The first instruments to be used for WIMP searches were solid
state germanium ionisation type detectors [28]. These recorded high-resolution background energy spectra,
which were then compared to the expected WIMP recoil spectra to
establish upper limits on interaction cross-sections (assuming
the Galactic dark matter was indeed made of WIMPs in a
straightforward spherical virialised distribution). Figure
7
shows examples of such spectra [28] and the coherent limits [1
] obtained from a number of experiments of this type. The
background spectra can be coarsely characterised by two
parameters, which are the threshold and the count rate just below
threshold. These approximately determine how low in WIMP mass the
instrument sensitivity extends and how low a cross-section limit
can be set respectively. This can be seen by comparing the Cosme
and Twin curves in the two panels of figure
7
. The difference between the Cosme and Twin background spectra is
due to the use of freshly mined germanium in the production of
Twin, which consequently does not show the cosmogenically
activated line just below 9 keV. An alternative way of
achieving the same suppression of the cosmogenic lines is to use
enriched germanium as done by the Heidelberg/Moscow
experiment [13]. The Sierra Grande curve in figure
7
is from a long exposure germanium experiment in which a search
for both daily and annual modulation has been performed [1
,
2], and the results from the daily modulation search are shown in
figure
8
. No significant signals are seen. An example of an annual
modulation search is shown in the right-hand panel of the figure.
This is actually from a scintillator experiment [16
] and this will be discussed later. The next advance expected
from germanium detectors of this type will be from the Heidelberg
group [12] who are developing a high-purity natural germanium crystal
surrounded by an active veto that also uses natural germanium.
This will exploit the fact that any WIMP scattering events will
be single-site due to the very low scattering cross-section,
while most other background events will be multi-site (e.g.
multiple elastic neutron scattering or multiple compton
scattering for
-rays).
Another advantage of some scintillators over germanium is that
it is much easier to make large mass detectors out of them. This
increases the event rate and makes it feasible to look for any
annual modulation signals, assuming experiment systematics can be
kept under control. This is the approach of the DAMA group [19,
17,
20], who currently have some of the lowest axial and coherent
limits, and who have claimed a positive annual modulation
result [16
] (see right-hand panel in figure
8
and later discussion). Other `simple' scintillators that are in
use include
and liquid xenon. Various other effects in scintillators are
also being studied as a means to provide additional
discrimination against non-nuclear recoil backgrounds. These
include using the ratio of visible to UV light emitted by cooled
undoped NaI [127], looking for directional nuclear recoil effects in
stilbene [128], and using pulse-shape analysis from a mixed scintillator
system(with fine grains of
in an organic liquid scintillator) to take advantage of the
recoil range difference between electrons and nuclei [126].
Figure
11
shows one proposed type of configuration for a two-phase system
in which photomultipliers are used to record two scintillation
signals for each event, S1 and S2 [35]. S1 is the primary scintillation signal from the liquid volume,
which occurs as a direct result of the WIMP/
-ray scattering interaction. In addition to scintillation, the
interaction will also produce localised ionisation in the liquid.
An applied electric field is then used to drift the ionisation
electrons towards and into the gaseous xenon. In the gas there is
a region in which the applied electric field is strong enough to
produce secondary scintillation, or electroluminescence, which
produces signal S2. S1 and S2 are thus separated in time. At low
electric field the S1 signal itself will be amenable to pulse
shape analysis as described above for NaI. The S2 signal
amplitude will depend on how many ionisation charges are drifted
into the gas volume. This will depend on how many are produced in
the initial interaction and on what fraction of those immediately
recombine. The level of recombination is expected to be higher
for events with a higher linear energy density deposit
dE
/
dx, and so nuclear recoil type events are expected to show a much
lower fraction of surviving drifting electrons. Hence, the ratio
of S2 to S1 should be much lower for nuclear recoils compared to
say
-ray deposits of the same amount. This effect has been
demonstrated in low field operation [35
,
69], and the left-hand panel of figure
12
shows some results from the chamber of figure
11
. A 30 kg detector is being constructed [35
] in which nuclear recoil events are identified by the lack of a
secondary signal. An alternative scheme uses high-field operation
in which ionisation from nuclear recoils can also be seen, and in
which discrimination relies on the finite ratio of S2 to
S1 [7]. This should give much higher background rejection and a
8 kg instrument is underway [135].
The potential discrimination power available using the various
techniques can be described by a figure of merit [117] as shown in figure
13
. The top curves show the situation using pulse shape
discrimination in NaI, and the two lower curves then show what
improvement might be expected from using pulse height ratios from
cooled NaI (UVIS) and a two-phase xenon system. In this figure,
the performance improves as the figure of merit decreases and the
potential advantage of liquid xenon over NaI is significant.
A variant on the above scheme is to try to `image' the
ionisation charge distribution using TEA (or TMA) added to the
liquid xenon, which will convert scintillation photons into
electrons [142,
101]. The idea here is that for nuclear recoil events there will be
relatively few direct ionisation electrons left, due to the high
dE
/
dx, and most drifting electrons will have been produced by photon
absorption in the TEA/TMA. This should give an exponential
spatial distribution (scale length around 2 cm) of electrons
drifting into the gas region. Whereas, for background
-rays, there will be a significant core of electrons left over
from the primary interaction in addition to those created by
photon absorption, giving a more centrally peaked image.
If semiconductor target materials are used, it is possible
also to extract ionisation signals from bolometer
experiments [121,
33]. Nuclear recoils produce less ionisation compared to thermal
energy than x-ray and
-ray background events. For events initiated well away from
surfaces, this allows for good discrimination power. Surface
events, from external electrons for example, can be problematic
as the ionisation can be inefficiently collected compared to the
thermal energy, which mimics nuclear recoil signals. The
ionisation signals are collected using charge-sensitive
preamplifiers in the usual way for semiconductor diodes.
If scintillator target materials are used it is possible also
to extract scintillation signals [39]. The situation is analogous to the simultaneous ionisation
measurement in that nuclear recoil events are much less efficient
at producing ionisation and excitation than typical background
events. In this case it is even possible to use SPTs deposited on
light absorbers (e.g.
silicon) as the scintillation signal channel [39].
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Experimental Searches for Dark Matter
Timothy J. Sumner http://www.livingreviews.org/lrr-2002-4 © Max-Planck-Gesellschaft. ISSN 1433-8351 Problems/Comments to livrev@aei-potsdam.mpg.de |