Abstract Body

A Transition Edge Sensor (TES) is a kind of microcalorimeter or bolometer utilizing super-conducting film as a thermometer. Attached with an absorber of small heat capacitance at low temperature (< a few x 100 mK), a small heat input caused by an irradiated particle can be measured accurately. As an example, an energy resolution ΔE<2 eV for X-ray is achieved[1 and reference therein], and optical photon counting with 67 meV is reported[2]. This presentation will introduce many applications of this device that are being used and planned. After a sounding rocket experiment[3], future X-ray telescopes (ex. Athena[4], LEM[5]) and will adopt a TES microcalorimeter array and a Cosmic Microwave Background (CMB) mission LiteBIRD[6] will use TES bolometers. In particle physics, many dark matter search experiments (ex. CRESST[7], ALPS[8]) use TES high sensitivity and accurate energy determination by TES are required to measure energy levels in Kaonic atom[9] and 229 Th[10]. We conducted a Solar axion direct search experiment with a TES array with a57Fe absorber[12] and a test run was done in 2024.

To expand the field of application, a large format array and its readout method is essential. The original TES readout was done by a DC-SQUID amperemeter coupled with a TES. Multiplexing methods such as frequency-domain multiplexing (FDM) [13] and time-domain multiplexing (TDM) [14] are proposed to reduce the number of harnesses in refrigerators. ISAS and AIST collaborated to develop a microwave SQUID multiplexing (MWMUX) method with RF-SQUID and succeeded in reading 38 pixels by one channel[15]. These methods will be used in future applications, such as plasma diagnostics, and material analysis in combination with electron microscopes.

References

[1] Gottardi, L. and Nagayoshi, K. ,Appl. Sci., 11(9), 3793 (2021_

[2] Hattori, K. et al., Superconod. Sci. Technol., 35, 095002 (2022)

[3] Adams, J.S. et al., J. Low Temp. Phys 199, 1062(2019)

[4] Barret, D. et al., Experimental Astronomy 55, 373 (2023)

[5] Bandler, S.et al., J. of Astronomical Telescopes, Instruments and Systems, 9, 041002 (2023)

[6] Westbrook et al., Proc. SPIE Int. Soc. Opt. Eng. 11443, 11435Q (2020)

[7] Rothe, J et al. J. of Low Temp Phys, 193, 1160(2018)

[8] Gimeno, A. et al. NIM A, 1046, 167588 (2023)

[9] Hashimoto, S. et al., Phys. Rev. Lett, 128, 112503 (2022)

[10] Yamaguchi, A., et al., Phys. Rev. Lett, 123, 222501 (2019)

[12] Yagi, Y. et al., IEEE Tran. App. Supercon, 33(5), 2100805 (2023)

[13] Akamatsu, H. et al. Applied Physics Letters 119(18), 182601(2021)

[14] Doriese, W.B. et al. J. of Low Temp Phys, 184, 389(2016)

[15] Nakashima, Y. et al. App. Phys. Lett, 117 122601 (2020)

Acknowledgment

This work was supported by JSPS KAKENHI Grant Number 20H05857. Our research was partially preformed in the nano-electronics clean room in Institute of Space and Astronautical Institute (ISAS) and CRAVITY (Clean Room for Analog and digital superconductiVITY) at AIST.

Abstract Body

Superconducting detectors have been applied to observations in cosmology and astroparticle physics. In this talk, I will not cover all such projects, but instead, focus on two specific projects in which I have been involved.

First, I will discuss observations of the cosmic microwave background (CMB). These observations have provided profound insights into the nature of the universe, including the composition and age of the cosmos, by analyzing the anisotropy of intensity and E-mode polarization. Recent and upcoming CMB measurements are concentrating on detecting B-mode polarization. Large angular scale measurements of B-mode polarization could reveal signatures of primordial gravitational waves, which are believed to have been generated by quantum fluctuations of space during cosmic inflation, prior to the hot Big Bang. Current and future CMB experiments utilize arrays of superconducting detectors—ranging from several thousand to hundreds of thousands—to suppress the power fluctuations caused by the shot noise of incoming photons. These observations not only measure the CMB, originating from the farthest reaches of the universe, but also detect microwaves emitted by nearby galaxies due to synchrotron and dust emissions (foregrounds). To accurately measure CMB B-mode polarization, it is essential to remove these foregrounds, which can be done by exploiting their different frequency dependencies. Consequently, future B-mode polarization observations will involve wide frequency band measurements. I will describe a proposed space mission [1] designed to detect primordial gravitational waves with a precision capable of distinguishing between most major single-field inflation models.

