hall d tagger and beamline review, nov. 19-20, 2008, newport news1 question question: how can you...

Hall D Tagger and Beamline Review, Nov. 19-20, 2008, Newport News 1

QuestionQuestion: 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 drawa smooth curve for theincoherent part underthe peak

post-collimated coincidence spectrumwith the microscope

Answer: the microscope Answer: the microscope alonealoneis not designed to tag a sufficientlyis not designed to tag a sufficientlylarge slice of the spectrum tolarge slice of the spectrum toadequately constrain the fit.adequately constrain the fit.


So then, how cancan 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 107 /s


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.4. The microscope also can be run in this fashion, by enabling

all rows, but we don’t intend to do that…but we don’t intend to do that…raw counting rate in a column of microscope fibers

microscope energy window

central fiber row


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.4. The microscope also can be run in this fashion, by enabling

all rows, but we don’t intend to do that…but we don’t intend to do that…coincidence counting rate in a column of microscope fibers

microscope energy window

central fiber row


But what if things don’t always work as expected?

The microscope can be switched instantly to enable as The microscope can be switched instantly to enable as many rows as are needed to fully contain the coincidence many rows as are needed to fully contain the coincidence stripe.stripe.

In each energy channel, the 5 fibers in a column subtend some fraction ffii of the total collimated flux for that channel, for i=1…5.

All 500 values of ffii are measured during setup, and monitored continuously on the 4 columns with individual readout of all 5 fibers.

Once the alignment is such that all fibers in a given row have ffii > 9X%, all other rows are set to zero gain except that one.

If this doesn’t happen, there is a problem with the If this doesn’t happen, there is a problem with the collimation system. Fix it before trying to take physics collimation system. Fix it before trying to take physics data.data.


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 incoherentbremsstrahlung integral

Shows that smearing from momentum transfer to atomin incoherent bremsstrahlungis negligible compared to m/E,and also the virtual spot size 0.2 m/E.


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hall d tagger and beamline review, nov. 19-20, 2008, newport news1 question question: how can you...

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