[Frost] Target polarization questions
Christopher D. Keith
ckeith at jlab.org
Fri Mar 18 08:48:47 EDT 2011
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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