Rumore termico e sistemi di raffreddamento

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Rumore termico e sistemi di raffreddamento
Corso di Rivelatori per lo Spazio

• Dr. Emanuele Pace
• Giugno 2006

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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

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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)

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Calcolo della corrente di buio

• Partiamo dallequazione 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

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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

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Dark current e temperatura

• La corrente di buio cresce con la temperatura,
  poiché la concentrazione di portatori intrinseci
  aumenta in modo proporzionale a

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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

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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

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• BREVE INTRODUZIONE AI
• SISTEMI DI RAFFREDDAMENTO
• UTILIZZATI NELLO SPAZIO

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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.

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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.

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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.
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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.

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Riscaldamento della sonda
Solar radiation 1371 W/m2
Albedo blackbody emission 200 W/m2
X
X
Solar wind 2 x 105 K
Atmosphere 103 K
Rate di collisioni e riscaldamento trascurabili
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Schermi termici
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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.

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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.

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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.

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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.

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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.

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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.

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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.

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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.
•    

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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.

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Advantages/disadvantages of different types of
cooler technology
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Some examples of missions using the above coolers
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Ideal UV detector for space
Radiation hardness
REQUESTS
High sensitivity

• Very low noise

Large area
Solar blindness
Chemical inertness
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Alternative diamond materials
Diamond is an appealing materials for XUV photon
detection. Their main properties are hereafter
summarized

• Eg 5.5 eV ? dark current lt 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

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Why diamond

Higher performances No cooling Less
optics no filters No coatings No
radiation shielding Mechanical hardness
Low power Light system Long durability Clean
environment
SPACE SYSTEM IMPROVEMENT
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Diamond detectors
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Detector technology
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Dark current
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Time response
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Electro-optical performance
pCVD
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Quantum efficiency
scCVD
pCVD
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Comparison
1
2
1 Naletto, Pace et al, 1994 2 Wilhelm et
al.,1995
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Minimum detectivity
NEP
l 210 nm EQE 300
NEP 5 x 10-11 erg s-1 cm-2nm-1
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Fluxes Sensitivity
NEP 5 x 10-11 erg s-1 cm-2 nm-1
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Diamond trackers
RD42 Collaboration, NIMA 436 (1999) 326-335
RD42 Collaboration, NIMA 434 (1999) 131-145
Particle detectors _at_ CERN ATLAS CMS
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Diamond imagers
Synchrotron beam profiler
C. Schulze-Briese et al., NIMA 467468 (2001)
230234
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Electronic structures
DM17
DP129
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Proposed devices
E. Pace et al., ESA Proceedings, SP-493 (2001)
311-314. E. Pace et al., SPIE Proc. 4498 (2001)
121-130.