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Re: Position calculations on start/stop scintillators [message #18126 is a reply to message #18119] Thu, 09 April 2015 15:16 Go to previous messageGo to previous message
miree is currently offline  miree
Messages: 71
Registered: June 2014
continuous participant
From: *ikp.physik.tu-darmstadt.de
Hi,

I guess it is not optimally calibrated. The ToF position determination is very dependent on a good calibration of all PMTs.
By PMT calibration I mean position correction, i.e. the dependence of delta_t(delta_x), where delta_t is the time delay between particle hit and PMT signal, and delta_x is the distance between the particle hit position and the PMT position.

The calibration goes like this: Plotting delta_t over delta_x and determine the slope and offset of the resulting structure
How this structure should look like a straight line (see Figure 1 of this GSI report: http://repository.gsi.de/record/52088/files/PHN-ENNA-EXP-55.pdf)
The offset (intersection of the line with y-axis) and slope (of the line) for each PMT have to be copied into the calibration file for the ToF processor "ToFStart.cal"

This calibration might change between experiments (mainly because of differences in PMT voltages, I guess).

In the prespec analysis script, these corellation plots are created in this section:

########################################################################
# Time of flight start detector
########################################################################
processor Lycca/ToFStart/Preproc   UTILS.MhTdcPreprocessor
	input[0:15]     <-   LyccaTargetTofCrate.mhtdc0[0:15]
	input[16:31]    <-   LyccaTargetTofCrate.mhtdc1[16:31]
	# ...
end
processor Lycca/ToFStart/Membrane         LYCCA.CircularMembraneScintillator
	pmt_time[0:31]   <-    Lycca/ToFStart/Preproc.output[0:31]
	x_hit            <-    Frs/S4tracking.xs[5]
	y_hit            <-    Frs/S4tracking.ys[5]
        #....
end
# create the delata_t vs. distance corellation plots for all 32 PMTs
# Membrane.dist[i] is the distance of PMT_i to the particle impact point
# Membrane.T_Tp[i] is the time of PMT_i minus the estimated time of particle impact
for $i in [0:31]
	processor Lycca/ToFStart/Diagnostics/Tdist0$i     UTILS.Pair
		first   <-   Lycca/ToFStart/Membrane.dist[$i]
		second  <-   Lycca/ToFStart/Membrane.T_Tp[$i]
		display first:second 256,0,300:512,-35,55       in   TofStart_Tdist_corrlation
	end
end


Note that the corellation plot depends on the calibration parameters, the complete process is iterative:
1)start with initial values for offsets and slopes in the calibration file: 0 and 0.01 are good starting values
2)determine offsets and slopes from the corellation plot
3)go to 1)

(2 to 3 iterations are usually enough)

Michael
 
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