X-ray microcalorimeters

Microcalorimeters offer a promising technology for high throughput, high resolution X-ray spectroscopy. A microcalorimeter is, in essence, an ultra sensitive thermometer. It measures the energy of each incident X-ray photon by sensing a tiny increase in temperature of the absorber and then converts the temperature pulse to a measurable electrical pulse, typically by making use of the fact that its electrical resistance of a microcalorimeter is a strong function of temperature. It recovers and is ready for the next X-ray photon, when the heat generated leaks from the absorber to the heat bath of constant temperature; the more quickly the leakage occurs the faster the pulse decays. The accuracy to which a microcalorimeter measures the photon energy is fundamentally limited by thermodynamic fluctuations in the exchange of thermal energy between the absorber and the heat bath, which is proportional to the temperature squared and also to heat capacity. To achieve good spectral resolution, therefore, it is necessary to operate the microcalorimeter at cryogenic temperatures (typically < 100 mK). We are developing X-ray microcalorimeters based on superconducting transition-edge sensor technologies, for a concept mission, known as the Hot Universe Baryon Surveyor (HUBS). The scientific thrust of HUBS is to see the “missing baryons” in circumgalactic and intergalactic media.

Contact: Wei Cui
X-ray polarimetry

Astronomical X-ray polarimetry at energies of a few keV has been an unexplored area for more than 40 years, since the last successful experiment in 1975 with a Bragg polarimeter onboard the OSO-8 satellite. Due to the limited sensitivity of past technologies, however, no useful constraints can be placed for other X-ray sources. High degree of linear polarization is expected from high energy astrophysical objects where magnetic field or asymmetric geometry plays a key role in radiation or radiation transfer. X-ray polarimetry will allow us to measure the magnetic fields in supernova remnants, pulsars, relativistic jets and help constrain the emission models and particle acceleration mechanism, or shed light onto the geometry of the accretion flows around black holes. It can also help test fundamental physics such as the QED effect vacuum birefringence caused by extremely strong magnetic fields. Sensitive X-ray polarimetry will open a new window in X-ray astronomy in addition to imaging, spectroscopy and timing. Here at Tsinghua we are focusing on the development of novel technologies that enable high sensitivity X-ray polarimetry, including the gas pixel detector (GPD) for photoelectric polarimetry and the multilayer Bragg polarimeter for soft X-ray polarimetry. We are able to assemble space qualified GPD detectors in house. We participated in several future space-borne projects such as the X-ray Imaging Polarimetry Explorer (XIPE), which is selected for phase A study by ESA, and the Chinese future mission X-ray Timing and Polarization (XTP).

Contact: Hua Feng
Dark matter directional detectors

When conducting direct dark matter (DM) detection experiments, gas detectors are often used. They measure the energy and other key properties of nuclear recoils coming from elastic collisions with DM candidate particles—WIMPs (Weakly Interacting Massive Particles). It is important to discriminate the isotropic background from WIMP like events. This is possible because the angular distribution of WIMPs is pointing to the Cygnus constellation. MIMAC (MIcro-tpc MAtrix of Chambers) detector is used to identify the elusive dark matter events by simultaneously measuring nuclear recoil energies and obtaining WIMP directional information. The direction of nuclear recoils is determined based on the 3D track reconstruction technique and a dedicated statistical method developed to optimize the exclusion limit from a given set of data. STPC (Spherical TPC) detector is another promising gas detector in DM directional detection. It is a new detector based on the radial geometry with spherical proportional amplification read-out. The detector has the advantage in using a single electronic channel to read-out a large volume. This single information can still show the radial coordinate of the interaction point. Large gains can be obtained, with the condition of low energy threshold in the sub-keV region.

Contact: Charling Tao