The Picasso Experiment

As we have seen, droplet detectors are threshold counters where each individual superheated droplet acts as an independent bubble chamber detector. This threshold depends on temperature and pressure. At higher temperatures, the energy threshold for nuclear recoils is lower.  We determined the precise dependence of the recoil energy threshold on temperature and pressure through extensive calibration runs with mono-energetic neutron beams at the Montréal tandem accelerator facility.
It was found that the efficiency to detect nuclear recoils with energy Er at a given temperature is not a step function, but rises gradually from threshold to full efficiency. Fig.1 shows the temperature dependence of the energy threshold for 19F recoils in superheated C4F10 for two detection efficiencies (50% and 90%) at ambient pressure (i.e. 1.23 bar at the depth of the underground laboratory).
Since PICASSO detectors are operated in the temperature range from 47o down to 20o, this translates into sensitivities to 19F recoils from 6 to 500 keV. Neutralino induced recoil energies are expected to be smaller than 100 keV and therefore become detectable above 30oC.
Figure 1: Evolution of the energy threshold for 19F recoils in C4F10 as a function of temperature. At around 15oC the detector becomes sensitive to alpha particles from U/Th contaminations. Above 55oC recoils with energies below one keV can be detected, but at the same time the detector becomes sensitive to gamma-rays and minimum ionizing particles. 


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