# Applications for Microwave Kinetic Induction Detectors in Advanced Instrumentation
## 5 MKIDs Applications in Material Science with Synchrotrons  
Another field where kinetic inductance detectors are being used is in the use of synchrotron radiation for material science. Synchrotron radiation is emitted by electrons radially accelerated to speeds close to the speed of light in the vacuum of a particle accelerator. In the reference frame of the electron, this emission is at radio frequencies, while in the reference frame of the detectors, it is shifted into the X-ray range. A periodic series of magnets, called undulators, cause transverse undulations in the electron. The wavelength of the emitted radiation is a function of the period of these undulators. Synchrotrons have increased the brightness of X-ray sources by 26 orders of magnitude since the 1970s. Using MKIDs as detector element in synchrotron beam lines allows to exploit their major advantages of energy resolution and their ability to be multiplexed to large arrays, and only having to deal with the disadvantage of requiring advanced cryogenics to cool them down to mK temperatures [143]. [PERSON] et al. [110] describe developing MKIDs for synchrotron X-ray spectroscopy, citing their ability to scale up to large arrays. They list particular uses of this X-ray spectroscopy as synchrotron experiments, laboratory tools and astronomical satellites. They proposed WSi\({}_{x}\) for these detectors due to it having a short attenuation length for soft X-rays and controllable \(T_{c}\) in the range of 1 to 3 K, and it being able to be sputtered from separate W and Si targets with good control. Exact values for \(T_{c}\) can be tuned by changing the W to Si ratio. Initial results show quality factors greater than \(10^{6}\)[144]. They later instead suggest [112; 145] using TKIDs in a thermal quasi-equilibrium state to detect X-ray photons. They use TKIDs as they do not require as strict limitations on the absorber design, as regular MKIDs for X-rays would. In order to assess the ability of TKIDs to be used for X-ray spectroscopy they have simulated their operation with a range of parameters, relevant to measuring X-ray photons, and examined the detector design parameter space. The response of the simulated TKID was then tested with several noise sources, and these simulated results were then used to design new devices. Based on results from these simulations, optimized TKIDs were designed for 6 keV X-rays. This optimized TKID is proposed to have a \(T_{c}\) of 1 K, heat capacity of 1 pJ/K, and a resonator bandwidth of 6 kHz FWHM. They suggest that further improvements could be made with the use of a parametric amplifier, as the HEMT amplifier noise was the limiting factor. [PERSON] et al. [109] have also developed TKIDs for X-ray imaging spectroscopy, stating potential applications of both X-ray astronomy as well as X-ray spectroscopy at synchrotron radiation sources. While diffractometers can already realize energy resolution of \(\sim\)0.5 eV for 5.9 keV [146] photons, they have to be scanned across their energy range, which TKIDs do not. While TESs are also being used for spatially resolving X-rays, showing energy resolutions of 2.4 eV at 5.9 keV, and pixel counts of over a thousand, they run into difficulties when scaling up to larger arrays. In [109] TKIDs have been fabricated from sub-stoichiometric TiN\({}_{\mathrm{x}}\) films, a Nb feedline and ground plane, and a 500 nm thick Ta absorber, isolated from the inductor by a free standing Si\({}_{3}\)N\({}_{4}\) membrane. With a critical temperature of 1.6 K, and an operating temperature of 75 mK to 200 mK, temperature rises of 200 mK to 300 mK were measured from 6 keV photons. Exciting the fabricated TKID with an Fe\({}^{55}\) source, 4970 photons were plotted as a histogram. From this, an energy resolution of 75 eV was calculated at 5.9 keV. [PERSON] et al. [109] conclude that further optimization should be able to improve the TKID energy resolution to that achieved by TES calorimeters. [PERSON] et al.