Position-, Time-, and Energy-sensitive planar Ge(i)- and Si(Li)-Detectors

During the last decade, the development of position and energy sensitive solid-state detectors has experienced a tremendous progress. This progression is mainly motivated by demands of nuclear physics and astronomy (satellite missions) experiments for highly efficient gamma-ray spectrometers and in particular by unique advantages of such detection devices which arises for applied research. One also may anticipate that position sensitive germanium detectors will play an important role for future X-ray spectroscopy in atomic physics dealing with highly charged heavy ions, such as H-like uranium.
 

The unique properties of such structured planar detectors are millimeter to sub-millimeter spatial resolution as well as a good time (e.g. typically 50 ns, down to 10ns depending on geometry) and excellent energy resolution for the hard X-ray energy regime (about 1%-2% below 100keV and about 0.2% at 1 MeV). This allows for designing sensors (photo lithographically defined Ge(i)- or Si(Li)-crystals) for various applications. Our main focus in applications is on precision experiments with highly charged and heavy ions at particle accelerators like GSI/FAIR or with intense x-ray sources like DESY/Petra3. For these types of experiments we develop in a long-lasting collaboration with the detector lab at IKP/FZ-Jülich dedicated detector systems with optimized geometries for the use as imaging sensors or as Compton-Polarimeter to analyze the linear polarization of x-rays as experiment observable in the energy regime from 50 keV to 1 MeV.

In combination with a focusing crystal spectrometer, a position sensitive Ge(i)-detector permits the measurement of an energy spectrum wide enough to investigate the whole interesting energy regime simultaneously. Along with a a focusing crystal spectrometer (e.g. FOCAL project) this detector may play a key role for a precise test of quantum electrodynamics (QED) in the heaviest one-electron systems.

The pictures above show microstrip detector systems (left:1-dimensional; right:2-dimensional), developed at the Forschungszentrum Jülich, with a position resolution (defined by the contract size) of roughly 250 μm and timing resolution of 50 nsec. A position resolution of less than 50µm by applying interpolation techniques has been shown with these devices.

We have to stress the importance of germanium pixel and of two-dimensional strip detectors (DSSD) for the study of the dynamics of heavy ions colliding with electrons or low-density gaseous matter. Such collisions are strongly affected by electron–ion recombination processes, such as radiative electron capture (REC, the time-reversed photoionization process in ion–atom collisions), processes which are of plasma and astrophysical relevance. Because for high-Z ions and fast collisions, electron–ion recombination in general produces strongly polarized X-rays in the energy regime between 50 and 500 keV, the polarization sensitivity of thick two-dimensional germanium detectors via the Compton effect provides an important key to reveal the physics of these processes.

In the most recent polarimeter project we address the enhancement to 1 MeV by the combination of a thick planar Si(Li)-DSSD with a thick planar Ge(i)-DSSD in telescope configuration. This results in a significantly increased detection efficiency for high energetic photons scattered in the Si(Li) and detected by the Ge(i), while we maintain still a high efficiency for low energy photon detected in the Si(Li). For this system we achieved a two times better energy resolution (800eV for 60keV) by a redesign of the 128 preamplifiers to bring the most noise sensitive components closer to the detector crystal and by decreasing their operation temperature below -100°C.

An alternative approach to x-ray spectroscopy is the use of novel microcalorimeter detectors. Their working principle is based on the detection of the heat increase in a small absorber upon getting hit by an x-ray. Microcalorimeters are ideally suited for use at low photon rates, as background radiation of higher energy is hardly absorbed due to the small detector volume. By achieving a spectral resolution one to two orders of magnitude better than typical semiconductor detectors while providing a broad bandwidth acceptance, this type of detector offers unique experimental possibilities x-ray spectroscopy at GSI/FAIR. Moreover, the quantum efficiency of microcalorimeters can be matched to the energy range to be measured by selecting the absorber material and the absorber geometry. However, the spectral resolving power scales inversely with the square root of the absorber volume.

Nowadays, microcalorimeters of the TES (Transitions Edge Sensor) and MMC (Metallic Magnetic Calorimeter) types achieve spectral resolutions of close to 1 eV full width at half maximum (FWHM) for incident photons energies of 6 keV. In the particular the MMC detector technology, which was largely developed in the group of Prof. Enss at Heidelberg University, is characterized by excellent energy resolution and linearity and allows signal rise times of less than 100 ns, which are two to three orders of magnitude shorter than those of other microcalorimeter concepts and are particularly attractive for coincidence measurements. Based on this pioneering work, within the SPARC collaboration currently the maXs (Micro-Calorimeter Arrays for High Resolution X-ray Spectroscopy) detector design is developed and applied to precision x-ray spectroscopy of heavy ions. Currently, this type of detector features an array of 64 individual pixels, that each consist of an absorber for the incident radiation and a paramagnetic sensor where the heat increase is converted into a change in the magnetization, see Fig. 1. The operation temperature of less than 20 mK is provided by ³He/⁴He dilution refrigerators.

In 2021, the first dedicated microcalorimeter-based experiment was carried out to record radiative transitions in the heaviest two-electron ion, i.e. helium-like uranium (U⁹⁰⁺). To this purpose two maXs detectors were placed at the 0° and 180° view ports of the electron cooler of the CRYRING@ESR storage ring, see Fig. 2. By achieving a spectral resolution of better than 100 eV FWHM at photon energies of about 100 keV, this measurement allowed for a first time to disentangle the x, y, z, and w transitions that form the Ka₁ and Ka₂ peaks. This provided for the first time a highly accurate test of the interaction of electrons with the quantum vacuum and with each other in the presence of the extremely strong field of the uranium nucleus. Overall, the measurement confirms the predictions of bound-state quantum electrodynamics for the heaviest two-electron ion. Such high accuracy tests in the so far largely unexplored regime of extreme electromagnetic field provided by heavy nuclei are a prerequisite for using QED predictions as a solid foundation in the search for physics beyond the standard model.