[Frost] Target polarization questions

Michael Dugger dugger at jlab.org
Fri Mar 18 10:01:54 EDT 2011


Chris,

I like the idea of using the time dependent function P(t) for the HF and 
comparing that to the LF data. The FROST group will have to discuss the 
situation and come up with a consensus as to haw we want to deal with 
correcting the polarization measurement. Roughly, this looks to be, on 
average, about a -3% correction in the polarization. It will just take a 
bit of time and thought to get the measurement fine tuned.

Thanks a bunch.

Take care,
Michael

On Fri, 18 Mar 2011, Christopher D. Keith wrote:

> Hi Michael,
> You are correct that the polarization determined with the LF coil with
> beam-on is p=c*a (lower case for LF).  And it should probably be corrected
> so that it better matches the HF polarization at the end of a Frozen Spin
> run (i.e just before repolarizing).
>
> You could for example estimate the polarization as a function of time
> using only the HF polarization and assume a simple exponential decay,
>  P(t) = P * exp(-t/T1)
> and compare this to the LF measurements that were taken during the run.
> T1 is the spin-lattice time constant and is a complicated function of
> field and temperature.  The relaxation rate is going to be a bit faster at
> the beginning of a frozen spin run because it takes the target about 8
> hours to reach its base temperature of 30 mK.  You can see this by looking
> at the LF NMR data which was measured every 30 minutes.
>
> I guess your options are:
> 1) Use the LF data (taken every 30 minutes) only;
> 2) Use the HF data only (taken only at beginning and end of a frozen spin
> cycle);
> 3) Use some combination of 1) & 2);
>
> #1 will give the highest average polarization and #2 the lowest.
>
> If my idea about charged-particle heating is correct, the discrepancy
> between HF and LF measurements started off small (or zero), and grew
> larger the longer beam was on target.
>
> Chris
>
>
>>
>> Chris,
>>
>> Thanks for the explanation :)
>>
>> Now, if we are using the equation P = c_{LF1}*A, where A is measured
>> using the LF coil throughout the run, then there needs to be some sort of
>> modification to the equation for P. Unfortunately, this will result in the
>> average polarization being lower then what we have now, and I very much
>> would like it to be higher :(
>>
>> This should be figured out because it is not a small effect. If what you
>> propose is correct, then the high field data is to be trusted and need
>> only a single calibration constant, whereas the low field data will have
>> a c_{LF1} value that changes throughout the run.
>>
>> Unless, of course, I don't understand how the polarization data we use is
>> put together and the equation we use is NOT P = c_{LF1}*A !
>>
>> Take care,
>> Michael
>>
>>
>> On Thu, 17 Mar 2011, Christopher D. Keith wrote:
>>
>>> 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
>>>>
>>>> _______________________________________________
>>>> Frost mailing list
>>>> Frost at jlab.org
>>>> https://mailman.jlab.org/mailman/listinfo/frost
>>>>
>>>
>>>
>>> ______________________________________________________________________
>>> 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
>>> ______________________________________________________________________
>>>
>>
>
>
> ______________________________________________________________________
> 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
> ______________________________________________________________________
>


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