Laser calibration of  SVT

                              Jana Bielcikova, Yale University

The aim of my study is to use time changes of the laser spot positions to remove temperature variations
coming from a non-uniform temperature cooling of  SVT.

We have 3 laser spots located at:



Fig.1 Anode (top) and time (bottom) position of the laser spots. Left side corresponds to B3L15W7,
right side to B3L7W1.

P.S. You can notice that in hybrid 2 ( B3L7W1) (right panel), the anode where is the laser shining to is dead and
the signal is collected on the two neighboring anodes, but it has still the same timebucket position.


Time variations of the laser spots position

First, I have looked at the change of the radial position of  the laser spots with event number
and there is indeed an oscillation present.


Fig.2 Radial position of the laser spots vs event number (top B3L15W7, bottom B3L7W1).

But these are not really good variables to look at, let's have a look at the change in the drift
distance of the laser spots with time. The drift distance I have obtained from the transformation
routine StSvtCoordinateTransform::GlobaltoLocal  in StSvtCoordinateTransform.cc
The time of each event is stored in the StEvent as a number of seconds since January 1, 1970.
Below you can see the change of the drift distance with time.


Fig. 3 Drift distance of  laser spots vs time.


Peak-to-peak the laser spot drift positions are changing by about 60 microns.

Then I have tried to correct the time variations of the drift distance by varying the drift velocity.
In order to avoid fluctuations, I keep a running average over last 10 events to calculate the new
drift velocity. I use one of the laser spots at B3L15W7 around timebucket 96 to calculate the
drift velocity and apply it to other to spots. Here is the result:



Fig. 4 Drift distance of  laser spots vs time. The laser spot from the middle figure was used to correct
for the temperature changes.


Fig. 5  Drift velocity changes with time