5.4 Neutralino direct detection techniques

5.4.1 Ionisation detectors

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Figure 7: Background energy spectra for two Ge detectors of the PNL/USC/Zaragoza group taken from [28Jump To The Next Citation Point] (a – upper panel). Coherent cross-section upper limits from Ge detectors taken from [2Jump To The Next Citation Point] (b – lower panel).

The first instruments to be used for WIMP searches were solid state germanium ionisation type detectors [28Jump To The Next Citation Point]. 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 7View Image shows examples of such spectra [28] and the coherent limits [2Jump To The Next Citation Point] 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 7View Image. 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 7View Image is from a long exposure germanium experiment in which a search for both daily and annual modulation has been performed [2Jump To The Next Citation Point1], and the results from the daily modulation search are shown in Figure 8View Image. 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 [16Jump To The Next Citation Point] 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 γ-rays).

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Figure 8: Background rate from 428.1 days of data binned in 10-minute intervals and folded to look for daily modulation [2] (a – upper panel). Results of an annual modulation search using ∼ 4 years of data from the DAMA experiment [16Jump To The Next Citation Point] (lower panel).

5.4.2 Scintillation detectors

The next most common type of instrument in use is the scintillator, either as a solid crystal or as a liquid. NaI has been used most effectively to date. The predominantly non-zero nuclear spin of both natural Na and I make these detectors much more sensitive to axial couplings than natural germanium. While their energy thresholds and resolutions tend to be significantly worse than for germanium detectors, scintillators offer an additional discriminatory power in that the individual scintillation signals from nuclear recoil type WIMP interactions are expected to show a different time profile from that of the background. This has been studied in some detail by various authors [1155]. Figure 9View Image shows measured comparative differential pulse shapes from the UKDMC NaI group [105Jump To The Next Citation Point118Jump To The Next Citation Point]. Figure 10View Image compares γ-ray and neutron induced nuclear recoil calibration time constant distributions (using simple single exponential fits to each pulse) with the background obtained from their working experiment. The closer statistical match of the measured background to the γ-ray distribution allows the upper limits to be reduced accordingly.

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Figure 9: Differential pulse shapes from NaI for various radiation types. There is a clear difference between the functional form for high dE ∕dx interactions, such as nuclear recoil and alpha tracks, and γ-ray induced electron tracks.
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Figure 10: Differential time constant distributions from the UK NaI experiment [105118] showing the measured background (solid line + data points), and neutron and γ-ray calibration distributions.

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 [201918Jump To The Next Citation Point], who currently have some of the lowest axial and coherent limits, and who have claimed a positive annual modulation result [16Jump To The Next Citation Point] (see right-hand panel in Figure 8View Image and later discussion). Other ‘simple’ scintillators that are in use include CaF2 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 CaF2 in an organic liquid scintillator) to take advantage of the recoil range difference between electrons and nuclei [126].

5.4.3 Hybrid detectors using both scintillation and ionisation

All the previous techniques make use of only one diagnostic signal channel, i.e. pulse shape discrimination, annual modulation, pulse height ratio, or directional dependence. A technique that makes use of two distinct signal channels using a two-phase (liquid/gas) xenon detector has been demonstrated and is under development by the UKDMC group. Xenon is particularly attractive as a dark matter detector target for several reasons. It has a nuclear mass that is well matched to the preferred neutralino mass range. It scintillates in both the liquid and gas phases. It has a useful electron drift lifetime in both liquid and gas phases and can be used in a proportional ionisation mode. Two separable isotopes exist, one with spin and one without. However, it does have some disadvantages, such as: one needs a high level of purity, liquid xenon is more difficult to handle than a crystal scintillator, its scintillation signals are well in the UV (∼ 1750 Å), and its scintillation signals are very fast (< 50 ns).

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Figure 11: The two-phase xenon test chamber used by Wang [35Jump To The Next Citation Point].

Figure 11View Image 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 [35Jump To The Next Citation Point]. S1 is the primary scintillation signal from the liquid volume, which occurs as a direct result of the WIMP/γ-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 γ-ray deposits of the same amount. This effect has been demonstrated in low field operation [35Jump To The Next Citation Point69], and the left-hand panel of Figure 12View Image shows some results from the chamber of Figure 11View Image. A 30 kg detector is being constructed [35Jump To The Next Citation Point] 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].

