Gamma Calibration Using A New Test Apparatus At Queen’s University And Optimization Analyses For The PICASSO Experiment
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Authors
Levy, Cecilia
Date
2009-09-26T20:33:23Z
Type
thesis
Language
eng
Keyword
PICASSO , Dark Matter
Alternative Title
Abstract
The PICASSO experiment located 2 km underground in SNOLAB is directly
searching for dark matter signals by looking for interactions between dark matter
particles and an active target made of superheated droplets of freon C4F10. During
the interaction, energy is deposited to the freon triggering a phase transition,
inducing pressure waves which are detected by piezo-electric sensors.
A temperature dependent analysis of the amplitudes of the signals for detector
71 showed that, above 25 ◦C, between 20 and 80 % of the events were saturated
implying that the preamplifiers had too high a gain. Decreasing this gain by a fixed
factor was not found to be a suitable solution to the problem. Ideally, a temperature
dependent gain should be established. In addition, some channels have intrinsic
problems and should be repaired.
A threshold analysis was used to establish the trigger efficiency which was found
to be 90% above 25 ◦C but only 50% at lower temperatures with the current setting
of the threshold. A temperature dependent threshold setting has been proposed.
A new setup at Queen’s University has been built and a gamma calibration using
three different radioactive sources (22Na,137Cs,57Co) was undertaken leading to a
new detector response curve for gammas. For a proper analysis, new and more
appropriate cuts were implemented. The analysis confirmed the expectation that
the PICASSO detectors are mostly blind to gammas below 50 ◦C. However, the
detectors appear to be more sensitive to 122 keV gammas than to 622 keV gammas
by a factor of about 10. The sensitivity for 22Na also differs by a large factor from
what was expected from old calibrations on detectors with much smaller bubbles.
The rate plots exhibit a strong exponential increase in rate above 40 ◦C which is not
due to any of the gamma sources used, but could be due to neutrons or low energy
x-rays. This remains under investigation.
Description
Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2009-09-24 18:28:21.308
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