[Frost] Target polarization questions
Christopher Keith
ckeith at jlab.org
Fri Mar 18 11:36:05 EDT 2011
I think the comparison would be interesting. Jo and I discussed this
a long time ago. Let me know what you come up with.
Chris
Michael Dugger wrote:
>
> 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
>> ______________________________________________________________________
>>
--
______________________________________________________________________
Christopher D. Keith
Jefferson Lab, MS 12H email: ckeith at jlab.org
12000 Jefferson Ave. ph: 757-269-5878
Suite #6 fax: 757-269-5235
Newport News, VA 23606
______________________________________________________________________
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