Re: BCAL timing

[David Lawrence <davidl@jlab.org>]

Hi Blake,

    Nice! Can you give us an update on the BCAL analysis at next 
Tuesday's software meeting?

Regards,
-David

Blake Leverington wrote:
> Hi David,
>
> I did a quick fit to your corrected resolution_#deltat vs Ephoton plot I.
>    TF1 *f1 = new TF1("f1","sqrt(([0]*1/sqrt(x/1000))^2 + ([1])^2)  
> ",150,650); The result is in the plot with the energy dependent term 
> being 152ps and the constant term is 53 ps. This is for the sigma of 
> (tn8+ts8)/2.0 - (tn7+ts7)/2.0 so the resolution is sqrt(2) smaller 
> than that as I understand it for reading one end.
>
> -Blake
>
> David Lawrence wrote:
>>
>> 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
>
> ------------------------------------------------------------------------
>

-- 

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  David Lawrence Ph.D.
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