GEMTPC Resolution Studies in Magnetic Fields

1
GEM-TPC Resolution Studies in Magnetic Fields
Laser System
University of Victoria / Victoria Canada D.
Karlen, T. Michailopoulos, C. Nell, P.
Poffenberger, G. Rosenbaum
The Time Projection Chamber

• 30cm drift
• Double GEM amplification
• Quartz window for UV laser
• STAR front end electronics

Our laser delivery system has been designed to
deliver either one or two beams into the TPC.
The beam positions in the transverse and
longitudinal directions are controllable remotely
and independently.
Movable mirror
Single Track Laser Scans
Laser Tests of Resolution Estimates
To determine the single row resolution in cosmic
ray events, first a track fit is done which is
used as a reference track. All of the parameters
except for the transverse position of the this
reference track are fixed while a track fit is
done using only the row being studied. The
distribution of residuals between the single row
fit and the reference track are fit to a Gaussian
to estimate the standard deviation. If the row
being studied is included in the reference fit
the resolution is too optimistic if it is not
included the resolution is too pessimistic.
Resolution is taken to be the geometric mean of
the standard deviations of the two
distributions. This method is checked using laser
tracks, first using the above method and then
using the method described below. The two
results are in very good agreement Example (with
2mm pads)
Laser Two Track Resolution Studies
Two track resolution is measured by putting two
parallel beams into the chamber at the same drift
distance. By adjusting the transverse separation
of the two beams the two track resolution can be
determined. Each beam is sent in independently
and the mean and standard deviations of the track
coordinates are measured. The beams are then
sent in together and the mean and standard
deviations of the coordinates for each of the two
tracks are found and compared to the original
results.
Track separations of 2mm 4mm showed no bias and
only a slight degradation of resolution. For
separations greater than 4mm no degradation was
seen.
Cosmic Ray Track Resolution
With 2mm x 7mm pads, the resolution was found to
be better than 100 ?m for all drift distances at
magnetic fields greater than 1.5 Tesla, with a
good agreement between Monte Carlo simulation and
data. The MC simulation however, suggests an
improvement in resolution for fields greater than
1.5T which was not seen in the data. A pad array
with narrower (1.2 mm wide) pads was constructed.
This pad array produced an improvement in
resolution.
reference fit excludes row 95.2 ?m reference fit
includes row 61.6 ?m
resolution (geometric mean) 76.6 ? 0.6 ?m
The second estimate of the resolution comes from
fitting the distribution of track coordinates for
repeated events with the laser at a fixed
location.
Results for P5 gas
2 mm pads
8 row fit 28 ?m resolution single
row resolution (8)1/2 x 28 ?m
Open circles Monte Carlo Closed shapes Data
Mean 6.7751 mm Sigma 0.027760 mm
transverse resolution (mm)
B 0 tesla
1.2 mm pads
B 0.9 tesla
resolution 78.5 ?m
B 1.5 tesla
300 mm
30 mm
drift time (50 ns bins)
With 1.2mm pads resolution of 80 ?m achieved