Authors: M. D’Andrea, C. Macculi, S. Lotti, L. Piro, A. Argan, G. Minervini, G. Torrioli, F. Chiarello, L. Ferrari Barusso, E. Celasco, G. Gallucci, F. Gatti, D. Grosso, M. Rigano, D. Brienza, E. Cavazzuti, A. Volpe
First Author’s Institution: INAF/IAPS, Via del Fosso del Cavaliere 100, 00133 Rome, Italy
Status: Published in the Journal of Low Temperature Physics [open access]
Looking at the sky in different parts of the electromagnetic spectrum allows us to learn about different processes in astronomy. X-rays are towards the shorter-wavelength, higher-energy end of the spectrum. They are produced by really hot things, like gas undergoing violent collisions or possible hot gas between galaxy clusters, and by really high-energy things, like supermassive black holes. X-rays are absorbed by Earth’s atmosphere, so they must be viewed from space.
ATHENA is a planned X-ray space telescope from the European Space Agency. One of its instruments will be a spectrometer, which can tell apart X-rays with different energies, like telling apart the colors of visible light. Today’s paper tells how the spectrometer team is approaching a tricky problem with this kind of instrument where non-X-ray particles can masquerade as X-rays.
From thermometer to spectrometer
ATHENA’s spectrometer, called X-IFU, is designed to find the energy of X-ray photons using devices called transition-edge sensors (TES). TESs are made of special materials known as superconductors whose resistance drops abruptly to zero when cooled past a (very cold!) temperature called the critical temperature. But while the drop in resistivity is very fast as a function of temperature, it isn’t instantaneous. Therefore, during the transition, the superconductor is a very sensitive thermometer. If you change the temperature by just a tiny amount, the resistance changes a lot (see Figure 1)!
In a TES instrument like ATHENA X-IFU, absorbers facing the sky turn incoming X-ray photons into heat. The absorbers are attached to TESs, which are kept in the steep transition between their normal and superconducting states and have a known voltage (V) applied to them. By Ohm’s law, V=IR, the change in resistance (R) from the absorbed photon’s heat causes a pulse in the current (I). The size of the current pulse tells you what the energy of the X-ray photon was.
Real X-ray or impostor?
The problem is that X-ray photons aren’t the only energetic particles whizzing through space! High-energy ions called cosmic rays are plentiful in space, and they can deposit energy in the absorbers too. What’s an astronomer who only wants to look at X-rays to do?
Because the absorbers are designed to match X-ray energies, if the particle was more energetic than an X-ray, it will still have energy left over after hitting the absorber. If you put another absorber underneath, you’ll detect it a second time, and you’ll know to remove that signal from your X-ray science data (see Figure 2). The second detector, called an anti-coincidence detector, is necessary for X-IFU and similar instruments.
Testing the prototype
In this paper, the authors report on a prototype anti-coincidence detector for X-IFU, which also uses TESs. To filter cosmic rays out of your data, you don’t need to know where in the sky they came from, only when they arrived, so the anti-coincidence detector pixels are much bigger than the science detector pixels. The prototype is one pixel, which consists of a big silicon absorber on which a big TES and a heater for calibrating the TES are placed. A previous prototype used many connected small TESs spread across the big absorber, but using one big TES is simpler, provided it can do the job.
First, the authors aim two radioactive sources emitting at known energies around 6 keV and 60 keV at the prototype. The 6 keV source is pointed at the corner of the absorber away from the TES and can be measured well, so the one-big-TES strategy is working. Pulses of energy injected from the heater match the known energies from the 6 keV and 60 keV sources to 3%, so the calibrator is also working. The authors determine from the 6 keV-source data that the minimum energy the prototype can detect is 1.4 keV. X-IFU is meant to detect X-rays with energies between 0.2 to 12 keV. A typical background particle will deposit 150 keV in the anti-coincidence detector, but there is a wide spread of energies. In order to filter out a high enough percentage of the cosmic rays to get good X-ray data, the anti-coincidence detector needs to be able to detect event energies as least as low as 20 keV. A lower threshold of 1.4 keV not only passes the test but overachieves.
Finally, the new anti-coincidence detector prototype is set up next to an older prototype to look at natural background radiation in the lab, which includes both X-ray photons and cosmic rays. Similar to how it will work in practice, background X-ray photons should be counted in just one detector, where they are fully absorbed, while background cosmic rays should ping both detectors. The count rates and energies match expectations.
To make a sensitive measurement of anything, you not only have to be good at detecting the signal you want, but also good at eliminating background signals you don’t want. Today’s paper shows what that looks like in practice for an exciting upcoming X-ray instrument.
Astrobite edited by Kasper Zoellner and Archana Aravindan
Featured image credit: Fig. 1, D’Andrea et al. (2024)