Next, I will briefly introduce a smaller, future project aimed at detecting solar electron neutrinos. The Sun emits light through the fusion of hydrogen into helium at its core. This fusion process begins with the collision of two protons, producing a deuteron, a positron, and an electron neutrino (pp reaction). Neutrinos generated by this reaction are known as pp neutrinos. Measuring these pp neutrinos can provide invaluable information about the Sun's internal conditions, which cannot be observed by electromagnetic waves alone. Additionally, it could offer clues about new physics, including dark matter. Previous experiments have observed pp neutrinos using targets weighing between ten and one hundred tons. In 1976, Raghavan proposed using Indium-115 [2] as a target, which could significantly reduce background events through the delayed coincidence of de-excitation gamma rays (Fig. 1), allowing the target mass to be reduced to one ton. I will present an idea to use a target including indium combined with superconducting detector arrays.

References

[1] LiteBIRD Collaboration, Prog. Theor. Exp. Phys. 2023, 042F01 (2022).

[2] R. S. Raghavan, Phys. Rev. Lett. 37, 259 (1976).

Acknowledgment

The authors acknowledge the CRAVIT in AIST. The authors are supported by KAKENHI 23K25880 and 21H00077, and RECTOR in Okayama University

Figure 1. Solar electron neutrino reaction with In-115 produce the inverse beta rays and excited Sn which emits two gammas in the de-excitation process. The two gammas emitted with a delay of 3.3 microseconds can be used to have delayed coincidence to reduce background events.

Keywords: Cosmic Microwave Background, Solar neutrino

Abstract Body

Introduction
X-ray absorption spectroscopy (XAS) is a method of materials analysis which provides nanostructure information around specific element in a sample via observing high resolution XAS near the absorption edge. Soft X-rays (SX) are important for XAS, since measurements of light element dopants are only possible there. When measuring XAS of bulk materials with SX, the fluorescence yield is measured instead of the absorption due to the low X-ray transmittance. Because the fluorescence yield of SX is low and the characteristic X-rays of other elements overlap on the fluorescent X-ray spectrum, a spectrometer with high sensitivity and high resolution is required. Superconducting tunnel junction (STJ) detectors have attracted attention because they can simultaneously achieve higher resolution than semiconductor detectors, higher sensitivity than analyzing crystals, and high counting rate required to detect rare events due to dopants.

For these reasons, XAS using STJ detector array were developed in the past two decades [1-3]. We constructed an XAS equipped with a 100-pixel, 100-micron square STJ detector. The performance achieved was an area of ​​1 mm2, energy resolution of 10-20 eV for X-ray energy below 1000 eV, and count rate of 0.5 M cps with energy resolution of 20 eV at the C-K line [4-5]. The instrument was used to analyze compound semiconductor and structural materials [6-8]. The lowest dopant concentration was 80 ppm nitrogen in silicon carbide [8]. However, to analyze real materials for industries, it is necessary to be able to measure lower concentrations of dopants and to measure a large number of samples. Therefore, we developed an XAS apparatus equipped with a 200-pixel STJ detector to achieve the sensitivity twice. Since the new 200-pixel chip introduced problems with magnetic flux trapping, geomagnetic shielding methods are reconsidered. After solving these issues, we conducted performance tests at synchrotron radiation beamline BL-11A at the Photon factory of the High Energy Accelerator Research Organization. This paper reports the implementation of a 200-pixel STJ detector array and the performance of the XAS system using the STJ array.

Experiment
The STJ spectrometer used is as follows: The STJ chip was fabricated at Clean Room for Analog-digital superconductiVITY (CRAVITY) at the National Institute of Advanced Industrial Science and Technology. The area of ​​one pixel detector is 100 microns square, and the layer structure is Si substrate / 100 nm Nb / 70 nm Al / AlOx / 70 nm Al / 300 nm Nb, the passivation layer is SiO2, and the wiring is Nb. The characteristics of a detector with the same structure have already been reported [9]. Two hundred pixels are placed on a 17 mm square silicon die (Figure 1a). The layout is almost the same as in the literature [3-5]. The difference from the previous setup is that the material for the magnetic shield has been changed from permalloy C (PC) to A4K. The reason for changing the material was that the magnetic permeability of the PC magnetic shield decreases at low temperatures, causing fluxoid traps and resulting in variations in the current-voltage characteristics of each pixel. Tests at room temperature have shown that the A4K shield can reduce the effects of the earth's magnetic field to less than 50 nT, which is approximately one-thousandth of the earth's magnetic field.

