[ Mass Spectroscopy ] [ Fracture Processes ]

The CRESST detector development has led to two interesting by-products enabled by the unique properties of low temperature detectors. Both of these were suggestions originating from our group.

Mass Spectroscopy

The first one, suggested in 1991, and under continuous development since then by various groups, concerns the investigation of large complex molecules, such as proteins, by mass spectrometry. Study of such macromolecules is an important tool in biology. In such studies the molecule is typically accelerated to an energy of some tens of keV. Due to the great mass of the molecule (usually that of many thousands of hydrogen atoms) and the fact that energy is proportional to the mass (E = 1/2·M·v2) the molecule is moving rather slowly compared to a particle or nucleus with the same energy.
Now most, if not all, familiar detectors have as their basic initiating process the interaction of the projectile with an electron in the detector. However, according to basic atomic physics, interactions with electrons depend on the velocity of the projectile in such a way that they become inefficient at low velocity. This leads to the situation that for large, slow biomolecules the detection process stops working well and there are problems for mass spectrometry.
However with the cryodetector the situation is different. Being a thermal detector, it responds simply to energy directly, and not to velocity. Thus in principle a 20 keV electron or a 20 keV protein molecule looks the same to the cryodetector. Remarkably, this simple observation seems to actually work well in practice. Furthermore, due to certain technical advantages the cryodetector promises a higher accuracy. Furthermore, the measurements are sensitive to single molecules. Results (European Journal of Mass Spectrometry 10, 469-476 (2004)) attained sensitivities below 1 attomole absolute amount on target with proteins in the 6000 hydrogen atom mass range (Insulin).

A view in the lab

A view in the lab with the mass spectrometer setup

Fracture Processes

Another by-product of the CRESST Dark Matter search concerns the study of fracture processes in materials. In the early stages of running of the first CRESST detectors, unexpected signal pulses were seen. Their origin was traced to fracture events in the sapphire detector crystals. This was due to very tight clamping with hard contacts, and these pulses were soon eliminated by the use of somewhat softer supports. During this investigation, there were extensive runs recording the energy and time of each event. This provides large low background data sets, recording many microfracture events with high sensitivity. The energy threshold of some keV corresponds to the breaking of only a few hundred covalent bonds in the sapphire crystal, a sensitivity orders of magnitude greater than that of previous technique. This suggests the possible development of a technology capable of studying microfracture at the atomic level.
Study of the microfracture events reveal a number of interesting features. Their energy spectrum is like that for earthquakes and the time series of the events shows features of fractal statistics. Two papers have been published:
  • Physics Letters A 356 (2006) 262-266
  • Nuclear Instruments and Methods in Physics Research A 559 (2006) 754–756; LTD-11 Conference Proceedings
  • See also our comment physics/0612081.


Picture taken by conformal microscopy; credit Jussi Timonen (email: )

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