Presentation on theme: "Rumore termico e sistemi di raffreddamento Dr. Emanuele Pace Giugno 2006 Corso di Rivelatori per lo Spazio."— Presentation transcript:
Rumore termico e sistemi di raffreddamento Dr. Emanuele Pace Giugno 2006 Corso di Rivelatori per lo Spazio
E. Pace - Rivelatori per lo spazio2 Dark current La dark current è la corrente di perdita dei fotorivelatori, i.e., la corrente non indotta da fotogenerazione Si chiama dark current perché corisponde ad una corrente ottenuta senza illuminare Limita la dinamica dei fotorivelatori: Riduce lampiezza del segnale Introduce un rumore (shot) non eliminabile con densità spettrale Può variare molto da punto a punto in un rivelatore dimmagini causando il fixed pattern noise
E. Pace - Rivelatori per lo spazio3 Calcolo della corrente di buio La corrente di buio non può essere determinata analiticamente o usando strumenti di simulazione; può essere determinata solo sperimentalmente Per comprendere gli effetti di alcuni parametri sulla corrente di buio, si può derivare unespressione per la corrente di buio dovuta ai difetti di bulk (non necessariamente la sorgente principale)
E. Pace - Rivelatori per lo spazio4 Calcolo della corrente di buio Partiamo dallequazione di continuità: Con portatori minoritari fotogenerati costante di diffusione (cm 2 /s) rate di fotogenerazione rate di ricombinazione Supponendo R(x) trascurabile, di misurare la corrente di buio e di essere in uno stato stazionario, si ha
E. Pace - Rivelatori per lo spazio5 Calcolo della corrente di buio La soluzione generica è con condizioni al contorno: al contatto ohmico al bordo della regione di svuotamento Si ha perciò La densità di corrente di lacune è quindi data da:
E. Pace - Rivelatori per lo spazio6 Dark current e temperatura La corrente di buio cresce con la temperatura, poiché la concentrazione di portatori intrinseci aumenta in modo proporzionale a
E. Pace - Rivelatori per lo spazio7 Rumore termico Il rumore termico è generato dal moto degli elettroni indotto dalla temperatura in regioni resistive ha valor medio nullo, banda spettrale larga e piatta, distribuzione dei valori gaussiana e densità spettrale
E. Pace - Rivelatori per lo spazio8 Raffreddare…. La corrente di buio e il rumore termico dipendono fortemente dalla temperatura. Per ridurne il contributo è necessario e sufficiente raffreddare il sensore. La temperatura di raffressamento dipende dalle caratteristiche strutturali ed elettriche del rivelatore
E. Pace - Rivelatori per lo spazio9 BREVE INTRODUZIONE AI SISTEMI DI RAFFREDDAMENTO UTILIZZATI NELLO SPAZIO
E. Pace - Rivelatori per lo spazio10 Raffreddamento passivo Passive coolers require no input power. There are two types: Radiators. Radiators are panels radiating heat according to Stefan's Law and are the workhorse of satellite cooling due to their extremely high reliability. They have low mass and a lifetime limited only by surface contamination and degradation. It is important, however, to prevent the surface from viewing any warm objects and the device must therefore be carefully designed taking the vehicle's orbit into account. They also have severe limitations on the heat load and temperature (typically in the milliwatt range at 70K). Multiple stages are often used to baffle the lowest temperature stage, or patch, and it has been shown that efficiency is nearly optimized with three stages, although two is often enough. In this case the first stage consists of a highly reflective baffle (e.g. a cone), to shield the patch from the spacecraft, Earth or shallow-angle sun-light. Stored cryogens. Dewars containing a cryogen such as liquid helium or solid neon may be used to achieve temperatures below those offered by radiators (heat is absorbed by either boiling or sublimation respectively). These provide excellent temperature stability with no exported vibrations but substantially increase the launch mass of the vehicle and limit the lifetime of the mission to the amount of cryogen stored. They have also proved to be of limited reliability. Passive coolers have been used for many years in space science applications due to their relatively high reliability and low vibration levels.
E. Pace - Rivelatori per lo spazio11 Liquidi refrigeranti Sistemi ad elio liquido permettono temperature di funzionamento a 3K e a 4.2K. Esistono anche dewar a diluizione che consentono temperature di funzionamento intorno a 1K, ma sono utilizzati per lo più in esperimenti a bordo di palloni stratosferici Sistemi ad azoto liquido permettono temperature di funzionamento maggiori o uguali a 77K Sistemi misti con preraffreddamento ad azoto liquido e raffreddamento ad elio liquido. Questi sistemi permettono una durata maggiore dellesperimento in quanto il refrigeratore principale, lelio, evapora più lentamente e dura quindi di più.
