Cryogenic Charged Particle Detectors

Conventional charged particle detectors work on the principle of collecting electric charges induced by the passage of a charged particle through either an ionizing medium, such as a gas in a gas ionization chamber, or by passage through a semi-conductor in which electron-hole pairs are produced.  Subsequent amplification of the charges results in an electronic signal, the voltage of which is measured and is proportional to the energy of the chaged particle.

The group is exploring the development of a new type of charged particle detector, different from silicon and gas ionization detectors.  Instead of collecting charge in through the processes described above, our devices generate their electronic signal by phonon collection.  The detectors consist of a substrate of sapphire crystal, 300 microns thick, onto which are manufactured 4 arrays of super-conducting Josephson junctions (STJ).  These were manufactured using the techniques of photolithography, in collaboration with collaborators at the RIKEN national laboratory, Japan.

Under operating conditions, the detector is cooled down to 300 mK.  When an energetic charged particle enters the crystal, some fraction of its kinetic energy is convered into heat.  Within a crystal, heat is manifested as vibrations of the lattice, called phonons.  The STJ arrays interact with the phonons.  When a phonon arrives at an STJ, it is capable of breaking a Cooper pair within the STJ.  When broken, these are, crudely speaking, something analogous to the electron-hole pairs formed within a conventional silicon detector.  The electrons from the broken Cooper pair form a voltage pulse which, like in the case of a conventinal detector, has a height proportional to the energy deposited in the crystal by the charged particle.  However, unlike a silicon detector, the energy required to break a Cooper pair is 100 to 1000 times smaller than the few eV required to produce a particle-hole pair in silicon.  For this reason, the number statistics of charge carriers from an STJ device is ≈10 to 30 times higher than that of silicon, and thus the energy resolution of these devices should be significantly better than conventional detectors.

The Detector