystals (4 in the  
present case). The size of the  image for a 
point source at infinite distance along the  
dispersion direction would be 2.1 mm ( F W H M ) ,  
bet ter  than the  design goal. These results 
indirectly demonstrate tha t  t he  energy 
resolution of 1000 at 7 keV is obtained 
together with a spatial resolution of 2.5 min of 
arc. This will be confirmed won  by 
measurements along the  dispersion direction in 
a parallel X-ray beam. In these conditions 
point 2 appears to be t he  most severe 
limitation of the system but i t  can be 
overcome if we use staggered or alternative 
rows of crystals which produce the  right 
overlapping for a nearly uniform energy range 
coverage. 
t 
Future tests will extend measurements 
to LiF 22C, PET and TIAp. 
The results obtained so far a r e  
surprisingly good and should be still bet ter  
with an improved bending technique (increased 
curing t ime of the  lue, optimization of the  
bending matrix shape!. 
References 
Cantin, M., Koch L., Mougin 8. and Rocchia 
R., 198 I, CEA Report DPh.EP.Ap./8 I - I I R. 
European Space Agency Assessment Study, 
1980, SCI (8015 
Schnopper H.W., Taylor P.O., 1980, App. Opt. 
19, 3306 
Schnopper H.W., Koch L., Mougin B., Rocchia 
R., Bethelsdorf R.F. and Culhane J.L. 1981, 
15th ESLAB SYmposium X Ray Astronomy, 
Amsterdam. 


Laboratory studies on a spherically curved Bragg spectrometer for cosmic X-ray spectroscopy

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