17-06-2010, 06:23 PM
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HIGH TEMPERATURE SUPERCONDUCTING-SENSORS
Submitted by :
E.Sai Sandeep
Y.Subba Reddy
THIRD YEAR
ELECTRONICS & COMMUNICATION ENGINEERING
SIR.C.R.REDDY COLLEGE OF ENGINEERING
ELURU “534007
ABSTRACT:
Scientific and industrial photon sensors, in contrast, peer into the electromagnetic realms beyond that of visible light-into the low frequency (long- wavelength, low energy) world of infrared and microwaves and into the high frequency regime of X-rays and gamma rays. But they have their limitations too. A revolution in photon detection is now under way, with the advent of sensors based on High-Temperature superconductivity that are capable of fine measurements and other prodigious feats. These new tools are dramatically improving the sensitivity of measurements across the electromagnetic spectrum. This paper presents an overview on High-Temperature superconducting sensors that come in photon detection regime. The superconducting sensors have myriad scientific and technological applications, including homeland security(detection of materials that could be used in a nuclear weapon), analysis of defects in microchips, astronomy, chemical analysis and particle physics. The important two superconducting sensors 1.Thermal sensor, 2.pair-breaking sensors are discussed in this paper.
INTRODUCTION:
The attractive property of superconductors is the zero resistance. This promoted their use in such application as power transmission. The same feature that renders them valuable in photon detectors. Superconductivity, the flow of electric current without resistance, arises when electrons in a suitable material bind together in what are called cooper pairs, which then flow en masse like a super fluid. A fragile quantum-mechanical effects, superconductivity occurs in a material only when it is cooled below an extremely low temperature, termed its critical transition temperature. Cooling a material reduces the vibrations of its atoms. If the temperature rises above that threshold, the thermal energy of the vibrations knocks apart the parteners in the cooper pairs, eliminating the superconductivity( show in below fig1). fig1: occurance of superconductivity.
Because of this sensitivity to heat, many superconducting devices have to be refrigerated to within a few degrees above absolute zero(0 kelvins). Some types require temperatures as low as a few hundredths of a kelvin. These extreme temperatures can be achieved with commercially available refrigerators that use liquid helium or a process called adiabatic demagnetization, but the need for such cooling prohibits many applications. Scientists have struggled for years to develop materials with more robust superconductivity that survives at higher temperatures.
PRINCIPLE:
Superconducting photon sensors rely on the ability of the energy in a single photon to disrupt thousands of cooper pairs. The change in the superconductivity state can be measured in a number of ways to reveal the energy deposited by the photon. Because a photonâ„¢s energy is proportional to its frequency, which is the key to learning about the object that the photon came from.
DETECTION OF LIGHT BY A SUPERCONDUCTOR:
Superconductor ¢s reaction to incident photon is in two ways:
1. The incident radiation may increase the temperature in superconductor.(or)
2. It dispenses the energy to cooper pairs and disrupt its coordination of pairs. In both cases the disruption of superconductivity takes place. The first type is called thermal sensor and the other kind, is called pair-breaking sensor.
THERMAL SENSORS:
The thermal type of sensors relies on the fact that the electrical resistance of a superconducter rises sharply from zero to its normal value in the very narraow temperature range in which the materical switches from its superconducting to its normal state. The abrupt change in resistance allows the superconducter to act as an ultrasensitive thermometer. A sensor that uses the superconducting phase transition this way is called a Transition-Edge Sensor(TES).
When a TES absorbs a photon, the photonâ„¢s energy gets converted into heat, which increases the temperature and thus the resistance of the material in proportion to the deposited energy. A superconducting device known as SQUID detects the corresponding momentary drop in the current and converts this signal into a voltage pulse that can be further amplified by conventional electronics before data collection.
A schematic diagram (fig2) gives the thermal sensing detection. Depending on the photon-absorbing material used, a TES can serve as a spectrometer for measuring the energy of X-rays and gamma rays. It can also act as photon counter for near-infrared to visible wavelengths. Hence it can be used as total power sensor for radiation at infrared to millimeter wave bands.
The first TES sensor was developed in the 1940s but were
impractical for many years. The problem was that the superconducting transition is often less than a thousandth of a degree wide, and it was very hard to keep the temperature of the device within that range. These variations in transition temperature made TES impossible to operate as an array of sensors . A technique called voltage biasing helps to solve the problem. A constant voltage applied across the sensor results in an electric current through the TES, which heats the sensor. When the transition temperature is reached, the resistance goes up, decreasing the current and stopping the heating. The self- heating thus acts as a negative feedback that tends to keep the temperature of the film within its transition. In an array of voltage-biased sensors, each sensor will self-heat into its own transition, even if the transition temperatures vary slightly. The negative feedback also speeds the response of the sensors. The introduction of voltage biasing has led to an explosive growth in the development of TES sensor arrays worldwide.
COUNTING QUASIPARTICLES:
UNLIKE A THERMAL SENSOR, a superconducting pair- breaking sensor cannot rely on a change of electrical resistance to signal the absorption of a photon. An incoming photon breaks cooper pairs and creates quasiparticles, which for most purposes can be thought of as free electrons in an otherwise superconducting metal. The number of quasiparticles created is proportional to the photonâ„¢s energy. But because the sensor is cooled to well below its transition temperature, a sea of intact cooper pairs is still present, so the electrical resistance remains zero. A superconducting pair-breaking sensor therefore must be able to distinguish between cooper pairs and quasiparticles.
