Re: BCAL timing (fwd from David)

[Elton Smith <elton@jlab.org>]

---------- Forwarded message ----------
Date: Thu, 05 Apr 2007 07:03:30 -0400
From: David Lawrence <david.lawrence1@cox.net>
Reply-To: davidl@jlab.org
To: Blake Leverington <leverinb@uregina.ca>
Cc: halld-cal@jlab.org
Subject: Re: BCAL timing


Hi Blake,

    To minimize the contribution of the timewalk in tn8 to the tn7-tn8
time difference, I cut on n8>1000. Likewise, for the timewalk functions
of the other tubes, I cut on the "reference" channel's ADC value being
large. Granted, this will not remove it completely, but it does reduce
it down to a level that is not bigger than the intrinsic spread of the
timing distribution of the PMT. Of course, the procedure could be
iterated using the previous iteration's timewalk functions to correct
the reference channel's time. But again, since the first iteration had
so little influence from the timewalk of the reference channel, I don't
believe you will see any difference in the final resolution.

    You are right. The distributions I produced will have the intrinsic
resolution of both cells and so are sqrt(2) larger than the intrinsic
resolution of a single cell. In principle, one mat be able to use the
timing and energy info from multiple cells to improve the resolution
even further. That has to be thought about how to do it correctly.

    After talking with Elton and Simon about this current technique, out
biggest concern is that using the 2 cells from the BCAL may be canceling
some systematic part of the error giving a misleading-ly lower
resolution than we would otherwise be able to obtain. I think this
depends on how valid it is to assume cell 7 and cell 8 are independent
detectors.

Regards,
-David

Blake Leverington wrote:
> Hi David,
>
> I had done something like this previously except for the last step
> (ts8+tn8)/2 - (ts7+tn7)/2 but I'm wondering how good the walk
> corrections are when you plot say n7 vs tn7-tn8. There is a walk in
> tn8 as well that does not completely correlate with n7 though they
> appear to be corrected nicely. This is my only concern, otherwise
> everything looks good from what I see. I guess this a naive question
> but since you are taking the difference of 2 sums of the PMT's, does
> this still give you the intrinsic resolution or is it 2 cells folded
> together? I suppose I can check this myself too.
>
> I'll use your code to see what I see in the other cells.
> Thanks!
>
> -Blake
>
> David Lawrence wrote:
>>
>> Hi Blake,
>>
>>    I have just added a page to the wiki under BCAL analysis and
>> today's date. The URL is:
>>
>> http://www.jlab.org/Hall-D/software/wiki/index.php/BCal_Beam_Test_Plots%2C_April_4%2C_2007
>>
>>
>> It shows some plots and describes *a* method of extracting the timing
>> resolution from the BCAL without using the tagger timing (we're still
>> working on that). I would urge you to take a look and think about how
>> valid the method is. I also uploaded the ROOT macros used to make the
>> plots. They are linked at the bottom of the page. You should also
>> take a look and make sure I didn't make any mistakes!
>>
>> Let me know what you think.
>> Regards,
>> -David