X Ray Detectors Final

8/6/2019 X Ray Detectors Final

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XX--ray detectorsray detectors

In astrophysicsIn astrophysics

by Ciprian Cretu ,

Physics MSc, Iasi University

2010


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How Astronomers Observe XHow Astronomers Observe X--rays Emitted by Cosmicrays Emitted by CosmicSourcesSources

Although the more energetic XAlthough the more energetic X--rays (E > 30rays (E > 30 keVkeV) can penetrate the air for) can penetrate the air fordistances of at least a fewdistances of at least a few metersmeters (otherwise,(otherwise, RntgenRntgen would never havewould never have

observed them, and medicalXobserved them, and medica

lX--ray machines wou

ld not work), the Earth'sray machines would not work), the Earth'satmosphereatmosphere is thick enough that virtually none are able to penetrate fromis thick enough that virtually none are able to penetrate from

outer space all the way to the Earth's surface. Xouter space all the way to the Earth's surface. X--rays in the 0.5rays in the 0.5 -- 55 keVkeVrange, where most celestial sources give off the bulk of their energy, can berange, where most celestial sources give off the bulk of their energy, can bestopped by a few sheets of paper; ninety percent of thestopped by a few sheets of paper; ninety percent of the photonsphotons in a beam ofin a beam of33 keVkeV XX--rays are absorbed by traveling through just 10 cm of air!rays are absorbed by traveling through just 10 cm of air!

To observe XTo observe X--rays from the sky, the Xrays from the sky, the X--ray detectors must be flown aboveray detectors must be flown above

most of the Earth's atmosphere. There are at present three methods of doingmost of the Earth's atmosphere. There are at present three methods of doingso:so:

Rocket flightsRocket flights A detector is placed in the nose cone section of the rocket and launchedA detector is placed in the nose cone section of the rocket and launched

above the atmosphere. This was first done at White Sands missile range inabove the atmosphere. This was first done at White Sands missile range inNew Mexico with a V2 rocket in 1949. XNew Mexico with a V2 rocket in 1949. X--rays from the Sun were detected byrays from the Sun were detected by

the Navy's experiment on board. Anthe Navy's experiment on board. An AerobeeAerobee 150 rocket launched in June of150 rocket launched in June of1962 detected the first X1962 detected the first X--rays from other celestial sources. The experimentrays from other celestial sources. The experimentpackage contained in this rocket is pictured at right. The largest drawback topackage contained in this rocket is pictured at right. The largest drawback torocket flights is their very short duration (just a few minutes above therocket flights is their very short duration (just a few minutes above theatmosphere before the rocket falls back to Earth) and theirlimited field ofatmosphere before the rocket falls back to Earth) and theirlimited field ofview. A rocket launched from the United States will not be able to seeview. A rocket launched from the United States will not be able to seesources in the southern hemisphere sky; a rocket launched from Australiasources in the southern hemisphere sky; a rocket launched from Australia

will not be able to see sources in the northern hemisphere sky.will not be able to see sources in the northern hemisphere sky.


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BalloonsBalloons

Balloon flights can carry instruments to altitudes of 35Balloon flights can carry instruments to altitudes of 35kilometerskilometers above sea level, where they are above theabove sea level, where they are above the

bulk of the Earth's atmosphere. Unlike a rocket wherebulk of the Earth's atmosphere. Unlike a rocket wheredata are collected during a brief few minutes, balloonsdata are collected during a brief few minutes, balloonsare able to stay aloft for much longer.are able to stay aloft for much longer.

However, even at such altitudes, much of the XHowever, even at such altitudes, much of the X--rayrayspectrumspectrum is still absorbed. Xis still absorbed. X--rays with energies less thanrays with energies less than

3535 keVkeV cannot even reach balloons. One ballooncannot even reach balloons. One balloon--borneborneexperiment was called theexperiment was called the High Resolution GammaHigh Resolution Gamma--rayrayand Hard Xand Hard X--ray Spectrometer (HIREGS)ray Spectrometer (HIREGS). It was. It waslaunched in 1994 from the Antarctic where steady windslaunched in 1994 from the Antarctic where steady windscarried the balloon on a circumpolar flight lasting forcarried the balloon on a circumpolar flight lasting for

almost two months!almost two months! A picture of the launch of HIREGS can be seen. TheA picture of the launch of HIREGS can be seen. The

instrument is at the bottom end of the balloon tether.instrument is at the bottom end of the balloon tether.


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SatellitesSatellites

Adetector is p

laced on a

Adetector is p

laced on a sate

llitesate

llite which is takenwhich is takenup to anup to an orbitorbit well above the Earth'swell above the Earth's

atmosphere.atmosphere.

Unlike balloons, instruments on satellites areUnlike balloons, instruments on satellites areable to observe the full range of the Xable to observe the full range of the X--rayrayspectrum.spectrum.

Unlike rockets, they can collect data for as longUnlike rockets, they can collect data for as longas the instruments continue to operate.as the instruments continue to operate.

In one instance, theIn one instance, the Vela 5BVela 5B satellite (in thesatellite (in thepicture), the Xpicture), the X--ray detector remained functionalray detector remained functionalfor over ten years!for over ten years!


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The Kinds of Objects in the Universe that XThe Kinds of Objects in the Universe that X--rayrayAstronomers ObserveAstronomers Observe

There are a variety of different kinds of astronomicalThere are a variety of different kinds of astronomical

sources which emitsources which emit electromagnetic radiationelectromagnetic radiation in the Xin the X--ray regime. These include:ray regime. These include:

Active GalaxiesActive Galaxies

Binary Star SystemsBinary Star Systems

Black HolesBlack Holes

Cataclysmic VariablesCataclysmic Variables Dark MatterDark Matter

Diffuse BackgroundDiffuse Background

GammaGamma--ray Burstsray Bursts

Neutron StarsNeutron Stars

PulsarsPulsarsStarsStars The SunThe Sun

Supernovae and their RemnantsSupernovae and their Remnants

White DwarfsWhite Dwarfs

XX--ray Transientsray Transients


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Active Galaxies and QuasarsActive Galaxies and Quasars

A Monster in the MiddleA Monster in the Middle

Most largeMost large galaxiesgalaxies have ~1011have ~1011 MoMo ofofstarsstars, ~109, ~109--10 Mo of10 Mo ofinterstellar gas, and ~1012 Mo of darkinterstellar gas, and ~1012 Mo of dark mattermatter. But at least 5%. But at least 5%of galaxies, though it may be all of them, also have somethingof galaxies, though it may be all of them, also have somethingelse lurking inside...a monster in the middle!else lurking inside...a monster in the middle!

