Dear Colleagues-
I apologize for the late notice but I will not be attending the
analysis meeting today. With regard to the transparencies I showed at
last weeks meeting I have been trying to determine the source of the high
value for the tdc offsets. After much sorting through code I went back to
the basics. Using the root file commis-871.root generated by ntuple.C I
looked at one of the paddles that I had used as an example last week. I
plotted the raw tdc data for paddle Left 3 (the last forward angle TOF).
The tdc peaks for top and bottom come at about 1000. With our ASSUMED
calibration of 50 psec/ch this places the tdc peak 50000 psec or 50 nsec
after the trigger. Right? (Please let me know if I am wrong here.) It
turns out this is indeed approximately the offset that is calculated.So
are my numbers really off? I am continuing to look into this and also to
format the output so that the file that gets written essentially becomes
an update of the calibration file blast.sc_cal and not just some text file
which must be copied over by hand.
Also:
There is some low bin content stuff below the tdc peak that I don't
understand. The width of each tdc peak at the base (i.e. not FWHM) is
about 160 channels.
Please tell me if you agree with the following:
As documented in TBLDetRecon.cc, the propagation time for light in the
TOF was calculated as follows. With an index of refraction of 1.58 we have
Vsc = c/1.58 = 1.89E8. Also some light will reach the PMT directly and
some after reflecting off of the scintillator walls. The limiting factor
on reflection is the critical angle. For n=1.58 the critical angle is
about 39 degrees. Sin(39) is about .63 (this is worst case). Thus we end
up with a factor in between .63 and 1. A factor of .78 was attained by
iterating the position plot until a scintillator length of 180cm (for a
backward angle TOF) was achieved.
So our effective light propagation time is Vsc = 0.0189*.78 = 0.0147,
Units are (cm/picosec)
Now for the 160 channel width described above we have 160 ch * 50 psec/ch
= 8000 psec
And 8000 psec * 0.0147 cm/psec = 118 cm *** This is not bad as our
forward angle TOFs are about 119 cm.
So I have some confidence in this.
I must therefore ask: is the code right but the method wrong? (referring
to last week's handout)
I should be at the next meeting-
Pete
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