detectors for x-ray astronomy

Detectors for X-ray astronomy

49

Proportional Counters

Workhorses of X-ray astronomy for >10 years 1962-1970: Rockets and Balloons

1962 Sco X-1 and diffuse X-ray sky background discovered by Giacconi sounding rocket

Limited by atmosphere (balloons) and duration (rockets)

• 1970 → Satellite era Uhuru: First dedicated X-ray Satellite e.g. Ariel V, EXOSAT e.g. Ginga e.g. XTE e.g. ROSAT

Adapted from Hill, Urbino 08

50

Proportional countersSimplest proportional counter is made of a gas chamber with an anode to collect charges

Gas Detectors (Ar, Xe) Incident X-ray interacts with a gas atom and a photoelectron is ejected Photoelectron travels through the gas making an ionisation trail Trail drifts in low electric field to high E-field In high E-field multiplication occurs (Townsend avalanche) Charge detected on an anode


"Proportional counter avalanches" by Dougsim - Own work.Licensed under CC BY-SA 3.0 via Wikimedia Commons

51

One can add 2nd chamber to discriminate between X-ray photons and cosmic rays

52

Duration about 1 ns → short pulse!Gain of 105 or more

53

Quenching molecules The positive atom moves slowly to the cathode wall of the chamber. They

are neutralized by gaining an electron

This causes an energy release which can further ionize the gas, creating a charge pulse!

To eliminate (or quench) this signal, one uses a 10-15% proportion of organic gas (e.g., methane), which has low electron affinity

Thus, the positive ions will take electrons from the quenching gas molecules, and only the ionized molecules will reach the cathode

The recombination of the ionized molecules does liberate some energy, but insufficient to ionize the gas, but it breaks the molecules

Such proportional counters have, thus, a limited time life, unless they are constantly replenished by quenching molecules (gas flow counters)

54

Escape peak If the incident X-ray photon has higher energy, E0, than the K-shell

fluorescence energy of gas, Ef, an escape peak can appear at E’=E0-Ef

Fluorescence X-ray photon can escape the counter, leaving a smaller energy to be measured

Ar-filled PC

Ef(Ar)=3 keV

No escape peak, E0<Ef

NB: Ef(Ne)=0.85

keV

No escape peak as the fluorescence energy is almost completely absorbed in the counter

55

Typical Characteristics

Energy Resolution is limited by: The statistical generation of the charge by the photoelectron By the multiplication process

Quantum Efficiency: Low E defined by window type and thickness High E defined by gas type and pressure

Townsend Avalanche

0.4


56

Position sensitivity Non-imaging case: Limited by source confusion to 1/1000 Crab Imaging case: track length, diffusion, detector

depth, readout elements Timing Resolution

Limited by the anode-cathode spacing and the ion mobility: ~ µsec

Timing variations: High Gain: 103-105

Sensitivity Area

Sensitivity Area


Typical Characteristics

57

Background origin

Charged particles Cosmic rays, sub-relativistic electrons, particles

from interactions with detector/telescope Photons

Forward Compton scattering of gamma rays in the gas can deposit 0.1-10 keV

X-rays and gamma-rays created by cosmic rays Fluorescent X-rays created in structure

Background is generally flat

58

Background rejection techniques

Energy Selection Reject events with E outside of band pass

Rise-time discrimination Rise time of an X-ray event can be characterised. The

rise-time of a charged particle interactions have a different characteristic.

• Anti-coincidence Use a sub-divided gas cell with a shield of plastic

scintillator Co-incident pulses indicate extended source of

ionisation


59

CapellaROSAT PSPC ( ~ 1-2)

60

Micro-channel plates

61

Chandra HRC Micro-channel plate

Gain is very high 106-108

CGCD=crossed grid charge detector

62

Fine position determination

63Low QE and no energy response

64

X-ray CCDs1977 –

ASCA XMM Chandra Swift Suzaku

Swift XRT CCD


65

CCDs

Charge Coupled Devices invented in the 1970s Sensitive to light from optical to X-rays In practice, best use in optical and X-rays CCDs make use of silicon chips The CCD consists of (1) a p-type doped silicon

substrate, (2) the charge storage (depletion) layer, which is covered by (3) a SiO2 insulating layer; upon this is (4) an array of closely spaced electrodes, which can be set to pre-defined voltage value

66

Si array

Si arrayp-type

Si arrayn-type

Also Sb,P

Also Al,Ga

67

Electrons in a lattice do not have discrete energies. They form energy bands:

Valence band Conduction band

For semi-conductors, the Fermi level is just in the middle of the conduction and valence bands. At finite temperature, some electrons of the valence band can jump into the conduction band (current noise)

EG(Si)=1.1 eV (IR), EG(Ge)=0.72 eV

EG(C)=5.5 eV (insulator)

Reminder of solid state physics

photon phot

on

Hole Electron(From http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html)

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

http://hyperphysics.phy-astr.gsu.edu/hbase/hframe.html

68

The pn junction

69

Forward-biased pn junction

Reverse-biased pn junction

70

Transverse cut of CCD with buried channelBlecha (cours instrumentation)

4.1 eV

2.5 eV

1.8 eV

1.2 eV

71

The electrode has positive potential to attract the generated photoelectrons in a potential well

The above MOS capacitor is 1 pixel

72

Front-illuminated CCDs

p

Inco

min

g p

ho

ton

s

625m

n

They have a low Quantum Efficiency due to the reflection and absorption of light in the surface electrodes. Very poor blue response. The electrode structure prevents the use of an anti-reflective coating that would otherwise boost performance.