The performance was evaluated by measuring the current-voltage characteristics, X-ray spectrum, and XAS at the synchrotron beamline, BL-11A. The current-voltage characteristics were measured by the two-terminal method. The X-ray measurements were carried out as follows. A set of charge amplifier and digital multichannel analyzer was used for each pixel. The energy resolution was evaluated using the full width at half maximum of the characteristic X-ray peak. The X-ray absorption spectrum was measured as the ratio of the characteristic X-ray intensity to the incident light intensity, with the energy of the incident light as a parameter.

Results and Discussion
The results of measuring the current-voltage characteristics of the 200-pixel STJ detector are as follows. Of the 200 pixels, 189 pixels showed a current value of 10 nA or less at a sub-gap voltage of 0.3 mV. For seven pixels, the sub-gap current was 20-30 nA. In the preliminary experiments, the distribution of　pixels showing high current values ​​was different. This fact indicates that random flux quantum traps are the cause of the excess current, and suggests that the magnetic field at the superconducting transition exceeds 200 nT, which is the flux quantum of 100 microns square. In contrast, according to the magnetic shielding characteristics measured at room temperature, the magnetic field around the STJ chip is less than 50 nT. In order to determine the cause of magnetic flux trapping and eliminate the adverse effects of the magnetic field, in-situ magnetic field measurements are required.

Figure 1c shows the X-ray spectra for some samples. The energy resolution of each pixel for the N-K line (392 eV) is 10.1 eV ± 0.9 eV in full width at half maximum. Figure 1d shows an example of measuring the N-K absorption edge of polyimide. By using a 200-pixel array, the time required to acquire data was cut in half.

Summary
XAS using STJ detector arrays is required for the analysis of light element dopants in advanced materials. In order to improve the detection sensitivity, we attempted to use a 200-pixel STJ detector. By improving the magnetic shielding and suppressing flux quantum traps, over 90% of the pixels are usable. Tests were performed at a synchrotron radiation beamline, and an energy resolution of 10.1 ± 0.9 eV for N-K lines was achieved, enabling XAS. The overall sensitivity is twice that of our previous 100-pixel system.

References

[1] M. Ohkubo et al., Nucl. Instrum. Methods. in Phys. Res. A 559-2 (2006) 731-733, doi:10.1016/j.nima.2005.12.120

[2] S. Friedrich, J. Synchrotron Rad.13 (2006) 159–171, doi:10.1107/S090904950504197X

[3] S. Shiki et al., AIP Conf. Proc. 1185 (2009) 409–412, doi:10.1063/1.3292365

[4] S. Shiki et al., J Low Temp Phys 167 (2012) 748–753, doi:10.1007/s10909-012-0526-6

[5] S. Shiki et al., J Low Temp Phys 176 (2014) 604–609, https://doi.org/10.1007/s10909-013-1074-4

[6] M. Ohkubo et al., Sci Rep 2 (2012) 831, doi:10.1038/srep00831

[7] N. Isomura and Y. Kimoto, J. Synchrotron Rad. 28 (2021) 1114–1118, doi:10.1107/S1600577521004008

[8] Y. Maeda et al., J. Phys. Chem. C 124-20 (2021) 11032-11039, doi:10.1021/acs.jpcc.0c02491

[9] G. Fujii et al., J Low Temp Phys 184 (2016) 194–199, doi:10.1007/s10909-015-1433-4

Acknowledgment

The authors thanks to Tomoaki Ishizuka in AIST for his support in the experiments. The device was fabricated in the CRAVITY in National Institute of Advanced Industrial Science and Technology (AIST). A part of this work was supported by "Advanced Research Infrastructure for Materials and Nanotechnology in Japan (ARIM)" of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), grant number JPMXP1222AT5041. This work was performed under the approval of the Photon Factory Program Advisory Committee (Proposal No. 2021G666).

Figure 1. (a) Microphotograph of a 200-pixel, 100 micoron square STJ detector array. (b) Ssetup of experiment. (c) Fluorescence X-ray spectrum of polyimide, teflon, aluminium, and stainless steels taken by STJ detector array (d) Partial fluorescence yield X-ray absorption spectrum of polyimide at nitrogen K edge obtained using the 200-pixel STJ array.