These monsters aren't any of the typicalThese monsters aren't any of the typical typicaltypical horror film terrors, thoughhorror film terrors, thoughthey might appear in one of your favorite science fiction movies. In realitythey might appear in one of your favorite science fiction movies. In realitythey arethey are supermassivesupermassive black holes that spew forth tremendous amounts ofblack holes that spew forth tremendous amounts ofenergy from jets on their tops and bottoms. How can these incredible objectsenergy from jets on their tops and bottoms. How can these incredible objects

be explained?be explained? Long ago when galaxies were young, the stars in their coresLong ago when galaxies were young, the stars in their cores

were very closely packed. Star collisions and mergers occurred,were very closely packed. Star collisions and mergers occurred,giving rise to a single massivegiving rise to a single massive black holeblack hole (MBH) with perhaps(MBH) with perhaps106 to 109 Mo. Gas from the galaxy's interstellar medium, from106 to 109 Mo. Gas from the galaxy's interstellar medium, froma cannibalized galaxy, or from a star that strays too close, fallsa cannibalized galaxy, or from a star that strays too close, fallsonto the MBH. As inonto the MBH. As in XX--rayray binary star systemsbinary star systems, an, an accretionaccretiondiskdisk forms, emitting huge amounts offorms, emitting huge amounts oflightlight across theacross theelectromagnetic spectrumelectromagnetic spectrum ((infraredinfrared to gammato gamma--rays). The MBHrays). The MBHplusplus accretionaccretion disk produces the phenomena seen indisk produces the phenomena seen in activeactivegalactic nuclei (AGN)galactic nuclei (AGN). Below you see optical and radio. Below you see optical and radio imagesimages

of the active galaxy NGC 4261. The centra

lobject, accretionof the active ga

laxy NGC 4261. The centra

lobject, accretiondisk, and lobes are all visible.disk, and lobes are all visible.


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A diagram of an active galaxy, showing the primary components


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Ground Based and Hubble Space Telescope imagesGround Based and Hubble Space Telescope images

of the ActiveGalaxy NGC 4261of the ActiveGalaxy NGC 4261


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The different types ofAGN are variations on this theme. ManyThe different types ofAGN are variations on this theme. Manygalaxies today (including our Galactic center??) may have agalaxies today (including our Galactic center??) may have aquiet MBH which happens not to have recently accreted gas.quiet MBH which happens not to have recently accreted gas.SeyfertSeyfert galaxies have accretion onto a moderategalaxies have accretion onto a moderate--mass MBH,mass MBH,while the more luminous quasiwhile the more luminous quasi--stellar objects (i.e. quasars)stellar objects (i.e. quasars)have accretion onto a highhave accretion onto a high--mass MBH.mass MBH.

In ~10% of the AGN, the MBH + accretion disk somehowIn ~10% of the AGN, the MBH + accretion disk somehowproduce narrow beams of energetic particles andproduce narrow beams of energetic particles and magneticmagneticfieldsfields, and eject them outward in opposite directions away from, and eject them outward in opposite directions away from

the disk. These are thethe disk. These are the radioradio jetsjets, which emerge at nearly the, which emerge at nearly thespeed oflightspeed oflight. Radio galaxies,. Radio galaxies, quasarsquasars, and, and blazarsblazars are AGNare AGNwith strong jets, which can travel outward into large regions ofwith strong jets, which can travel outward into large regions ofintergalactic space. Many of the apparent differences betweenintergalactic space. Many of the apparent differences betweentypes ofAGN are due to our having different orientations withtypes ofAGN are due to our having different orientations withrespect to the disk. Withrespect to the disk. With BlazarsBlazars and Quasars, we are lookingand Quasars, we are looking

down the jet. Fordown the jet. For SeyfertsSeyferts, we are viewing the jet broadside., we are viewing the jet broadside. Considerable uncertainties remain. Exactly how are jetsConsiderable uncertainties remain. Exactly how are jets

produced and accelerated? Where do the clouds producing theproduced and accelerated? Where do the clouds producing thebroad emission lines come from? Can we empirically confirmbroad emission lines come from? Can we empirically confirmthat a MBH is actually present?that a MBH is actually present?


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An artists concept of an active galactic nuclei


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SeyfertSeyfert GalaxiesGalaxies

Of the two types ofActive Galactic Nuclei (AGN) whichOf the two types ofActive Galactic Nuclei (AGN) whichemit gamma rays,emit gamma rays, SeyfertSeyfert galaxies are the lowgalaxies are the low--energyenergygammagamma--ray sources.ray sources.

SeyfertSeyfert galaxies typically emit most of their gamma raysgalaxies typically emit most of their gamma raysup to energies of about 100up to energies of about 100 keVkeV and then fade as weand then fade as weobserve them at higher energies. Early gammaobserve them at higher energies. Early gamma--rayray

observations ofobservations of SeyfertSeyfert galaxies indicated thatgalaxies indicated that photonsphotonswere detected up towere detected up to MeVMeV energies, but more sensitiveenergies, but more sensitiveobservations have cast doubt on this possibility. At theseobservations have cast doubt on this possibility. At theselow gammalow gamma--ray energies, the emission is usually aray energies, the emission is usually asmooth continuation of the Xsmooth continuation of the X--ray emission from suchray emission from suchobjects. This generally indicates that the physicalobjects. This generally indicates that the physical

processes creating the gamma rays are thermalprocesses creating the gamma rays are thermalprocesses similar to those responsible for emission fromprocesses similar to those responsible for emission fromgalactic black hole sources. As a result, gammagalactic black hole sources. As a result, gamma--rayraystudies of the highstudies of the high--energy spectrum and variability canenergy spectrum and variability cangive scientists important information about the physicalgive scientists important information about the physicalenvironment in the AGN.environment in the AGN.


