[Frost] target depolarization

[Christopher Keith]

Hi Michael,
It's interesting that can see a polarization gradient in the target.  If 
it's real, I'd expect to see it get worse the longer beam is on target.

Here's a photo showing the LF NMR coil:
http://www.jlab.org/~ckeith/Frozen/Photos/Mixer.jpg
You can see that it's arranged to try to cover the entire length of the 
cell.  So it's not correct to ask "where the LF NMR measurements are 
made".   We try to measure the average polarization along the entire 
length.  Same with the HF coil, which is placed opposite the LF one.    
It looks similar, only one turn, not two, for lower inductance.  Hence, 
it's unlikely that they sample the polarization exactly equivalently.  
For the transverse running, I decided to use the same coil for both HF 
and LF measurements.  That opened another can of worms though.

I don't think the HF NMR plots are indicative of a polarization 
gradient, but I could be wrong.  The "ugly" ones come about from thermal 
drifts of the NMR cable inside the cryostat.  The temperature profile of 
the cryostat changes a lot when we start/stop polarizing, and we aren't 
always able to adjust for it.

Hope this helps
Chris



Michael Dugger wrote:
>
> Hi,
>
> I have some results regarding the target polarization as a function of 
> z-vertex that might be of interest. In short: it looks like we can see 
> the downstream de-polarization of the target using the pi+ n reaction, 
> and this de-polarization is a large effect.
>
> Method:
> 1) Isolate regions of the gamma p -> pi+ n reaction where the E 
> observable appears to be always less than -0.5. By looking at Brian's 
> slides 
> (http://www.jlab.org/Hall-B/secure/g9/morrison/brian_E_obs_2-10-2011.pdf), 
> I chose the region in cos(theta_cm) between -0.4 and +0.7 for E_gamma 
> between 1500 and 1600 MeV, and cos(theta_cm) between -0.7 and +0.8 for 
> E_gamma between 1600 MeV and 1750 MeV.
>
> 2) Slice the data into 32 z-vertex bins from z-vertex = -4.0 to +4.0 cm.
>
> 3) Find the |E| observable, with no carbon subtraction or other 
> background subtraction, for the missing mass (assumed reaction gamma p 
> -> pi+ X) between 0.89 and 0.99 GeV/c^2. This was performed assuming 
> that our current values of the beam*target polarization are correct.
>
> 4) Plotted the results.
>
> You can see the results here:
> http://www.jlab.org/~dugger/g9/g9a/polZ1.gif
>
> Top panel is z-vertex of the denominator of E.
> Bottom panel is the absolute value of E as a function of z-vertex.
>
> This looks to be a huge effect. We need to figure out how to get an 
> average polarization.
>
> Questions: Can we quantify the region in z where the low field NMR 
> measurement took place? Is there any pictures, or diagrams the show 
> the placement of the low-field NMR coils?
>
> We can try and use the high field NMR data but there are a lot of 
> "anomalous" measurements that would leave gaps in our polarization 
> tables. See the entries (red implies anomalous measurement) in the 
> list given on 
> http://clasweb.jlab.org/rungroups/g9/wiki/images/6/66/Clf_spreadsheet3_landscape.jpg 
> .
> Moreover, in general, the the high field NMR measurements made 
> previous to re-polarization might be questionable.
>
> When I look at the baseline subtracted high field NMR signals on the 
> FROST
> webpage:
> http://clasweb.jlab.org/rungroups/g9/wiki/images/c/c1/P4_2_areas_edited.jpg 
>
> they look bad.
>
> Question: Are these a fair representation of what the baseline subtracted
> NMR signal looks like for the high field just previous to 
> re-polarization?
>
> All the other baseline subtracted NMR plots look OK:
>
> High field after polarization ->
> http://clasweb.jlab.org/rungroups/g9/wiki/images/c/ca/P1_1_areas_edited.jpg 
>
>
> Low field after right after polarization ->
> http://clasweb.jlab.org/rungroups/g9/wiki/images/d/d4/P2_1_areas_edited.jpg 
>
>
> Low field just before polarization ->
> http://clasweb.jlab.org/rungroups/g9/wiki/images/9/90/P3_2_areas_edited.jpg 
>
>
> Question:
> Could the poor quality of the baseline subtracted high-field NMR plots be
> due to the large gradients in the butanol polarization with respect to
> the beam?
>
> Thanks for your time.
>
> -Michael

-- 
______________________________________________________________________
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
______________________________________________________________________