# Patent

## Title
Methods for detecting photons, radiations or neutrons using superconductors and methods for obtaining two-dimensional images thereof

## Abstract

In the improved neutron image detector, MgB2 enriched in a constituent element 10B of wide energy gap is used as a neutron detection plate, which is provided at the center and the four corners with a phonon sensor comprising an insulation layer overlaid with Mg11B2 enriched in 11B of narrow energy gap in order to detect phonons resulting from the generation of α-rays which occurs in the detection plate upon incidence of neutrons, and sensor's signal intensity and signal propagation time are used to detect the incident position of neutrons.

## Background
This application is based upon and claims the benefit of priority from Japanese Patent Application No. 195150/2001, filed Jun. 27, 2001, the entire contents of this application are incorporated herein by reference.
BACKGROUND OF THE INVENTION
This invention relates to methods for detecting photons, radiations or neutrons using superconductors and methods for obtaining two-dimensional images thereof. The invention is particularly characterized by using MgB2 as a superconductor having isotopes that become adequately superconducting at the temperature of liquid helium. The invention also realizes imaging with high position resolution by using a two-dimensional imaging method that detects phonons with a superconductor sensor as they are generated from a superconducting or single-crystal detection medium. The invention is therefore applicable to the fabrication of sensors for use in experiments or analyses with photons shorter than a few microns, X-rays or neutrons. It also finds utility in analytical techniques that take advantage of its ability to detect both the incident energy and position of photons, radiations or neutrons.
An X-ray detector of high energy resolution using a Nb/Al/Al2O3/Al/Nb tunnel junction device has been developed by Matsumura et al. (Matsumura et al., Nucl. Instrum. & Methods, A329 (1993) 227) and other researchers and used as a superconductor-based radiation detector. An imaging detector for detecting the incident energy and position of a radiation with the intermediary of phonons generated from a superconducting or single-crystal radiation detection medium has been developed by Kurakado et al. (Kurakado et al., Rev. Sci. Instrum. 62 (1991) 156) and other researchers.
In the contemplated prior art technology, Nb has been used as a superconductor; particularly, in the case of detectors using a superconducting tunnel junction, it was necessary to cool them to a temperature about a tenth of the critical temperature before use but cooling to about 0.4 K was time-consuming and required an expensive apparatus. It has therefore been desired to develop a superconducting detector that can be operated at higher temperatures. In addition, neutron imaging detectors were scanty that used superconductors. What is more, it has been very difficult to develop an imaging detector capable of detecting X-rays or neutrons at high position resolution.
SUMMARY OF THE INVENTION
The invention provides a superconducting detector device that uses MgB2 with a critical temperature above 30 K which has isotopes 10B and 11B with different energy gaps. The device has an incomparable detecting function and can be operated at the temperature of liquid helium. If MgB2 is enriched in 10B rather than 11B, a neutron detector and a neutron imaging detector can be constructed.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a photon and radiation detector using a superconductor tunnel junction device that operates on an isotope effect for providing two superconductors with different energy gaps;
FIG. 2 is a neutron detector using a superconductor tunnel junction device which operates on the isotope effect of MgB2 containing boron as a neutron converter in order to provide two superconductors with different energy gaps;
FIG. 3 is a neutron detector having neutron sensitivity only on one side and using a superconductor tunnel junction device which operates on the isotope effect of MgB2 containing boron as a neutron converter in order to provide two superconductors having different energy gaps;
FIG. 4 is a three-layered neutron detector having neutron sensitivity only on one side and using a superconductor tunnel junction device which operates on the isotope effect of MgB2 containing boron as a neutron converter in order to provide two superconductors with different energy gaps;
FIG. 5 is a photon, radiation and neutron detector using a superconductor tunnel junction device which incorporates a gradient superconductor with a gradual compositional transition in the direction of thickness from a superconductor enriched in 10B of wide energy gap to a superconductor enriched in 11B of narrow energy gap;
FIG. 6 is a photon, radiation and neutron image detector using a superconductor tunnel junction device which incorporates as a detection medium a gradient superconductor with a gradual compositional transition in a two-dimensional plane from a superconductor enriched in 10B of wide energy gap to a superconductor enriched in 11B of narrow energy gap;
FIG. 7 is a photon, radiation and neutron image detector using a superconductor tunnel junction device which incorporates as a quasiparticle trapping layer a gradient superconductor with a gradual compositional transition in a two-dimensional plane from a superconductor enriched in 10B of wide energy gap to a superconductor enriched in 11B of narrow energy gap;
FIG. 8 is a radiation image detector using a superconducting tunnel junction device to detect the incident position of photons or radiations with the intermediary of phonons generated from an isotopic superconductor of wide energy gap as a detection plate;
FIG. 9 is a neutron image detector using a superconducting tunnel junction device to detect the incident position of neutrons with the intermediary of phonons generated from a 10B enriched isotopic superconductor as a neutron detection plate;
FIG. 10 is a neutron image detector using a superconducting tunnel junction device to detect the incident position of neutrons by means of a three-layered superconducting sensor with the intermediary of phonons generated from a 10B enriched isotopic superconductor as a neutron detection plate;
FIG. 11 is a neutron image detector using a superconducting tunnel junction device to detect the incident position of neutrons by means of a superconducting sensor of an SIS structure with the intermediary of phonons generated from a 10B enriched isotopic superconductor as a neutron detection plate;
FIG. 12 is a neutron image detector using a superconducting tunnel junction device to detect the incident position of neutrons by means of a superconducting sensor of an SIS structure with the intermediary of phonons generated from a 10B containing single LBO crystal as a neutron detection plate;
FIG. 13 is a fast neutron image detector using a superconducting tunnel junction device to detect the incident position of neutrons by means of a superconducting sensor of an SIS structure with the intermediary of phonons generated from a hydrogen (H) containing plastic material as a fast neutron detection plate;
FIG. 14 is a neutron image detector using a superconducting tunnel junction device to detect the incident position of neutrons by means of a phonon detector device of a bolometer structure with the intermediary of phonons generated from a 10B enriched isotopic superconductor as a neutron detection plate; and
FIG. 15 is a neutron image detector using a superconducting tunnel junction device to detect the incident position of neutrons by means of a phonon detector device of a calorimeter type with the intermediary of phonons generated from a 10B enriched isotopic superconductor as a neutron detection plate.



