It is important to note that TEs take time and depending on the immediate goals, having a very accurate polarization measurement may not be needed.  For example, if you're just testing equipment or running a functionality test or doing some training.  When taking physics production data it's important to have high-quality TEs.  However, doing TEs costs valuable beam time which we generally can't afford.  So, it good to learn how to do TEs well without wasting too much time.  During beam time you will need at least three separate TE measurements taken at different times for each coil for every new target load.  One single TE can be taken at multiple temperatures to reduce error but this takes more time so it good to plan these things with other beam downtime.   

                       
                                                                                                                                                         Figure 1

  • NMR System is tuned and a high statistics baselines (5K sweeps) for the relevant channels have been taken. See How to take a baseline and then take data on UVA-NMR system
  • Once you have taken a baseline check the scale of the NMR noise by selecting the baseline you just made and taking data on the NMR (a good level is 5X10-5 V, see Figure 1).
  • Magnet is a full field and in persistent mode, leads are ramped down
  • The nose is full and the level is above the lower heat exchanger
  • The run valve is set to replenish nose boil-off (most recent studies indicated 0.52 turns with no microwaves)
  • The gate valve is closed and the roots backing pump is running with the gate-valve bypass on PID control set to the desired pressure (usually around 20 Torr but this can change depending on the preferred temperature)

The minimum time to wait for the spin temperature to reach the lattice temperature is 3t1, but this depends on what state the target material was in before when you begin monitoring. If cryostat conditions where already pretty stabilized, watch the fridge pressure for at least 20 minutes. Use the following page to analyze:

https://e906-gat1.fnal.gov/data-summary/e1039/target-NMR-event.php


Click read file with events.csv selected.  The type in for Event number range: Last "50" Events

Note: Always click "Read File" button on the top of the https://e906-gat1.fnal.gov/data-summary/e1039/target-NMR-event.php page to reload the most recent file before selecting the last 50 events.


At this point you can start to take TE data and see what type of error you get.  The error (standard deviation) will be based on both the variation in area and the variation in pressure.  You want both to be as stable as possible but if there is a trend in area as a function of time this is a clear indication that you are not yet at TE.  If it's purely stochastic variation over 20 minutes at high temperatures then you are at TE but you may have to wait for 30 or 40 minutes to confirm depending on the temperature of the TE you are doing.  Warmer temperatures reach TE faster but have smaller signals to integrate.  You get reduced integration error with larger area at lower temperatures but they take much longer.  The Q-meter can only measure polarization to just below a 1% relative error so trying to get a TE with error far smaller than this is not a good use of time when beam is available.  If you have time on your hands getting error below the 1% level is totally feasible but its better to take multiple TEs at different temperature.  You can do this by setting the pressure PID between 3.5 and 75 Torr.  The tolerable scale of fluctuations in area is dependent on the cryostat conditions.  You can obtain better stability by overfilling the nose to at least bottom heat exchanger and having the run valve closed more during lower temperature TE measurements.


The standard steps are the following

1) Set the number of NMR sweeps to 500 and start taking data

2) For a quick and dirty TE you need at least 15 points once the area has stabilized but for a high quality TE you want more like 50 points at no less than 3 different temperatures in the pressure range mentioned above.

3) After you load the event file (press read file) and have selected the required number of points (press render events), start monitoring the area.  Area for the proton will be on the scale of 10^-2 so you want stability on the level of 10^-3.

4) When you press calculate the polarization at the corresponding temperature and the new calibration constant will appear below with errors.

5) You may want to do this a few times to see if the data you are collecting is truly stable in area and the calculated calibration constant is not changing much.

Here is an example of a decent TE (notice the level of stability in not just pressure but area as well):


6) Record all of the Calculation Result output in the FNAL elog and post it to the discord channel (and put the final calibration constant in PDP if needed).

7) Do the same thing for the other two coils if needed.


If for some reason you get a very different calibration constant at a different temperature, its generally a sign that something shifted in the cable or Q-meter system or in the target cell itself and you may have to start from the beginning.  If this happens let the Target Expert know.

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