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Rumore termico e Rumore termico e sistemi di raffreddamentosistemi di raffreddamento
Dr. Emanuele Pace
Giugno 2006
Corso di Rivelatori per lo SpazioCorso di Rivelatori per lo Spazio
E. Pace - Rivelatori per lo spazio 2
Dark currentDark 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 l’ampiezza del segnale Introduce un rumore (shot) non eliminabile con
densità spettrale
Può variare molto da punto a punto in un rivelatore d’immagini causando il fixed pattern noise
E. Pace - Rivelatori per lo spazio 3
Calcolo della corrente di buioCalcolo 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 un’espressione per la corrente di buio dovuta ai difetti di bulk (non necessariamente la sorgente principale)
E. Pace - Rivelatori per lo spazio 4
Calcolo della corrente di buioCalcolo della corrente di buio
Partiamo dall’equazione di continuità:
Con portatori minoritari fotogenerati
costante di diffusione (cm2/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 spazio 5
Calcolo della corrente di buioCalcolo 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 spazio 6
Dark current e temperaturaDark 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 spazio 7
Rumore termicoRumore 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 spazio 8
Raffreddare….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 spazio 9
BREVE INTRODUZIONE AI BREVE INTRODUZIONE AI
SISTEMI DI RAFFREDDAMENTOSISTEMI DI RAFFREDDAMENTO
UTILIZZATI NELLO SPAZIOUTILIZZATI NELLO SPAZIO
E. Pace - Rivelatori per lo spazio 10
Raffreddamento passivoRaffreddamento 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 spazio 11
Liquidi refrigerantiLiquidi 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 dell’esperimento in quanto il refrigeratore principale, l’elio, evapora più lentamente e dura quindi di più.
E. Pace - Rivelatori per lo spazio 12
Esempio: ASTRO FEsempio: 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 spazio 13
Esempio: ASTRO FEsempio: 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 spazio 14
Riscaldamento della sondaRiscaldamento della sonda
Solar radiation
1371 W/m2
Albedo + blackbody emission
200 W/m2
Solar windSolar wind
2 x 102 x 1055 K K
AtmosphereAtmosphere
101033 K KX XRate di collisioni e riscaldamento trascurabili
E. Pace - Rivelatori per lo spazio 15
Schermi termiciSchermi termici
E. Pace - Rivelatori per lo spazio 16
Criogeneratori per bassissime temperatureCriogeneratori 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 spazio 17
Active coolersActive 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 spazio 18
Stirling cycleStirling 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 spazio 19
Joule-ThompsonJoule-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 spazio 20
Reverse Brayton cycleReverse 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 spazio 21
Esempio: NICMOSEsempio: 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 spazio 22
Effetto PeltierEffetto 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 spazio 23
Raffreddamento termo-elettricoRaffreddamento 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 spazio 24
Cella PeltierCella 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 spazio 25
Typical Typical
Cooler temp. heat lift Advantages Dis-advantages
Radiator 80 K 0.5 W Reliable, low vibration, long lifetime
Complicates orbit
Cryogen 4 K 0.05 W Stable, low vibration Short lifetime, out-gassing, massive, complex
Stirling -- 1 stage
80 K 0.8 W Efficient, heritage Vibrations
Stirling -- 2 stage
20 K 0.06 W Intermediate temp Under development
Pulse tube 80 K 0.8 W Lower vibrations Lower efficiency than Stirling
Peltier 170 K 1 W Lightweight High temp, low efficiency
Joule-Thompson
4 K 0.01 W Low vibrations Requires hybrid design
Sorption 10 K 0.1 W Low vibrations Under development
Rev. Brayton 65 K 8 W High capacity Complex
ADR 0.05 K 0.01 mW Only way to reach these temps.
