HDMS Experiment

Introduction

There is strong observational and theoretical evidence that non-baryonic dark matter accounts for about a third of the critical density of the Universe. Many candidates for this kind of Dark Matter have been proposed: among the most favoured ones there are slow thermal relics born in an early phase of the Universe, stable or very long lived. These weakly interacting, massive (1 GeV - 1 TeV) particles (WIMPs) arise independently from cosmological considerations in supersymmetric models as neutralinos - the lightest supersymmetric particles. Direct detection of neutralinos can occur in very low background experiments, where the elastic neutralino scattering off target nuclei is observed. The HDMS experiment is aiming to search for WIMPs by the scattering of those particles on Germanium nuclei.

HDMS operates two ionization HPGe detectors in a unique configuration (see Fig.1).

A small, p-type Ge crystal is surrounded by a well-type Ge crystal, both being mounted into a common cryostat system. To shield leakage currents on the surfaces, a 1 mm thin insulator made from vespel is placed between them.

Fig. 1 Schematic view of the HDMS detector

Two effects are expected to reduce the background of the inner detector:

The anticoincidence between the two detectors, which acts as an effective suppression for multiple scattered photons
The detection crystal is surrounded by Ge, which is one of the radio-purest known materials.

The HDMS experiment started in 1998 with a prototype phases: the HDMS-prototype (for which both inner and outer detector were made of natural Ge) took successfully data over a period of about 15 month in the Gran Sasso Underground Laboratory. The full scale experiment was installed at LNGS in August 2000: the main difference was the use of enriched ⁷³Ge for the inner detector, but also a new, low level Cu-crystal holder was used. The enrichment ⁷³Ge consists at the same time in a de-enrichment in the isotope ⁶⁸Ge, whose X-rays are a known source of background in the low energy region.

Fig. 2 Reduction of the HDMS background through anticoincidence

The reduction of the background due to the anticoincidence between the two detectors can be seen in Fig.2; one can also appreciate the reduction of the two gamma-peaks around 10 keV (X-rays from ⁶⁸Ge), due to the decay of the isotope.
Moreover, the isotope ⁷³Ge has a spin different from zero and can therefore be used to look for spin-dependent (SD) WIMP-nucleus interactions.

The first results from the HDMS experiment in the final setup are shown in Fig.3.

Fig. 3 Latest results from HDMS on Spin independent WIMP-proton coupling

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