| The superconducting Transition Edge Sensor (TES) microcalorimeters in development in Blas Cabrera's research group at Stanford University are extremely sensitive particle detectors capable of measuring the energy and arrival time of a single photon with high accuracy. Baising the TES at a constant voltage sets up a negative electrothermal feedback mechanism that keeps the pixel in its superconducting to normal transition region. |
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How does ETF work?
Voltage baising the pixel sets up a stable equilibrium between energy dissipated in the TES by the current passing through it (P=V2/R) and heat loss to the substrate (P~T5TES-T5Sub). If the TES temperature is raised above its equilibrium value, its resistance rises as well, lowering the power dissipated in TES. This causes the temperature of the TES to fall. Conversely, if the TES temperature is lowered, the power dissipated increases, thereby raising the TES temperature.
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Particle Detection When a particle's energy is absorbed in the TES, the TES is warmed, causing it to move up in the transition region. Increased resistance at a constant voltage means that the current passing through the TES is lowered. The TES is coupled, through an inductor in series, to a low noise SQUID amplifier. Although TES quickly returns to its equilibrium condition, we can measure the current pulse and reconstruct the energy of the incident photon. |
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SQUID stands for Superconducting QUantum Interference Device. A SQUID is a clsoed superconducting ring with two Josephson junctions in parallel. A closed superconducting ring has the property that the magnetic flux through the ring must remain constant. According to Faraday's Law, any change in the magnetic flux will be countered by a change in the current flowing around the ring such that the total magnetic flux is constant. The Josephson junctions are "gates" that will allow magnetic flux to escape the SQUID ring. If the current increases to the critical current of the junctions, they will temporarily lose superconductivity and allow a flux quantum to escape the loop, lowering the current around the ring. We operate the SQUIDS in a flux-locked loop, meaning that we feedback a correction signal to the SQUID to keep the current in it constant. The feedback current will then be proportional to the current change in the TES.
In short, the SQUID is a very sensitive current to voltage converter with very low impedance. Using a series array of SQUIDs amplifies the output voltage linearly. We use arrays of 100 series SQUIDs per pixel, turning the microamp current TES signal into a millivolt voltage signal that can be measured with conventional electronics. For more information about SQUIDs check out MR. SQUID, a sweet introductory guide to SQUIDs from Star Cryoelectronics.
Why use TESs?
Why use TESs when there are a number of photon detectors out there already for almost any energy range of photon you might want to detect? TESs have several advantages over many types of detectors, namely single photon time and energy resolution, no false counts (no dark current) and high quantum efficiency.
For example, suppose one wants to construct a color image of a faint, rapidly varying astronomical source, such as a pulsar, using a CCD and a telescope. As a CCD has no inherent color sensing properties, one would have to take 3 different images, each through a red, green or blue filter, and then digitally combine the images. With a TES, the color of each incident photon is measured. To image a faint source, one would have to expose the CCD for a long time (several minutes or more), obscuring changes in the target occurring on a short time scale. With a TES detector, one is able to measure the time-of-arrival of each photon to one microsecond, allowing direct observation of rapid variations in the source object.