E. Pace - Rivelatori per lo spazio12 Esempio: ASTRO F ASTRO-F satellite consists of a cryostat and a bus module. A telescope and scientific instruments are stored in the cryostat and cooled by liquid Helium and mechanical coolers. The bus module takes care of house keeping of the satellite, attitude control, data handling, and communication with the ground system. The height and weight of the satellite are 3.7 meters and 952 kg, respectively. The cryostat and the bus module have independent structures so as to decrease heat inflow into the cryostat.
E. Pace - Rivelatori per lo spazio13 Esempio: ASTRO F 170 liter of superfluid liquid Helium (at launch time) is loaded into the tank of the cryostat and cools the instruments and the telescope down to a very low temperature. Two sets of Stirling-cycle mechanical coolers are incorporated in addition to the liquid Helium. The addition of the mechanical coolers extends the Helium life and reduces the quantity of Helium to be carried into space. ASTRO-F will make observations for one and a half years keeping a very low temperature using both liquid Helium and the mechanical coolers.
E. Pace - Rivelatori per lo spazio14 Riscaldamento della sonda Solar radiation 1371 W/m 2 Albedo + blackbody emission 200 W/m 2 Solar wind 2 x 10 5 K Atmosphere 10 3 K X X Rate di collisioni e riscaldamento trascurabili
E. Pace - Rivelatori per lo spazio15 Schermi termici
E. Pace - Rivelatori per lo spazio16 Criogeneratori per bassissime temperature Adiabatic Demagnetization. Adiabatic Demagnetization Refrigeration (ADR), has been used on the ground for many years to achieve milli-Kelvin temperatures after a first stage cooling process. The process utilizes the magneto-caloric effect with a paramagnetic salt. These coolers are currently under development for space use. 3He coolers. In addition to its use as a stored cryogen the properties of can be used to achieve temperatures below 1K with closed cycle "Sorption coolers" (above 250mK), and dilution refrigerators (above 50mK). The former are scheduled for use in the SPIRE and PACS instruments aboard the Herchel satellite whereas the latter will be used on the Planck mission.
E. Pace - Rivelatori per lo spazio17 Active coolers Passive coolers are now joined by coolers requiring input power, so called active devices (also termed `cryocoolers'). Active coolers use closed thermodynamic cycles to transport heat up a temperature gradient to achieve lower cold-end temperatures at the cost of electrical input power. The first long-life active cooler system successfully operated in space was a cluster of four rhombic-drive, grease lubricated Stirling cycle machines launched in 1978 aboard DOD 1-78-1 developed by Phillips and cooling two gamma ray detectors. Although these showed significant performance degradation on-orbit they operated sufficiently well to keep the payload operating until it was destroyed during a successful test of an anti- satellite interceptor in 1985. Since this first generation the flexibility and reliability of active coolers have proved major contributors to the success of many missions.
E. Pace - Rivelatori per lo spazio18 Stirling cycle The major types of active cooler are: Stirling cycle. These coolers are based on causing a working gas to undergo a Stirling cycle which consists of two constant volume processes and two isothermal processes. Devices consist of a compressor pump and a displacer unit with a regenerative heat exchanger, known as a `regenerator'. Stirling cycle coolers were the first active cooler to be used successfully in space and have proved to be reliable and efficient. Recent years have seen the development of two-stage devices which extend the lower temperature range from 60- 80K to 15-30K. Pulse tube. Pulse tube coolers are similar to the Stirling cycle coolers although the thermodynamic processes are quite different. They consist of a compressor and a fixed regenerator. Since there are no moving parts at the cold-end reliability is theoretically higher than Stirling cycle machines. Efficiencies approaching Stirling cycle coolers can be achieved and several recent missions have demonstrated their usefulness in space.
E. Pace - Rivelatori per lo spazio19 Joule-Thompson Joule-Thompson. These coolers work using the well known Joule- Thomson (Joule-Kelvin), effect. A gas is forced through a thermally isolated porous plug or throttle valve by a mechanical compressor unit leading to isenthalpic cooling. Although this is an irreversible process, with correspondingly low efficiency, J-T coolers are simple, reliable, and have low electrical and mechanical noise levels. A J-T stage driven by a valved linear compressor, similar to those used for Stirling cycle and Pulse Tube coolers, will be flown on the planned Planck telescope mission (expected to be launched in 2007). Sorption. Sorption coolers are essentially J-T coolers which use a thermo-chemical process to provide gas compression with no moving parts. Powdered sorbent materials (e.g. metal hydrides), are electrically heated and cooled to pressurize, circulate, and adsorb a working fluid such as hydrogen. Efficiency is low but may be increased by the use of mixed working gases. Demonstration models have already been flown and they are expected to useful in long-life missions where very low vibration levels are required, such as the planned Darwin mission to image the atmospheres of extra-solar planets.
