Operation of the CESR-TA vertical beam size monitor at

We describe operation of the CESR-TA vertical beam size monitor (xBSM) with e± beams with Eb=4GeV. The xBSM measures vertical beam size by imaging synchrotron radiation x-rays through an optical element onto a detector array of 32 InGaAs photodiodes with 50μm pitch. The device has previously been successfully used to measure vertical beam sizes of 10–100μm on a bunch-by-bunch, turn-by-turn basis at e± beam energies of ~2GeV and source magnetic fields below 2.8kG, for which the detector required calibration for incident x-rays of 1–5keV. At Eb=4.0GeV and B=4.5kG, however, the incident synchrotron radiation spectrum extends to ~20keV, requiring calibration of detector response in that regime. Such a calibration is described and then used to analyze data taken with several different thicknesses of filters in front of the detector. We obtain a relative precision of better than 4% on beam size measurement from 15 to 100 μm over several different ranges of x-ray energy, including both 1–12keV and 6–17keV. The response of an identical detector, but tilted vertically by 60°in order to increase magnification without a longer beamline, is measured and shown to improve x-ray detection above 4keVwithout compromising sensitivity to beam size. We also investigate operation of a coded aperture using gold masking backed by synthetic diamond.