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The Von Hamos Spectrometer - a hard X- ray diagnostic


The Von Hamos Spectrometer - a hard X- ray diagnostic

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									Facility Developments - Instrumentation

The Von Hamos Spectrometer - a hard X- ray diagnostic
M Notley, A Damerell, J Leach, D Neely.
CLRC Rutherford Appleton Laboratory, Chilton, Didcot, Oxon., OX11 0QX
G Tallents, J Lin, R Smith, S J Pestehe.
Physics Dept., University of Essex, Colchester

Introduction                                                                Data collection from other regions of the x-ray spectrum is
A Von Hamos1) X-ray crystal spectrometer was designed and                   possible with the current spectrometer design by using x-ray
manufactured for use on an experiment to investigate the                    crystals with different lattice spacings as shown by figure 2 or
properties of laser produced plasmas. The experiment was                    by changing the radius of curvature to increase or decrease the
carried out using the Titania high power laser system and aimed             focal range.
to investigate the characteristics of a high density Iron plasma in
local thermodynamic equilibrium2). The targets used were
                                                                                                                                             KAP 2d = 25.9 Å
Aluminium doped Iron-Mylar coated slabs which were

                                                                              Distance from CCD detector
designed to give a tamped (short time) plasma containing the                                               500                               Mica 2d = 19.8Å
spectra of both Iron and Aluminium. The spectrum of                                                                                          PET 2d = 8.74 Å

                                                                                     to crystal (mm)
Aluminium is well known and can therefore be used to diagnose                                              400
the conditions under which the Iron spectra are produced. The
spectrometer was designed to collect information from the                                                  300
He−β and Lyman-α lines of the Aluminium spectrum. This
report describes to the design and performance of the

The Von Hamos Spectrometer Design                                                                          100
The important and significant difference between the Von
Hamos design compared with other crystal spectrometers                                                       0
currently employed at the CLF is the use of a curved rather than                                                 0              5      10     15       20      25
a flat crystal to significantly enhance collection efficiency. A                                                               Wavelength Range / Å
crystal of dimension 20 mm x 50 mm and radius of curvature of
65 mm was implemented in our design in order to collect                     Figure 2. Graph showing the wavelength ranges available using
information from the 2-8 Å region of the x-ray spectrum, with               different crystals and the distances necessary for focusing.
special interest in 6.05 Å and 6.65 Å - the He-β and Ly-α lines
                                                                            The radius of curvature used for the current design allowed a
of Aluminium. This shape of crystal allows 2 orders of
                                                                            specific spectral range between 6 and 6.8 Å to be detected by
magnitude greater collection of flux from the plasma source
                                                                            the CCD. The experiment required that the He-β and Ly-α
than that obtainable by a flat crystal of similar dimension. The
crystal used to obtain data from the specific region of interest            lines of Aluminium be detected simultaneously and they
was PET (Petraethylate) 100 which has a 2d lattice spacing of               therefore had to fit spatially within the 28 mm length of the
8.74 Å.                                                                     CCD chip and still be within focal range.

The instrument consists of an adjustable mounting for the                   Instrument Performance
crystal onto which a cleaved crystal of 0.2 mm was curved and               The spectrometer was installed and its performance assessed
attached. Also a platform holding a mounting plate for either               throughout its use in the experiment. The efficiency of the
film or CCD and a housing, prevent stray light and hot electrons            instrument was measured to be 2x10-4 in the wavelength region
from interfering with the data of interest, were incorporated.              of interest (see figure 3). This is dependent on the collected
The spectrometer forms 1-D image data due to the geometry of                solid angle (10-4 sr at a distance of 130mm from the source), the
the optics (figure 1). It can also record data on a single shot             integrated reflectivity of the crystal 4), 5) , the sensitivity of the
basis by the use of an 16 bit X-ray CCD camera3) or film.                   CCD Chip5) (~50% of incident photons) and the transmission of
                                                                            the filters used (99.6%).
                           20mm               m
                                          50 m
                                                                                Throughput Efficiency

                                                              Sou                                          0.0005
                                                  mm                                                       0.0004
                                              = 65
                                     Rad                                                                   0.0003
              ge P     h
           Ima velengt                                                                                     0.0001
                                                                                                                         2.5           4.5         6.5              8.5
  Figure 1. Diagram showing the geometry of the Von Hamos                                                                             Wavelength Å
  design of spectrometer using a cylindrically curved crystal.
                                                                                Figure 3. Graph showing the throughput efficiency of the
                                                                             instrument when used with a 16 bit CCD 3) & Beryllium Filter

