A photon beam position monitor for SSRL beamline 9 Page: 2 of 9
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SSRL is currently constructing Beamline 9, a hard x-ray beamline for use in structural
molecular biology research. It is generated in the SPEAR 3 GeV electron storage ring
by an 8 period hybrid wiggler magnet with a peak field of 1.9 Tesla, a period length of
26 cm, and a field integral over 1/2 period of -17T-cm. It produces a 16 mrad fan of
radiation which is split into three branch lines as depicted in Figure 1. The critical
energy of the radiation is 11400 eV.
Beam position control is a major consideration in the quality of research results
obtained on this and other synchrotron radiation beamlines. To achieve the best
control of the beam position at the experiment with a single position monitor, the
monitor should be located at a similar distance from the source. In such downstream
locations, monitors previously used at SSRL, (ion chambers and photo-electron
devices) often suffer intensity dependent drift as the result of bend magnet radiation
contamination, aperture vignetting, or spray from upstream components such as
mirrors and beam stoppers.
In order to avoid these problems we have devised a position monitor which utilizes
fluorescence radiation from a cooled copper target placed directly in the wiggler
beam. This monitor will be placed in the gap between 2 branch lines near the
horizontal centerline of the wiggler radiation fan.
A similar device was reported by Stefan et all in 1986, but it was detector limited and
somewhat complex in design.
PRINCIPLE OF OPERATION
For centuries, astronomers have measured the position of celestial objects using the
occultation of light sources.2 We employ that technique here to monitor beam
A water-cooled copper target is placed in the direct wiggler beam. The target surface
which faces the incoming radiation is planar and is sloped at ~100-150 to the median
plane of the incident beam in order to reduce the absorbed power density and
Incident x-rays having energy above the Cu k edge at 8979 eV will excite
fluorescence in the target which is viewed by a pair of silicon PIN photodiodes.3 The
field of view of each diode is occluded such that one diode views only the portion of
the target above the median plane and the other views the portion below. Vertical
motion of the beam increases the flux into one diode while reducing the flux into the
other. The difference signal can be fed back to the electron beam steering system to
maintain the beam position.
Scattered radiation spray from upstream objects such as mirrors and slits is
predominantly low energy, and hence will not excite fluorescence in the Cu target.
Thus, in contrast to photo-electron or ion chamber based monitors, the fluorescence
beam position monitor should be less sensitive to this spray.
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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/m1/2/: accessed May 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.