Re: Bcal read out options.

[Elton Smith <elton@jlab.org>]

HI George,

Thanks for your comments. Elton.




Elton Smith
Jefferson Lab MS 12H5
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elton@jlab.org
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On Fri, 17 Jul 2009, Georgios Voulgaris wrote:

> Hi Elton ,
>
>     I had a first reading of the document 1300 and I would like to comment about
> timing , paragraph 4.3 .
>        When we have signals with high dynamic rage, timing resolution is limited
> because of "slewing" i.e. a big pulse reaches threshold earlier then a small pulse,
> assuming that both of the pulses have the same rise time. There are several remedies
> to this:
>     a) Offline correction .
>             i) We can have a parametrization of time vs. pulse high and use tis to
> correct the timing.
>             ii) If we use Flash ADC we can fit the pulse data with a curve and then
> use the 50% point for timing.
>     For both cases the correction deteriorates as P.H. decreases. Unfortunately I
> can't give an estimate for this correction because its been a long time since I have
> used them.
>     a) Hardware:
>                 i) Modern leading edge discriminators can go down to 3 or 5 mV
> threshold so there is no need for extra amplification. ( Caen N844 ). One has to
> make sure that noise level from all sources is vary small.
>                 ii) Constant fraction discriminator, ( internal pulse forming and
> discrimination). These devices have rate limitation, they are used  more often in 
> Nuclear  physics,  but  it may be worth the trouble  to test  them. (Caen N842 or
> similar).
>                 iii) Amplification  plus pulse shaping so that a short pulse with 1
> ns rise time is created. One has to look for the electronics or develop them. Mostly
> "home brew" electronics.
>         Of course there are other reasons that affect the timing:
>        Size of the shower, hadrons or delta rays that can confuse the analysis. I
> believe that a FADC is needed to resolve  this  issue.
>           I suppose that several of these issues have been investigated already.
>
>           I will try to connect to the meeting, but usually we end up with a "broken
> telephone", so I'd rather use e-mail.
>
> Best Regards,
> George Voulgaris.
>
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