DESCRIPTION OF THE PREFERRED EMBODIMENTS
EXAMPLE 1
The first example of the invention as it relates to a photon, radiation and neutron detector is described below with reference to FIG. 1. In this example, MgB2 is used as a superconductor having a higher critical temperature than conventional superconductors such as Nb. The superconductor MgB2 can be prepared in two forms enriched in 10B and 11B, respectively. The isotope effect realizes two superconductors 10B and 11B having different energy gaps. This effect was used to fabricate a five-layered superconducting tunnel junction device comprising in the order written an isotopic superconductor Mg10B2 of wide energy gap, an isotopic superconductor Mg11B2 of narrow energy gap, an insulation layer, an isotopic superconductor Mg11B2 of narrow energy gap and an isotopic superconductor Mg10B2 of wide energy gap. The Mg10B2 layers were each about 500 nm thick and the Mg11B2 layers were each about 50 nm thick. The insulation layer was about 2 nm thick. The isotopic superconductor Mg11B2 of narrow energy gap formed a quasiparticle trapping layer which trapped the quasiparticles generated from the isotopic superconductor Mg10B2 of wide energy gap and this enabled efficient detection of photons or radiations. Since the superconductor layer working as the detection medium and the trapping layer were made of isotopic superconductors, they were easier to make than the conventional Nb/Al layer and their continuity assured more efficient device operation. The design of Example 1 allowed for fabrication of a radiation detector capable of measuring the energy of radiations.
EXAMPLE 2
The second example of the invention as it relates to a neutron detector is described below with reference to FIG. 2. Example 2 is identical to Example 1 except for the use of an isotopic superconductor Mg10B2 containing the neutron converter boron (10B) as a constituent element. By using the isotopic superconductor Mg10B2 to detect the α-rays emitted from 10B having a large neutron absorption cross-sectional area, we could fabricate a neutron detector using a superconducting tunnel junction device to be capable of efficient neutron detection.
EXAMPLE 3
The third example of the invention as it relates to a neutron detector is described below with reference to FIG. 3. In this example, an isotopic superconductor Mg10B2 containing boron (B) as a constituent element was used to fabricate a four-layered superconducting tunnel junction device comprising in the order written an isotopic superconductor Mg10B2 of wide energy gap, an isotopic superconductor Mg11B2 of narrow energy gap, an insulation layer and an isotopic superconductor Mg11B2 of narrow energy gap. Mg11B2 formed a quasiparticle trapping layer. Since the Mg11B2 on one side rendered insensitivity to neutrons, the neutron detector of Example 3 was sensitive to neutrons only on the other side. The individual layers of the detector had the same thicknesses as in Example 1.
EXAMPLE 4
The fourth example of the invention as it relates to a neutron detector is described below with reference to FIG. 4. In this example, an isotopic superconductor Mg10B2 containing boron (B) as a constituent element was used to fabricate a three-layered superconducting tunnel junction device comprising in the order written an isotopic superconductor Mg10B2 of wide energy gap, an insulation layer and an isotopic superconductor Mg11B2 of narrow energy gap. Since the Mg11B2 on one side rendered insensitivity to neutrons, the neutron detector of Example 3 was sensitive to neutrons only on the other side. The individual layers of the detector had the same thicknesses as in Example 1.
EXAMPLE 5
The fifth example of the invention as it relates to a photon, X-ray and neutron detector is described below with reference to FIG. 5. In this example, an isotopic superconductor MgB2 containing boron (B) as a constituent element has the isotope effect which realizes two superconductors having different energy gaps. This effect was used to fabricate a three-layered superconducting tunnel junction device comprising in the order written an Mg10B2/Mg11B2 gradient superconductor with a gradual compositional transition in the direction of thickness from a superconductor enriched in 10B of the wider energy gap to a superconductor enriched in 11B of the narrower energy gap, an insulation layer and an Mg10B2/Mg11B2 gradient superconductor with a gradual compositional transition in the direction of thickness from a superconductor enriched in 10B of the wider energy gap to a superconductor 11B of the narrower energy gap. The two gradient superconductor layers were each about 500 nm thick and the insulation layer was about 2 nm thick. The quasiparticles generated in each gradient superconductor would migrate to the isotopic superconductor of the narrower energy gap, thereby allowing for efficient detection of photons, radiations or neutrons by the superconducting tunnel junction.
EXAMPLE 6