One device capable of distinguishing cooper pairs and quasiparticles is the superconducting tunnel junction, which consists of two superconducting films separated by a thin layer of insulating material. If the insulator is thin enough(about two nanometers), electrons can cross from one side of the barrier to the other by a process known as quantum-mechanical tunneling.
By applying a small magnetic field, the tunneling of cooperpairs can be controlled across the junction. The quasiparticles can still cross the junction. A proper voltage applied to the device and can control the current. Current will flow only until one of the superconducting films absorbs a photon, generating quasiparticles. The resulting current pulse is proportional to the number of quasiparticles created and thus proportional to the incident energy and frequency of photon.
A microwave kinetic inductance sensor measure the number of quasiparticles in a superconducter. It takes advantage of the fact that a superconducting structure can have an electromagnetic resonance at a microwave frequency, much as a tuning fork has a mechanical resonance at an audio frequency. When photons create quasiparticles in the superconducter, the resonance becomes less sharp and wave propagation slows down, reducing the resonant frequency. The shifts in both the resonant frequency and the sharpness of the resonance are proportional to the number of quasiparticles. Initial results with these devices are extremely promosing.
Promissing Applications:
The superconducting sensors available today are 10 to 100 times more sensitive than conventional sensors operated at room temperature. These devices are improving measurements in a broad range of fields. Nuclear nonproliferation and homeland defense: One of the most pressing international priorities is to control the dissemination of the nuclear materials that could be used in attacks by terrorists. Nuclear materials contains unstable isotopes, which emit x-rays and gamma rays. The characteristic energies of these photons provide a finger print revealing which radioactive isotopes are present. Unfortunately, some isotopes that occur benign applications emit gamma rays with energies that are very similar to those emitted by materials used in weapons leads to ambiguous identifications and false alarms.
Thousands of radiation portal monitors are available today to detect the gamma rays emitted by nuclear materials carried by vehicles. One of worst fears is that terrorists might smuggle highly enriched (weaponsgrade ) uranium to build a crude Hiroshima “style atomic bomb. The primary signature of highly enriched uranium is the 185.7-kilo-electron volt(keV) gamma ray from uranium 235. This gamma ray, though, has almost the same energy as the 186.1- keV gamma ray emitted from the radium 226 in clay, in cat litter and other materials , making it very difficult to distinguish the two. This so-called kitty litter problem is the largest source of false alarms noticed in general.
The measurements with superconducting detectors are compared with conventional detector and are shown .Gamma-ray sensors based on TES technology that have more than 10 times better energy resolution than conventional sensors. These sensors can resolve more lines in the complicated gamma “ray spectra of nuclear materials, such as uranium and plutonium isotopic mixtures [see the graphs above]. The devices are being developed specifically to help in the verification of international nonproliferation treaties by determining the plutonium content of spent nuclear fuel. But they are also able to distinguish between radium 226 in cat litter and uranium 235 in highly enriched uranium. If a conventional handheld sensor or portal monitor were to detect a gamma-ray signal, one of the superconducting devices could be used as a follow-up tool to distinguish unambiguously between these two isotopes, thus eliminating many false alarms.
ANALYSIS OF MICROCHIPS:
An application that is important for the semiconductor industry is electron-probe micro analysis. When a scanning electron microscope images a sample, the electron beam causes the sample to emit X-rays . One can determine the chemical composition of the sample within the nanometer-scale region of the beam by measuring the energies of the varies X-rays emmitted. As the beam scans across the entire sample, the resulting image shows where different chemical compositions are present, mapping out the structures that define how a microchip functions.
At present the semiconducter industry uses semiconducter x-ray detectors to study structures and defects on micro chips.But as microchips use smaller features,new generations of micro analysis instruments with greater sensitivity are needed. By developing micro analysis systems based on TES sensors that have an energy resolution 50 times better than industrially available semiconductor detectors,enabling to resolve many important X-ray spectral peaks. Such micro analysis systems are now becoming commercially available.
OTHER USES:
In addition to the applications discussed in the main text, superconducting detectors are also used for the following :
- X-rays spectroscopy at synchrotrons, including chemical analysis of metals in proteins and other samples. ¢ Efficient detection of large biological polymers and DNA fragments in mass spectrometers, which has applications in genomics, proteomics, drug discovery and analysis of natural compounds.
- Photon counting at telecommunications wavelengths(infrared) for quantum cryptography.
- The search for weakly interacting massive particles postulated to make up dark matter in the universe.
- Submillimeter astronomy. ¢ Cosmology.
CONCLUSION:
The fragility of superconductivity that makes suitable for sensitive photon detectors is discussed. Because of their sensitivity to temperature when compared to the other detectors, superconducting material thus acts as superb sensors of photons and other energitic particles to fulfill scientific and commercial applications ranging from anti-terrorism to astronomy.
REFERENCES:
1. sciam.com
2. Transition-Edge sensors by K.D.Irwin and G.C. Hilton.
3. IEEE magazines.
4. PHYSICS for scientists and engineers. Vol2. 6th edition . By rayomnd
A.Serway and Jhon W.Jewett,Jr.
5. Low-Temperature Particle Detectors in Annual reviews of nuclear and
particle science.
