In the ESR we had thw whole day (saturday) a 400MeV/u bunched 238U92+ test beam (about 2 MHz). The RF signal is cabled as input to the DAQ (seperate post/entry).
Intensity is artificially reduced to a few 10^6 (with some tests alsodone at 10^5).
In all the tests the both particle detectors were moved in pus the empty pocket on the inside to mimick the background conditions during the run (aka no other isoopes, no other charge state, clearing of orbit fromdonw-charged ions.
Noise background
With the bunched beam all three PMTs and the channeltron show an ~20MHz "noise beat" below the actual signal that seems to be phase stable. The "noise train" is repeated with about 2 MHz (very likely the bunch revolution frequency). The amplitude of the noise is rather low and can be easily discriminated for the three PMTs, but not so easy for the channeltron, as the channeltron signal is more "continuous" and has no clear gap for the threshold.
A working hypothesis could be that Al mirrors or something similar act as pickup electrodes for the real beam.
2024-06-10: Correction : Signal is not "phase-stable" - and 200 kHz (wrong reading and triggering of scope)
Channeltron
The signal of the CT is rather small. In order to get the singal of the CT out of the noise it needed to be opreated at about 2600V. With an according threshold setting the CT sees a signal that is clearly from the beam and absent with "no-beam". The signal is 6-7 kHz even at the lowest intensitiies of ~10^5 ions in the ring. Worse, it doesn't show any bunch timing peak in the spctrum.
We decided that under these conditions the CT cannot be used during the 229Th beamtime.
PMTs
Voltages of the PMTs and thresholds of the CAEN CFD are adjusted with a bunched beam with an intensity of few 1 to 3*10^6 ions (and thus should be ok for lower intensities):
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PMT South ("braodband") ET9422 | 1500 V | thresh: 7
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PMT Middle ("smallband") ET9423 | 2550 V | thresh: 9
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PMT North ("smallband") ET9423 | 2600 V | thresh: 11
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Bunch signals in DAQ
Ion frequency (measured) used in DAQ1 in SetParameter.C: 1.9693 MHz, the divisiion factor is set to 88.
All three PMTs show very nice and small bunch-time signals with a very low background floor. Thresholds are adjusted to have at least "1CFD tick" extra margin.
The small-band PMTs have no-beam rates < 1Hz, PMT south < 10 Hz.
The clear visibility of the bunches on the small-band PMTs means, on the other hand, that we still have quite some background from collisions with the residual gas (albeit it seems to be lower that in the last beamtime, but no concrete (normalized) numbers are available. PMT North and Middle have about the same solid angle but PMT middle features the CaF2 window. The bunch singal rate of PMT middle is about twice the one of PMT north indicating that about half of the signal stems from hydrogen Ly-alpha (at 121.6 nm) that is cut-off by the CaF2-window (about 123 nm).
We should discuss a) if the vacuum can be further improved, e.g. using the turbo pump, b) if PMT north should get a CaF2 filter (not at the moment).
All these findings are for the 400 MeV/u 238U92+ beam and should be rechecked with decelerated 200 MeV/u beam.
For both DAQs data are accumulated with two data sets, the first at a a few 10^6 ions and the second at a few 10^5 ions:
files for run1 (mid int): 003.lmd (DAQ1), 006-006.lmd (DAQ2)
files for run2 (low int): xxx.lmd (DAQ1), yyy-yyy.lmd (DAQ2)
The timing signals can also be very nicely be seen in the 25 ps TDC of DAQ2.
DAQ2 records also th common Stop signal in the 25ps TDC. There are indications that the CS signal (derived from the bunch frequency) jitters. The discrimonation of the bunchsignal should be cheked.
In DAQ2 the thesholds and windows for Mesytec are not yet set properly, so thre is no ADC signal, and no "second" discriminated signal.
This is because the Mesytec MSFC16 module had to replaced by a different similar module but with non-ideal internal components. The module still needs ot be adjusted properly. |