E. Pace - Rivelatori per lo spazio20 Reverse Brayton cycle A Brayton-type engine consists of three components: A gas compressor A mixing chamber An expander In the original 19th century Brayton engine ambient air is drawn into a piston compressor, where it is pressurized; a theoretically isentropic process. The compressed air then runs through a mixing chamber where fuel is added, a constant-pressure process. The heated, pressurized air and fuel mixture is then ignited in an expansion cylinder and gives up its energy, expanding through a piston/cylinder; another theoretically isentropic process. Some of the work extracted by the piston/cylinder is used to drive the compressor through a crankshaft arrangement. Reverse Brayton. Reverse/Turbo Brayton coolers have high efficiencies and are practically vibration free. Coolers consist of a rotary compressor, a rotary turbo-alternator (expander), and a counterflow heat exchanger (as opposed to the regenerator found in Stirling or Pulse Tube coolers). The compressor and expander use high-speed miniature turbines on gas bearings and small machines are thus very difficult to build. They are primarily useful for low temperature experiments (less than 10K), where a large machine is inevitable or for large capacity devices at higher temperatures (although these requirements are quite rare). A recent application of this class of cooler was the Creare device used to recover the NICMOS instrument on the Hubble Space Telescope.
E. Pace - Rivelatori per lo spazio21 Esempio: NICMOS Cryocooler di NICMOS (HST) These cryogenic coolers allow longer operational lifetimes than is presently possible with expendable cryogenic systems. It is expected that with the NCS installed on HST, NICMOS's operational life will be extended by at least five years beyond SM3B. The NCS works using three fluid loops. The first loop is the circulator loop. When installed in the HST, gas will be circulated in this loop between the cooling system and the inside of the NICMOS cryostat. This carries heat away from the cryostat and keeps the detectors at their operating temperature (73 Kelvin or -200 C). For the HOST mission, the NICMOS cryostat was replaced by a simulator that contains an exact replica of the plumbing and interfaces in the NICMOS. This device is called the NICMOS Cooling Loop Simulator (NCLS). The second loop is the primary cooling loop. It contains a compressor, a turboalternator, and two heat exchangers. This loop implements a reverse-Brayton thermodynamic cycle, and provides the cooling power for the entire system. It is the heart of the NICMOS cryocooler. In generating this cooling power, a significant amount of heat is also generated (up to 500 Watts). This heat is carried away from the primary cooling loop by the third loop in the NCS, called the Capillary Pumped Loop. This loop connects the main heat generating component, the compressor, with the external radiator, which radiates the heat into space. The heat is carried by evaporating ammonia on the hot end, and recondensing it at the cold end of the CPL.
E. Pace - Rivelatori per lo spazio22 Effetto Peltier Peltier effect coolers. Solid-state Peltier coolers, or Thermo-Electric Converters (TECs), are routinely used in space to achieve temperatures above 170K (e.g. the freezers aboard the Interational Space Station). These devices work on the same principle as the Seebeck effect, but in reverse: the creation of a temperature difference between two dissimilar metals by application of a current.
E. Pace - Rivelatori per lo spazio23 Raffreddamento termo-elettrico A thermoelectric cooler (TEC), sometimes called a thermoelectric module or Peltier, is a semiconductor-based electronic component that functions as a small heat pump. By applying a low voltage DC power source to a TEC, heat will be moved through the cooler from one side to the other. One cooler face, therefore, will be cooled while the opposite face simultaneously is heated. Consequently, a thermoelectric cooler may be used for both heating and cooling by reversing the polarity (changing the direction of the applied current). This ability makes TECs highly suitable for precise temperature control applications as well as where space limitations and reliability are paramount and CFCs are not desired.
E. Pace - Rivelatori per lo spazio24 Cella Peltier A typical single stage cooler consists of two ceramic plates with p- and n-type semiconductor material (bismuth telluride) between the plates. The elements of semiconductor material are connected electrically in series and thermally in parallel. When a positive DC voltage is applied to the n-type thermoelement, electrons pass from the p- to the n-type thermoelement and the cold side temperature will decrease as heat is absorbed. The heat absorption (cooling) is proportional to the current and the number of thermoelectric couples. This heat is transferred to the hot side of the cooler, where it is dissipated into the heat sink and surrounding environment.
