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Re: GlueX-doc-926




Elton:

Actually, the way you should treat pedestals depends very much on the
goals that you chasing in your analysis. If the counts in your ADC
spectrum correspond to the energy depositions of the real particles, and
IF YOUR PEDESTAL IS NARROW ENOUGH, the event-by-event subtraction of the
mean pedestal value is perfectly correct. If the pedestal distribution is
wide, the simple event-by-event subtraction of the mean pedestal value
will lead (most probably) to the measured energy distortion, but (in the
most cases) you have nothing else to do.

Our goal is quite different: We suspect that our center-panel ADC spectrum
contains both real-particles amplitudes AND some "pedestal" contamination
(viz., "empty-SiPM" events) that leaks through not-very-strong analysis
cuts (our choise in the purity/statistics trade). Our analysis task is to
answer the question: "What will be the ADC spectrum shape after removal of
the "pedestal contamination"? To have such an answer, you must subtract
normalized "pedestal" spectrum from the "real particles+pedestal-mixure"
spectrum (i.e., you should remove from the spectrum NOT just the constant
shift but the SHAPE). And this is very important, otherwise you will be
confused by the remaining half (above the mean pedestal value) of the
pedestal contamination.

Surely, in your result of "shape subtraction", the "zero" amplitude will
be not located in "zero" histogram channel, so you might want to make this
additional shift on the mean pedestal value. Or you may not do this final
shift, and just keep in the mind that "zero" amplitude sits (lets say) in
the channel 90, and make your conclusions accordingly (viz., you should
study that is the separation of your spectrum from the channel 90). I do
not think that this last shift is a "matter of of life and death" IN SUCH
AN ANALYSIS.

Thank you,
Andrei



On Thu, 13 Dec 2007, Elton Smith wrote:

>
> Hi George,
>
> I have a question about your report on Si PMs regarding Fig 11. It seems
> that you have taken the middle spectrum and subtracted the fitted shape of
> the pedestal from the top spectrum to produce the bottom spectrum. The
> resulting spectrum shows a displacement from the origin. But this is not
> the way one should subtract pedestals. These need to be subtracted event
> by event (not in a distribution) and the result will be a spectrum
> identical to the middle one but with the peak (currently approximately in
> channel 90) centered on zero. This distribution does not show a clear
> separation of signal from pedestal.
>
> Am I missing something?
>
> Thanks, Elton.
>
>
>
>
> Elton Smith
> Jefferson Lab MS 12H5
> 12000 Jefferson Ave
> Suite # 16
> Newport News, VA 23606
> elton@jlab.org
> (757) 269-7625
>
>