First results of the soft X-ray microfocus beamline

Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 485–487
First results of the soft X-ray microfocus beamline U41-PGM
Ch. Jung*, F. Eggenstein, S. Hartlaub, R. Follath, J.S. Schmidt, F. Senf,
M.R. Weiss, Th. Zeschke, W. Gudat
Experimental Division, BESSY GmbH, Albert-Einstein-Strae 15, D-12489 Berlin, Germany
Abstract
We present the ﬁrst experimental data obtained from the new microfocus beamline U41-PGM recently put into
operation. The optical layout of the beamline has been designed to conserve the high spectral brillance of the undulator
source. The beamline consists of a collimated plane grating monochromator (PGM) and a subsequent refocusing stage.
A toroidal mirror is used for refocusing onto the sample surface. With the 600 1/mm grating an absolute photon ﬂux
between 1012 and 1013 photons per second has been recorded over the whole energy range. Although the
monochromator is not designed to deliver highest energy resolution, a resolving power of up to 6000 at the nitrogen
K-edge has been obtained recently. The ﬁxfocus constant cff is variable in a wide range. According to the experimental
requirements it can be set to select either high photon ﬂux or energy resolution. # 2001 Elsevier Science B.V. All rights
reserved.
Keywords: Microspectroscopy; Collimated PGM X-ray ﬂuorescence
In a comparative monochromator study for a
high ﬂux microspectroscopy beamline at BESSY
II, the authors presented the concept of a beamline
split into a monochromatisation stage and a
subsequent refocusing stage [1]. The advantage of
this concept is that the refocusing stage can be
easily modiﬁed according to the experimental
requirements without changing the monochromator performance. Second, at least two diﬀerent
refocusing stages can optionally be used with
diﬀerent demagniﬁcation factors. The concept is
realised in two steps: the ﬁrst step was the
construction of the monochromator together with
a refocusing stage for moderate spatial resolution
*Corresponding author. Tel.: +49-30-56392-2944; fax: +4930-56392-2944.
E-mail address: [email protected] (C. Jung).
of the order of 10–20 mm. This beamline is under
operation now since March 2000. In the second
step, a refocusing stage is under construction,
which will provide a microfocus of 1 mm in
diameter. Since the strong demagniﬁcation makes
it necessary to place the optical element close to
the sample position, the refocusing stage thus
becomes more or less part of the experimental
setup.
The beamline is served by the U41 undulator.
The U41 is installed in a low beta section of the
BESSY-II storage ring [2]. It has a period length l0
of 41 mm and 80 periods. The emittance corrected
source size of the undulator is about 200 mm
horizontally and 60 mm vertically (FWHM), depending on the photon energy. For monochromatisation, a collimated Petersen-type plane grating
monochromator (PGM) is used [3]. A toroidal
0168-9002/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 8 - 9 0 0 2 ( 0 1 ) 0 0 3 7 6 - X
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C. Jung et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 485–487
mirror collimates the synchrotron light in the
dispersion plane. In the horizontal plane the
mirror images the source onto the exit slit. The
monochromator thus provides a moderate horizontal demagniﬁcation of a factor of about 2.2.
The prefocusing torus is then followed by the wellknown combination of plane mirror and plane
grating [4]. This combination enables to vary the
ﬁxfocus constant cﬀ, deﬁned as the ratio of the
cosine of the diﬀraction angle and the cosine of the
incidence angle at the grating, in a wide range. To
conserve the high brillance of the undulator source
it is required to place the grating as close as
possible behind the source and to operate the
monochromator at cff -values51. Consequently,
the plane grating needs to be placed in front of the
plane mirror to reduce slope and ﬁgure error
contributions of optical elements in front of the
grating. A detailed optimisation of the grating
parameters for the desired range of photon
energies from 170 to 1500 eV delivered best results
for a blaze grating with an angle of 0.88 and
600 1/mm. In addition to the degree of freedom in
setting the cﬀ-value, the choice of a blaze grating
enables to optimise the photon ﬂux if desired by
staying on blaze when the photon energy is
scanned.
The monochromatic collimated beam is focused
onto the exit slit by a cylindrical mirror. For a
collimated PGM, the demagniﬁcation in the
dispersion plane is determined by the source
distance of the collimating mirror, the image
distance of the focusing mirror, and the cff -value
selected. A toroidal mirror refocuses the exit slit
with a demagniﬁcation of 4 into the sample area.
In the vertical plane the spot thus depends on the
exit slit width. For the design value of 40 mm, a
vertical spot size of about 10 mm can be expected.
Performance tests can easily be carried out by
use of a gas cell which is permanent part of the
beamline. The gas cell is placed between exit slit
and refocusing stage, and it allows to record
absorption spectra independent of the experimental setup attached to the beamline. Additionally, a
solid state detector (GaAsP-diode) is available
which can be used to determine the absolute
photon ﬂux. A second detector unit is placed right
behind the refocusing mirror.
