Re: Cross talk

["Fernando J. Barbosa" <barbosa@jlab.org>]

Hi Yves,


Yves Van Haarlem wrote:
> Hi Fernando,
>
> About the cross talk: yes of course, but at least the plots show that 
> something is going on. I need to run over the data again for the 
> scatter and that takes some time. The plots I showed I had already 
> available.
OK.
>
> For the peak amplitude plots I go over the fADC time spectra and take 
> the time sample with the maximum ADC value. I (my program) subtract 
> the pedestal mean (650) of this (ADC) value and if it is above 
> pedsigma*5 it is dumped into the peak-amplitude histogram. I do this 
> for every event. By changing the pedestal (threshold cut) the 
> amplitude distribution shape will not change, you will only add/cut 
> away the lowest peak-amplitudes but I can do it anyway.
You don't need to do it as you only take the one sample (maximum 
amplitude) from every event. I was thinking of something else.
>
> I do not understand your proposition to make a coincidence between two 
> adjacent straws. The Fe-55 source is collimated (2*r = 0.5mm) in order 
> to keep the rate down (the VME computer crashes when the rate is 
> higher then 30 events/s) I could point it to a region where two straws 
> are overlapping but do you really expect coincidences? I would think 
> the x-ray is completely absorbed in one or the other straw, not in 
> both. I believe you can also see this in the cross talk plots on the 
> wiki: the source is placed on channel 2 but also pointing to channel 
> 5. The trigger is on channel 2 and I do not see something in channel 5.
>
You are right, I was thinking of cosmics for the coincidence trigger or 
perhaps Sr90 and the coincidence to remove accidentals/background. Did 
you ever get a Sr90 source?

Regards,
Fernando




>
> Cheers,
>     Yves
>
>                           /--------------------------------
>                          /   Yves Van Haarlem
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> On Thu, 19 Jun 2008, Fernando J. Barbosa wrote:
>
>> Hi Yves,
>>
>> I looked at the plots you just posted and I have a  few questions.
>>
>> For crosstalk, you are plotting the peak amplitude distributions for 
>> the whole data run, I believe. You need to compare the peak 
>> amplitudes on an event-by-event basis, e.g., compare peak amplitude 
>> on ch1 to ch2 for event 1, and repeat for the other events. The best 
>> way to do this is to get a scatter plot. Applying a fit to the 
>> scatter plot, should yield a straight horizontal line (or vertical) 
>> [meaning independence between channels] for zero crosstalk or a line 
>> with a slope for any crosstalk. The slope is the crosstalk, say volts 
>> in ch1 to volts in ch2. You will need a plot for each channel 
>> adjacent to the straw with the source.
>>
>> For the raw ADC data, one event per plot, it seems that the noise is 
>> very low and that is a good thing. Now, when you take this data and 
>> plot the amplitude distribution do you apply a threshold cut? For 
>> instance, I see that the baseline or pedestal is at about ADC channel 
>> 650 - do you exclude all the samples below this value? It would also 
>> be interesting to see how the amplitude distributions change with 
>> different  threshold cuts.
>>
>> For the "energy" plots, it seems that these plots follow the same 
>> trend as those plots under  "Different-HV" which is good. But I still 
>> don't understand why the shape - it should look like a Poisson 
>> distribution or more properly a Landau distribution. What I see is 
>> that the low level amplitude tail gets proportionally smaller with 
>> increasing HV, as a result of perhaps higher S/N ratio. This may be 
>> artificial, of course, due to higher occurrence of limited streamer 
>> pulses or higher gain (more signals above the trigger threshold).
>>
>> It would be nice if you could form a trigger by using the coincidence 
>> of two overlapping straws, as I mentioned in a previous email.
>>
>> Regards,
>> Fernando
>>
>>
>>
>>
>> Yves Van Haarlem wrote:
>>> Hi All,
>>>
>>> As a follow up from the meeting this after noon. I posted some 
>>> peak-amplitude spectra on the wiki concerning cross talk. One can 
>>> see that there is some measurable electronic cross talk between 
>>> adjacent channels on the HV-board.
>>>
>>> http://www.jlab.org/Hall-D/software/wiki/index.php/ASIC_design_optimizations#Cross_talk 
>>>
>>>
>>> Cheers,
>>>     Yves
>>>
>>>                           /--------------------------------
>>>                          /   Yves Van Haarlem
>>>        _--~~--_         /
>>>      /~/_|  |_\~\      /      Carnegie Mellon University
>>>     |____________|    /        Department of Physics
>>>     |[][][][][][]|:= /          Wean Hall room 8404
>>>   __| __         |__ \            Pittsburgh, PA 15213
>>>  |  ||. |   ==   |  | \           USA
>>> (|  ||__|   ==   |  |) \
>>>  |  |[] []  ==   |  |   \           Tel.:   +1 412 268-6949
>>>  |  |____________|  |    \                  +1 412 641-9252
>>>  /__\            /__\     \          Fax.: +1 412 681-0648
>>>   ~~              ~~       \-----------------------------------
>>>
>>
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