The sixth example of the invention as it relates to a neutron image sensor is described below with reference to FIG. 6. In this example, an isotopic superconductor MgB2 containing boron (B) as a constituent element has the isotope effect which realizes two superconductors having different energy gaps. This effect was used to fabricate a three-layered superconducting tunnel junction device comprising in the order written an Mg10B2/Mg11B2 gradient superconductor with a gradual compositional transition in a two-dimensional plane from a superconductor enriched in 10B of the wider energy gap to a superconductor enriched in 11B of the narrower energy gap, an insulation layer and an Mg10B2/Mg11B2 gradient superconductor with a gradual compositional transition in a two-dimensional plane from a superconductor enriched in 10B of the wider energy gap to a superconductor enriched in 11B of the narrower energy gap. The two gradient superconductor layers were each about 500 nm thick and the insulation layer was about 2 nm thick. On account of the Mg10B2/Mg11B2 gradient superconductor, different energy gaps, hence, varying pulse peaks would be output depending upon the incident position of photons, radiations or neutrons; as a result, photons or radiations incident with specified energy or neutrons emitting a specified energy of α-rays could be detected for their image at high position resolution. This design enables the fabrication of a photon, radiation and neutron image detector using a superconducting tunnel junction device.
EXAMPLE 7
The seventh example of the invention as it relates to a neutron image sensor is described below with reference to FIG. 7. In this example, there is fabricated a five-layered superconducting tunnel junction device comprising in the order written an isotopic superconductor Mg10B2 as a detection medium that is enriched in 10B of wide energy gap, an Mg10B2/Mg11B2 gradient superconductor with a gradual compositional transition in a two-dimensional plane from a superconductor enriched in 10B of the wider energy gap to a superconductor enriched in 11B of the narrower energy gap, an insulation layer, an Mg10B2/Mg11B2 gradient superconductor with a gradual compositional transition in a two-dimensional plane from a superconductor enriched in 10B of the wider energy gap to a superconductor enriched in 11B of the narrower energy gap, and the other detection medium made of Mg10B2. The two layers of the isotopic superconductor Mg10B2 enriched in 10B of wide energy gap were each 500 nm thick, the gradient superconductor layers were each about 50 nm thick, and the insulation layer was about 2 nm thick. The Mg10B2/Mg11B2 gradient superconductors worked as quasiparticle trapping layers, so when the quasiparticles generated within Mg10B2 were detected in the superconducting tunnel junction, different energy gaps, hence, varying pulse peaks would be outputted depending upon the incident position of photons, radiations or neutrons; as a result, photons or radiations incident with specified energy or neutrons emitting a specified energy of α-rays could be detected for their image at high position resolution. This design again enables the fabrication of a photon, radiation or neutron image detector using a superconducting tunnel junction device.
EXAMPLE 8
The eighth example of the invention as it relates to a radiation image detector is described below with reference to FIG. 8. The isotopic superconductor Mg10B2 enriched in 10B of wide energy gap has a different energy gap than the isotopic superconductor Mg11B2 enriched in 11B of narrow energy gap. To take advantage of this effect, the isotopic superconductor Mg10B2 of wide energy gap was used as a detection plate which was provided with a phonon sensor at the center and the four corners in order to detect phonons generated in the plate which it was illuminated with a radiation. The phonon sensor had a two-layered structure consisting of an insulation layer overlaid with the isotopic superconductor Mg11B2 enriched in 11B of narrow energy gap. Both the intensity and the propagation time of signals from the phonon sensors were both used to detect the incident position of a radiation. The detection plate made of the isotopic superconductor Mg10B2 measured 2 mmL×2 mmW×0.2 mmT; the Mg11B2 layer was 50 nm thick and the insulation layer was 2 nm thick. This design enables the fabrication of a radiation image detector using a superconducting tunnel junction device. The energy of an incident radiation could be measured by analyzing the output signal from each phonon sensor.
EXAMPLE 9
The ninth example of the invention as it relates to a neutron image detector is described below with reference to FIG. 9. This example is identical to Example 8. Since the isotopic superconductor Mg10B2 containing the neutron converter boron (10B) as a constituent element is used as the neutron detection plate, α-particles emitted from 10B having a large neutron cross-sectional area are detected in the isotopic superconductor Mg10B2 layer formed in a thickness of 0.2 mm and efficient neutron detection can be accomplished by the neutron image detector of Example 9.