E. Pace - Rivelatori per lo spazio25 Typical Coolertemp.heat liftAdvantagesDis-advantages Radiator80 K0.5 W Reliable, low vibration, long lifetime Complicates orbit Cryogen4 K0.05 WStable, low vibration Short lifetime, out-gassing, massive, complex Stirling -- 1 stage 80 K0.8 WEfficient, heritageVibrations Stirling -- 2 stage 20 K0.06 WIntermediate tempUnder development Pulse tube80 K0.8 WLower vibrationsLower efficiency than Stirling Peltier170 K1 WLightweightHigh temp, low efficiency Joule- Thompson 4 K0.01 WLow vibrationsRequires hybrid design Sorption10 K0.1 WLow vibrationsUnder development Rev. Brayton65 K8 WHigh capacityComplex ADR0.05 K0.01 mW Only way to reach these temps. Large magnetic field Advantages/disadvantages of different types of cooler technology
E. Pace - Rivelatori per lo spazio26 Some examples of missions using the above coolers MissionCoolerTempHeat liftMassLifetime UARS/ISAMS2 x Stirling80 K0.5 W5 kg3 years IRASHelium cryogen4 KN/A70 kg300 days STS/BETSESorption10 K100 mW10 mins Cassini/CIRSRadiator80 K200 mW2.5 kgUnlimited EOS/AIRS2 x Pulse tube55 K1.63 W35 kg50,000 hrs HST/NICMOSRev. Brayton65 K8 W2 years
E. Pace - Rivelatori per lo spazio27 Ideal UV detector for space Very low noise Radiation hardness Solar blindness Chemical inertness High sensitivity REQUESTS Large area
E. Pace - Rivelatori per lo spazio28 Alternative: diamond materials E g = 5.5 eV dark current < 1 pA visible rejection (ratio 10 -7 ) high XUV sensitivity Highly radiation hard Chemical inertness Mechanically robust High electric charge mobility = fast response time Low dielectric constant = low capacitance Diamond is an appealing materials for XUV photon detection. Their main properties are hereafter summarized :
E. Pace - Rivelatori per lo spazio29 Why diamond Low young's modulus Low young's modulus Small band gap Small band gap Reactive surface Reactive surface Weak Bonding Difficult to thin Dark current Dark current Unstable UV response Unstable UV response Bulk radiation damage Bulk radiation damage Visible light response Visible light response Shielding Hybrid More optics Phosphor, coating Phosphor, coating Back support Cooling Magnetic torque on spacecraft Magnetic torque on spacecraft Severe cleanliness Requirements Severe cleanliness Requirements Power hungry Heavy Vibration problems Vibration problems MATERIAL PROPERTY IMAGER PROBLEM SYSTEM SOLUTION SYSTEM PENALTY SPACE SYSTEM IMPROVEMENT Higher performances No cooling Less optics & no filters No coatings No radiation shielding Mechanical hardness Low power Light system Long durability Clean environment
E. Pace - Rivelatori per lo spazio30 Diamond detectors hν Coplanar geometry hν Transverse geometry
E. Pace - Rivelatori per lo spazio31 Detector technology Diamond layer Interdigitated electrodes
E. Pace - Rivelatori per lo spazio32 Dark current
E. Pace - Rivelatori per lo spazio33 Time response
E. Pace - Rivelatori per lo spazio34 Electro-optical performance 200400600800 1000 1E-8 1E-7 1E-6 1E-5 1E-4 1E-3 0,01 0,1 1 10 100 E = 2.8 V/ m UV/VIS > 10 8 External quantum efficiency Wavelength (nm) E. Pace et al., Diam. Rel. Mater. 9 (2000) 987-993. pCVD
E. Pace - Rivelatori per lo spazio35 Quantum efficiency scCVDpCVD
E. Pace - Rivelatori per lo spazio36 Comparison  Naletto, Pace et al, 1994  Wilhelm et al.,1995  
E. Pace - Rivelatori per lo spazio37 Minimum detectivity = 210 nm EQE = 300 NEP = 5 x 10 -11 erg s -1 cm -2 nm -1 NEP
E. Pace - Rivelatori per lo spazio38 Fluxes & Sensitivity NEP = 5 x 10 -11 erg s -1 cm -2 nm -1
E. Pace - Rivelatori per lo spazio39 Diamond trackers RD42 Collaboration, NIMA 436 (1999) 326-335 RD42 Collaboration, NIMA 434 (1999) 131-145 Particle detectors @ CERN: ATLAS & CMS
E. Pace - Rivelatori per lo spazio40 Diamond imagers C. Schulze-Briese et al., NIMA 467–468 (2001) 230–234 Synchrotron beam profiler J. Schein, Rev. Sci.Instrum., 73, 1 (2002), 19-22 Laser beam profiler
E. Pace - Rivelatori per lo spazio42 Proposed devices Incident radiation Diamond layer Interdigitated electrodes E. Pace et al., ESA Proceedings, SP-493 (2001) 311-314. E. Pace et al., SPIE Proc. 4498 (2001) 121-130. Diamond layer Grounded Mesh Back electrodes