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S800 Tuning (XDT)
This page gives the steps that are followed when tuning the S800. The two possible tuning modes are included, namely, the Focus mode and the Dispersion-matching mode.
Before proceeding, it is mandatory to complete all the steps of the to-do list necessary to prepare the S800 for tuning. The preparation of the S800 for tuning is covered by the A1900 prior to every experiment.
Focus Mode
For most of the experiments in the S800, the analysis line is run in focus mode. In this optics, the analysis line is achromatic, i.e. the dispersive position of the beam focused in the target area (pivot point) does not depend on the momentum. Thus, this mode provides the biggest momentum acceptance (4%). On the other hand, since the spectrograph focal plane is chromatic, the resolution is limited to about 1 part in 1000 in energy.
Unreacted beam
In the first part of the XDT, the rigidity of the S800 is typically set to match the value of the fragment beam (selected in the A1900) after passing through the S800 target. This is where the term “unreacted beam” comes from.
Send beam to FP
Bias S800 FP scintillator photomultipliers
Remove stops to look for beam at S800 FP with
scalers and adjust beam rate with attenuators
Look at FP scintillator scalers (E1 up, E1 down)
There are typically a few scaler counts without beam
Object scintillator setup
Use
scope to look at signal patched out to data U6 (channel #54 in data U6 patch panel)
This signal is sent to the CANBERRA 454 Quad CFD in data U6
One of the output from this CFD is sent (via patch panel #62) to the TAC and scaler (channel OBJ.Scint) in S3. The other output goes through a passive delayed, and is sent (via patch panel #67) to the Phillips TDC
Check raising time and amplitude. Good signal: ~10 ns raising time; 400-500 mV amplitude
Adjust MCFD threshold:
Open configuration file MCFD16.tcl in /user/s800/operations/daq/usb/Configs
The OBJ signal feeding this module is not patched out to data U6
The OBJ signal from MCFD-16 module goes to the Mesytec MTDC32 module and scaler (channel OBJ.MCFD.Scint)
Make sure that the threshold of the XFP MCFD channel is reasonable. Rates in scaler channels XFP.Scint and XFP.MCFD.Scint should be comparable
Adjust MCFD OBJ threshold looking at scalers. The ratio of OBJ to XFP scaler rates (channels OBJ.MCFD.Scint and XFP.MCFD.Scint) should reflect the transmission of the cocktail beam
Save new threshold in configuration file MCFD16.tcl
FP scintillator setup
Select
Spectcl window
S800_SCINT.win
Adjust
bias looking at 2D spectra
e1.deup_e1.dedown (showing the parameters s800.fp.e1.de_down vs. s800.fp.e1.de_up) for the FP E1 scintillator
See spectrum below. Changing bias of this detector stretches the curve (i.e. shifts the blob in the middle of the curve corresponding to typical unreacted beam)
Different particles with different energy loss will shift the curve corresponding to particles covering whole FP
Ionization Chamber setup
Adjust pad gains
There are 16 pads each providing energy-loss information in the beam direction
The idea is to make sure the dynamic range is OK so that heavy particles do not saturate the spectra; the pad gains do not have to be matched
Use summary 2D spectra IC.raw (see spectrum below)
Gains are controlled in
s800shpini.tcl file in directory
s800/operations/daq/usb/Configs (an example of the content of this file can be seen
here).
CRDCs setup
-
Look at anode signal on
scope while biasing drift and anode
Patched to data-U6 on labeled connector
200 – 500 mV signals are good
CRDC1 anode is noisier (digital noise) than CRDC2
Bias CRDC1 and CRDC2. Typical starting values:
For He-3 @ ~130 MeV/u: CRDC1 (Anode=1120 V, Drift=1000 V); CRDC2 (Anode=1120 V; Drift=1000 V)
For Be-12 @ ~30 MeV/u: CRDC1 (Anode=970 V, Drift=1000 V); CRDC2 (Anode=970 V; Drift=1000 V)
For Ar-34 @ ~80 MeV/u: CRDC1 (Anode=860 V, Drift=1000 V); CRDC2 (Anode=840 V; Drift=1000 V)
For Zn-58 @ ~44 MeV/u: CRDC1 (Anode=740 V, Drift=1000 V); CRDC2 (Anode=740 V; Drift=1000 V)
For Rb-74 @ ~40 MeV/u: CRDC1 (Anode=650 V, Drift=1000 V); CRDC2 (Anode=650 V; Drift=1000 V)
For U-238 (88+) @ ~70 MeV/u: CRDC1 (Anode=570 V, Drift=1000 V); CRDC2 (Anode=570 V; Drift=1000 V)
Gate bias:
~20-30 V (adjust to optimize signal height compared to noise)
Should see counts on scalers
Count rate is a little higher than on scintillator due to noise or thresholds
Check
Spectcl window
S800_CRDCS.win (see figure below) to verify the good performance of the detectors. (The spectra for each CRDC can be checked separatelly in windows
s800_CRDC1.win and
S800_CRDC2.win)
Timing setup
Checking Particle ID and rate at S800 FP
Analysis line classic PPAC setup (Focus optics only)
Checking PPACs with beam
Scalers do not provide reliable diagnostic information because of noise
Bias PPACs while looking at patched out anode signal on scope to check for sparking
Typical starting value: 400V
A sample HV sample for 48K @ 95 MeV/u biases: PPAC1=580V, PPAC2=540V
Look at spectra of raw up, down, left, right, anode to decide on bias
Run with smaller p-acceptance (e.g., 0.5%)
Efficiency against Focal plane CRDCs should be 100%
Optimize bias setting based on raw signals
Check that position spectra look reasonable
Run with larger p-acceptance (e.g., 2%)
Efficiency against Focal plane CRDCs should be 100%
Record run showing tracking
Confirm angular dispersion: ~50 mrad/% (not an absolute measurement)
Confirm correlations between dispersive angle at intermediate image and p in FP (e.g., crdc1x). This correlation will be somewhat washed out by straggling in the target; in principle, this should be checked without the target, but the benefit vs. cost in time to remove the target is not worth it.
