[Hps-ecal] HPS Ecal signals parametrization

[Andrea Celentano]

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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