Re: [BLAST_ANAWARE] sorry!

From: Tancredi Botto (tancredi@mitlns.mit.edu)
Date: Wed Aug 21 2002 - 15:36:40 EDT


Hi peter

> 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?

no, they are not off, but you should have told us it was in picoseconds..
let's use ns

> 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.

I of course assume that you do all testing in some directory of yours

I do not see anyhting wrong with your handout. Some of the comments we
made last time:
you should use the value you measure for the total swing of propagation
times in a tof (not a calculated values). The offsets of course only
change the value of the center channel and care must be taken to define
the center channel. You should use the median value (which you know once
you now the width of the distribution).

now *for the future* there is an issue in how to ensure backward
compatibility for the analysis of new/old data for which some channels
end up being plugged to different new/old modules... (different offsets)
It could happen anytime (in the next 3 years?) and in either case we have
to make sure we use the right offsets. Will you put this in the database ?

ciao,
tancredi

 
> 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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