Setup beamline
Start scalers
Setting Optimization
Focused optics
Matched optics
Reaction Setting
Setting up Reaction Settings
Reaction setting to FP
Start with Attenuator setting of unreacted beam and step up in intensity
Set up beam blocker, if necessary
Expect to see unreacted beam if reaction setting is within +/- 3% of unreacted beam setting
Should have to move only one of the two blockers unless charge states are present
A graphic tool is available to help (not yet calibrated)
Try to cut only as much as necessary; depends on
Move blocker, decrease attenuator, repeat
Coincidences
Overview
Most experiments at the S800 involve setting up an auxiliary detector system (e.g. SeGA, HiRA, etc) to be used in coincidence with the standard detectors of the S800.
The auxiliary detector provides a secondary trigger that is fed into the S800 trigger system
A key part of setting up the S800 for such experiments is getting proper timing setup between the S800 and any auxiliary detectors
For cases where the Secondary detector has a slow response relative to the S800, the coincidence timing must be reset to the S800 timing by delaying the S800 trigger using the third gate and delay generator on the trigger
GUI
An example of experiments where auxiliary detectors are not used and, thus, setting up coincidence timing is not an issue are the experiments with tritons run by the charge exchange group
It is not clear whether coincidence setup gets logged as “XDT” or “EXR”
Setup
Coincidence signals are usually visible on scope without running scope in acquire mode
Adjust the width of the early signal (S800 or secondary) should be wide enough to catch coincidences with the late signal (width of late signal is not critical)
Readjust TDC delays based on changes made to S800 trigger delay
Experimenters will need to adjust their delays based on delay made to S800 trigger
Have experimenters record a run with coincidences on their account
S800 trigger TDC channel should show a peak (which corresponds to coincidences)
“secondary” TDC channel should have a peak
Length of run required is typically about 10-15 minutes
To be resolved: whether or not to this run copied from experiment account for documentation of device tuning
Follow-up
More detail needed
Minimum rates required for coincidence setup
Selection of appropriate substitute reactions for coincidence setup
How to feel comfortable that there will not be a problem with FP detector gases running out
Starting alarms
Starting logging
Dispersion Matching tuning
In the dispersion-matching optics, the S800 focal point is achromatic, i.e. the position of the beam in the dispersive direction does not depend on the momentum. As a consequence, the beam is momentum-dispersed on the target area (pivot point) with a dispersion of about 10 cm/%. The main goal of the tuning is to ensure that the position and angle dispersion are cancelled at the focal plane, thus maximizing the resolution at that point. We also want a good image in the object position, which will also contribute to increase the resolution at the focal plane.
Charge-exchange experiments require typically this optics. In some cases, the beam used is 3H, which has a rather high rigidity (around 4.8 Tm). This imposes a serious constrain, because the maximum rigidity of the spectrograph is 4 Tm. Thus, in this case, the tuning of the S800 is done with 3He, produced with a CH2 target.
Select
Spectcl window
S800_DISPMATCH.win
The parabolas seen in the above spectra correspond to reactions with H in the target. The blurred lines on the right of the parabolas correspond to reaction with C. It is hard to see clearly this lines, so we need to make several gates
Open spectrum E1.DE_TOF.RF and define a gate around 3He, and call it foi (Fragment Of Interest). (Note that unlike other experiments, where the energy loss is measured by the IC, we use here the E1 sinctillator.
This gate is used to fill spectra S800.FP.TRACK.XFP_TRACK.AFP!FOI and CRDC1.XG_CRDC1.TAC!FOI. As can be seen in the figures below, this gate “cleans” the spectra significantly. Indeed, one can now see the lines from reaction with C; the leftmost one corresponds to the ground-state, the next one to the right is the first excited state
Define rectangular gates in this spectra, making sure that it is narrow enough to select a vertical section of the parabolas, but wide enough to get enough statistics. Call them afp (in spectrum CRDC1.XG_CRDC1.TAC!FOI) and bfp (in spectrum S800.FP.TRACK.XFP_TRACK.AFP!FOI). After applying these new gates, the kinematics spectra are very clean
The pre-defined gate allgates is made by the AND condition of all the gates defined above (foi, afp, and bfp). This gate is used to fill the spectrum CRDC1.XG!FOI-AFP-BFP, which will be our diagnostics tool
The leftmost peak corresponds to reactions with H. The central peak are reaction with C. The goal of the tweak is to make these peaks as narrow as possible
Open the NCS application QtKM in the Applications Menu. Open file BLSetup_A1900.gkm. The magnetic elements that are typically tweaked with the knob box seating on the left side of u6pc5 are I232TA, I236TC, and I245TC which can be found on page S800 BLine+Spectrograph. Other elements used to improve the focusing in the object point, and the transmission are I172QA and I174QB. The goal of the dispersion-matching tuning is to find a compromise between transmission and resolution.