Hall D Tagger and Beamline Review, Nov , 2008, Newport News1 Question Question: How can you reliably extract the polarization of the collimated beam tagged by the microscope by fitting to the spectral shape, if the microscope sees only a narrow window around the coherent peak? different ways to draw a smooth curve for the incoherent part under the peak post-collimated coincidence spectrum with the microscope Answer: the microscope alone is not designed to tag a sufficiently large slice of the spectrum to adequately constrain the fit.

Hall D Tagger and Beamline Review, Nov , 2008, Newport News2 can So then, how can we reliably extract the polarization of the collimated beam tagged by the microscope? 1.spectrum of post-collimated beam measured in PS alone. 2.coincidence spectrum of PS with 190 counters in fixed array. 3.coincidence spectrum of PS with microscope counters. running at 10 7  /s

Hall D Tagger and Beamline Review, Nov , 2008, Newport News3 How can we be sure that these spectra will all agree? 1.The broad-band tagger hodoscope will have full post- bremsstrahlung electron acceptance. 2.Only individual counter gains, discriminator thresholds, etc. contribute to this difference. 3.These are calibrated out using tagging efficiency correction. but we don’t intend to do that… 4.The microscope also can be run in this fashion, by enabling all rows, but we don’t intend to do that… raw counting rate in a column of microscope fibers microscope energy window central fiber row

Hall D Tagger and Beamline Review, Nov , 2008, Newport News4 How can we be sure that these spectra will all agree? 1.The broad-band tagger hodoscope will have full post- bremsstrahlung electron acceptance. 2.Only individual counter gains, discriminator thresholds, etc. contribute to this difference. 3.These are calibrated out using tagging efficiency correction. but we don’t intend to do that… 4.The microscope also can be run in this fashion, by enabling all rows, but we don’t intend to do that… coincidence counting rate in a column of microscope fibers microscope energy window central fiber row

Hall D Tagger and Beamline Review, Nov , 2008, Newport News5 But what if things don’t always work as expected?  The microscope can be switched instantly to enable as many rows as are needed to fully contain the coincidence stripe. f i  In each energy channel, the 5 fibers in a column subtend some fraction f i of the total collimated flux for that channel, for i=1…5. f i  All 500 values of f i are measured during setup, and monitored continuously on the 4 columns with individual readout of all 5 fibers. f i  Once the alignment is such that all fibers in a given row have f i > 9X%, all other rows are set to zero gain except that one.  If this doesn’t happen, there is a problem with the collimation system. Fix it before trying to take physics data.

Hall D Tagger and Beamline Review, Nov , 2008, Newport News6 Possible vertical angle   –  e correlation spoiler Confidence that small losses of tags outside the central row of the microscope can be easily modeled relies on the simple observation that the virtual spot size dominates the spread in the   –  e correlation – means it is energy independent. integrand inside incoherent bremsstrahlung integral Shows that smearing from momentum transfer to atom in incoherent bremsstrahlung is negligible compared to m/E, and also the virtual spot size 0.2 m/E.

Hall D Tagger and Beamline Review, Nov , 2008, Newport News7 backup slides