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QuasarsQuasars

In the 1960s, some radio sources seemed to beIn the 1960s, some radio sources seemed to be

associated with 'stars', and were calledassociated with 'stars', and were called quasiquasi--

stellar radio sourcesstellar radio sources ororquasarsquasars. But they had. But they had

spectraspectra similar tosimilar to SeyfertSeyfert galaxygalaxy nuclei! Itnuclei! It

became clear that they arebecame clear that they are SeyfertsSeyferts and radioand radio

galaxies where the nucleus out shines all of thegalaxies where the nucleus out shines all of the

stars by factors of 10stars by factors of 10--1000. The luminosity of1000. The luminosity of

quasars can reach 1012 Lo. They al

so tend toquasars can reach 1012 Lo. They al

so tend tobe farther away than eitherbe farther away than either SeyfertSeyfert Galaxies orGalaxies or

BlazarsBlazars


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The region of the sky containing one of the highThe region of the sky containing one of the high--energy quasars, PKS 0528+134, isenergy quasars, PKS 0528+134, is

shown at two different times using the EGRET instrument on theshown at two different times using the EGRET instrument on the

Compton GammaCompton Gamma--Ray ObservatoryRay Observatory. These active galaxies are highly variable,. These active galaxies are highly variable,

strongly emitting gammastrongly emitting gamma--rays sometimes, disappearing at other timesrays sometimes, disappearing at other times


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BlazarsBlazars

Active Galactic Nuclei observed at high (>100Active Galactic Nuclei observed at high (>100 MeVMeV) energies form) energies forma subclass known asa subclass known as blazarsblazars; a; a blazarblazar is believed to be an AGNis believed to be an AGNwhich has one of itswhich has one of its relativisticrelativisticjets pointed toward the Earth sojets pointed toward the Earth so

that the emission we observe is dominated by phenomenathat the emission we observe is dominated by phenomenaoccurring in the jet region. Amongst allAGNs,occurring in the jet region. Amongst allAGNs, blazarsblazars emit overemit overthe widest range ofthe widest range offrequenciesfrequencies, being detected from radio to, being detected from radio togammagamma--ray.ray.

Specifically, to be classified as aSpecifically, to be classified as a blazarblazar an AGN must be seenan AGN must be seenwith one of the following properties:with one of the following properties:

high radiohigh radio--brightness accompanied by flatness of the radio spectrumbrightness accompanied by flatness of the radio spectrum

high opticalhigh opticalpolarizationpolarization,,

strong optical variability on very short timescales (less than few days).strong optical variability on very short timescales (less than few days).

In the class of objects selected according to these criteria, thereIn the class of objects selected according to these criteria, there

appear to be two subgroups:appear to be two subgroups: (1) sources showing strong and broad emission lines, such as those of(1) sources showing strong and broad emission lines, such as those of

quasars (called Flat Spectrum Radio Quasars), andquasars (called Flat Spectrum Radio Quasars), and

(2) sources showing a featureless optical spectrum (called BL Lac objects).(2) sources showing a featureless optical spectrum (called BL Lac objects).There are additional important differences between these subclasses suchThere are additional important differences between these subclasses suchas they show different luminosity andas they show different luminosity and redshiftredshift distributions, and a differentdistributions, and a different

morphology of the extended radio emission.morphology of the extended radio emission.


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XX--ray Binary Starsray Binary Stars

If your eyes could seeIf your eyes could see XX--raysrays rather than opticalrather than opticallightlight, you would see a very different and unusual, you would see a very different and unusualsight when you looked up at the sky. You would besight when you looked up at the sky. You would beoverwhelmed by a few hundred very brightoverwhelmed by a few hundred very bright starsstars,,mostly concentrated towards the center of ourmostly concentrated towards the center of ourGalaxyGalaxy. Most of these stars would in fact be X. Most of these stars would in fact be X--rayray

binaries, where abinaries, where a black holeblack hole ororneutron starneutron starisisdevouring material from its companion star.devouring material from its companion star.

A basic quest of science is to test the laws ofA basic quest of science is to test the laws ofphysics under all conditions. Unexpectedphysics under all conditions. Unexpecteddiscoveries can lead to breakthroughs in ourdiscoveries can lead to breakthroughs in ourunderstanding of the laws of nature. Xunderstanding of the laws of nature. X--rayray

observations of neutron stars and black holesobservations of neutron stars and black holesprovide a unique probe into how theprovide a unique probe into how the UniverseUniverseoperates under extreme physical conditions.operates under extreme physical conditions.


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XX--ray Binaries (in yellow) near the Galactic Centerray Binaries (in yellow) near the Galactic Center


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Black Holes: What Are They?Black Holes: What Are They?

Black holes are the evolutionary endpoints of stars at least 10 to 15 times asBlack holes are the evolutionary endpoints of stars at least 10 to 15 times asmassive as the Sun. If a star that massive orlarger undergoes amassive as the Sun. If a star that massive orlarger undergoes a supernovasupernovaexplosion, it may leave behind a fairly massive burned out stellar remnant.explosion, it may leave behind a fairly massive burned out stellar remnant.With no outward forces to opposeWith no outward forces to oppose gravitationalgravitational forces, the remnant willforces, the remnant will

collapse in on itself. The star eventually collapses to the point of zerocollapse in on itself. The star eventually collapses to the point of zerovolume and infinitevolume and infinite densitydensity, creating what is known as a ", creating what is known as a " singularitysingularity ".".Around the singularity is a region where the force of gravity is so strong thatAround the singularity is a region where the force of gravity is so strong thatnot even light can escape. Thus, no information can reach us from thisnot even light can escape. Thus, no information can reach us from thisregion. It is therefore called a black hole, and its surface is called the "region. It is therefore called a black hole, and its surface is called the " eventeventhorizonhorizon ".".

But contrary to popular myth, a black hole is not a cosmic vacuum cleaner. If our SunBut contrary to popular myth, a black hole is not a cosmic vacuum cleaner. If our Sun

was suddenly replaced with a black hole of the same mass, the Earth's orbit aroundwas suddenly replaced with a black hole of the same mass, the Earth's orbit aroundthe Sun would be unchanged. (Of course the Earth's temperature would change, andthe Sun would be unchanged. (Of course the Earth's temperature would change, andthere would be no solar wind or solar magnetic storms affecting us.) To be "sucked"there would be no solar wind or solar magnetic storms affecting us.) To be "sucked"into a black hole, one has to cross inside theinto a black hole, one has to cross inside the Schwarzschild radiusSchwarzschild radius. At this radius, the. At this radius, theescape speed is equal to theescape speed is equal to the speed oflightspeed oflight, and once light passes through, even it, and once light passes through, even itcannot escape.cannot escape.

The Schwarzschild radius can be calculated using the equation for escapeThe Schwarzschild radius can be calculated using the equation for escape

speed:speed: v esc = (2GM/R)1/2v esc = (2GM/R)1/2 For photons, or objects with noFor photons, or objects with no massmass, we can substitute c (the speed oflight), we can substitute c (the speed oflight)

forfor V escV escand find the Schwarzschild radius,and find the Schwarzschild radius, RR, to be, to beR = 2GM/c2R = 2GM/c2

If the Sun was replaced with a black hole that had the same mass as theIf the Sun was replaced with a black hole that had the same mass as theSun, the Schwarzschild radius would be 3 km (compared to the Sun's radiusSun, the Schwarzschild radius would be 3 km (compared to the Sun's radius

of nearly 700,000 km). Hence the Earth wou

ld have to get very c

lose to getof near

ly 700,000 km). Hence the Earth wou

ld have to get very c

lose to getsucked into a black hole at the center of our Solar Systemsucked into a black hole at the center of our Solar System


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Since stellar black holes are small (only a few to a few tens of kilometers inSince stellar black holes are small (only a few to a few tens of kilometers insize), and light that would allow us to see them cannot escape, a black holesize), and light that would allow us to see them cannot escape, a black holefloating alone in space would be hard, if not impossible, to see. For instance,floating alone in space would be hard, if not impossible, to see. For instance,the photograph above shows thethe photograph above shows the opticaloptical companion star to the (invisible)companion star to the (invisible)black hole candidateblack hole candidate CygCyg XX--1.1.

