TRISTAN

The TRISTAN Experiment

TRISTAN, the TRItium Sterile Anti-Neutrino experiment, is part of the worldwide search for the constituents of dark matter. Although the existence of dark matter is evident from cosmological observations, its nature is yet unknown. The TRISTAN experiment is managed by the Max-Planck-Institute for Physics in Munich. It targets at the experimental confirmation of the so-called sterile neutrino, which has been predicted in several extensions of the standard model and is treated as a potential contribution to dark matter. If sterile neutrinos with a mass in the keV-range exist, they would be indirectly visible by a kink-like distortion in the electrons’ energy spectrum of a β‑decaying isotope. As the sterile neutrinos’ spectral signature is expected to be of the order of 10^-6, the experiment depends on statistical sensitivity and on a strong β‑source. Therefore, TRISTAN is foreseen to profit from the tritium β‑source of the KATRIN (KArlsruhe TRItium Neutrino) facility. The aim of KATRIN is the direct determination of the electron anti-neutrino’s rest mass by measuring the maximum electron energy of the tritium β‑decay. For a precise measurement of the spectrum’s high-energy end KATRIN filters low-energy electrons by a retarding electric field and counts the number of electrons with energies above the threshold. KATRIN starts data taking in 2018 and is supposed to run over a period of five years.

To make use of the KATRIN source for the TRISTAN experiment the setup will be upgraded by a detector system capable of measuring the full energy spectrum at challenging boundary conditions. The detector has to cover an area with a diameter of 20 cm. To handle the total count rate of 10⁹/sec the detector must be segmented with several thousand cells. The required energy resolution is 300 eV FWHM at 20 keV, corresponding to an equivalent noise charge of 20 el. As backscattering and reflection of electrons will be a major source of uncertainty in the analysis, the insensitive thickness of the radiation entrance window has to be of the order of 10 nm. The sum of requirements decided in favour of a multi-channel Silicon Drift Detector (SDD) for the TRISTAN experiment.

The TRISTAN Detector

In a first prototype run small-size SDDs with a format of seven hexagonal cells and cell diameters from 0.25 mm to 2 mm had been fabricated at HLL and characterised by the project partners MPP (Max-Planck-Institute for Physics), KIT (Karlsruhe Institute of Technology), and CEA (Commissariat à l’Énergie Atomique, France). Tests in lab environment and a precursor experiment at the NuMass facility in Troitsk, Russia have been performed.

In a further step towards the final TRISTAN detector hardware, a second prototype production of SDDs including devices with the final format has been initiated. To cope with high count rates expected in the TRISTAN experiment and to make the sensor more robust with respect to electronic pick-up and microphonic noise the new SDDs will be equipped with the integrated first transistor of the amplifying electronics. Because of the area requirement of the experiment setup and the high count rate per unit area, the TRISTAN detector chip fills a large sensitive area of 36 x 36 mm² and is divided into 166 hexagonal SDD cells (fig. 1). The diameter of an SDD cell is 3 mm. In the TRISTAN experiment, 21 of these SDD arrays will form the focal plane (fig. 2). To minimise the insensitive area in the detector plane, a four-side buttable SDD module has been designed (fig. 3). Thermal and mechanical interfaces, power connections, and readout electronics are arranged perpendicular to the detector plane. The SDD cells are read out in a continuous and simultaneous mode, i.e. each cell has an amplifying electronics chain of its own. The readout ASIC and the module concept are being developed in a collaboration with the Politecnico and the company XGLab, both in Milan, Italy.

The integration of the TRISTAN SDDs in the KATRIN facility is scheduled in 2023.

Figure 1

Layout plot of the TRISTAN multi-channel Silicon Drift Detector (SDD).

The sensor area is composed of 166 hexagonal SDDs. The bond pads for power and signal connections are placed at the top and bottom chip edges. The SDD cells are 3 mm in diameter. The total sensitive area is 11.6 cm², the chip format is 38 x 40 mm².

Figure 2

Focal plane of the TRISTAN experiment.

The detector plane consists of 21 modules with almost 3.500 SDD channels and covers a circle of 21 cm diameter (drawing by Politecnico di Milano).

Figure 3

TRISTAN SDD module.

The TRISTAN SDD module is four-side buttable. Mechanical and thermal interfaces, power connections and readout electronics are arranged perpendicular to the detector plane (drawing by Politecnico di Milano).