Keywords: X-ray spectroscopy, superconducting tunnel junction, synchrotron radiation, materials analysis

Abstract Body

Several types of superconducting detectors have successfully been utilized in various significant fields such as cosmology, dark matter, quantum communications, etc. due to their high performance [1,2,3,4,5]. The imaging by the superconducting detector was supposed to be limited at 20,000 pixels at most due to the heat inflow from room temperature to cryogenic detector through the readout wires. However, a paper claiming a 400,000-pixel camera with a delay-line technique was published in 2023 with a lot of attention [2]. A current-biased kinetic inductance detector CB-KID is a superconducting delay-line detector proposed by us to observe a local stimulus [1,5]. It uses a versatile 50-ohm matching impedance in signal transportation in the inside of the detector. A superior merit of CB-KID was demonstrated by realizing a neutron imager, i.e., it simultaneously realized a delay-line neutron transmission imaging system (an imaging function named as Role-I) and a time-of-flight energy dispersive spectroscopy system (a function named as Role-II) [1,6]. This must be useful in material sciences. CB-KID consists of orthogonal XY superconducting meanderlines and a neutron conversion layer, with a meander period of 1.5 μm (waveguide width 900 nm), a waveguide length of 151 m, and a sensing area of 15 mm × 15 mm. Initially, a signal arriving time to an end electrode (X or Y) was measured by a Kalliope-I readout circuit equipped with a 1ns time-to-digital converter TDC and the position was allocated to a certain segment as the smallest unit by the delay-line method. A higher spatial resolution requires a higher-temporal resolution. Since the propagation velocity along the waveguide of the signal is about 20% of the speed of light, and a position resolution remained at a level of 7 - 15 times of the meanderline period 1.5μm [1,6]. A rising time of the signal to a threshold value is slightly delayed from a nuclear-event-occurrence time. A time over threshold (ToT) acquisition of our Kalliope-I readout circuit has a dead zone, i.e., ToT is discarded when it is less than 8ns. Therefore, it could not be used for delay-time correction of the event time. ToT is a lasting time, where a signal amplitude stays over the TDC threshold. We developed a new 30ps operating Kalliope-II circuit to aim for a resolution limit down to 1.5μm. The Kalliope-II readout circuit utilized a high-resolution TDC (HR-TDC) and a front-end main circuit for continuous readout DAQ (AMANEQ) [7]. High-temporal 30ps resolution measurement of ToT enabled us to conduct a delay-time correction rather satisfactory. This significantly improved the positional resolution (Role-I). Thanks to delay-time correction, a typical data acquisition time of a high-resolution image with the Kalliope-II system effectually became faster than the Kalliope-I system. A Role-II function is also realized with HR-TDC. By referring to operating principle of CB-KID, one notices that the measurement object can be expanded not only to neutrons but also to other sorts of stimuli. The pixel size of the Kalliope-II imager becomes as small as 1.5 μm × 1.5 μm when it is determined by 30ps operating readout circuit. It is impressive to note that this corresponds to an untrodden 100,000,000-pixel camera of the 15 mm × 15 mm sensitive area as a Role-I CB-KID [1,6]. This is in marked contrast with performances achieved by other superconducting detectors [2,3,4,5]. The time-dependent spectroscopy with CB-KID becomes possible to find wide applications in other fields too as a Role-II CB-KID.

Acknowledgements: This research was supported by Grant-in-Aid for Scientific Research (A) (JP16H02450, JP21H04666), Grant-in-Aid for Early-Career Scientists (JP21K13566, JP23K13690) from Japan Society for the Promotion of Science, and J-PARC Project Research (2024P0501). We thank for the support provided by R. Honda, who developed the HR-TDC and AMANEQ circuits, in constructing the measurement system.

References

1. T. Ishida, Superconducting Neutron Detectors and Their Application to Imaging, IEICE Trans. Electron. E103.C(5) (2020) 198-203.

2. B. G. Oripov et al., Large-scale nanowire camera with a single-photon sensitivity, Nature 622(2023) 730-734.

3. C. Nones et al., High-impedance NbSi TES sensors for studying the cosmic microwave background radiation, Astron. Astrophys. 548 (2012) A17.

4. J. Chiles et al., New constraints on dark photon dark matter with superconducting nanowire detectors in an optical haloscope, Phys. Rev. Lett. 128 (2022) 231802.

