WO1991006016A1

WO1991006016A1 - Superconductive radiation detector

Info

Publication number: WO1991006016A1
Application number: PCT/CH1990/000244
Authority: WO
Inventors: Damian Twerenbold
Original assignee: Damian Twerenbold
Priority date: 1989-10-19
Filing date: 1990-10-19
Publication date: 1991-05-02

Abstract

A radiation detector with a crystalline body (10) of low specific heat has a temperature measurement element (4) and a cryostatic temperature regulator (9) for setting a predetermined operating temperature. The crystalline body (10) ist further provided with a superconductive layer (1) that is not connected with an external electric potential. To control the device, the cryostatic temperature regulator (9) is set below the transition temperature of the superconductor (1). The superconductive layer (1) is exposed to a micro-wave field (1). The thickness of the superconductive layer (1) corresponds approximately to the LONDON depth of penetration of the micro-wave radiation applied to that layer. A second superconductive layer (2) is preferably arranged between the crystalline body (10) and the first superconductive layer (1). This second layer is thicker than the first layer (1). This detector is less limited than known detectors as far as its dimensions and working temperature are concerned, and has therefore an extraordinarily wide range of aplications. Besides being useful to measure particle radiation, such as gamma or X-ray radiation, the detector may also be used to measure the low radiant flux of ultraviolet radiation, as well as an extremely long wave infra-red radiation with approximately 1 mm wavelength.

Description

Superconducting radiation detector

The invention relates to a superconducting radiation detector as defined in the preamble of claim 1.

In order to achieve a particularly high response sensitivity, radiation detectors are known which are operated at temperatures near absolute zero. A known embodiment is the bolometer, in which the energy of absorbed particles is measured via the temperature rise induced in the process. In addition to this thermal measurement method, there are also superconducting radiation detectors, which are based on a non-thermal measurement principle in that the absorbed energy to be measured by the sensor is converted into surplus quasi-particles by breaking up Cooper pairs. The excitation energy of a quasi-particle is given by the energy gap "delta" of the superconductor used, which corresponds to half the binding energy of a Cooper pair. The minimum resolution of this sensor is determined by the smallest ionization energy of the detector material used. This smallest ionization energy is proportional to the energy gap "delta". In the practical example, it is on the order of 1 meV. The good intrinsic energy resolution is advantageous in both versions.

However, the practical applications for both types of detectors are limited, since in the case of the bolometer the operation at extremely low temperatures of less than 10 mK or in the case of the superconducting tunnel contacts to small dimensions, namely to a layer thickness of less than 1 μm and thus to one insufficient usable volume is limited. It is an object of the present invention to develop a radiation detector which has at least the same good resolution as known sensors, but is less restricted in terms of dimensions and operating temperature.

This object is achieved by a detector with the features of patent claim 1. The decisive advantage of this detector consists in a useful signal that is several times larger than known detectors with comparable sensitivity and in the extremely wide range of possible applications with regard to the response energy, which is ultimately limited by the value 2 "delta". In addition to measuring particle radiation, such as gamma radiation or X-rays, the detector is also suitable for measuring a low radiation flow from ultraviolet radiation to extremely long-wave infrared radiation of about 1 mm wavelength.

Details and further advantages of the invention result from the following description of exemplary embodiments.

The figures show:

1 shows the basic structure of a radiation detector according to the invention,

2 shows the representation of different energy levels in different areas of the detector according to FIG. 1,

3 shows the example of a large-volume gamma radiation detector, and

Fig. 4 shows the example of a small-volume detector for the

X-ray measurement, with the possibility of training as a detector-A ray. The basic structure of the detector is shown first with reference to FIG. 1. Similar to a bolometer, the base is a crystal body 10 made from an electrically non-conductive crystal with low specific heat, for example from a silicon or germanium crystal or from beryllium, sapphire or diamond. At least one layer 1 of superconducting material is applied to the surface of the crystal body. Alternatively, a second layer 2 of superconducting material can be arranged between the crystal 10 and the layer 1. For reasons explained later, the superconducting energy gap “delta” _{2 of} the second layer 2 should be greater than the energy gap “delta” ι of the first layer 1.

In the crystal cell 10 there is a temperature sensitive element 4, e.g. a thermo-electrical or optical converter, such as a thermistor or a ferroelectric dielectric, or a superconducting bolometer, the taps 5A, 5B of which are led to the outside to a measuring sensor 6. The converter 4 is e.g. formed by heavily doping an area within the semiconductor crystal 10. A ferroelectric dielectric can also be applied epitaxially.

In a departure from a conventional bolometer, the coated crystal structure 10 described is arranged in the effective range of a microwave field 8 emanating from a microwave generator 7. A cryostat 9 keeps the assembly at the desired operating temperature, e.g. to a temperature in the range between 0.1 and 1.0 K.

The effect of microwave radiation on the resistance behavior of superconductors is basically described in: F. London: "Superfluids, Macroscopic Theory of Superconductivity", Vol. 1, Dover Publications, New York 1961. There is a "L0ND0N penetration depth" of the microwaves into a superconducting layer Are defined. In the configuration described above, the first superconducting layer 1 is now applied in a thickness which is not much greater than the LONDON penetration depth.

The trapped energy can now be detected in two ways: In the first case (a), the energy particles to be measured or the radiation flow are absorbed directly by the superconducting layer 2. Cooper pairs are broken up, which in turn generates excess quasi-particles. In the second case (b), the radiation to be measured penetrates the crystal body and generates non-thermal phonons, ie lattice vibrations. The phonons then migrate into the superconducting layer 2, and those with an energy greater than 2 As in the first case, "delta" in turn generate excess quasi-particles. The first case (a) provides the higher resolution or the better measurement sensitivity. In contrast, in case (b) a larger detector volume is available, which ensures a high absorption rate, in particular for high-energy particle radiation.

The basic function of the detector device is explained below with reference to FIG. 2. First, the above case (a) is described. The radiation absorbed in the first layer 1 generates an excessive number of quasiparticles by breaking up the Cooper pairs. Those quasiparticles which are within the London penetration depth of the microwave radiation applied from the outside are excited to a higher energy level by absorption of microwaves. However, this excitation energy is immediately released again by the emission of a phonon of the same energy. Since the superconducting layer or layers 1, 2 are transparent to phonons of this energy, the phonons will get into the crystal body 10 and heat it. It is assumed that after this process of combined microwave absorption and phonon emission the state of the quasiparticles is not changed, so that the over- massive quasiparticles during their entire lifespan "tau" _x of almost 100 μsec can absorb energy catalytically from the microwave field and release it to the crystal body 10 in the form of thermal energy. The result is a sharp rise in temperature due to the low specific heat value for the crystal material. The combination of the crystal body with the superconducting layer and the microwave field results in a large signal gain for the detector.

In the case (b) mentioned above, the radiation to be measured is absorbed directly in the crystal body 10 and transported to the surface in the form of non-thermal phonons. Those phonons that get into the superconducting layer or layers 1, 2 break up Cooper pairs and generate an excessive number of quasi-particles. The rest of the process is then identical to case (a).

Since the quasiparticles can only interact with the microwaves at the LONDON penetration depth, the thickness of the layer 1 must be comparable to the LONDON penetration depth. The thicker second superconducting layer 2, whose energy gap "delta" 2 is greater than the energy gap "delta" _{} of} the first layer 1, is used to prevent transmission of microwaves directly into the crystal body. As a result, the excessive quasiparticles within the first layer 1 held. An additional temperature rise in the crystal body 10, which would influence the measurement result, can thereby be avoided. Microwave losses in the cavity are also reduced.

With regard to the type of radiation to be measured by the detector, the measuring device can be optimized according to the following criteria: radiation quanta with greater energy (> 10 keV) have a long absorption length and therefore require large-volume detectors. An interpretation according to the mentioned case (b) is suitable for this. On the other hand, if low-vegetarian X-rays or optical radiation are to be measured, training is sufficient

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Case (a), i.e. the simple thickness of the superconducting layers of less than 1 μm. In order to achieve the desired mode of operation of the device described, the operating temperature T of the device to be maintained by the cryostat 9 must in any case be at most approximately 10% of the transition temperature of the first layer 1. This ensures that there is practically no recombination with thermal quasiparticles in layer 1.

Since the temperature rise for a given deposited energy to be measured increases with decreasing cryostat temperature T ₀ and T ~ ^ and is inversely proportional to the detector volume, the signal amplification of the detector can be used to obtain a large-volume detector for low temperature or to operate a small volume detector at a higher temperature.

The exploitation of the long life of the quasiparticles and their function as a "catalyst" in the absorption of microwaves and in the emission of phonons is the decisive step in achieving the aforementioned gain and thus the achievable high signal / noise ratios. In this regard, the detector can be understood as a type of FET transistor. A large part of the microwave energy conducted into the crystal is controlled by a small number of quasiparticles on the surface of the detector cell. At rest, microwave energy is only absorbed by thermal quasiparticles, the density of which decreases exponentially as the temperature drops. The microwave power is now set so that the temperature of the detector cell rises slightly when microwave energy is supplied. The detector signal then depends on the ratio of the excess quasiparticles to the thermal quasiparticles.

Another advantage over the known bolometer is the additional utilization of the non-thermal phonon energy that is converted into thermal phonons by the described process of breaking up Cooper pairs and absorbing microwave energy.

A microwave cavity having an opening for feeding in the microwave field is suitable for better application of the microwave field. Preferably the cavity body is also made of a superconducting material, e.g. made of niobium. In this way, energy losses from the supplied microwave field are kept extremely low. In the example, they are 100 μW at an operating temperature between 0.1 and 1.0 K.

On the basis of the following figures, some preferred exemplary embodiments of different detector types are shown in more detail. However, it is expressly pointed out that the examples can be modified and adapted depending on the desired application, so that the examples shown are not to be understood as restrictive.

3 shows the implementation of a large-volume (1000 cm *) gamma radiation detector for measuring energies greater than 1 MeV at a low temperature of 100 mK. Because of the high microwave fluxes required and the reflection of the microwaves on the superconducting layer, a rectangular microwave cavity 3 is used. The transducer 4 is inserted in a wall 11 of the cavity, which is not directly exposed to the microwave radiation. The crystal body 10 is thermally decoupled from the microwave cavities by a thermal decoupling 14. On the cavity side, the decoupling 14 must have superconducting properties, the energy gap of this superconductor having to be smaller than the energy gap of the second superconducting layer 2. For this purpose either a third superconducting layer 12 or the same material as for the thermal decoupling 14 is suitable, so that there is a continuous decoupling barrier made of superconducting material, in the example of NbN. The use of NbN is advantageous in that no excessive quasiparticles on the detector surface can get into the superconducting Nb cavity because the energy gap of NbN is greater than that of Nb. Another side 13 within the cavity is open to the radiation to be measured.

A plurality of cavities 3 can be arranged on the surface of the crystal body 10. For the use of the temperature measuring element 4, part of the surface of the crystal body 10 remains accessible at any suitable location. In principle, any point on the crystal surface can remain freely accessible for the radiation to be measured.

Fig. 4 shows the implementation of a small-volume detector for e.g. 6 keV X-ray or IR radiation with high energy resolution at 0.9 K, i.e. the lowest temperature that can be reached with a 4He cryostat. In principle, the same detector can be used to detect small flows of optical photons up to the far infrared region. In this example too, the crystal body 10 is installed in the wall of a microwave cavity 3 made of Nb. In the example, the crystal body 10 consists of Be or Si. Two detector elements are shown in this illustration, which is intended to indicate that detector arrays can also be implemented according to the principle shown, the individual crystal bodies being separated by thermal decoupling 14, e.g. also made of Be or Si.

Since, in this example, the areas which form the detector cells, as well as the thermal decoupling areas 14 located between them, can consist of the same crystal material, in particular of a single piece, this arrangement is particularly suitable for an automated manufacturing process how this is applied to integrated circuits. On the cavity side, there is a third superconducting layer 12 on the thermal decoupling 14, which consists, for example, of Nb-Ge. The energy gap "delta" 3 of the third superconducting layer is larger than the energy gap "delta" 2 of the second layer 2.

A heat sensor 4 suitable for this example can be designed as a thermistor, which is connected to a measurement sensor with a current source and a voltage amplifier as essential elements. Alternatively, the heat sensor can consist of a measuring dielectric, to which a measuring sensor with a voltage source and a charge amplifier is connected.

A gamma radiation detector for 1 MeV consists, for example, of 1000 cm ³ germanium. The crystal body is surrounded by 1.5 μm Nb and 0.1 μm AI. The quasiparticle lifetime "tau" _x is 100 μsec, and the detector is coupled to the heat bath at T = 0.1 K with a heat conduction time "tau" _c = 1 msec. The microwave frequency is 35 GHz. The detector achieves the best possible energy resolution of 45 eV at 1 MeV, with a microwave power flow of 0.1 MWcm ^"2. This energy resolution is determined by the fluctuations in the gain, which are caused by the energy gap in the first layer 1, in this case AI. The calculated Q value of the cavity is 10 ^, which corresponds to a power loss of 100 μW with a microwave power flow of 0.1 MWcm ~ 2 Microwave generator supplied and removed from the cryostat at 0.1 K.

An X-ray detector for 6 keV consists of a 1 mm x 1 mm x 0.1 mm Be crystal, which is surrounded by 1.5 μm NbN and 0.1 μm PbB. The quasiparticle lifetime "tau" _x is 50 μsec. The detector is coupled with a heat conduction time "tau" _c = 1 msec to the heat bath at T = 0.9 K. The microwave frequency is 70 GHz. The detector achieves the best possible energy resolution of 7.5 eV at 6 keV, with a microwave power flow of 100 MWcm ^"2. This energy resolution is determined by the fluctuations in the gain, which are caused by the energy gap in the first layer 1, in this case PbB. The calculated Q value of the cavity is 10 * ³ , which corresponds to a power loss of 100 μW for this cavity with a microwave power flow of 100 MWcm ^"2 . This must be supplied by the microwave generator and removed by the cryostat at 0.9 K.

According to a modified example, the X-ray detector for 6 keV consists of a 0.5 mm × 0.5 mm × 0.1 mm Si crystal which is surrounded by 1.5 μm NbN and 0.1 μm PbB. The quasiparticle lifetime "tau" _χ is 100 μsec in this case. The detector is coupled with a heat conduction time "tau" _c = 1 msec to the heat bath at T = 0.8 K. The microwave frequency is 35 GHz. The detector achieves the best possible energy resolution of 10 eV at 6 keV, with a microwave power flow of 1 MWcm ~ 2. The calculated Q value of the cavity is 10 ^, which corresponds to a power loss of 100 μW with a microwave power flow of 1 MWcm ^"2. This must be supplied by the microwave generator and dissipated by the cryostat at 0.8 K. .

In the design as an infrared detector, for example, a 1 mm x 1 mm x 0.1 mm Be crystal is surrounded by 1.5 μm NbN and 0.1 μm PbB. The quasiparticle lifespan "tau" _χ is 50 μsec. The detector is coupled with a heat conduction time "tau" _c = 1 msec and the heat bath at T = 0.9 K. The microwave frequency is 70 GHz. Values of 10 ^"19 W /, / Hz can be reached before the detector becomes unstable. The calculated Q value of the cavity is Q = 10 ¹³ , which corresponds to a power loss of 100 μW for a cavity with a power flow of 100 MWcm ^"2. This must be supplied by the microwave generator and dissipated by the cryostat at 0.9 K.

Finally, an infrared detector consists of a 0.5 mm x 0.5 mm x 0.1 mm Si crystal, which is surrounded by 1.5 μm NbN and 0.1 μm PbB. The quasiparticle lifetime "tau" _x is 100 μsec. The detector is coupled with a heat conduction time "tau" _c = 1 msec and the heat bath at T = 0.8 K. The microwave frequency is 35 GHz. Values of 10 ^"19 W / VHz can be reached before the detector becomes unstable. The calculated Q value of the cavity is Q = 10 * ² , which with a microwave power flow of 1 MWcm ^{" 2} results in a loss of power in this cavity of 100 μW corresponds. This must be supplied by the microwave generator and removed by the cryostat at 0.8 K.

Claims

1. Superconducting radiation detector with a crystal body (10) of low specific heat, with a temperature measuring element (4) connected to it and with a cryostat (9) for regulating a predetermined operating temperature, characterized in that the crystal body (10) is provided with a superconducting layer (1) that is not connected to external electrical potential, that the cryostat (9) is set to operate below the transition temperature of the superconductor (1), and that the superconducting layer (1) is exposed to a microwave field (8) is exposed.

2. Radiation detector according to claim 1, characterized in that the thickness of the superconducting layer (1) corresponds approximately to the LONDON penetration depth of the applied microwave radiation in this layer.

3. Radiation detector according to claim 1, characterized in that a second superconducting layer (2) is present between the crystal body (10) and the first superconducting layer (1), the second superconducting layer (2) being thicker than the first (1).

4. Radiation detector according to claim 3, characterized in that the second superconducting layer (2) consists of a material whose superconducting energy gap "delta2" is greater than that "delta ^" of the first superconducting layer (1).

5. Radiation detector according to claim 2, characterized in that the superconductor-coated crystal body (10) is coupled to a microwave cavity, and that between the superconductor-coated crystal body (10) and the cavity wall (3) a thermal decoupling ( 14) is attached, which is part of the cavity wall (3).

6. Radiation detector according to claim 5, characterized in that the thermal decoupling (14) on the cavity-facing side consists of a third superconducting layer (12), for example of NbN, the energy gap "delta ¹ ^ of the third layer is greater than the energy gap "delta" 2 of the second superconducting layer (2).

7. Radiation detector according to claim 6, characterized in that the thermal decoupling (14) completely from the material of the third superconducting layer (12), e.g. consists of NbN.

8. Radiation detector according to claim 2, characterized in that on the surface of the crystal body (10) one or more microwave cavity (s) (3) are arranged.

9. Radiation detector according to claim 8, characterized in that part of the surface of the crystal body (10) is accessible via an insert opening (11) for the temperature measuring element (4).

10. Radiation detector according to claim 8, characterized in that part of the surface of the crystal body (10) is accessible via an inlet opening (13) for the radiation to be measured.

11. Radiation detector according to claim 1, characterized in that the crystal body (10) made of germanium and the first superconducting layer (1) consist of aluminum.

12. Radiation detector according to claim 11, characterized in that for the design as a gamma radiation detector a germanium or silicon crystal body (10) at least in the range of microwave exposure with an Nb layer (2) with a thickness in the μm range and additionally with a thinner Al -Layer (l) is coated.

13. Radiation detector according to claim 1, characterized in that for design as an infrared or X-ray detector, a Berylliu crystal body (10) at least in the range of microwave exposure with an NbN layer (2) and additionally with the first superconducting layer (1 ) is coated from PbB, the layer thickness of the NbN layer (2) being in the μm range and the PbB layer (1) being thinner than the NbN layer (2). '

14. Radiation detector according to claim 1, characterized in that for the design as an infrared or X-ray detector, a silicon crystal body (10) at least in the range of microwave exposure with an NbN layer (2) and additionally with the first superconducting layer (1 ) is coated from PbB, the layer thickness of the NbN layer (2) being in the μm range and the PbB layer (1) being thinner than the NbN layer (2).

15. Radiation detector according to claim 2, characterized in that the superconductor-coated crystal body (10) is used in the wall of a microwave cavity (3) and that the layer structure is designed in the manner of integrated circuits.

16. Radiation detector according to claim 15, characterized in that a group of crystal bodies is arranged as an array in the wall of the cavity (3), the individual crystal bodies being decoupled from one another by thermal decoupling (14).

17. Radiation detector according to claim 16, characterized in that the thermal decoupling (14) consists of the same material as the crystal body (10).

18. Radiation detector according to claim 17, characterized in that the thermal decoupling (14) and the crystal body (10) consist of a single, identical piece.

19. Radiation detector according to claim 16, characterized in that the thermal decoupling (14) on the cavity side with a third superconducting layer (12), e.g. made of Nb-Ge, the energy gap "delta" 3 of the third layer being greater than the energy gap "delta" 2 of the second superconducting layer (2).