A photon beam position monitor for SSRL beamline 9 Page: 3 of 9
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Because the device uses secondary hard x-rays, only the stationary Cu target need
be inside the beam transport vacuum system. The detector diodes, slits, occulting
mask, and any adjustments can be outside of the vacuum, viewing the target through
a beryllium window. Figure 2 shows a schematic diagram of the position monitor.
A prototype position monitor was tested in Beamline 2-2 at SSRL, a bending magnet
beamline with a critical energy of 4700 eV; hence a useful flux for producing Cu
fluorescent x-rays approximately 10-2 that of Beamline 9 for a given horizontal
A fixture which carries a copper plate tilted at -100 to the beam median plane, the
diodes, and the slits was mounted on a vertical sample positioner in the experimental
hutch of the white radiation branch of Beamline 2.
The beam was defined by a horizontal slit to be -5 mm wide; corresponding to -0.25
mrad of beam. In the vertical it was limited only by the built-in apertures of the
beamline. In order to not disturb other users of the beamline, the beam was left in its
steered position and the position monitor was scanned vertically for the tests. The
diode signals were fed through Keithley current amplifiers.
The photodiodes delivered -10-7 A at 100 mA stored current in SPEAR. Figure 3 is a
plot of the normalized difference signal of the position monitor.
Figure 4 shows the response of the position monitor to motion in steps of 2.5 microns.
The noise on the signal is actual motion of the photon beam relative to the monitor
In order to take advantage of the relatively high output of the AXUV photodiodes (100-
1000 nA) available in our application, we are designing a low cost dual channel
preamplifier to replace a pair of Keithley current amplifiers.
Because we are using the photodiode pair to generate an error signal, we are more
concerned with inter-channel matching and stability than absolute linearity. Hence
we plan to use a dual channel op-amp selected for its precision and small voltage
offset rather than superior input bias current specification.
Since the signal to noise ratio is best when the gain is at its highest, we require a
variable gain amplifier.
The preamp is located a few inches from the diodes to minimize noise pickup so it is
not readily accessible for adjustment. In order to avoid sending the gain loop to a
switch over a long cable, we plan to use a dual channel analog multiplexer to select
the appropriate gain resistors remotely. In this way we can send a relatively noise
immune 2-bit control signal to the multiplexer to switch among four different gain
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Cerino, J.A.; Rabedeau, T. & Bowen, W. A photon beam position monitor for SSRL beamline 9, article, October 1, 1995; Menlo Park, California. (https://digital.library.unt.edu/ark:/67531/metadc671421/m1/3/: accessed May 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.