[Frost] Target polarization questions

[Christopher D. Keith]

Michael,
It's important to remember that different coils were used for the high
field and low field NMR systems, and it is unlikely that they sampled the
equal parts of the target with equal weight.  The LF system was intended
to only be a monitor of the polarization during the beam-on, frozen spin
runs.  Unlike the HF system it was never accurately calibrated in a
dedicated set of polarization measurements.  Instead, it was cross
calibrated against the HF measurements prior to, and following each frozen
spin run.  These are the type 2 and 3 measurements you describe.

Ideally these cross-calibrations should have been consistent, ie
c_{LF1}/c_{LF2} = 1.  This is the same as saying A(1)/A(4)=A(2)/A(3).
Why aren't they more consistent?  It's my belief that forward-going
charged particles produced by the photon beam depolarized the downstream
part of the target faster than the upstream.  The geometry of the LF coil
was such that it sampled the downstream part of the target a little more
heavily than the upstream.  So it "registered" more polarization loss
following a frozen spin run (type 3 measurement) than did the HF coil
(type 4).

Does this make sense?

Chris


>
> Hi,
>
> I'm trying to understand the target polarization and have a couple of
> questions.
>
> Here is what I understand. Please correct me if I am wrong:
>
>>From what I can tell by looking at the "Target Polarization" web page at
> http://clasweb.jlab.org/rungroups/g9/wiki/index.php/Target_Polarization
> To find the calibration constants for the polarization by cutting the
> data up into 4 types:
>
> 1) High B-field after polarization and just before going to low B-field
>
> 2) Low B-field right after the switch between low and high B-field
>
> 3) Low B-field right before the switch between low and high B-field
>
> 4) High B-field just after switch from low B-field and just before
> repolarization
>
> We assume that the polarization can be calculated in the form p = c*A,
> where p is the polarization, c is a calibration constant, and A is the
> area under the peak of a frequency deviation plot. The c calibration
> constant is determined using only high B-field data. For low field data,
> the equation is p = c_{LF1}*A, where the c_{LF1} calibration constant is
> determined by using the c calibration constant and comparing the area A
> between the type 1 and type 2 data (i.e. c_{LF1} = c*A1/A2).
>
> There is a calculation made for c_{LF2}, where c_{LF2} = c*A4/A3. There
> are some measurement where c_{LF2} are deemed as "anomalous", but even
> where the anomalous distinction is not made, there are fairly large and
> systematic differences between c_{LF1} and c_{LF2}. The values of the
> ratio between c_{LF1} and c_{LF2} (for non-anomalous measurements) are
> given as:
> 1.025
> 1.036
> 1.093
> 1.080
> 1.037
> 1.171
> 1.030
> 1.074
> 1.049
> 1.013
>
> These value of c_{LF1}/c_{LF2} are systematically high and have an average
> value of 1.061.
>
> Question: We are using c_{LF1} and the area A throughout the low B-field
> data to determine the polarization, so I must assume that the High-B field
> type 4 determination of P is not as accurate as type 1. Am I looking at
> this wrong?
>
> Question: Do we use both high B-field types (types 1 and 4) to determine
> c? If so, how is it possible to have a systematic uncertainty in c be only
> 1.6% when the c_{LF1}/c_{LF2} ratio is systematically high by 6.1% ?
>
> Thanks for your time.
>
> -Michael
>
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>


______________________________________________________________________
Christopher D. Keith
Jefferson Lab, MS 12H                        email: ckeith at jlab.org
12000 Jefferson Ave.                         ph: 757-269-5878
Newport News VA 23606                        fax: 757-269-5235
______________________________________________________________________