T. Oshima

Multi-pixel readout of transition-edge sensors using a multi-input SQUID

Abstract We propose a new method to read out signals from a TES (superconducting Transition-Edge Sensor) calorimeter array with a single or a small number of SQUIDs. Since phonon noise and Johnson noise of a calorimeter can be made very small in these calorimeters, an increase in noise, thus a degradation of energy resolution by adding signals from plural pixels together at some stage of signal processing may be acceptable for some applications of TES calorimeters. We propose to use a SQUID with multi-input coils which will sense signals from different pixels. The input coils of a SQUID are electrically well-isolated from each other. The pixel that generates a signal can be identified by utilizing additional information, such as the pulse shape. We studied the feasibility of this method with analysis and simulations, and show for example, that a 16×16 pixel array can be read out with 16 SQUIDs.

AC calorimeter bridge; a new multi-pixel readout method for TES calorimeter arrays

In order to realize a large format (e.g. ∼32×32) calorimeter array, it is essential to multiplex calorimeter signals at cryogenic temperatures without losing signal to noise ratio. For this purpose we propose a brand-new readout method, the CABBAGE (Cal¯orimeter B_ridge B_iased by an A_C Ge¯nerator) where an AC biased calorimeters are placed in resistance bridges. In this paper we first describe the principles of CABBAGE and investigate its response and noise. We propose the large format calorimeter array readout using CABBAGEs, and discuss the new TES microcalorimeter readout method without using SQUIDs.

Status of X-ray microcalorimeter development at ISAS

A superconducting phase-transition microcalorimeter is a promising detector for high resolution X-ray spectroscopy. We are developing such a detector for future Japanese X-ray astronomy missions. In our design, a tin absorber is electrodeposited as a mushroom structure to achieve a high covering fraction. We have succeeded in detecting X-rays with the first model of our calorimeter with an electrodeposited absorber, though the energy resolution was limited due to residual resist. We are also developing a phase-transition microcalorimeter with a low transition temperature. We describe the results of these experiments, and discuss the limiting factors of their performance.

Fabrication of an X-ray microcalorimeter with an electrodeposited X-ray microabsorber

For the next generation of astronomical X-ray imaging detectors, arrays with large numbers of microstructures (∼1000 pixels) will be required. To meet this requirement, a tin absorber for an X-ray microcalorimeter, which has a so-called “mushroom” shape, is fabricated by electrodeposition and polishing. This method enables the fabrication of a large number of arrayed microcalorimeters. Details of the fabrication process, the characteristics of the absorber and the fabricated microcalorimeters are reported.

Improvements of an X-ray microcalorimeter for detecting cosmic rays

An X-ray microcalorimeter that consists of an x-ray absorber to transfer the incident photon energy to the temperature rise, a temperature sensor to detect the temperature change and suspending beams for thermal isolation from the substrate have been fabricated. Titanium/Gold thin film transition edge sensor (TES) is used as the temperature sensor. We fabricated and tested the first prototype in the previous study and obtained the transition temperature of 0.52 K, energy resolution of 550 eV (FWHM) for 6 keV radiation. These values were smaller than that of expected. We applied a Sn absorber and redesigned the microstructure of the x-ray microcalorimeter. Consequently, we have obtained 158 eV at 5.9 keV radiation of the energy resolution, which is about 4 times higher than that of the first prototype. This value is nearly equal to the conventional X-ray CCD. The highest energy resolution of the x-ray microcalorimeter of our design is estimated to approximately 5 eV at the operating point of 0.2 K. To realize such a good energy resolution calorimeter array, we are going to improve the sensitivity of the TES by optimizing the process condition. A Sn absorber formed by electroplating is also under evaluating simultaneously. It is necessary to fabricate uniform array structures.

Development of a high-energy-resolution x-ray microcalorimeter using Ti/Au TES

A prototype of an x-ray microcalorimeter using Titanium/Gold transition edge sensor (TES) for detecting cosmic rays is fabricated and tested. This paper reports first experimental result of the prototype. By using silicon bulk micromachining, freestanding microstructure suspended with fine beams are obtained to achieve thermal isolation from the substrate. A superconductor, Ti in this case, can be used as a very sensitive temperature sensor at the narrow temperature range around its transition temperature. At the low temperatures below 1K, the microstructure with very small heat capacity is expected to be thermally detecting single photons. Design consideration to realize radiation detection with extremely good energy resolution has been taken place. Our tentative goal is to obtain the energy resolution of 20eV for 10keV radiation at 0.5K. We have fabricated a test device of the TES. The sensitivity of it is larger than 1000, which is enough for this purpose. The energy resolution of the prototype of the x-ray microcalorimeter was 550eV for 6keV radiation at approximately 0.5K. This value is smaller than that expected. An optimization of the TES features is still necessary for a good energy resolution.

Impedance Measurement of a Gamma‐Ray TES Calorimeter with a Bulk Sn Absorber

We performed complex impedance measurements with a Ti/Au‐based gamma‐ray TES calorimeter with a bulk Sn absorber. Excellent energy resolution of 38.4±0.9eV at 60 keV was observed. The impedance of the calorimeter can be well explained by a two‐body thermal model. We investigated the behavior of the parameters of the calorimeter during the superconducting‐to‐normal transition. We confirmed that C and Ga are in good agreement with the predicted values. We performed a noise analysis and found several excess noise components, as well as internal thermal fluctuation noise (ITFN) term due to the thermal conductance between the Sn absorber and the Ti/Au TES. Dominanting the noise is an excess noise having a similar frequency dependence to the phonon noise and the ITFN noise.

A via hole based superconducting wiring method for enhanced X-ray image sensors

In this paper we developed through-wafer interconnections using only a simple fabrication process. The interconnection was successfully transformed into the superconducting state. A current density of 13 cm/sup 2//mA was obtained in the superconducting state. We realize high energy-resolution X-ray imaging using the superconducting through-wafer interconnections.

Transition edge X-ray sensors for industrial applications

Abstract We present the performance of transition edge sensors (TES) using Au/Ti bilayer (operating temperature 113 mK ) with fast decay time for industrial applications. The energy resolution was 9.2 eV for Mn Kα1 (designed value 3.5 eV ) and the decay time 71 μs , yielding the count rate 2.3 kcps (count per sec). We found that the energy resolution was limited by excess noise. The excess noise increased in proportion to the inverse of TES resistance as decreasing the TES resistance from normal resistance. The source of excess noise might exist in the TES.