Dr. Masataka Ohkubo, leader and his colleagues of the Super-Spectroscopy System Research Group (SSSRG), the Research Institute of Instrumentation Frontier (RIIF), the National Institute of Advanced Industrial Science and Technology (AIST), an independent administrative institution, have implemented the installation technology for multi-cable (involving 100 coaxial cables) leading fast pulse signals out of 0.3 K cryogenic environment to room temperature one, needed for the development of advanced mass spectrometers (Fig. 1).

The superconducting detectors have excellent performance of detecting soft X rays and giant macromolecules, which was not available with the conventional semiconductor-based detectors in the X-ray spectrometry and the mass spectrometry. However, the system requires a cryogenic environment of temperature as low as 0.3 K and as the effective area of a single superconducting detector is as small as hundreds of µm at the largest, an array of 100 superconducting detectors is required for the practical application of mass spectrometer with 100 % particle detecting efficiency.

In order to use the 100 ssuperconducting detectors, a fast communication technology to link up the 0.3 K cryogenic environment with the room temperature ambient is needed. However, heat flow through the cable has been too great to implement the system through the conventional technology.

In the present study, the heat flow has been suppressed to less than 5.4 µW by using 100 coaxial cables as thin as 0.33 mm diameter, and making conductor from lower heat conduction metal. The feasibility of installation has been verified through the measurement of thermal conductivity with the coaxial cable under the cryogenic environment.

On the basis of these results, the prospect has been opened for practical use of the time-of-flight mass spectrometer (TOF MS) with ultimate sensitivity and 100 % particle detection efficiency covering particles from atoms to macromolecules such as protein, irrespective of molecular weight and molecular species.

Fig. 1 Outer view and interior of a cryostat with 100 coaxial cables (middle), an array of superconducting particle detectors for TOF mass spectrometer under development (left), and an actual cooling curve (right)

Under the 0.3 K cryogenic environment, thermal noise is reduced to as low as 0.026 meV in comparison to 26 meV at room temperature, making it possible to implement the measuring performance not available with the room temperature operation, and to develop the noiseless measuring instrument practically not affected by thermal noises. Particularly, in the time-of-flight mass spectrometer (TOF-MS) where molecular species is identified based on the time of flight of ionized and then accelerated molecules, the use of superconducting detector under the cryogenic environment makes it possible to achieve 100 % particle detecting efficiency for particles ranging from atoms to giant macromolecules of protein, irrespective of molecular weight and molecular species.

In the TOF-MS, it is essential to exactly determine time information of ions arriving at the detector at an accuracy of a few ns (1 ns = 1 nano-second = 10^-9 second). However, no technology has been available so far, to install a cryogenic high frequency multi-cable system linking the superconducting detector array with the semiconductor-based electronics signal processing system working at room temperature. The most serious problem for the installation of the 100 coaxial cables is to suppress heat flow to the superconducting sensors placed in the 0.3 K cryogenic environment. For instance, along a single copper conductor of 100 µm diameter and 30 cm length heat flow occurs as much as around 100 µW. With 100 conductors, and with inevitably thicker coaxial cable, the flow will amount to dozens of mW or more, spoiling the use of array detectors.

The RIIF-AIST demonstrated earlier that the superconducting detector had the performance surpassing the limit of conventional technology. The application of such an excellent performance to the actual measuring instruments had to face certain hurdles such as 0.3 K cryogenic environment and leading signals from the cryogenic environment to room temperature. The outcome of the R&D efforts on these matters was obtained under the AIST's original project "Development of Multi-dimensional Information Time-of-Flight Mass Spectrometry (Super-TOF), fiscal years 2002-05".

The superconducting detectors have excellent performance of detecting soft X rays and macromolecules, which was not available, in principle, with the conventional semiconductor-based detectors in the X-ray spectroanalysis⁽¹⁾ and the mass spectrometry⁽²⁾. However, the system requires a cryogenic environment of temperature as low as 0.3 K, which is realized by the use of a cryostat based on ³He, an isotope of helium, for instance.

On the other hand, the effective area of a single superconducting detector is as small as hundreds of µm at the largest. Accordingly, an array of 100 superconducting detectors is required for the practical application of mass spectrometer with 100 % particle detecting efficiency. Besides, in the TOF MS, it is necessary to determine time for particles arriving at the detector with a resolution as fine as a few ns, and to achieve this, taking fast pulse signals out of the cryogenic environment to room temperature is needed. For the array of 100 superconducting detector devices, 100 coaxial cables are to be led out of the cryogenic environment for fast signal transmission, and in order to keep the cryostat in stable operation, it is essential to hold heat flow at less than 100 nW per coaxial cable. It has been considered impossible to install so many coaxial cables in such a scale.

In the present study, the heat flow per cable has been suppressed to less than 54 nW by reducing cable size to 0.33 mm in diameter and by using metal conductor of lower thermal conductivity. In parallel with this, the feasibility of installing the coaxial cable to the superconducting detector has been confirmed by testing the thermal conductivity of the cable under the cryogenic environment. While the thermal conductivity under the cryogenic conditions has been measured with metal materials and other raw materials for the coaxial cable up to now, no data of ultra-fine coaxial cables after the final fabrication has been reported.

The thermal conductance, an index for heat transfer, is affected by the microscopic structure of the materials. It may be expected, therefore, the thermal conductivity is changed extensively through the preparation of the ultra-fine coaxial cables involving large strain. For this reason, the thermal conductivity data for raw materials is to be regarded inadequate, and the value after the cable fabrication is to be measured under the actual conditions.

Fig. 2 Measurement of thermal conductance of ultra-fine coaxial cable of 0.33 mm diameter

For this purpose, a new tool has been developed to determine small heat flow. Fig. 2 shows the measurements of thermal conductance for two kinds of coaxial cables. If the cable length and the difference in operating temperatures are known, it is possible to estimate the exact heat flow with the measurement data. In the present study, the "coaxial cable 1" was installed in consideration of high frequency characteristics. The heat flow of a single cable of this type was found to be 54 nW, indication the feasibility of installing up to 100 coaxial cables to the cryostat for the TOF-MS.

The outcome of this R&D work will be utilized for the implementation of superconductivity time-of-flight mass spectrometer for the practical application. It is expected that if cables labeled with thermal conductivity data for temperatures from 0.3 K to the room temperature can be supplied, the user can estimate the exact heat flow without doing preliminary experiments, contributing to the spread of superconducting devices working under the cryogenic environment and making the ultra-high accuracy measurement practicable.