[d2n-analysis-talk] S2m Timewalk & MIP Value

[David Flay]

Hi all,

I just wanted to share my e-mail exchange with Vince on the subject of the
S2m timewalk and the MIP value.

He describes how the MIP refers to cosmic particles, where their ADC
amplitudes are ~60-70 channels.

He mentions that the RF time vs (1/sqrt(LADC) + 1/sqrt(RADC)) should be
examined for the timewalk, followed by a re-optimization of the time
offsets.

I'm intending on shifting this to more of a 'background study' and focus
more heavily on optics...


"Hi Vince,

I have a question regarding the scintillator timewalk correction.

For S1, I first set all the DB coefficients to zero, then plotted TDC vs.
1/sqrt(ADC).  I then obtained the slope of a linear fit to that plot, and
put those coefficients into the DB.  This corrected the timewalk issue in
S1.

However, if I do the same for S2m, it does not work.  I believe it's
because the 'MIP' value is set to 50 for S2m, whereas it's set to a much
larger value (~1300) in S1.  This affects how the timewalk correction is
carried out.  From what I can tell in the source code of the
THaScintillator class, the MIP value refers to an offset point at which
the timewalk is taken to be zero -- for minimum ionizing particles (is
this correct?)  Is this MIP value for S2m supposed to be that different
from S1?  If it is not a large value, then how do I correct the timewalk
effect in S2m?

Here's my most recent discussion on the timewalk:
http://www.jlab.org/~flay/analysis/2010/LHRS_4_15_10.pdf

Thanks for any insight,

Dave"

"On Fri, Apr 16, 2010 at 10:50 AM, Vincent Sulkosky <vasulk at jlab.org> wrote:

Hi David,


You should check this link out from a talk Chiranjib made:

http://www.jlab.org/~vasulk/RF.pdf

The 'MIP' value is just an offset.  It really does not matter.  The reason
it is smaller for S2m is that the amplitude is smaller.  There are a few
reasons for this.  But you will notice that the ADC amplitude for
electrons is probably somewhere between 70-80 channels after pedestal
subtraction.  If you look at cosmics, it will probably be around 60-70
channels.  Technically cosmics (muons) and high energy (rel.) electrons
are MIPs.

So yes the value SHOULD BE different compared to S1!  Like you noticed,
when you change this value, it 'appears' to affect the offsets as well. If
you look at Chiranjib's talk, he mentions that he reoptimzed the offsets
after calibrating the time-walk correction.

Your problem is probably related to the fact that the right side of S2m is
self-timed.  Hence like you noticed, there appears to be no correction on
the right-hand side.  But this is an artifact of self-timing.  In reality
it exists.  What Chiranjib did was he plotted the RF time versus the sum
of 1/sqrt(ADC_L) and 1/sqrt(ADC_R) on pages 14-15.  Perhaps he can comment
on exactly what he did to extract a left and right time-walk coefficient?

btw, the time-walk correction for S2m should be very small.  Chiranjib
found only a few 10's of ps affects.  This is also due to the small
amplitude and a small threshold on the discriminator.  Hence, I would not
spend too much more time on this issue.

Cheers,
Vince"


- Dave


-------------------------------------------------
David Flay
Physics Department
Temple University
Philadelphia, PA 19122

office: Barton Hall, BA319
phone: (215) 204-1331

e-mail: flay at jlab.org
            flay at temple.edu

website: http://www.jlab.org/~flay
              http://quarks.temple.edu
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