However, if a black hole passes through a cloud of interstellarHowever, if a black hole passes through a cloud of interstellarmattermatter, or is, or isclose to another "normal" star, the black hole canclose to another "normal" star, the black hole can accreteaccrete matter into itself.matter into itself.As the matter falls or is pulled towards the black hole, it gains kinetic energy,As the matter falls or is pulled towards the black hole, it gains kinetic energy,heats up and is squeezed by tidal forces. The heatingheats up and is squeezed by tidal forces. The heating ionizes the atomsionizes the atoms, and, andwhen the atoms reach a few millionwhen the atoms reach a few million KelvinKelvin, they emit, they emit XX--raysrays. The X. The X--rays arerays aresent off into space before the matter crosses the Schwarzschild radius andsent off into space before the matter crosses the Schwarzschild radius andcrashes into thecrashes into the singularitysingularity. Thus we can see this X. Thus we can see this X--ray emission.ray emission.

Binary XBinary X--ray sources are also places to find strong black hole candidates. Aray sources are also places to find strong black hole candidates. Acompanion star is a perfect source ofcompanion star is a perfect source of infallinginfalling material for a black hole. Amaterial for a black hole. Abinary systembinary system also allows the calculation of the black hole candidate's mass.also allows the calculation of the black hole candidate's mass.Once the mass is found, it can be determined if the candidate is aOnce the mass is found, it can be determined if the candidate is a neutronneutronstarstaror a black hole, since neutron stars always have masses of about 1.5or a black hole, since neutron stars always have masses of about 1.5times the mass of the Sun. Another sign of the presence of a black hole is itstimes the mass of the Sun. Another sign of the presence of a black hole is itsrandom variation of emitted Xrandom variation of emitted X--rays. Therays. The infallinginfalling matter that emits Xmatter that emits X--raysraysdoes not fall into the black hole at a steady rate, but rather moredoes not fall into the black hole at a steady rate, but rather moresporadically, which causes an observable variation in Xsporadically, which causes an observable variation in X--ray intensity.ray intensity.Additionally, if the XAdditionally, if the X--ray source is in a binary system, and we see it fromray source is in a binary system, and we see it fromcertain angles, the Xcertain angles, the X--rays will be periodically cut off as the source israys will be periodically cut off as the source is eclipsedeclipsedby the companion star. When looking for black hole candidates, all theseby the companion star. When looking for black hole candidates, all thesethings are taken into account. Many Xthings are taken into account. Many X--rayray satellitessatellites have scanned the skieshave scanned the skies

for Xfor X--ray sources that might be bl

ack hol

e candidates.ray sources that might be bl

ack hol

e candidates.


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CataclysmicCataclysmic VariablesVariables

Cataclysmic variablesCataclysmic variables (CVs) are binary star systems which(CVs) are binary star systems which

have ahave a white dwarfwhite dwarfandand a normalstara norma

lstarcompanion. They arecompanion. They aretypically smalltypically small -- the entirethe entire binary systembinary system usually has the size ofusually has the size of

the Earththe Earth--Moon systemMoon system -- with anwith an orbitalorbital period in the range 1period in the range 1--10 hrs.10 hrs.

The white dwarf is often referred to as the "primary" star, andThe white dwarf is often referred to as the "primary" star, andthe normal star as the "companion" or the "secondary". Thethe normal star as the "companion" or the "secondary". The

companion star, a more orless normal starlike our Sun, losescompanion star, a more orless normal starlike our Sun, losesmaterial onto the white dwarf viamaterial onto the white dwarf via accretionaccretion..

Since the white dwarf is very dense, theSince the white dwarf is very dense, the gravitationalgravitational potentialpotentialenergy is enormous, and some of it is converted intoenergy is enormous, and some of it is converted into XX--raysraysduring the accretion process.during the accretion process.

There are probably over a million such cataclysmic variables inThere are probably over a million such cataclysmic variables inthethe GalaxyGalaxy, but only those close to our Sun (several hundreds), but only those close to our Sun (several hundreds)have been studied in Xhave been studied in X--rays so far. This is because CVs arerays so far. This is because CVs arefairly faint in Xfairly faint in X--rays; they are just above therays; they are just above the coronalcoronal XX--rayraysources and far below the Xsources and far below the X--ray binaries in terms of howray binaries in terms of howpowerful their Xpowerful their X--ray emissions are.ray emissions are.


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A diagram of a Cataclysmic Variable, showing the normal star,A diagram of a Cataclysmic Variable, showing the normal star,

the accretion disk, and the white dwarf. The hot spot is where matter from thethe accretion disk, and the white dwarf. The hot spot is where matter from the

normal star meets the accretion disk.normal star meets the accretion disk.


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Dark MatterDark Matter

When theWhen the UniverseUniverse was young, it was nearly smooth andwas young, it was nearly smooth andfeatureless. As it grew older and developed, it becamefeatureless. As it grew older and developed, it became

organized. We know that our solar system is organizedorganized. We know that our solar system is organizedinto planets (including the Earth!)into planets (including the Earth!) orbitingorbiting around thearound theSun. On a scale much larger than the solar systemSun. On a scale much larger than the solar system

(about 100 million times

larger!),(about 100 mi

llion times

larger!), starsstars co

llect themse

lvesco

llect themse

lvesintointo galaxiesgalaxies..

