The superheated droplet technique
The degree of metastability of a superheated liquid, shortly called superheat, depends on the difference of the temperature dependent vapor pressure and the applied external pressure. At a given temperature, bubble formation on the track occurs, if within a region of critical size lcrit the deposited energy, Edep, exceeds a threshold energy Emin
In this relation dE/dx is the mean energy deposited per unit distance. Although this detector is a threshold device, recoil energy spectra can be recorded by ramping the temperature and thus varying the detector threshold.
Since large specific energy losses are characteristic for nuclear recoils, the operating conditions can be tuned such that the detector is fully sensitive to nuclear recoils, but essentially blind to gamma rays and cosmic muon induced events with small dE/dx. For example when operating such a device at a recoil threshold of 5 KeV, gamma-ray induced events are rejected by more than a factor of ten million! This is a very important aspect for dark matter detection, since dark matter induced events are very rare and all kinds of precautions have to be made to assure suppression of any parasitic radiation present in our detector environment.
This feature of "background blindness" of droplet detectors is also exploited in neutron spectroscopy and for personal neutron dosimeters, since neutron induced recoils can be detected unambiguously even in presence of a large gamma radiation field (see e.g. Bubble Technology Industries).
In contrast to bubble chambers used in high energy physics, which are based on the same principle, droplet detectors are basically continuously active, since only one droplet at a time undergoes phase transition. Only occasionally, e.g. every 30 hours the detector medium is set under pressure in order to transform gas bubbles back to liquid droplets. Moreover droplet detectors can be calibrated easily at high count rates with radioactive sources.
...and here is how a PICASSO detector looks like!
Fig.3: A liquid is in an unstable, superheated state when it is kept at a temperature above its boiling point and when its vapor pressure exceeds the ambient pressure.
Fig.4: An ionizing particle triggers a phase transition in a superheated liquid if it deposits an amount of energy larger than the critical energy Ecrit over a critical lenght lcrit .