Large magnetic field
Advantages/disadvantages of Advantages/disadvantages of different types of cooler technology different types of cooler technology
E. Pace - Rivelatori per lo spazio 26
Some examples of missions using Some examples of missions using the above coolers the above coolers
Mission Cooler Temp Heat lift Mass Lifetime
UARS/ISAMS 2 x Stirling 80 K 0.5 W 5 kg 3 years
IRAS Helium cryogen 4 K N/A 70 kg 300 days
STS/BETSE Sorption 10 K 100 mW 10 mins
Cassini/CIRS Radiator 80 K 200 mW 2.5 kg Unlimited
EOS/AIRS 2 x Pulse tube 55 K 1.63 W 35 kg 50,000 hrs
HST/NICMOS Rev. Brayton 65 K 8 W 2 years
E. Pace - Rivelatori per lo spazio 27
Ideal UV detector for spaceIdeal UV detector for space
Very low noise
Radiation hardness
Solar blindness
Chemical inertness
High sensitivity REQUESTS
Large area
E. Pace - Rivelatori per lo spazio 28
Alternative: diamond materialsAlternative: diamond materials
Eg = 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 spazio 29
Why diamondWhy diamond
Low young'smodulus
Low young'smodulus
Small band gap
Small band gap
Reactive surface
Reactive surface
Weak BondingWeak Bonding
Difficult to thinDifficult to thin
Dark current
Dark current
Unstable UV response
Unstable UV response
Bulk radiation damage
Bulk radiation damage
Visible lightresponse
Visible lightresponse
ShieldingShielding
HybridHybrid
More opticsMore optics
Phosphor, coating
Phosphor, coating
Back supportBack support
CoolingCooling
Magnetictorque on spacecraft
Magnetictorque on spacecraft
Severe cleanliness
Requirements
Severe cleanliness
Requirements
Power hungryPower hungry
Heavy Heavy
Vibrationproblems
Vibrationproblems
MATERIALPROPERTY
IMAGER PROBLEM
SYSTEM SOLUTION
SYSTEMPENALTY
SPACE SYSTEM IMPROVEMENTSPACE 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 spazio 30
Diamond detectorsDiamond detectors
hνCoplanar geometry hν
Transverse geometry
E. Pace - Rivelatori per lo spazio 31
Detector technologyDetector technology
Diamond layer
Interdigitated electrodes
E. Pace - Rivelatori per lo spazio 32
Dark currentDark current
-5 -4 -3 -2 -1 0 1 2 3 4 5-200
-100
0
100
200
scCVD
Electric Field (V/ m)
Cur
rent
(fA
)
E. Pace - Rivelatori per lo spazio 33
Time responseTime response
E. Pace - Rivelatori per lo spazio 34
Electro-optical performanceElectro-optical performance
200 400 600 800 10001E-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 > 108
Ext
erna
l qua
ntum
effi
cien
cy
Wavelength (nm)
E. Pace et al., Diam. Rel. Mater. 9 (2000) 987-993. pCVD
GL
E
q
h
P
IEQE
ott
f
E. Pace - Rivelatori per lo spazio 35
Quantum efficiencyQuantum efficiency
scCVDpCVD
140 160 180 200 220 240
0,1
1
10
100
EQ
E (
e- / ph
)
Wavelength (nm)
1.2 V / m
E. Pace - Rivelatori per lo spazio 36
ComparisonComparison
100 120 140 160 180 200 220 240 26010-8
10-7
10-6
10-5
10-4
10-3
10-2
10-1
100
101
102
103
EQE CVD Diamond EQE UV enhanced CCD EQE MCP + KBr
EQ
E (e
- / ph
)
Wavelength (nm)
[1] Naletto, Pace et al, 1994
[2] Wilhelm et al.,1995
[2]
[1]
110 120 130 140 150 160 170 180 190 200 210 220 230 240 250 2601E-4
1E-3
0,01
0,1
1
10
100
1000
Wavelength(nm)
EQ
E(e
lect
rons
/pho
ton)
HS148 5V/um HS148 1V/um DF865 2.8V/um
E. Pace - Rivelatori per lo spazio 37
Minimum detectivityMinimum detectivity
= 210 nm
EQE = 300NEP = 5 x 10-11 erg s-1 cm-2nm-1
NEP
E. Pace - Rivelatori per lo spazio 38
Fluxes & SensitivityFluxes & Sensitivity
NEP = 5 x 10-11 erg s-1 cm-2 nm-1
E. Pace - Rivelatori per lo spazio 39
Diamond trackersDiamond 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 spazio 40
Diamond imagersDiamond 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 spazio 41
Electronic structuresElectronic structures
DM17
DP129
(8,4 ± 0,4) µm
(52,9 ±0,4) µm
(8,4 ± 0,4) µm
(6,8 ± 5) µm (18 ± 1) µm
(54 ± 1) µm
(15 ± 1)µm
250 µm
650 µm
E. Pace - Rivelatori per lo spazio 42
Proposed devicesProposed 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