Our Sun is an average star in an average galaxy calledOur Sun is an average star in an average galaxy calledthe Milky Way. The Milky Way contains about 100 billionthe Milky Way. The Milky Way contains about 100 billionstars. Yes, that's 100,000,000,000 stars! On stilllargerstars. Yes, that's 100,000,000,000 stars! On stilllarger

scales, individua

lga

laxies are concentrated into groups,sca

les, individua

lga

laxies are concentrated into groups,or whator what astronomersastronomers callcallclusters of galaxiesclusters of galaxies


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The cluster includes the galaxies and any material which is inThe cluster includes the galaxies and any material which is inthe space between the galaxies. The force, or glue, that holdsthe space between the galaxies. The force, or glue, that holdsthe cluster together is gravitythe cluster together is gravity ---- the mutual attraction ofthe mutual attraction ofeverything in the Universe for everything else. The spaceeverything in the Universe for everything else. The space

between galaxies in c

lusters is fi

lled with a hot gas. In fact, thebetween ga

laxies in c

lusters is fi

lled with a hot gas. In fact, thegas is so hot (tens of millions of degrees!) that it shines ingas is so hot (tens of millions of degrees!) that it shines in XX--

raysrays instead ofinstead ofvisible lightvisible light. In the image above, the hot X. In the image above, the hot X--rayraygas (shown in pink) lying between the galaxies is superimposedgas (shown in pink) lying between the galaxies is superimposedon anon an anan optical picture of the cluster of galaxies. By studyingoptical picture of the cluster of galaxies. By studyingthe distribution and temperature of the hot gas we can measurethe distribution and temperature of the hot gas we can measure

how much it is being squeezed by the force ofhow much it is being squeezed by the force ofgravitygravity from allfrom allthe material in the cluster. This allows scientists to determinethe material in the cluster. This allows scientists to determinehow much total material (how much total material (mattermatter) there is in that part of space.) there is in that part of space.

Remarkably, it turns out there is five times more material inRemarkably, it turns out there is five times more material inclusters of galaxies than we would expect from the galaxies andclusters of galaxies than we would expect from the galaxies andhot gas we can see. Most of the stuff in clusters of galaxies ishot gas we can see. Most of the stuff in clusters of galaxies isinvisible and, since these are the largest structures in theinvisible and, since these are the largest structures in theUniverse held together by gravity, scientists then conclude thatUniverse held together by gravity, scientists then conclude thatmost of the matter in the entire Universe is invisible. Thismost of the matter in the entire Universe is invisible. Thisinvisible stuff is called 'invisible stuff is called 'dark matterdark matter'. There is currently much'. There is currently muchongoing research by scientists attempting to discover exactlyongoing research by scientists attempting to discover exactly

what this dark matter is, how much there is, and what effect itwhat this dark matter is, how much there is, and what effect itmay have on the future of the Universe as a whole.may have on the future of the Universe as a whole.


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Soft XSoft X--ray Diffuse Backgroundray Diffuse Background

The nature of the softThe nature of the soft XX--rayray diffuse background (SXRB) variesdiffuse background (SXRB) variesconsiderably over its energy range. At the lowest energies, 0.1considerably over its energy range. At the lowest energies, 0.1 --

0.30.3 keVkeV, nearly a

llof observed SXR

Boriginates as therma

l, near

ly a

llof observed SXR

Boriginates as therma

lemission from hot (~106 K)emission from hot (~106 K) plasmaplasma. There are two major. There are two major

components of this hot plasma.components of this hot plasma.

First, it is contained in a hot bubble in theFirst, it is contained in a hot bubble in the diskdisk of theof the GalaxyGalaxywhich surrounds the Sun (but was not created by the Sun) andwhich surrounds the Sun (but was not created by the Sun) andextends from ~50 pc to ~200 pc in different directions.extends from ~50 pc to ~200 pc in different directions.

Second, there is an extensive distribution of this plasma in theSecond, there is an extensive distribution of this plasma in thehalo of our Galaxy. Above 1halo of our Galaxy. Above 1 keVkeV, most of the SXRB is not, most of the SXRB is notactually diffuse in origin but is rather the superposition of manyactually diffuse in origin but is rather the superposition of manyunresolved discreteunresolved discrete extragalacticextragalactic sources, such assources, such as activeactivegalactic nuclei (AGN)galactic nuclei (AGN) andand quasarsquasars. (We know this because with. (We know this because with

verylong Xverylong X--ray observations we can identify the individua

lray observations we can identify the individualsources.) Between 0.5 and 1sources.) Between 0.5 and 1 keVkeV the situation is considerablythe situation is considerably

more confused. Both extragalactic discrete sources andmore confused. Both extragalactic discrete sources andGalactic emission from hot plasma contribute to the observedGalactic emission from hot plasma contribute to the observedfluxflux. As extragalactic objects are discussed in other places, this. As extragalactic objects are discussed in other places, thistext will concentrate on the lowtext will concentrate on the low--energy Galactic diffuseenergy Galactic diffuse

emission.emission.


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Neutron starsNeutron stars

Neutron starsNeutron stars are compact objects that are created in the coresare compact objects that are created in the cores

of massiveof massive starsstars duringduring supernova explosionssupernova explosions. The core of the. The core of thestar collapses, and crushes together every proton with astar collapses, and crushes together every proton with a

correspondingcorresponding electronelectron turning each electronturning each electron--proton pair into aproton pair into a

neutron. Theneutron. The neutronsneutrons, however, can often stop the collapse, however, can often stop the collapse

and remain as a neutron star.and remain as a neutron star.

Neutron stars are fascinating objects because they are theNeutron stars are fascinating objects because they are the

most dense objects known. They are only about 10 miles inmost dense objects known. They are only about 10 miles in

diameter, yet they are more massive than the Sun. One sugardiameter, yet they are more massive than the Sun. One sugar

cube of neutron star material weighs about 100 million tons,cube of neutron star material weighs about 100 million tons,

which is about as much as a mountain.which is about as much as a mountain.


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Like theirless massive counterparts,Like theirless massive counterparts, white dwarfswhite dwarfs, the, the

heavier a neutron star gets the smaller it gets. Imagine ifheavier a neutron star gets the smaller it gets. Imagine ifa 10 pound bag of flour was smaller than a 5 pound bag!a 10 pound bag of flour was smaller than a 5 pound bag!

Neutron stars can be observed occasionally, as withNeutron stars can be observed occasionally, as with

PuppisPuppis A above, as an extremely small and hot starA above, as an extremely small and hot star

within a supernova remnant. However, they are morewithin a supernova remnant. However, they are more

likely to be seen when they are alikely to be seen when they are a pulsarpulsaror part of anor part of an XX--

ray binaryray binary..


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PulsarsPulsars

A pulsar is aA pulsar is a neutron starneutron starwhich emits beams of radiation thatwhich emits beams of radiation that

sweep through the earth's line of sight. Like asweep through the earth's line of sight. Like a black holeblack hole, it is, it isan endpoint to stellar evolution.an endpoint to stellar evolution.

The "pulses" of highThe "pulses" of high--energyenergy radiationradiation we see from a pulsar arewe see from a pulsar are

due to a misalignment of the neutron star's rotation axis and itsdue to a misalignment of the neutron star's rotation axis and its

magnetic axis. Pulsars pulse because themagnetic axis. Pulsars pulse because the rotationrotation of theof theneutron star causes the radiation generated within the magneticneutron star causes the radiation generated within the magnetic

field to sweep in and out of ourline of sight with a regularfield to sweep in and out of ourline of sight with a regular

period.period.

