It is well-known that superior energy resolution can be realized with radiation detectors that use superconducting tunnel junctions, as compared with semiconductor radiation detectors using semiconductors. The energy gap of a superconductor is small, on the order of 1/1000th of the energy gap of a semiconductor or less, and is considerably smaller than the maximum phonon energy; therefore, when radiation is absorbed in a superconductor, a 1000-times greater number of electrons than in the case of a semiconductor detector are excited above the energy gap. In a superconducting tunnel junction, the electrons excited above the energy gap can be extracted as signal charge via a tunneling effect. Therefore, a radiation detector using a superconducting tunnel junction can have extremely high sensitivity and high energy resolution (see Non-Patent Literature 1).
However, with a detector of the type in which radiation is directly absorbed in a single superconducting tunnel junction to measure its energy, because the superconducting tunnel junction has a small surface area on the order of several hundred micrometers×several hundred micrometers or less and a thickness on the order of several hundred nanometers, the detection efficiency is extremely low, at one part in several thousand compared with that of a semiconductor detector.
To overcome these drawbacks of radiation detectors using superconducting tunnel junctions, a superconducting series-junction detector has been developed (for example, see Patent Literature 1 and Patent Literature 2). The superconducting series-junction detector is provided with a superconducting series junction in which a large number of superconducting tunnel junctions are connected in series or in parallel on at least one surface of a single-crystal insulator or semiconductor substrate. The superconducting series junction mentioned here includes not only junctions connected in a single series, as in Patent Literature 1, but also includes multiple series junctions connected in parallel. The substrate thickness is, for example, about 400 micrometers. In the superconducting series-junction detector, radiation is absorbed in a radiation absorber, such as the substrate, where the energy thereof is converted to phonons, the phonons are absorbed in the superconducting tunnel junctions formed on the surface of the radiation absorber, exciting electrons in the superconductor, and those excited electrons are extracted as signal charge via the tunneling effect. The energy of the radiation is measured from the magnitude of that signal charge. Because the phonons are absorbed in a large number of superconducting tunnel junctions, the effective area of the detector can be made large. In addition, because the thickness of the radiation absorber is the thickness of the detector, the radiation absorption efficiency can also be increased.
A superconducting tunnel junction is also effective for an optical sensor. When a superconducting series junction is used as an optical sensor, the sensor effective area is large; therefore, in this case too, an advantage is afforded in that the detection efficiency is high. In the present invention, light such as ultraviolet light, visible light, infrared light, which are electromagnetic waves similar to X-rays etc., are also included in the definition of radiation.
With regard to radiation detectors making use of the fact that a large number of electrons are excited in a superconductor by radiation, besides superconducting tunnel junction detectors, for example, kinetic induction detectors (Kinetic Inductance Detector) are also well-known (for example, see Non-Patent Literature 2). Kinetic inductance detectors are also known as microwave kinetic inductance detectors (Microwave Kinetic Inductance Detector). In a kinetic inductance detector, when electrons are excited in a superconducting film of a resonator by radiation, the kinetic inductance of the superconductor resonator changes. The resonance frequency and Q factor of the superconducting resonator change due to this change in the kinetic inductance. The amount of radiation or the energy per individual unit of radiation is measured from the magnitude of the changes in resonance frequency and Q factor (for example, see Non-Patent Literature 3).
In other words, in a kinetic inductance detector and a superconducting tunnel junction detector, although the signal extraction methods are different, what they have in common is that the basis of the signal is excited electrons in the superconductor.
In the case of a kinetic inductance detector, similar to the case of a superconducting series-junction detector, radiation can be absorbed in a semiconductor or insulator single-crystal radiation absorber, where the energy thereof is converted to phonons, these phonons are absorbed in a phonon-absorbing superconducting film provided on the surface of the radiation absorber, exciting electrons, and the radiation can also be detected using changes in the resonance frequency and Q factor of a resonator formed to include this superconducting film (for example, see Non-Patent Literature 4 and Non-Patent Literature 5).
