Gamma Calibration Using A New Test Apparatus At Queen’s University And Optimization Analyses For The PICASSO Experiment
MetadataShow full item record
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.