[Halld-tagger] active collimator signal vs translation stage displacement

[Hovanes Egiyan]

Hi Richard,

thanks for the plot. Indeed, this is the plot I asked about. I have a 
couple questions about it:

o How are the error bars calculated? Are these the spec uncertainties on
the ADCs and amplifiers propagated into this signal ratio?

o What is the origin of the vertical shift of the curve, and apparently 
a scale
change as well? The limits are not -1 and +1, which means that the 
zero-crossing does not
indicate the beam position.  I assume this can be calibrated out. But then
if these signals are not well calibrated,  can the 60Hz, 120Hz etc 
oscillation components that you see
in the FFT be interpreted as fast beam current fluctuations resulting in 
the asymmetry variations
rather than fast position motion?

o Is this the finest step scan that was done?

o My understanding of the sensitivity of this device to the beam 
position was the
uncertainty on the location of the "zero-crossing" of a linear fit  in 
the central region
(inversely related to the uncertainty of the gain in the central region).
Do you have an estimate for that number from a simple fit of this curve 
in the
central region?

Hovanes.



On 10/31/2011 02:58 PM, Richard Jones wrote:
> Hovanes and all,
>
> I have attached a plot of what I think you wanted to see: the 
> difference over the sum of currents in the two inner wedges as a 
> function of displacement of the translation stage carrying the active 
> collimator across the beam.  Notice that in the central linear region 
> of high sensitivity, the error bars are much larger than outside this 
> region, where the sensitivity to position decreases.  The error bars 
> in the low slope regions on either side of the plot are a good 
> indication of the intrinsic resolution of the device.  The gain of the 
> central linear region is about 1.8/cm.
>
> In this plot you can see slow drifts in the beam position as bumps and 
> wiggles in the otherwise smooth transition from the region of -1 to 
> +1.  I estimate the intrinsic resolution at 15nA electron beam current 
> on a 10^-4 radiation length target to be less than 50 microns at 600 
> Hz bandwidth.
>
> -Richard Jones
>
>