In other words, with the phonon-mediated kinetic inductance detector and the superconducting series-junction detector, although the signal extraction methods differ, what they have in common is that the basis of the signal is excited electrons that are excited in the superconducting film on the substrate surface by phonons generated by radiation absorbed in the substrate. Note that, in the case of the superconducting series junction, the superconducting film on the substrate surface is a superconducting lower electrode of the superconducting tunnel junction.
In the case of the phonon-mediated kinetic inductance detector, it is necessary to provide the superconductor for absorbing phonons on the surface of the radiation absorber substrate; however, with regard to the entire resonator, in some cases the entire resonator is provided on that substrate surface (for example, see Non-Patent Literature 4), and in some cases, some elements of the resonator are provided on another substrate close to that substrate (for example, see Non-Patent Literature 5). FIGS. 1 to 4 show example structures of these kinetic inductance detectors. FIG. 1 and FIG. 2 are an example in which the entire resonator is provided on the surface of the radiation absorber substrate, where FIG. 1 is a plan view, and FIG. 2 is a sectional view taken through part A-A in FIG. 1. FIG. 3 and FIG. 4 are an example in which part of the resonator is provided on another substrate close to the radiation absorber substrate. FIG. 3 is a plan view of the other substrate close to the radiation absorber substrate, and FIG. 4 is a sectional view of the entire resonator, corresponding to part A-A in FIG. 3, including both a radiation absorber substrate 4 and another substrate 5 close to it. 1 represents a through-line formed of a superconducting film for supplying microwaves and measuring the Q factor and resonance frequency, 2 represents a phonon-absorbing superconducting film, and 3 represents a high-Q-factor superconducting resonator line. The through-line 1 and the high-Q-factor superconducting resonator line 3 are capacitively coupled. In addition, in this case also, the phonon sensor 2 is bonded to the surface of the radiation absorber 4.
In the superconducting series-junction detector and the kinetic inductance detector, radiation is absorbed in the radiation absorber, such as a semiconductor or insulator substrate, where the energy thereof is converted to phonons, the phonons are absorbed in the superconducting tunnel junction or phonon-absorbing superconductor formed on the surface of the radiation absorber, exciting electrons in the superconductor, and these excited electrons are extracted as signal charge via the tunneling effect, or a change in the characteristics of the superconducting resonator due to the excited electrons is extracted as a signal. The radiation energy is measured from the magnitude of this signal. However, this does not mean that all of the radiation energy is directly converted to phonons. This can be understood from the principle of a semiconductor detector that does not use phonons. In the case of a semiconductor detector, an electric field is applied to the detector to collect electrons and holes excited by radiation, and the radiation energy is measured from the magnitude of this signal charge. On the other hand, in the case of a radiation detector in which a signal is generated by phonons, such as the superconducting series-junction detector, the electrons and holes in the radiation absorber cannot contribute to the signal charge unless their energy is converted to phonons.
The processing time for the signal from the superconducting series-junction detector and the kinetic inductance detector is normally from several microseconds to several tens of microseconds. Therefore, in the case of these superconducting radiation detectors, the radiation energy expended in generating electrons and holes in the radiation absorber, such as a semiconductor or insulator, by the radiation does not contribute to the signal unless the electrons and holes recombine within the time for extracting and processing the signal, and the energy is thereof is converted to phonons. However, the lifetime of excited electrons in a high-purity silicon substrate, in other words, the electron-hole recombination time, is from several tens of microseconds to 1 millisecond or longer.
In addition, for example, in the case where the radiation absorber is semiconductor silicon and where the radiation is X-rays, it is well-known that about 30% of the X-ray energy is imparted to electrons, and the remaining about 70% is expended in generating phonons. However, this energy distribution ratio statistically fluctuates for each unit of radiation even though the radiation energy is constant, and this statistical fluctuation degrades the energy resolution.
Unless the excitation energy possessed by electrons excited by radiation such as X-rays and light is released in the form of phonons within the signal measurement time, there is a problem in that, not only does the energy resolution of these detectors deteriorate, but also the sensitivity is reduced by a corresponding amount.