Operation of the CESR-TA vertical beam size monitor at \(E_{\rm b}\)=4 GeV

We describe operation of the CESR-TA vertical beam size monitor (xBSM) with \(e^\pm\) beams with \(E_{\rm b}\)=4 GeV. 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 50 \(\mu\)m pitch. The device has previously been successfully used to measure vertical beam sizes of 10-100 \(\mu\)m on a bunch-by-bunch, turn-by-turn basis at \(e^\pm\) beam energies of \(\sim\)2 GeV and source magnetic fields below 2.8 kG, for which the detector required calibration for incident x-rays of 1-5 keV. At \(E_{\rm b}=4.0\) GeV and \(B\)=4.5 kG, however, the incident synchrotron radiation spectrum extends to \(\sim\)20 keV, 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 15-100 \(\mu\)m over several different ranges of x-ray energy, including both 1-12 keV and 6-17 keV. The response of an identical detector, but tilted vertically by 60\(^\circ\) in order to increase magnfication without a longer beamline, is measured and shown to improve x-ray detection above 4 keV without compromising sensitivity to beam size. We also investigate operation of a coded aperture using gold masking backed by synthetic diamond.