CLF Annual Report 1997/98                                             166
                                                                                                                                          Facility Developments - Instrumentation

Figure 4 shows a typical spectrum obtained with the
spectrometer. Via comparison of the Aluminium spectrum with
that of Germanium lines of known width, the wavelength
resolution of the instrument was ascertained to be better than
10 mÅ (although resolution was not a major factor concerned in
the design of the instrument).
                                                 #070202 time-15
                                                Te=380eV Ne=5e22                       Experimental

Intensity (a.u.


                                                                                                                   Figure 6. Spectrum as seen by the CCD from the second PET
                                                                                                                     crystal - the imperfections causing a dispersed spectrum.
                                                                                                                  Crystal quality has a large part to play in the image quality of
                  0.5                                                                                             the data and there are many factors in this including surface
                                                                                                                  quality, how smoothly curved the crystal is, reflectivity,
                                                                                                                  rocking curve effects and quality of the internal structure.
                        6   6.05   6.1   6.15            6.2       6.25   6.3   6.35                  6.4

                                                  Wavelength (A)                                                  Limitations of the instrument arise from factors such as the
                                                                                                                  source size, crystal resolving power and crystal quality.
Figure 4. Graph showing a typical spectrum obtained using the                                                     Initially the image quality and resolution of the instrument were
        spectrometer including a theoretical code plot                                                            not as high as seen in the figure 4.
A spectrum as detected by the CCD is shown in figure 5. This                                                      Theoretically the crystal is capable of easily resolving the Al
image was recorded by diffraction from the first PET crystal                                                      spectrum (dependence of rocking curve, lattice structure and
used which was found to be striated causing multiple 1-D                                                          position of the reflective layer).
                                                                                                                  The spectrometer performed very well and recorded the results
                                                                                                                  as per specification enabling single shot collection of data from
                                                                                                                  the region of interest. In the future it will be possible to adapt
                                                                                                                  the design for different spectral regions by use of different
                                                                                                                  crystals and by changing the radius of curvature of the mount.
                                                                                                                  The initial design included mounting capabilities for either a
                                                                                                                  CCD detector or a film back providing versatiltiy in data
                                                                                                                  The crystal quality is difficult to perfect and is the factor which
                                                                                                                  can most affect the images. Future developments should
Figure 5. Spectrum as seen by the CCD from an imperfect PET                                                       concentrate on finding a method to successfully bend crystals
  crystal - the striations causing multiple spectral recordings.                                                  without introducing any deformation into internal structure.

The imperfections in the structure of this crystal severely                                                       1. L. Von Hamos,
limited the performance of the spectrometer. Therefore another                                                       Z. Krystallogr 101 17 (1939)
crystal was installed which gave considerably better results (see                                                 2. R.Smith , G. Tallents, S.J Pestehe, G. Hirst, J.Lin, S. Rose,
figure 6 ). It was cleaved in-house and is estimated to have a                                                       M. Tagviashvili
thickness of 0.2 mm. To prevent the stresses caused by                                                               “A spectroscopic analysis of near solid density plasmas”
attaching the crystal along its whole length to the mount it was                                                     CLF Annual Report 1998
curved and fixed only at its edges which avoided the problem of
striation, but as curvature is still not perfect the data can be seen                                             3. Instaspec CCD camera including a 1024x256 pixel chip
to contain a dispersed spectral recording.                                                                           supplied by ANDOR Technology, Belfast

A perfect spectrogram would contain just one single row of                                                        4. A. Burek, University of California
intensity peaks due to the spectral lines. The dispersed                                                             “Crystals for Astronomical X-ray Spectroscopy”
spectrum in figure 6 may arise from the curvature of the crystal                                                     Space Science Instrumentation 2, 53 1976
being different at the points where it was fixed to the mount                                                     5. N.G. Alexandrop & G. Cohen
from the rest of the crystal causing the one row of peaks to                                                         “Crystals for Stella spectrometers”.
spread out.                                                                                                          Applied Spectroscopy Vol 28 No. 2 1974
                                                                                                                  6. A.Macphee
                                                                                                                     PhD Thesis 1997, QUB, Belfast

                                                                                                            167                                       CLF Annual Report 1997/98

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