last update: 29.07.2019
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PICO calibration efforts
The novel technologies employed in dark matter detectors require extensive calibrations in addition to the actual construction and operation of the dark matter search detector. It is only the hardest collisions from the fastest dark matter particles in our galaxy that are detectable, especially if dark matter particles have little mass. Miscalibrations of either the efficiency of the detector can dramatically reduce our ability to discover dark matter.
PICO deploys several bubble chambers for calibrations at above ground laboratories in Chicago and Montreal to calibrate our sensitivity. While each bubble chamber is different, they are all designed to have smaller active volumes, improved temperature control, and less material around the active volume than the dark matter search bubble chambers. This allow us to operate with stronger sources, reduced threshold uncertainty, and with radiation penetrating the active volume at a known energy. We also deploy chambers to test new software, hardware components, or to measure the temperature stability and acoustics of our detectors.
At Fermilab, the COUPP Iodine Recoil Efficiency (CIRTE) experiment exposed a bubble chamber to a 30 GeV pion beam to calibrate the efficiency of iodine recoils in CF3I. The resolution of the CIRTE experiment relied on keeping the amount of material in the beam to a minimum. This bubble chamber has thin walls and a water bath with a beam tube so that scattering of the pions was be minimized. A water bath around the active volume was constantly circulated, and could thermalize with the active volume within a minute. By tracking the direction of the pions as they entered and left the bubble chamber, we could measure the energy of the recoiling iodine nuclei from which most of the pions scattered. The number of bubbles produced was compared to the number of recoils at a given energy and found to be consistent with the maximum efficiency that could be expected from theory.
The CIRTE bubble chamber is currently at the University of Chicago filled with C3F8 to test it's response to low energy neutrons from 88Y/Be and 124Sb/Be sources. Neutrons act similarly to dark matter particles in our detectors; they bounce off nuclei. By exposing a superheated liquid detector to neutrons of a known energy, we can simulate a dark matter signal and measure our detection efficiency. Since we don't know the mass of the dark matter particle we might find, we need to measure our efficiency using a variety of neutron energies. The 88Y/Be and 124Sb/Be sources produce fluorine nucleus recoils with maximum energies of 29 keV and 4 keV respectively. By varying the threshold of our detector, we can try to extrapolate our efficiency other energies, but only if we assume how detector efficiency changes with our calculated threshold.
At the University of Montreal, we have access to a tandem Van de Graff accelerator that hits a very thin Vanadium-51 target with protons. At proton energies above 1.57 MeV, a nuclear reaction can occur to produces neutrons. By changing the proton energy, we can adjust the neutron energy and measure our efficiency. The PICO-0.1 bubble chamber is currently taking data to measure the detection efficiency using these neutrons. This bubble chamber tests a prototype hydraulic control design that PICO-2L now uses and PICO-250L will used.
We also perform calibrations on our dark matter bubble chambers using high energy neutrons. While the nuclear recoils produced by these neutrons are of higher energy than dark matter recoils, this data can confirm that our sensitivity remains stable over time. By studying the probability of these neutrons to bounce many times and produce multiple bubbles, we can obtain a rough in-situ measurement to confirm our dark matter sensitivity calibrations.
Figure 3: PICO-0.1 in action.
Figure1: An example event from the CIRTE bubble chamber. The top plot shows the relative timing of the tracker's trigger and of the acoustic signal, one camera image of the bubble, and the positions of where the pion went through the tracker.
Figure 2: Results from the COUPP Iodine Recoil Threshold Experiment (CIRTE). PRD 88 021101. The data points represent the measured iodine nucleation efficiency as a function of iodine equivalent recoil energy, where the contributions from carbon, fluorine and inelastic scatters have been subtracted. The gradual turn on is predominantly due to the angular resolution of the experiment. The red region shows the expectation from a step to 100% efficiency at threshold with uncertainty bands. The dashed blue curve shows the best fit step function at a slightly higher threshold. The black curve is a fit assuming a slightly slow turnon in efficiency. The inset shows two standard deviations confidence contours for a fit to the slow turnon model with the threshold allowed to float (pink) or constrained by the theory (solid cyan). The colored dots represent the corresponding curves in the main plot.
Figure 4: An event in the PICO-2L dark matter search bubble chamber while exposed to an AmBe neutron source. Can you count the number of bubbles? High multiplicity events such as this one can be used to calibrate our detection efficiency.