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Figure 12: Relative signal amplitudes for γ-rays and neutrons for two hybrid type experiments. On the left is shown the primary to secondary scintillation signal amplitudes for a two-phase xenon instrument [35]. Neutrons have large S1/S2 ratios. The right hand panel shows the ionisation versus phonon performance of the CDMS germanium bolometer [3Jump To The Next Citation Point]. γ-rays populate the plot above the solid line with nuclear recoils below it. The circled points are experimental data thought to originate from neutron recoils.

The potential discrimination power available using the various techniques can be described by a figure of merit [117Jump To The Next Citation Point] as shown in Figure 13View Image. 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 [142100]. 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 γ-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.

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Figure 13: Relative figures of merit for the discrimination potential in NaI, cooled NaI and two-phase liquid xenon [117]. In this plot a lower figure of merit implies proportionately better performance.

5.4.4 Phonon detectors – bolometers

Most of the energy imparted to a recoiling nucleus during a WIMP scattering will ultimately end up as phonons. These can be detected as a temperature rise. For crystalline target materials the specific heat at low temperatures varies as T 3, so the lower the temperature the greater the temperature rise as ΔT ∝ 1∕T 3. In principle this should yield very good energy resolution limited by the statistical fluctuations in the numbers of phonons produced. At a temperature of 20 mK, a 1 kg detector could achieve 100 eV resolution, with a correspondingly low threshold. However, in practise, the resolution is limited partly by the efficiency of the phonon ‘cooling’ process, whereby the initial non-thermal phonons with energies of 10–3 to 10–2 eV become degraded into thermal phonons of around 10–5 eV. Once thermalised, the phonons then need to be coupled into the temperature sensors, which tend to be separate components bonded onto the target materials. For a 1 kg detector the temperature rise would be ∼ 10–7 K/keV, dependent on the Debye temperature of the material, and temperature sensors with this level of sensitivity at such very low temperatures are difficult to achieve. The earliest sensors used were doped semiconductors, such as NTD germanium. There are now more sensitive sensors available. These rely on superconducting transitions and two types are in use. One is the superconducting phase transition (SPT) thermometer [39Jump To The Next Citation Point], the other is the superconducting transition edge sensor (TES) [34]. In both cases the temperature rise is measured by monitoring movement along the transition from superconductor to normal metal. For the TES this provides a sensitivity to the higher energy phonons. As these have not suffered the extensive scattering needed to thermalise them, some positional information can be recovered.

5.4.5 Hybrid detectors using phonons and scintillation/ionisation

The use of a separate temperature sensor bonded onto a target allows a range of different material choices for the target.

If semiconductor target materials are used, it is possible also to extract ionisation signals from bolometer experiments [12133]. Nuclear recoils produce less ionisation compared to thermal energy than x-ray and γ-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 [39Jump To The Next Citation Point]. 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].

5.4.6 Other techniques

Three other techniques are worthy of mention here. Two are techniques currently being developed while the third has been in use for some time. The first is the use of a gas target within a time projection chamber. The aim here is to image tracks of interactions within the time projection chamber and measure the range of the ionisation track and the energy deposition [24]. Nuclear recoil events have already been successfully recorded in a prototype device, and these have much shorter track lengths than an electron recoil of the same energy. This technique offers the prospect of realising a fully direction-sensitive detector that would not only enable use of all the directional WIMP signatures in attempting to extract signals, but would also allow the local WIMP velocity distribution to be measured. The second technique in this section is the use of superheated droplet detectors in which events leaving a high dE ∕dx deposit are capable of vapourising the droplets [3765]. Such detectors operate close to room temperature, exhibit low thresholds, and are insensitive to γ-rays that do not leave a sufficiently high density track. Readout can be either optical or acoustic. Finally, the ancient mica technique has already been used to derive upper limits to interaction cross-sections [10]. Ancient mica contains an historical record of nuclear recoil interactions over exposure times of Gyrs. The defects left in the crystal can be etched and examined using an atomic force microscope. Defects left from natural radioactive processes will tend to leave much more pronounced etch pits than expected for a WIMP, so the technique involves looking at the size distribution of the etch pits.

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