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 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:
