Superconducting Multilayer Architecture for Radiative Transient Discrimination

This invention relates generally to radiation detectors and, in particular, relates to (a) a Frequency domain Integrating Resonant Superconducting Transmission line (FIRST) detector having a superconducting meander line electrode interposed between two RF ground plane electrodes and also to (b) a stacked arrangement of two such detectors for discriminating against noise due to gamma radiation and/or Compton electron effects.

In the above referenced commonly assigned U.S. Pat. No. 4,962,316, entitled "Frequency Domain Integrating Resonant Superconducting Transmission Line Detector" Michael D. Jack discloses a FIRST detector that is comprised of a folded superconducting transmission line coupled at an input port to a narrow band microwave source and coupled at an output port to a microwave power monitor. An optically induced change in an inductance of the transmission line shifts the transmission line phase velocity and resonant frequency. This shift in resonant frequency attenuates the propagating wave amplitude proportionally to the product of the Q and the frequency shift. The FIRST detector is disclosed as a three layer structure having a bottom ground plane electrode and a top, superconducting radiation absorbing electrode. Interposed between the top and bottom electrodes is a dielectric layer. The FIRST detector is fabricated as a folded transmission line and is operated within a frequency range from several hundreds of KHz (10.sup.3 Hz) to one thousand GHz (10.sup.9 Hz). The device provides a realization of an optical detection dynamic range of eight orders of magnitude.

It is thus one object of the invention to provide another embodiment of a FIRST detector constructed as a five layer device wherein the superconducting folded transmission line electrode is interposed between a top and a bottom RF ground plane electrode and is insulated therefrom by adjacent dielectric layers, and wherein radiation that is detected is absorbed within the top, superconducting, RF ground plane electrode.

Advanced IR sensors that are required to operate in an ionizing radiation environment are typically further required to generate high rate, multiple samples per dwell. Such sensors generally also require complex spatial/temporal signal processing algorithms to discriminate against sample data that is corrupted due to gamma radiation or Compton electron events. Unfortunately, these demanding requirements may adversely impact signal processing rates, power consumption and the I/O capability of the sensor by as much as an order of magnitude over those sensors designed for operation in a more benign environment.

It is thus another object of the invention to provide apparatus enabling low power and compact focal plane sensors, based on superconductive detectors, to operate in an ionizing radiation environment with minimal gamma discrimination processing.

It is a further object of the invention to eliminate current or proposed focal plane gamma discrimination algorithms and electronics by utilizing the gamma discrimination provided by a pair of stacked FIRST detectors.

The foregoing problems are overcome and the objects of the invention are realized by a novel five layer FIRST detector device and also by a superconductive radiation detector structure that includes two resonant transmission line detectors stacked one on top of the other. The two stacked detectors share a common RF ground plane interposed between the two superconducting transmission line electrodes. Radiation is fully absorbed in an upper superconducting RF ground electrode causing a shift only in the resonant frequency of the upper transmission line detector. However, ionizing radiation originating in the environment or Compton secondary electrons emanating from the detector substrate or package cause frequency shifts in both the upper and the lower resonant detectors. Circuitry that detects a shift in the lower resonant transmission line frequency identifies noise corrupted samples without recourse to extensive gamma discrimination circuitry employed with prior art detectors.

In greater detail the invention concerns a radiation detector that includes a superconductive transmission line electrode having an input port for coupling to a frequency signal and an output port for coupling to a sensor for detecting a variation in the frequency signal. The transmission line electrode is interposed between a first electrode and a second electrode each of which is coupled to a common RF ground potential. The transmission line electrode is responsive to incident radiation that is absorbed within one of the superconducting RF ground electrodes for having induced therein a change in a resonant frequency of the transmission line electrode.

The invention further concerns a frequency domain radiation detector having at least two transmission line electrodes each having a resonant frequency and an input port for coupling to a frequency source. A first one of the transmission line electrodes is responsive to incident radiation that is absorbed in an overlying RF ground plane electrode for having induced therein a detectable change in the resonant frequency. Each of the transmission line electrodes has an output port for coupling to an event detector for detecting a substantially simultaneous occurrence of a change in output frequency of each of the transmission line electrodes, thereby indicating an occurrence of a ionizing radiation-induced noise event.

The above set forth and other features of the invention are more fully described in the ensuing Detailed Description of the Invention when read in conjunction with the attached Drawing, wherein:

FIG. 1 is a cross-sectional view, not to scale, of a superconducting transmission line having a ground plane, a dielectric layer and a superconducting top electrode in accordance with the disclosure of commonly assigned U.S. Pat. No. 4,962,316;

FIG. 2a is a representative top view, not to scale, showing radiation detectors each of which is comprised of a folded superconducting transmission line having an output coupled to a wideband non-resonant transmission line;

FIG. 2b is a representative top view, not to scale, and a circuit diagram showing radiation detectors arranged as differential pairs of illuminated and nonilluminated folded superconducting transmission lines;

FIG. 3 is a cross-sectional view, not to scale, of a further embodiment of the FIRST detector having a superconducting transmission line electrode interposed between a top and a bottom RF ground plane electrode and insulated therefrom by dielectric material;

FIG. 4 is a cross-sectional view, not to scale, illustrating a sensor arrangement having a pair of FIRST detectors stacked one on top of the other and sharing a common RF ground plane electrode that is interposed between the two transmission line electrodes;

FIG. 5a is a graph that illustrates resonant transmission line characteristics, specifically a frequency and an amplitude shift as a function of illumination; and

FIG. 5b is a graph which illustrates the temperature dependence of the resistive (R.sub.s) and inductive (.omega.L.sub.s) components of the surface impedance of a superconductor.

Reference is made first to FIG. 1 where there is illustrated a cross-sectional view of a superconducting transmission line 10 of the type disclosed in commonly assigned U.S. Pat. No. 4,962,316, filed July 31, 1989, entitled "Frequency Domain Integrating Resonant Superconducting Transmission Line Detector" by Michael D. Jack of which this application is a continuation-in-part.

The transmission line 10 is comprised of a RF ground electrode, or plane, 12 having a magnetic penetration depth .lambda..sub.L and a thickness b, a dielectric layer 14 of thickness d, and a top superconducting electrode 16 having a magnetic penetration depth .lambda..sub.L and a thickness b. Electromagnetic waves propagate along the transmission line with a velocity given by: ##EQU1##

For reasonable values of b.congruent..lambda..sub.L, d.congruent.0.2.lambda..sub.L, .lambda..sub.L =1400 Angstroms, and the dielectric constant .epsilon. approximately nine times that of free space; the group velocity Vg and phase velocity Vph can be shown to be approximately 3.times.10.sup.9 cm/sec.

The superconducting electrode 16 is preferably fabricated as a well-coupled high current density polycrystalline or nominally single crystalline film layer. Materials suitable for the fabrication of the electrode 16 include conventional "low temperature" superconductors such as Pb, Nb, NbN, the superconducting compounds having the A-15 crystal structure, Nb.sub.3 (M) where M is Sn, Al or Ge, as well as high temperature superconductors including compounds based on compositions containing (Y, Ba, Cu, O), (Bi, Sr, Ca, Cu, O), (Tl, Ba, Ca, Cu, O) and elemental replacements or substitutions thereof. An appropriate thickness of the upper superconducting electrode 16 is tailored to optimize performance in the desired wavelength range from 10.sup.-3 to 10.sup.-6 centimeters. Appropriate dielectrics include amorphous polycrystalline or crystalline insulators such as silicon dioxide, silicon nitride, aluminum, magnesium, zirconium, rare-earth or niobium oxides or epitaxially grown layers comprised of compounds related to the underlying superconductors. The dielectrics may also be thinned single crystal substrates such as oxides of Mg, Al, Strontium Titanate, Lanthanum Gallate, or Lanthanum Aluminate on which high quality superconductors may be grown. Thicknesses of dielectrics are chosen with respect to desired resonant frequency, dielectric losses and transmission line geometry and may vary in the approximate range between 10.sup.-2 to 10.sup.-6 centimeters. Dielectric thicknesses are comparable to or smaller than the spacing between adjacent folds of the upper electrode 16 to reduce crosstalk. Typical widths of the electrode 16 are approximately 0.1 micron to approximately 1000 microns. Typical overall lengths may range from approximately 0.001 cm to approximately 100 cm. The resonant frequency of the electrode 16 may vary between approximately several hundred KHz to approximately 1000 GHz and is, as indicated in Equation (2) below, inversely proportional to the length of the electrode 16.

Such a superconducting transmission line, having the preferred topological configuration including two or more ports, is advantageously applied to the detection of infrared or visible radiation. It is noted that the use of the invention is not limited to only these wavelengths but can be employed to detect radiation having wavelengths from the centimeter range up to and including x-rays and gamma rays. That is, the detector of the invention permits operation within a spectral band of from one or more centimeters to one or more Angstroms. The specific wavelength detected is a function of the bandgap of the material. For example, materials having a bandgap of a few tenths of a millivolt such as thallium, arsenic and zinc are adaptable to the detection of centimeter length electromagnetic waves. Also, the material bandgap can be "tuned" to a specific wavelength through the use of an induced energy bandgap resulting from the exploitation of the superconducting proximity effect. The use of the proximity effect is disclosed in commonly assigned copending U.S. patent application Ser. No. 07/377,698, filed July 10, 1989, entitled "Proximity Effect VLWIR Radiation Detector" by J. A. Wilson.

An example of one preferred topological configuration is shown in FIG. 2 where a transmission line electrode 18 can be seen to be folded between an input port 18a and an output port 18b. The folded transmission line electrode 18, also referred to as a "meander line", forms a portion of a radiation detector 20 that resonates at multiples of frequencies inversely proportional to the time for a wave to propagate from one end of the transmission line to the other and back:

An on-resonance probe pulse is simultaneously applied to the input ports 18a of the plurality of detectors 20 while the output ports 18b are sequentially coupled to an output transmission line 21 that is coupled to a detector, such as the square law diode detector 22. Power gain is provided by reading out the shift in resonant frequency through attenuation of the probe pulse power (P.sub.IN). A large change in P.sub.OUT is achieved due to the high Q of the resonant transmission line 18.

In the embodiment disclosed in commonly assigned U.S. Pat. No. 4,962,312 radiation is absorbed within the electrode 16 of the FIRST detector 10, the electrode 16 being comprised of a superconducting film which may be less than 1000 Angstroms in thickness an which absorbs substantially all optical quanta within a spectral band of approximately 0.3 micron to approximately 30 microns or more. Cooper pair breaking by incident photons results in a change in the kinetic inductance and a consequent shift in the resonance frequency of the superconducting transmission line 18. For a high Q structure such as the transmission line 18 a small shift in resonance results in a large change in the amplitude of the propagated on-resonance probe pulse. Furthermore, a large probe pulse amplitude may be propagated along the superconducting transmission line 18 with minimal loss or power dissipation, thereby reducing the sensitivity requirements for the probe pulse detector(s) 22.

Upon optical irradiation of the FIRST detector superconducting paired electrons, or Cooper pairs, are broken thereby creating excess normal state quasiparticles. This reduction in electron pairs participating in the superconducting ground state reduces the critical current density that can screen magnetic fields thereby increasing the screening or magnetic penetration depth, .lambda..sub.L. The penetration depth is proportional to the inverse square root of the number of electron pairs and can be expressed as:

where n.sub.qp is the quasiparticle density, n.sub.s is the electron pair density and .lambda..sub.L o is the magnetic penetration depth in the absence of light.

The number of broken electron pairs induced in the upper electrode 16 is proportional to the photon flux, yield per photon (r) and quasiparticle lifetime (.tau.) and is inversely proportional to the thickness (b) as given by the expression

Assuming a flux of 10.sup.11 /cm.sup.2, a per photon yield of six quasiparticles for each 10 micron photon, a lifetime of one millisecond and b=400 Angstroms a quasiparticle density of 1.5.times.10.sup.14 /cm.sup.3 is obtained.

The steady state density of superconducting pairs is given approximately by

where N(o) is the single spin density of states and E.sub.g is the superconducting energy gap. For a density of states of 10.sup.20 and an energy gap of 16 meV there is obtained a change in penetration depth and, hence, resonant frequency of approximately 1 part in 10.sup.4 (.DELTA.f/f=n.sub.qp /n.sub.s). This change in resonant frequency due to illumination corresponds to a resonant frequency shift of approximately 0.4 MHz. As shown in FIG. 5a this change in resonant frequency also results in a corresponding attenuation of the narrow band microwave source which propagates through the resonant transmission line 18 by more than a factor of five. In FIG. 5a the term P(.omega.) is given by the expression

The temperature dependence of the resistive (R.sub.s) and inductive (.omega.L.sub.s) components of the surface impedance of a superconductor are shown in FIG. 5b. As can be seen convergence of the reactive component, or inductance, rapidly approaches an equilibrium value at approximately 70% of the critical temperature. Thus, operation of the FIRST detector is achieved at up to approximately 70% to approximately 80% of critical temperature. For 100K HTS material this beneficially corresponds to operation at liquid nitrogen temperature.

Referring to FIG. 2b there is shown an embodiment of the FIRST detector wherein the detector 18 comprises differential pairs of matched transmission line electrodes 18c and 18d, one line of the pair (18c) being illuminated while the other line (18d) is maintained in a nonilluminated or dark condition. Detectors 18c and 18d are alternatively coupled to a wide bandwidth non-resonant stripline 24 by an associated switch S. Stripline 24 is coupled to a detector 22 and, via a switched capacitor 26, to a differential amplifier 28. Enhanced dynamic range and offset correction are achieved by multiplexing the output from the optically exposed element (18c) and the shielded element (18d) and integrating the difference in transmitted microwave power onto a differential element, such as the switched capacitor 26. The difference signal is amplified utilizing a wideband amplifier 28. Switches S and those coupling to C 26 are shown being alternately energized by timing signals .phi. and .phi.. Alternatively the difference between transmitted power from a single element sequentially illuminated and exposed to radiation may be amplified utilizing the same circuitry, with the same effect.

In the embodiments of FIGS. 1, 2a and 2b each of the meander line electrodes 18a-18d is fabricated as a narrow width linear element that overlies the low loss dielectric layer 14 and the bottom RF ground plane 12. As an example, a one micron width line, when folded such that it densely fills a 100 micron.times.100 micron pixel area, provides an effective length of approximately 0.5 cm. Such a transmission line 18, assuming a typical dielectric constant value, resonates at multiples of approximately 4.0 GHz, a frequency that is compatible with commercially available, high performance microwave Schottky diode detectors. The narrow band transmission line 18 is coupled to the sensor 22 and to the narrow band microwave source by, for example, resistive or capacitive inputs that are integrated with the line 18. A sharp resonance characteristic having a Q value in excess 10.sup.5 is achieved. A detector to detector uniformity of one part in 10.sup.5 is limited primarily by lithographic variations in the length of the folded transmission line electrode 18. In other embodiments the region within which the line is folded may have dimensions of approximately 10 microns by 10 microns to approximately 1000 microns by 1000 microns or greater. Of course the region need not be square in shape.

The transmission line electrodes 18 are coupled to readout circuitry such as that described in the before mentioned commonly assigned U.S. Pat. No. 4,962,316, the disclosure of which has been incorporated herein in its entirety.

Having thus reviewed in detail the structure and operation of the FIRST detector embodiment of commonly assigned U.S. Pat. No. 4,962,316 there is now described a five layer FIRST detector embodiment wherein radiation is absorbed within a top superconducting RF ground plane electrode instead of within the superconducting meander line. It is to be understood that the five layered embodiment now described may be used in place of the three layered embodiment previously described, either singularly or in arrays of same. Furthermore, the previous disclosure as to output circuitry, current scanners and the like applies equally to the five layered embodiment now described.

Referring now to FIG. 3 there is shown, in accordance with one aspect of the present invention, an embodiment wherein a radiation detector 30 is comprised of a bottom RF ground plane electrode 32, an overlying dielectric layer 34, a superconducting transmission line electrode 36, an overlying dielectric layer 38 and a top, radiation absorbing RF ground plane electrode 40. For this embodiment at least the radiation absorbing electrode 40, but preferably both of the RF ground electrodes 32 and 40, are comprised of a low temperature or a high temperature superconducting material. Radiation incident on the top surface of the detector 30 is absorbed, typically, within the first 200 Angstroms to 1,000 Angstroms of the top surface. The absorbed radiation breaks Cooper pairs causing an increase in the quasiparticle density. This increase in the quasiparticle density causes an increase in the magnetic penetration depth which results in a corresponding decrease in the resonant frequency of the transmission line electrode 36. The material compositions of the various layers of the detector 30 are the same as that disclosed above for the corresponding layers of the three layer FIRST detector 10 of FIG. 1. In this regard the additional dielectric 38 has the same composition as the dielectric layer 34, and may be continuous therewith, and the upper RF ground electrode 40 may have the same composition as the lower RF ground electrode 32, although this is not required for correct operation. Also, although the thickness of layer 34 may equal the thickness of layer 38 and the thicknesses of layers 32, 36 and 40 may all be equal to one another such is not a requirement. For example, it may be desirable to provide the layer 40 with a thickness that is selected based upon radiation absorption requirements resulting in the thickness of layer 40 differing from that of layer 32 and/or layer 36.

By example, one embodiment of the FIRST detector 30 includes a superconducting stripline electrode 36 extending over an area of 0.8 cm by 0.8 cm, the stripline electrode 36 being interposed between the two RF ground electrodes 32 and 40. The lower ground plane electrode 32 is comprised of Nb as is the material of the stripline electrode 36. The upper ground plane electrode 40 is comprised of an infrared radiation sensitive Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x high temperature superconductor. The detector is operated immersed in superfluid helium at 2.3K or is surrounded by a helium gas at 4.2K. At these temperatures a high "loaded" Q of approximately 10,000 has been achieved at a resonant frequency of approximately 700 MHz. The use of all high temperature superconducting material permits operation at correspondingly higher temperatures.

In accordance now with a further embodiment of the invention the FIRST detector architecture depicted in FIG. 3 is employed to provide intrinsic discrimination against noise resulting from ionizing radiation such as gamma radiation and noise resulting from scattered Compton electrons. As shown in FIG. 4 two FIRST detectors are stacked vertically. The lower detector (Detector B) is comprised of layers 52, 54, 56, 58, and 60. An upper detector (Detector A) is comprised of layers 60, 62, 64, 66, and 68. As such, Detector A and Detector B share a common RF ground plane electrode 60. Detector A and Detector B are disposed on a supporting substrate 70 which is preferably comprised of a substantially inactive, low Compton electron-generating material such as silicon (Si), sapphire (Al.sub.2 O.sub.3), or Lanthanum Aluminate. Radiation incident on the top FIRST detector (Detector A) is absorbed within the first 200 Angstroms to 1000 Angstroms of the top surface of the upper layer 68. The absorbed radiation breaks Cooper pairs causing an increase in the quasiparticle density, an increase in the magnetic penetration depth and a corresponding decrease in the resonant frequency of the meander transmission line electrode 64. As a result, a detectable change occurs in an RF probe pulse (f.sub.IN) that propagates through the transmission line 64 and which is detected by a monitor 72.

In that the incident radiation does not penetrate to the meander line electrode 64 the bottom FIRST detector (Detector B) is optically "blind". However, gamma radiation entering the top of the detector 50 and secondary Compton electrons scattered from the package and substrate 70 penetrate both Detector A and Detector B causing a substantially equal density of scattering events, that is a substantially equal density of generated quasiparticle pairs, resulting in simultaneous equal magnitude frequency shifts in both Detectors A and B. A coincidence or event detector 74 is coupled to both frequency outputs for detecting a simultaneous occurrence of a frequency shift (delta f) in each output. This effect occurs even for relatively low energy (.ltoreq.20 keV) gamma radiation. The gamma radiation loses only a small amount of energy penetrating these thin layers, which may be only .ltoreq.5.times.10.sup.3 Angstroms for the two detector stack illustrated. Lower energy gamma radiation is generally not observable in either Detector A or Detector B, resulting in substantially all gamma radiation and Compton electron events being readily distinguished from optical signals.

It is noted that as used herein a transmission line is considered to include at least two electrodes separated by a dielectric material wherein one of the electrodes conveys a radio frequency (RF) signal and has a resonant frequency associated therewith. The value of the resonant frequency is a function at least of the length of the electrode and the thickness and composition of the dielectric material. Such an arrangement can be considered to form a resonant cavity structure. The electrode or electrodes that do not convey the RF signal and which define the walls of the resonant cavity are preferably maintained at RF ground potential. Typically these other electrodes will also be maintained at an earth ground potential but such is not a requirement. That is, these other electrodes may be maintained at some DC potential relative to earth ground. In the embodiment of FIG. 1 the radiation absorbing electrode 16 also conveys the RF signal and is comprised of superconductive material. For the embodiment of FIG. 3 at least the top radiation absorbing electrode 40 and the RF signal conveying electrode 36 are comprised of superconductive material. For the stacked embodiment of FIG. 4 at least the electrodes 56, 60, 64 and 68 are comprised of superconductive material in order to achieve the desired goal of detecting the occurrence of noise events common to each of the RF conveying electrodes 56 and 64.

As such, and while the invention has been particularly shown and described with respect to presently preferred embodiments thereof, it will be understood by those skilled in the art that changes in form and details may be made therein without departing from the scope and spirit of the invention.