Promising results were achieved from the ﬁrst
spectra. With a limited opening of the front end
aperture, an absolute photon ﬂux of more than
1010 photons per second could be recorded for the
600 1/mm grating, an exit slit width of 100 mm and
a moderate cff -value (normalised to 100 mA beam
current). From this starting point a systematic
analysis of the beamline performance was carried
out. The ﬁrst step was to map the photon ﬂux as a
function of cﬀ-value and photon energy. For a
given undulator gap the undulator harmonic was
scanned for diﬀerent cff -values. The maximum of
each spectrum was determined and the corresponding photon ﬂux was derived from the data.
The result of this procedure is shown in Fig. 1 for
the ﬁrst and third undulator harmonic. The path
of optimal photon ﬂux is indicated by the dashed
line. The cff -value for the optimised path varies
Fig. 1. Contour plot of the normalized photon ﬂux of the U41PGM for the 600 1/mm grating and an exit slit width of 100 mm.
Front end apertures were set to 2 2 mm2 (horizontal vertical). Bottom: 1st undulator harmonic; top: 3rd undulator
harmonic. The gray scale code is identical for both plots (right).
White areas indicate ranges where the angular positions of the
optical elements are not accessible. The dashed line indicates the
cff -path of optimised photon ﬂux.
C. Jung et al. / Nuclear Instruments and Methods in Physics Research A 467–468 (2001) 485–487
Fig. 2. Energy resolution of the U41-PGM at the nitrogen Kedge for the 600 1/mm grating and an exit slit width of 10 mm.
The resolving power E=DE was determined by line shape
analysis. Solid symbols: cff ¼ 0:3; open symbols: cff ¼ 0:7.
from about 0.7 for low energies to about 0.6 for
high energies. This behaviour diﬀers to some
content from the calculated performance [1].
However, a small deviation of the blaze angle
from the design value of 0.88 can explain this
result.
Along the indicated cff -path, the photon ﬂux is
well above 1012 photons per second up to about
1300 eV of photon energy. If the 5th undulator
harmonic is used, a ﬂux of above 1012 photons
per second can even be obtained for energies above
1500 eV. The maximum of 4 1013 photons
per second appears at the onset of the undulators
working range at 170 eV.
In the second step the energy of the undulator
harmonic was determined as a function of the
undulator gap. From this, a set of data tables was
calculated which enables to scan undulator and
monochromator in parallel, staying in the maximum of the undulator harmonic for each energy.
The agreement between the photon ﬂux obtained
in this way and the ﬂux recorded for single
undulator harmonics, i.e. by scanning the monochromator at a ﬁxed undulator gap, was checked
at several energies within the working range of
487
each undulator harmonic. Up to the 5th harmonic
the deviation is negligible.
Although the beamline is not designed to deliver
highest possible energy resolution}the design goal
was a resolving power of about 2000 instead of
above 100,000 as reported for other collimated
PGM’s at BESSY [5]}it is of interest to check the
beamline performance also concerning its spectral
resolution capability. A series of absorption
spectra were recorded at the energy of the nitrogen
K-edge at about 400 eV, covering the available
range of cff -values. In addition, the experiments
were performed for diﬀerent orders of diﬀraction.
The result is shown in Fig. 2, where the resolving
power of the beamline is given for cff ¼ 0:3 and
cff ¼ 0:7 up to the third order of diﬀraction. The
resulting resolving power is well above the design
goal, the best value obtained is E=DE ¼ 6300.
The ﬁrst experimental data recorded at the
U41-PGM show that the predicted performance of
the high ﬂux microfocus beamline is met. Still, a
lot of work needs to be done to complete the
characterisation of the beamline by completing the
maps of photon ﬂux and resolving power, and by
determining the size of the focal spot at the sample
area. With the proved performance of the beamline, the requirements of microspectroscopy and
X-ray ﬂuorescence experiments concerning photon
ﬂux and ﬂux density are met.
References
[1] M.R. Weiss, R. Follath, F. Senf, W. Gudat, J. Electron
Spectrosc. Relat. Phenom. 101–103 (1999) 1003.
[2] J. Bahrdt, A. Gaupp, G. Ingold, M. Scheer, W. Gudat,
J. Synchrotron Radiat. 5 (1998) 443.
[3] R. Follath, F. Senf, Nucl. Instr. and Meth. A390 (1997) 388.
[4] H. Petersen, Ch. Jung, C. Hellwig, W.B. Peatman,
W. Gudat, Rev. Sci. Instr. 66 (1995) 1.
[5] R. Follath, Versatility of the collimated plane grating
monochromator design, Nucl. Instr. and Meth. A 467–468
(2001), these proceedings.

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