A diagram of a pulsar showing its rotation axis,A diagram of a pulsar showing its rotation axis,

its magnetic axis, and its magnetic field.its magnetic axis, and its magnetic field.


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Although all pulsars are neutron stars, not all pulsars shine inAlthough all pulsars are neutron stars, not all pulsars shine in

the same way. Xthe same way. X--ray pulsars in particular illustrate several waysray pulsars in particular illustrate several ways

in which pulsar emission can originate:in which pulsar emission can originate:

MagnetosphericMagnetospheric Emission:Emission: Like gammaLike gamma--ray pulsars, Xray pulsars, X--rayray

pulsars can be produced when highpulsars can be produced when high--energy electrons interact inenergy electrons interact in

the magnetic field regions above the neutron star magneticthe magnetic field regions above the neutron star magnetic

poles. Pulsars seen this way, whether in the radio, optical, Xpoles. Pulsars seen this way, whether in the radio, optical, X--

ray, or gammaray, or gamma--ray, are often referred to as "spinray, are often referred to as "spin--poweredpoweredpulsars," because the ultimate source of energy comes from thepulsars," because the ultimate source of energy comes from the

neutron star rotation. The loss of rotational energy results in aneutron star rotation. The loss of rotational energy results in a

slowing of the pulsar spin period.slowing of the pulsar spin period.

Cooling Neutron Stars:Cooling Neutron Stars: When a neutron star is first formed inWhen a neutron star is first formed in

aa supernovasupernova, its surface is extremely hot (more than, its surface is extremely hot (more than

1,000,000,000 degrees). Over time, the surface cools. While1,000,000,000 degrees). Over time, the surface cools. While

the surface is still hot enough, it can be seen with Xthe surface is still hot enough, it can be seen with X--rayray

telescopes.telescopes.


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If some parts of the neutron star are hotter than others (such asIf some parts of the neutron star are hotter than others (such as

the magnetic poles), then pulses of thermal Xthe magnetic poles), then pulses of thermal X--rays from therays from the

neutron star surface can be seen as the hot spots pass throughneutron star surface can be seen as the hot spots pass through

ourline of sight. Some pu

lsars, inc

ludingour

line of sight. Some pu

lsars, inc

luding GemingaGeminga (see above),(see above),show both thermal andshow both thermal and magnetosphericmagnetospheric pulses.pulses.

Accretion:Accretion: If a neutron star is in a binary system with a normalIf a neutron star is in a binary system with a normal

star, the powerfulstar, the powerfulgravitationalgravitational field of the neutron star can pullfield of the neutron star can pull

material from the surface of the normal star. As this materialmaterial from the surface of the normal star. As this material

spirals around the neutron star, it is funneled by the magneticspirals around the neutron star, it is funneled by the magneticfield toward the neutron star magnetic poles. In the process, thefield toward the neutron star magnetic poles. In the process, the

material is heated until it becomes hot enough to radiate Xmaterial is heated until it becomes hot enough to radiate X--

rays. As the neutron star spins, these hot regions pass throughrays. As the neutron star spins, these hot regions pass through

theline of sight from the Earth and Xtheline of sight from the Earth and X--ray te

lescopes see theseray telescopes see theseas Xas X--ray pulsars. Because the gravitational pull on the materialray pulsars. Because the gravitational pull on the material

is the basic source of energy for this emission, these are oftenis the basic source of energy for this emission, these are often

called "accretion powered pulsars."called "accretion powered pulsars."


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The SunThe Sun

The SunThe Sun

as seenas seen

in Xin X--raysrays

(from the(from the

YohkohYohkohsatellite)satellite)


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Supernova RemnantsSupernova Remnants

A supernova remnant (SNR) is theA supernova remnant (SNR) is the

remains of aremains of a supernova explosionsupernova explosion. SNRs. SNRs

are extremely important for understandingare extremely important for understanding

ourourGalaxyGalaxy. They heat up the. They heat up the interstellarinterstellar

mediummedium, distribute heavy, distribute heavy elementselements

throughout the Galaxy, and acceleratethroughout the Galaxy, and accelerate

cosmic rayscosmic rays


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SampleSample

SupernovaSupernova

R

emnantsR

emnants

Cygnus LoopCygnus Loop

in Xin X--raysrays


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Detecting XDetecting X--raysrays

XX--ray astronomy is a window on the hot universe, the universe of stellarray astronomy is a window on the hot universe, the universe of stellar

corona, hot interstellar gases, compact stars, and black holes. It is one ofcorona, hot interstellar gases, compact stars, and black holes. It is one of

severaldiscip

lines in astronomy that can on

ly be conducted in space,severa

ldiscip

lines in astronomy that can on

ly be conducted in space,outside of our protective atmosphere. Advances in xoutside of our protective atmosphere. Advances in x--ray astronomy areray astronomy are

therefore bound to advances in space flight.therefore bound to advances in space flight.

Interactions with MatterInteractions with Matter

Our ability to image the interior of our bodies with xOur ability to image the interior of our bodies with x--rays is sorays is so

familiar that the inability of xfamiliar that the inability of x--rays to travellong distancesrays to travellong distancesthrough Earth's atmosphere is startling. In fact, xthrough Earth's atmosphere is startling. In fact, x--rays travelrays travel

through the atmosphere over relatively short distances, lessthrough the atmosphere over relatively short distances, less

than ten kilometers, with the lowerthan ten kilometers, with the lower--energy xenergy x--rays beingrays being

absorbed by air before the higherabsorbed by air before the higher--energy xenergy x--rays. Therays. The

atmosphere absorbs the xatmosphere absorbs the x--rays of out space before they canrays of out space before they can

reach any point on Earth's surface; in fact, short of going toreach any point on Earth's surface; in fact, short of going to

space, the only craft that can reach an altitude high enough tospace, the only craft that can reach an altitude high enough to

observe cosmic xobserve cosmic x--rays is the highrays is the high--altitude balloon. Much earlyaltitude balloon. Much early

work on xwork on x--ray astronomy was done by balloon.ray astronomy was done by balloon.


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Observing from balloons, however, has some shortcomings, such as the shorObserving from balloons, however, has some shortcomings, such as the shor

duration and limited season for ballooningduration and limited season for ballooningwind condition must be right sowind condition must be right so

that a balloon doesn't drift over heavily populated areasthat a balloon doesn't drift over heavily populated areasand the interactionand the interaction

of xof x--rays and gammarays and gamma--rays with the thin atmosphere above the balloon. Todayrays with the thin atmosphere above the balloon. Today

balloons are primarily used to test new xballoons are primarily used to test new x--ray detector designs. Almost all xray detector designs. Almost all x--raratelescopes are now mounted on satellites.telescopes are now mounted on satellites.

