New X-ray pixel designs rival conventional square shape

ESA’s upcoming space telescope Athena will study the X-ray emission from galaxies with unprecedented energy resolution—the ability to distinguish different colors. Its X-ray cameras use for their pixels Transition Edge Sensors (TESs), which measure the energy of individual photons by balancing on the verge of being superconducting. When a TES absorbs a photon, this causes a small change in temperature leading to a dramatic reduction of the superconducting state and therefore a large increase in the resistance of the TES, proportional to the energy of the measured photon.

For Athena, nearly 4000 TES pixels have to be operated at the same time. But giving each pixel its own readout amplifier requires a lot of power, which is scarce in space. Therefore Athena uses a technique called multiplexing—many pixels are combined in a single readout chain with a single amplifier. Time Domain Multiplexing (TDM) is the conventional way, where each pixel in a chain is switched on, read out using a direct current (DC), and switched off very quickly before it’s the next pixel’s turn. SRON scientists are experts in an alternative way, Frequency Domain Multiplexing (FDM), where all pixels are read out simultaneously by using alternating currents (AC) at different frequencies. Both ways have weak and strong points, and both require well-designed pixels to work properly.

Historically, DC TDM has achieved the best energy resolution. But the SRON team, including Martin de Wit and Luciano Gottardi, has now optimized FDM by abandoning the traditional square shape and opting for long narrow pixels with high resistance. They have now for the first time reached the same energy resolution using an AC readout as the DC based TDM, measuring the energy of a 5.9 keV X-ray with an accuracy of 1.6 eV. This is the equivalent of looking at a rainbow and seeing more than 2,000 distinct colors.

Perhaps even more important is the understanding gained about how the design impacts the properties of the pixels. By comparing 5 different pixel designs, the SRON scientists were able to figure out how changes in the pixel’s width and length influence their critical temperature and thermal conductance. For future space missions a full control of these two properties is vital, since together they determine which type of radiation the pixels are optimized for, and how fast the pixels are, determining what objects can be studied. For bright sources, you want fast pixels, but for faint objects, slow pixels are better. The findings of the SRON team can play an important role in the design of future instruments.

M. de Wit, L. Gottardi, E. Taralli, K. Nagayoshi, M. L. Ridder, H. Akamatsu, M. P. Bruijn, M. D’Andrea, J. van der Kuur, K. Ravensberg, D. Vaccaro, S. Visser, J. R. Gao, and J.-W. A. den Herder, ‘High aspect ratio transition edge sensors for x-ray spectrometry’, Journal of Applied Physics