Rumore termico e sistemi di raffreddamento Dr. Emanuele Pace Giugno 2006 Corso di Rivelatori per lo...

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


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)



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



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:


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


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


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


BREVE INTRODUZIONE AI BREVE INTRODUZIONE AI

SISTEMI DI RAFFREDDAMENTOSISTEMI DI RAFFREDDAMENTO

UTILIZZATI NELLO SPAZIOUTILIZZATI NELLO SPAZIO


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.


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.


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.


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.


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


Schermi termiciSchermi termici


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.


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.


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.


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.


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.


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.


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.


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.


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.


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


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


Ideal UV detector for spaceIdeal UV detector for space

Very low noise

Radiation hardness

Solar blindness

Chemical inertness

High sensitivity REQUESTS

Large area


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 :


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


Diamond detectorsDiamond detectors

hνCoplanar geometry hν

Transverse geometry


Detector technologyDetector technology

Diamond layer

Interdigitated electrodes


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

)


Time responseTime response


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


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


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


Minimum detectivityMinimum detectivity

= 210 nm

EQE = 300NEP = 5 x 10-11 erg s-1 cm-2nm-1

NEP


Fluxes & SensitivityFluxes & Sensitivity

NEP = 5 x 10-11 erg s-1 cm-2 nm-1


Diamond trackersDiamond trackers

RD42 Collaboration, NIMA 436 (1999) 326-335

RD42 Collaboration, NIMA 434 (1999) 131-145

Particle detectors @ CERN: ATLAS & CMS


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


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


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

Rumore termico e sistemi di raffreddamento Dr. Emanuele Pace Giugno 2006 Corso di Rivelatori per lo...

Documents

Transcript of Rumore termico e sistemi di raffreddamento Dr. Emanuele Pace Giugno 2006 Corso di Rivelatori per lo...