XX--rays are readily absorbed by the inner, lowestrays are readily absorbed by the inner, lowest--energyenergy

electrons of an atom. When an electron in an atom absorbs an xelectrons of an atom. When an electron in an atom absorbs an x

ray, it either jumps to a higher energy level in the atom or it isray, it either jumps to a higher energy level in the atom or it is

freed from the atom. The energy absorbed when an electronfreed from the atom. The energy absorbed when an electronmoves to a higher atomic level is releases as the electron decaymoves to a higher atomic level is releases as the electron decay

to lowerlevels, which involved either the emission of optical andto lowerlevels, which involved either the emission of optical and

ultraviolet photons or the exchange of energy to another atomultraviolet photons or the exchange of energy to another atom

during a coll

ision.A

free el

ectronl

oses energy by scattering withduring a coll

ision.A

free el

ectronl

oses energy by scattering withatoms and other electrons until it is captured by an atom; theseatoms and other electrons until it is captured by an atom; these

processes distributes the original energy of the xprocesses distributes the original energy of the x--ray into manyray into many

optical and ultraviolet photons and into thermal energy that isoptical and ultraviolet photons and into thermal energy that is

shared among many atoms.shared among many atoms.


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Imaging with XImaging with X--raysrays

From the ways that xFrom the ways that x--rays interact with atoms, and from ourrays interact with atoms, and from our

experience with xexperience with x--ray machines, we would not think that xray machines, we would not think that x--raysrayscould be focused to a point by a telescope; in fact, the lowestcould be focused to a point by a telescope; in fact, the lowest

energy xenergy x--rays can be focused quite precisely by a mirror if therays can be focused quite precisely by a mirror if the

path of each xpath of each x--ray is nearly in the plane of the mirror. Theseray is nearly in the plane of the mirror. These

grazinggrazing--incidence mirrors are at the heart of xincidence mirrors are at the heart of x--ray telescopesray telescopes

such as NASA's Chandra Xsuch as NASA's Chandra X--ray Observatory.ray Observatory.

When opticallight is reflected by the metal coating of a mirror,When opticallight is reflected by the metal coating of a mirror,

the electromagnetic field of the light interacts with manythe electromagnetic field of the light interacts with many

electrons within the mirror, causing those electrons toelectrons within the mirror, causing those electrons to

accelerate. These accelerating electrons take energy from theaccelerate. These accelerating electrons take energy from theelectromagnetic wave of the incoming light and create a newelectromagnetic wave of the incoming light and create a new

wave traveling away from the mirror's surface, a mirror image ofwave traveling away from the mirror's surface, a mirror image of

the original wave.the original wave.


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Normally this interaction does not occur with xNormally this interaction does not occur with x--rays, becauserays, because

the wavelength of the xthe wavelength of the x--ray is too short to allow it to interactray is too short to allow it to interact

with more than the electrons bound tightly to an atom. But if thewith more than the electrons bound tightly to an atom. But if the

xx--ray travel

s al

most parall

el

to the surface of a metal

, itsray travel

s al

most parall

el

to the surface of a metal

, itselectromagnetic field will interact with many electrons, and aselectromagnetic field will interact with many electrons, and as

with opticallight, the xwith opticallight, the x--ray will be reflected.ray will be reflected.

For an xFor an x--ray to be reflected by a metal surface, it must approach the surfaceray to be reflected by a metal surface, it must approach the surface

at the angle of 85at the angle of 85 or more from the perpendicular. For a mirror with a widthor more from the perpendicular. For a mirror with a width

of one meter in the direction of travelof an xof one meter in the direction of trave

lof an x--ray, the co

llecting area, theray, the co

llecting area, thearea that the xarea that the x--ray sees has a width of only 8.7 cm; To create an xray sees has a width of only 8.7 cm; To create an x--rayray

telescope of a given collecting area requires a mirror with a surface area thattelescope of a given collecting area requires a mirror with a surface area that

is 11 times the collecting area.is 11 times the collecting area.

In practice, the mirrors in an xIn practice, the mirrors in an x--ray telescope are hollowray telescope are hollow

cylinders that are wider in the front than at the back. The xcylinders that are wider in the front than at the back. The x--raysraysreflect off of the inner surface to the focal point along the axis.reflect off of the inner surface to the focal point along the axis.

Because of the large incidence angle, the focallength of aBecause of the large incidence angle, the focallength of a

singlesingle--mirror telescope is extremely long For instance, if themirror telescope is extremely long For instance, if the

mirror has an opening radius of one meter, the focal point mustmirror has an opening radius of one meter, the focal point must

be about 5.8 meters from the opening of the mirror.be about 5.8 meters from the opening of the mirror.


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These properties of an xThese properties of an x--ray mirror present difficulties forray mirror present difficulties for

developing a spacedeveloping a space--born instrument, which must be compactborn instrument, which must be compact

and light if it is to be placed in orbit on a rocket. The area of aand light if it is to be placed in orbit on a rocket. The area of a

tel

escope can be increased by nesting mirrors of different radii.tel

escope can be increased by nesting mirrors of different radii.The focallength can be shortened by adding a second set ofThe focallength can be shortened by adding a second set of

mirrors behind the first; the costs is a loss of xmirrors behind the first; the costs is a loss of x--rays, since arays, since a

mirror does not reflect 100% of the xmirror does not reflect 100% of the x--rays that strike it. While allrays that strike it. While all

modern xmodern x--ray telescopes employ these strategies to increaseray telescopes employ these strategies to increase

the collecting area and shorten the focallength, an xthe collecting area and shorten the focallength, an x--rayraytelescope has a smaller collecting area and a longer physicaltelescope has a smaller collecting area and a longer physical

length than an equivalent radius optical telescope.length than an equivalent radius optical telescope.

Coded Aperture Mask TelescopesCoded Aperture Mask Telescopes

Above about 20Above about 20 keVkeV xx--rays cannot be imaged by mirrors. Forrays cannot be imaged by mirrors. Forthese energies, a less precise methodthese energies, a less precise method methodmethod is used to imageis used to image

the sky: the coded aperture mask. The idea is to let an objectthe sky: the coded aperture mask. The idea is to let an object

cast an xcast an x--ray shadow onto a detector. By knowing where theray shadow onto a detector. By knowing where the

object is relative to the detector, we know the direction to theobject is relative to the detector, we know the direction to the

source from the position of the shadow on the detector.source from the position of the shadow on the detector.


