[Hps-ecal] HPS Ecal signals parametrization

Fri Apr 11 03:38:48 EDT 2014

Sho,

I think the data speaks for itself, the 3-pole is much better! In 
particular the edge of the distribution will be much better, that is 
important to reproduce the time precision of the experiment in the 
monte-carlo! That might impact trigger algorithm, so it is critical not 
to miss this part of the distribution!

Best regards,

Le 04/10/2014 11:18 PM, Sho Uemura a écrit :
> Realized this discussion didn't make it to the ECal list.
>
> Would like more ECal experts to weigh in. I think we need to decide by 
> the end of the month for a parametrization to use in the mock data.
>
> On Fri, 4 Apr 2014, Sho Uemura wrote:
>
>> Mean chisq/DOF is 0.56 for 3-pole, 0.68 for double-Gaussian.
>>
>> http://www.slac.stanford.edu/~meeg/ecalpulsefit/3pole/rchisq.png
>>
>> http://www.slac.stanford.edu/~meeg/ecalpulsefit/2gaus/rchisq.png
>>
>> I think that in every fit the 2-Gaussian shape overestimates the 
>> rising edge; 3-pole looks better. As Andrea says, the pulse peak is 
>> definitely better fit by 3-pole.
>>
>> On Thu, 3 Apr 2014, Sho Uemura wrote:
>>
>>> chisq/dof plot attached - I'm using TGraph to fit, which assumes 
>>> errors of 1 mV per data point. I use the time range (-200, 60).
>>>
>>> On Thu, 3 Apr 2014, Gabriel CHARLES wrote:
>>>
>>>> Could you both provide an average value of chi square that the 
>>>> different parametrization can be compared easily, please ?
>>>>
>>>> Also, from the simulation it appears that the rising edge could be 
>>>> present. In attachment you will find a picture with two plots. The 
>>>> top one corresponds to the signal after the crystal and the APD, 
>>>> that is the input of the preamplifier.
>>>> It is obtained by the convolution of the signal of the crystal and 
>>>> the APD. The crystal response is composed of the sum of two 
>>>> decreasing exponential governed by different time constants. The 
>>>> APD transfert function is given by the bottom plot (sorry for the 
>>>> wrong Y axis units).
>>>>
>>>> There is no reason for the preamplifier to reduce the tail.
>>>>
>>>> I think that if there is no huge difference between the chi square 
>>>> it would be better to keep the two gaussian function.
>>>>
>>>> ---
>>>> Gabriel CHARLES
>>>> Institut de Physique Nucléaire d'Orsay
>>>>
>>>> On Thu, 3 Apr 2014 13:15:00 -0700 (PDT), Sho Uemura wrote:
>>>>> I tried two more parametrizations. These are parametrizations
>>>>> commonly used for the APV25 preamp that we use in the SVT.
>>>>>
>>>>> CR-RC: t*exp(-t/tp)
>>>>> 3-pole, or CR-RC-RC: t^2*exp(-t/tp)
>>>>>
>>>>> 3-pole seems to fit well, I think better than the asymmetric
>>>>> Gaussian. CR-RC seems no better than the Gaussian. Other
>>>>> parametrizations I tried (variations on CR-RC or 3-pole using more
>>>>> than one time constant) were degenerate with CR-RC or 3-pole, so I
>>>>> didn't include those plots.
>>>>>
>>>>> Plots attached are for 3-pole function. All plots for 3-pole and
>>>>> CR-RC, and the pyroot scripts I used, are online:
>>>>>
>>>>> http://www.slac.stanford.edu/~meeg/ecalpulsefit/
>>>>>
>>>>> I also see what you see, where there are 2 clusters in the
>>>>> distribution of shape parameters. I chose the center of the larger
>>>>> cluster (with the faster time constant) and refit all the events with
>>>>> this time constant fixed; those plots are named "fit2" and as 
>>>>> expected
>>>>> they fit the faster pulses well and the slower pulses poorly.
>>>>>
>>>>> More data will help.
>>>>>
>>>>> I plotted the three parametrizations we have, see plot4.pdf attached.
>>>>> If we agree that the Gaussian has an unphysical rising edge, I think
>>>>> we should use 3-pole.
>>>>>
>>>>> On Tue, 1 Apr 2014, Andrea Celentano wrote:
>>>>>
>>>>>> Dear all,
>>>>>> here are some results about HPS Ecal signals parametrization.
>>>>>> I took data with the crystal placed vertically, APD gain 150, 
>>>>>> room temperature. I put a threshold ~ 20 mV to keep only big 
>>>>>> enough signals, out of the noise.
>>>>>> I acquired data with a 2.5Gs/s oscilloscope, 1 GHz bandwidth, 50 
>>>>>> Ohm input impedance.
>>>>>>
>>>>>> I used the same* configuration employed at JLab for cabling: 8m 
>>>>>> 3M cable ---> passive splitter ---> 3m lemo cable.
>>>>>>
>>>>>> *actually I employed an 8 meters 3M cable instead of 7m because 
>>>>>> the latter is not available here in Genova.
>>>>>>
>>>>>> Attached you find a postcript file with the results. (outGood.ps 
>>>>>> shows the fit results covering some parts of the signal, 
>>>>>> outGood1.ps no)
>>>>>>
>>>>>> - Neglect first blank page
>>>>>> - Pages from 2 to 32 are the 31 signals I got, with superimposed 
>>>>>> the fit performed with the two-gaussians parametrization. Each 
>>>>>> chi2 fit is performed independently.
>>>>>> Signals are in mV and ns.
>>>>>> Note that near ~ 100 ns there is probably a reflection due to 
>>>>>> some impedance mismatch in the cables chain.
>>>>>> However,  I am not using those points to fit. I am fitting the 
>>>>>> data in between -200 ns and +80 ns. The function is then plotted 
>>>>>> in the full time range.
>>>>>>
>>>>>> - Last page is a summary of the fits performed. Two 1d-histograms 
>>>>>> are the distributions of the two time constants used in the 
>>>>>> parametrization. Then I am plotting also their correlation, as 
>>>>>> well as the correlation of the rise-time (par[1]) with the signal 
>>>>>> amplitude (from the fit).
>>>>>>
>>>>>> I noted that the fit parameters Trise, Tfall are not distributed 
>>>>>> as two gaussians. In particular, for Trise there is an 
>>>>>> accumulation of events at ~ 5 ns and ~ 7 ns, correlated with 
>>>>>> corresponding Tfall at ~ 15 and ~20 ns. Actually, I see that, 
>>>>>> other than the amplitude, signals do not have always the same 
>>>>>> shape: look, for example, at signals n.5 and n.6 (ps pages n.5 
>>>>>> and n.6).
>>>>>>
>>>>>> Attached you find also the C implementation of the signal 
>>>>>> parametrization, in form of a "double fun(double *x,double  
>>>>>> *par)" used by ROOT when fitting trough TF1.
>>>>>> Finally, I am attaching also the raw data for the 31 signals I 
>>>>>> got, so if you're interested you can play with different signal 
>>>>>> parametrizations.
>>>>>>
>>>>>> I am planning to take more data these days.
>>>>>>
>>>>>>
>>>>>> Bests,
>>>>>>
>>>>>> Andrea
>>>>>>
>>>>
>>>> ######################################################################## 
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>
>
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-- 

Raphaël Dupré
Institut de Physique Nucléaire d'Orsay
Université Paris-Sud

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