## 6 Future Instruments for Astrophysics and Fundamental Physics Research
### Instruments for Particle-Physics And Cosmology
Ideally, a fast detector with a rise time \(\tau_{res}<1\) us and low activity is preferred, considering the fact that the energy resolution is deteriorated by the increase of the heat capacity from the nuclei implanted into the \(Au\) absorber. Too low an activity, on the other hand, will require a large number of pixels, to meet the need of high statistics. This will become challenging for the multiplexing read-out, given the need of a large bandwidth per pixel [156,177]. The HOLMES approach is to use TESs coupled to Au absorbers, similar to the ones developed for the soft X-ray spectroscopy [94], with pulse rise time engineered to be about 10 us. Using tailored discrimination algorithms [178,179], a time resolution better than 3 us is expected. The total amount of implanted \({}^{163}\)Ho will be about \(6.5\cdot 10^{16}\) to give an activity of 300 dec/s/pixel. An energy resolution of 4.1 eV has been recently reported for the HOLMES TES calorimeters before the \({}^{163}\)Ho being implanted [180]. The k-pixel array is read out using GHz-FDM. Given the speed of the detectors, the separation among two adjacent GHz tones has to be large, \(\Delta f=14\) MHz, and 36 detectors could be simultaneously read-out per channel [156]. (ii) A relatively new field where large array of X-ray microcalorimeters could play an important role in the future is the search of axion-like particles. Axions have been originally predicted by the Peccei-Quinn theory [181,182] as a possible solution to the strong CP problem. Axion-like particles in general are proposed in several theories beyond the standard model [183]. The Sun could be a strong axions and axion-like particles source. When passing through a large (\(\sim\)9 T) magnetic field, solar axions can be converted to X-ray photons detectable by X-ray spectrometers. The solar axions spectrum is expected to be generated by axion-photon and axion-electron interactions, respectively. The former has a smooth shape with a peak at about 3 keV, corresponding to the inner solar temperature. The latter has a maximum at about 1 keV and shows many energy peaks up to 7 keV, which depend on the metal composition of the Sun. An instrument similar to X-IFU could contribute to the first axions detection. Thanks to the high resolving power and low energy threshold demonstrated by TES-based calorimeters, TES technology could become crucial, after discovery, to characterize the axions electrons process and to give a key contribution on the understanding of the stellar physics [184]. The International Axion Observatory (IAXO) is a new generation of axion helioscope that will improve the CAST state-of-the-art sensitivity [185] by more than a factor of \(10^{4}\). It will give access to probe a large fraction of the quantum chromodynamic axion models in the meV to eV mass band [186]. A smaller scale version of IAXO, called BabyIAXO, is currently under development [187]. Metallic magnetic calorimeters-based instrument has been proposed as one of the X-ray instruments for IAXO [188]. Low temperature X-ray calorimeters do have typically very high quantum efficiency, an essential feature for this application, and can be fabricated in large arrays. However, the most challenging requirement for an X-ray detector for IAXO is the ultra-low background set to be about \(10^{-7}\,\mathrm{keV}^{-1}\mathrm{cm}^{-2}\mathrm{s}^{-1}\). Experiments are on going at Heidelberg to assess the contribution on the radiation background of low temperature detectors. One of the major observed sources was the flourescence in the material surrounding the detector induced by cosmic muons and radioactive impurities. A level of \(1.2\times 10^{-4}\,\mathrm{keV}^{-1}\mathrm{cm}^{-2}\mathrm{s}^{-1}\), has been measured with their first laboratory prototype [188]. It is clear that a proper choice of low radioactivity materials and a careful simulation and design of the focal plane assembly is very important to be compliant with the extremely low background level requirement.