5. F. Grünenfelder _et a_l., Fast single-photon detectors and real-time key distillation enable high secret-key-rate quantum key distribution systems, Nat. Photonics 17(2023) 422–426.

6. T. Ishida et al., Neutron Transmission CB‑KID Imager Using Samples Placed at RoomTemperature, J. Low Temp. Phys. 214 (2024) 152-157.

7. R. Honda, M. Ikeno, M. Shoji, T. Takahashi, 2021 Autumn Meeting of the Physical Society of Japan, 16pV1-10 (in Japanese).

Keywords: pulsed neutrons, transmission imaging, high spatial resolution, readout circuit.

Abstract Body

A Microwave Kinetic Inductance Detector (MKID) [1] is a superconducting detector and consists of single microwave readout line, resonators and antennas. The MKID is fabricated by depositing and patterning a thin superconducting film on a dielectric substrate, e.g., silicon (Si) or sapphire (Al₂O₃). The Cooper pairs breaking due to the irradiating photon is detected as a change in the kinetic inductance, i.e., a change in the microwave transmission characteristics. A substrate absorption type superconducting detector has been proposed for particle physics experiments, e.g., searching for dark matter. This device uses a superconductor to detect phonons generated in a substrate by irradiation photon. In our past experiments with superconducting tunnel junctions [2], we experimentally found single-crystal lithium niobate (LiNbO₃:LN), generally used as piezoelectric materials and nonlinear optical materials, had high phonon propagation anisotropy. To utilize this material as a substrate, it is expected the phonon collection efficiency can be improved. In this work, to apply a LN substrate to a MKID, we study characteristics of the Nb thin film and the MKID on a LN substrate.

Fabricating a MKID, we used Z-cut single-crystal stoichiometric LN substrates with 10 mm×10 mm×500 μmt, produced by Oxide Corporation. The relative permittivity of Z-cut LN substrates was obtained about 24 (cf. the value of Si is about 11.8). Using this permittivity, we designed a microwave readout line and λ/2 resonators with a microstrip structure. A superconducting material Nb was deposited by DC magnetron sputtering. The MKID design was patterned by a photolithography technique, reactive ion etching process or liftoff process.

We measured properties of 200 nmt Nb thin film, such as crystalline, surface roughness, critical temperature (T_c) and residual resistivity ratio (RRR), on Si or LN substrates. We performed X-ray diffraction measurements and found the Nb thin films deposited on each substrate had same [1 1 0] orientation. The results of atomic force microscope with dynamic force mode showed the thin films had similar roughness. To evaluate the T_c and RRR of the Nb thin film, we mounted fabricated devices in an open dewar filled with liquid helium and cooled them to 4.2 K. During the cooling process, we measured the temperature dependence of Nb thin film resistivity using the 4 terminal method (Fig.1). This measurement showed the T_c and RRR of each thin film were much the same. These results suggested the properties of Nb thin film on LN substrate was equivalent to them on Si substrate.

After the Nb thin film evaluation, we mounted the fabricated MKID (Fig.2) in a dilution refrigerator and cooled them to about 150 mK. We measured microwave transmittance in the frequency domain with a vector network analyzer. We confirmed the MKID on the LN substrate worked as resonators as well as on the Si substrate. Fitting resonant peaks, we obtained the internal quality factor about 200 whereas it reached about 600,000 on Si substrate. Since the properties of Nb thin film are comparable, we have concluded the LN substrate limits the device performance. As mentioned in Ref.[3], we also observed overlapping bulk acoustic mode resonances with the microwave transmission. This mode was excited by overlapping electric field with piezoelectric materials, e.g., LN, and induced microwave loss. We will report details of the MKID evaluation with a LN substrate.

References

[1] P. K. Day et al., Nature, 425, 817, 2003
[2] T. Taino et al., IEEE Trans. Appl. Supercond., 15, 2, 2005
[3] L. Yang et al., Phys. Rev. Appl., 20, 054026, 2023

Acknowledgment

This work was supported by RIKEN Junior Research Associate Program and KAKENHI Grant Number 22K18991, 21K18150, 20H01937 and 19H05809. Equipment shared by RIKEN CEMS Material Characterizations Support Team and Semiconductor Science Research Support Team was used.

Fig.1 Temperature dependence of Nb thin film resistivity on Si or LN substrates.
Fig.2 Appearance of the LN substrate MKID mounted on oxygen-free copper jig.

Keywords: microwave kinetic inductance detector, substrate absorption type superconducting detector, lithium niobate