EXAMPLE 10
The tenth example of the invention as it relates to a neutron image detector is described below with reference to FIG. 10. This example is identical to Example 9, except that the phonon sensor has a three-layered structure consisting of the isotopic superconductor Mg11B2 enriched in 11B of narrow energy gap, an insulation layer and the isotopic superconductor Mg11B2 enriched in 11B of narrow energy gap. The phonon sensor having this configuration is capable of producing higher signal outputs than in Example 8 and, hence, the neutron image detector of Example 9 has even better sensitivity than that of Example 8. The above-described design enables the fabrication of a neutron image detector using a superconducting tunnel junction device.
EXAMPLE 11
The eleventh example of the invention as it relates to a neutron image detector is described below with reference to FIG. 11. This example is identical to Example 9, except that the phonon sensor has an SIS structure consisting of a superconductor (S), an insulator (I) and a superconductor (S). The SIS structure may be a Nb/Al/Al2O3/Al/Nb structure. The phonon sensor having this configuration has a narrower energy gap and, hence, is capable of producing higher signal outputs than in Example 8, provided that it operates at 0.4 K and below. The above-described design enables the fabrication of a neutron image detector using a superconducting tunnel junction device.
EXAMPLE 12
The twelfth example of the invention as it relates to a neutron image detector is described below with reference to FIG. 12. In this example, a 10B containing single-crystal LBO mass (LiB3O5 or Li2B4O7) is used as a neutron detection plate. In order to detect phonons generated when it is illuminated with neutrons, the detection plate is provided with a phonon sensor having a Nb/Al/Al2O3/Al/Nb structure as an SIS structure. This phonon sensor is provided at the center and the four corners of the detection plate and uses the intensity and the propagation time of output signals to detect the incident position of neutrons. The above-described design enables the fabrication of a neutron image detector using a superconducting tunnel junction device.
EXAMPLE 13
The thirteenth example of the invention as it relates to a fast neutron image detector is described below with reference to FIG. 13. In this example, a hydrogen (H) containing plastic material is used as a fast neutron detection plate. In order to detect phonons due to the generation of protons (p) in the (n,p) reaction which occurs when it is illuminated with neutrons, the detection plate is provided with a phonon sensor having a Nb/Al/Al2O3/Al/Nb structure as an SIS structure. This phonon sensor is provided at the center and the four corners of the detection plate and uses the intensity and the propagation time of output signals to detect the incident position of fast neutrons. The hydrogen (H) containing plastic detection plate measures 2 mmL×2 mmW×0.2 mmT. The above-described design enables the fabrication of a fast neutron image detector using a superconducting tunnel junction device.
EXAMPLE 14
The fourteenth example of the invention as it relates to a 2D neutron image detector is described below with reference to FIG. 14. This example is the same as Example 9, except that a bolometer device is used as the phonon sensor. The bolometer device is a neutron-producing nuclear transmutation Ge bolometer. The device is a sensor capable of highly sensitive detection of temperature elevations due to phonon signals through measurement of changes in resistance. The above-described design enables the fabrication of a neutron image detector using a superconducting tunnel junction device.
EXAMPLE 15
The fifteenth example of the invention as it relates to a 2D neutron image detector is described below with reference to FIG. 15. This example is the same as Example 9, except that a calorimeter device is used as the phonon sensor. The calorimeter device is a transition edge (TES) device using a tungsten superconductor. The device is a sensor capable of highly sensitive detection of temperature elevations due to phonon signals through measurement of changes in resistance. The above-described design enables the fabrication of a neutron image detector using a superconducting tunnel junction device.

## Claims

1. A photon or radiation detector using a superconducting tunnel junction device which, taking advantage of the isotopic effect providing two superconductors with different energy gaps, has a five-layered structure comprising in the order written an isotopic superconductor of wide energy gap, an isotopic superconductor of small energy gap, an insulation layer, an isotopic superconductor of narrow energy gap and an isotopic superconductor of wide energy gap.