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If more than one source exists, one can find the direction toIf more than one source exists, one can find the direction to

each source by finding the multiple shadows on the detector.each source by finding the multiple shadows on the detector.

As a practical matter, this method only works forlocating pointAs a practical matter, this method only works forlocating point

sources; a broad area of xsources; a broad area of x--ray emission cannot be imagedray emission cannot be imagedthrough this method.through this method.

In practice, an instrument designed to determine direction fromIn practice, an instrument designed to determine direction from

a cast shadow has a mask with holes over the instrument'sa cast shadow has a mask with holes over the instrument's

aperture. The pattern of holes in the aperture is carefullyaperture. The pattern of holes in the aperture is carefully

chosen to simply the problem of determining the direction tochosen to simply the problem of determining the direction toevery source in the field of view. This mask is called a codedevery source in the field of view. This mask is called a coded

aperture mask. Telescopes designed on this principle canaperture mask. Telescopes designed on this principle can

locate a source with an accuracy of several arc minutes.locate a source with an accuracy of several arc minutes.

Today's xToday's x--ray observatories use CCDs to observe xray observatories use CCDs to observe x--rays,rays,devices that are similar to the devices found in digital cameras.devices that are similar to the devices found in digital cameras.

The CCD is a solid state device that converts the energy of aThe CCD is a solid state device that converts the energy of a

photon into an electric charge.photon into an electric charge.


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In a single atom, the electrons orbit the nucleus at a fixed andIn a single atom, the electrons orbit the nucleus at a fixed and

precise set of energy levels; an electron cannot change its orbitprecise set of energy levels; an electron cannot change its orbit

unless it absorbs or releases enough energy to place it into oneunless it absorbs or releases enough energy to place it into oneof these other energy levels. The property of atoms has itsof these other energy levels. The property of atoms has its

origin in quantum mechanics. When groups of atoms are boundorigin in quantum mechanics. When groups of atoms are bound

in a material, as happens in a metal or crystal, many of thein a material, as happens in a metal or crystal, many of the

electrons are no longer bound to a single electron, but areelectrons are no longer bound to a single electron, but are

bound to the material as a whole. The energy levels for thesebound to the material as a whole. The energy levels for these

electrons cease to be precise value; instead one finds that theelectrons cease to be precise value; instead one finds that the

electrons are confined to energy bands, with any energy withinelectrons are confined to energy bands, with any energy within

a band available to an electron, and the energies between thea band available to an electron, and the energies between the

bands forbidden to the electron. The density of e

lectrons in anbands forbidden to the e

lectron. The density of e

lectrons in anenergy band is limited by the Pauli exclusion principle. Once aenergy band is limited by the Pauli exclusion principle. Once a

band is filled, electrons can only be placed into the nextband is filled, electrons can only be placed into the next--higherhigher

band.band.

I t l th hi h t b d i l ti ll fill d AI t l th hi h t b d i l ti ll fill d A


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In a metal, the highest energy band is only partially filled. As aIn a metal, the highest energy band is only partially filled. As a

consequence, when an electrical potential is placed across aconsequence, when an electrical potential is placed across a

conductor, the electrons can move to higher energies within theconductor, the electrons can move to higher energies within the

energy band, allowing the electrons to flow down the potential.energy band, allowing the electrons to flow down the potential.

In contrast, all of the energy bands containing electrons in anIn contrast, all of the energy bands containing electrons in an

insulator are filled, so when an electric potential is placedinsulator are filled, so when an electric potential is placed

across the insulator, the electrons are unable to change theiracross the insulator, the electrons are unable to change their

energy and move down the potential. The only way an electronenergy and move down the potential. The only way an electron

can move down the potential is to acquire enough energy tocan move down the potential is to acquire enough energy tojump to a higher energy band containing no electrons.jump to a higher energy band containing no electrons.

The designers of CCDs exploit this solid state behavior toThe designers of CCDs exploit this solid state behavior to

create a photon detector. In the case of an xcreate a photon detector. In the case of an x--ray detector, an xray detector, an x--

ray is absorbed by several electrons in a filled band, and theray is absorbed by several electrons in a filled band, and the

electrons jump to an energy band free of electrons. Byelectrons jump to an energy band free of electrons. By

measuring the amount of charge that collects into the free bandmeasuring the amount of charge that collects into the free band

over an interval of time, we can measure the amount of xover an interval of time, we can measure the amount of x--rayray

energy absorbed by the detector. The number of electrons inenergy absorbed by the detector. The number of electrons in

the band is a measure of the energy carried by a single xthe band is a measure of the energy carried by a single x--ray.ray.


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SemiconductorDetectors for (Imaging) XSemiconductorDetectors for (Imaging) X--ray Spectroscopyray Spectroscopy

Compared with other materials, semiconductors have uniqueCompared with other materials, semiconductors have unique

properties that make them very suitable for the detection ofproperties that make them very suitable for the detection of

ionizing radiation.ionizing radiation.

Furthermore, semiconductorsFurthermore, semiconductors especially siliconespecially silicon are theare the

most widely used basic materials for electronic amplifyingmost widely used basic materials for electronic amplifying

elements (transistors) and more recently for completeelements (transistors) and more recently for complete

microelectronics circuits.microelectronics circuits. Thus, part of the process technology that already existed inThus, part of the process technology that already existed in

(micro) electronics could be taken or adapted for detector(micro) electronics could be taken or adapted for detector

productionproduction

THE SIMPLEST SEMICONDUCTORTHE SIMPLEST SEMICONDUCTOR


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THE SIMPLEST SEMICONDUCTORTHE SIMPLEST SEMICONDUCTOR

DETECTOR STRUCTURE: THE DIODEDETECTOR STRUCTURE: THE DIODE

The most basic semiconductor structure is aThe most basic semiconductor structure is a

diode, a combination between ndiode, a combination between n-- and pand p--dopeddopedsemiconductors. It has electrically rectifyingsemiconductors. It has electrically rectifying

properties and may also serve as sensor forproperties and may also serve as sensor for

ionizing radiation.ionizing radiation. In that case it will usually be veryIn that case it will usually be very

asymmetrically doped similar to the case shownasymmetrically doped similar to the case shown

in figure where a lowly doped nin figure where a lowly doped n--type substratetype substrate

is connected to a highly doped thin pis connected to a highly doped thin p--layer.layer.

Schematic structure of a semiconductor diode with chargeSchematic structure of a semiconductor diode with charge


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Schematic structure of a semiconductor diode with chargeSchematic structure of a semiconductor diode with charge

density, electric field and potential for partial (solid line)density, electric field and potential for partial (solid line)

and full (dashed) depletionand full (dashed) depletion


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