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Metallic magnetic calorimeter (MMC) is one of the most promising x‐ray detector technologies for providing the very high energy resolution needed for future astronomical x‐ray imaging spectroscopy. For this purpose, we have developed micro‐fabricated 5×5 arrays of MMC of which each individual pixel has excellent energy resolution as good as 3.4 eV at 6 keV x‐ray. Here we report on the fabrication techniques developed to achieve good resolution and high efficiency. These include: processing of a thin insulation layer for strong magnetic coupling between the AuEr sensor film and the niobium pick‐up coil; production of overhanging absorbers for enhanced efficiency of x‐ray absorption; fabrication on SiN membranes to minimize the effects on energy resolution from athermal phonon loss. We have also improved the deposition of the magnetic sensor film such that the film magnetization is nearly completely that is expected from the AuEr sputter target bulk material. In addition, we have included a study of a posit...

We discuss materials characterization and integration considerations for large‐format, antenna‐coupled, low‐noise, cryogenic, bolometer arrays for far‐infrared space‐based observatories. The suitability of Bi as an antenna termination resistor and annealed gold for microstrip and groundplane, in the context of using each to optimize signal and bandwidth of the receiver is addressed. Bismuth is chosen as a candidate termination resistor material because films can be made with the 10–100 Ω/□ needed to impedance‐match the antenna in a compact, stable, and reproducible manner. We characterize both the DC electronic transport and the THz optical properties of thermally evaporated (TE) and ion assisted thermally evaporated (IAE) Bi. It is found that the residual resistance ratio of the IAE Bi is an order of magnitude higher than that of TE Bi in the relevant impedance range. Furthermore, we show that we have produced IAE Bi films with linear response across the THz band. We discuss the possibility of employing other, much lower volume, absorber materials including TiAu and PdAu alloys. A comparison of the expected noise in a standard coupling resistor for each material is made. A technique of fabricating 150 nm gold films having a low temperature resistivity equal to 3.5×10−9 Ωm via thermal annealing is also presented. We model the response of these candidate films in an antenna circuit for THz radiation.

We are developing arrays of position‐sensitive magnetic calorimeter (PoSM) X‐ray detectors for future astronomy missions. The PoSM consists of multiple absorbers thermally coupled to one magnetic sensor. Each absorber element has a different thermal coupling to the sensor. This results in a distribution of different pulse shapes and enables position discrimination between the absorber elements. PoSMs are motivated by the desire to achieve the largest possible focal plane area with the fewest number of readout channels without compromising on spatial sampling. Optimizing the performance of PoSMs requires careful design of key parameters such as the thermal conductances between the absorbers, magnetic sensor and the heat sink, as well as the absorber heat capacities. We report on the first experimental results from four‐absorber PoSMs, each absorber consisting of a two layer composite of bismuth and gold. The measured energy resolution (FWHM) was less than 5 eV for 6 keV X‐rays into all four absorbers. Straightforward position discrimination by means of rise‐time is also demonstrated.

We report on the sensitivity of a transducer which uses a low noise SQUID amplifier. The transducer is made of niobium and was electropolished and heat treated to improve the Qs. The mechanical Q was found to be 3.15×106 at 4 K and the electrical Q was 2.52×105. With a low noise SQUID designed for this transducer, the noise temperature of the transducer was found to be 1.1 mK, of which 0.7 mK was due to the SQUID. This is about 200× larger than the expected noise from the SQUID. Noise from flux trapped on the chip is the most likely cause.