Inco

min

g p

ho

ton

s

15m np

Anti-reflective coating

The QE can approach 100%. These thinned CCDs become transparent to near infra-red light and the red response is poor. Response can be boosted by the application of an anti-reflective coating on the thinned rear-side. These coatings do not work so well for front-illuminated CCDs due to the surface bumps created by the surface electrodes

Courtesy of S. Tulloch

Back-illuminated CCDs

73

The graph below compares the quantum of efficiency of a thick frontside illuminated CCD and a thin backside illuminated CCD.


Quantum Efficiency Comparison

Back-illuminated CCDs

Thinner dead layers higher low-E QE Thinner active region lower high-E QE Increased noise, charge transfer inefficiency

higher FWHM

BI CCD

FI CCD

From C. Grant, X-ray Astronomy School 2007

75

One pixel

Channel stops to define the columns of the image

Transparenthorizontal electrodesto define the pixels vertically. Also used to transfer the charge during readout

Plan View

Cross section

The diagram shows a small section (a few pixels) of the image area of a CCD. This pattern is repeated.

ElectrodeInsulating oxiden-type silicon

p-type silicon

Every third electrode is connected together. Bus wires running down the edge of the chip make the connection. The channel stops are formed from high concentrations of Boron in the silicon.


Structure of a CCD

76

RAIN (PHOTONS)

BUCKETS (PIXELS)

VERTICALCONVEYORBELTS(CCD COLUMNS)

HORIZONTALCONVEYOR BELT

(SERIAL REGISTER)

MEASURING CYLINDER(OUTPUT AMPLIFIER)


CCD Analogy

77

Exposure finished, buckets now contain samples of rain.


78

Conveyor belt starts turning and transfers buckets. Rain collected on the vertical conveyoris tipped into buckets on the horizontal conveyor.


79

Vertical conveyor stops. Horizontal conveyor starts up and tips each bucket in turn intothe measuring cylinder .


80

`

After each bucket has been measured, the measuring cylinderis emptied , ready for the next bucket load.


81 Courtesy of S. Tulloch

87

A new set of empty buckets is set up on the horizontal conveyor and the process is repeated.


105

Eventually all the buckets have been measured, the CCD has been read out.


106

pix

el

boun

dary

Charge packetp-type silicon

n-type silicon

SiO2 Insulating layer

Electrode Structure

pix

el

bou

nd

ary

inco

min

gp

hoto

ns

Photons entering the CCD create electron-hole pairs. The electrons are then attracted towards the most positive potential in the device where they create ‘charge packets’. Each packet corresponds to one pixel


Charge Collection in a CCD

107

In the following few slides, the implementation of the ‘conveyor belts’ as actual electronicstructures is explained.

The charge is moved along these conveyor belts by modulating the voltages on the electrodespositioned on the surface of the CCD. In the following illustrations, electrodes colour coded redare held at a positive potential, those coloured black are held at a negative potential.

1

2

3


Charge Transfer in a CCD 1.

108

1

2

3

+5V

0V

-5V

+5V

0V

-5V

+5V

0V

-5V

Time-slice shown in diagram

1

2

3



109

1

2

3

+5V

0V

-5V

+5V

0V

-5V

+5V

0V

-5V

1

2

3



110

1

2

3

+5V

0V

-5V

+5V

0V

-5V

+5V

0V

-5V

1

2

3



111

1

2

3

+5V

0V

-5V

+5V

0V

-5V

+5V

0V

-5V

1

2

3



112

1

2

3

+5V

0V

-5V

+5V

0V

-5V

+5V

0V

-5V

1

2

3



113

1

2

3

+5V

0V

-5V

+5V

0V

-5V

+5V

0V

-5V

1

2

3

Charge packet from subsequent pixel entersfrom left as first pixel exits to the right.



114

1

2

3

+5V

0V

-5V

+5V

0V

-5V

+5V

0V

-5V

1

2

3



115

Photoelectric Absorption in Silicon

Depth of active region in typical X-ray CCD

Best X-ray sensitivity


116

Photoelectric Absorption

Photoelectric interaction of X-ray with Si atoms generates electron-hole pairs.

On average: Ne = Ex/w Ne = number of electrons Ex = energy of X-ray photon w ~ 3.7 eV/e- (temperature dependent)

X-ray creates a charge cloud which can diffuse and/or move under influence of an electric field


118

Event Processing

CCD output rate (~10 Mbits/sec/CCD) exceeds telemetry resources

Raw CCD frames must be processed on-board to find prospective X-ray events

Event selection: CCD bias level determined and removed Pixel pulse height greater than threshold value (“event

threshold”) Pixel is a local maximum (3 x 3 pixels on ACIS)

For each event, record position, time and pulse heights of event island (3 x 3 pixels for ACIS)

Events are assigned a grade which characterizes the shape of the event.


Grading Events

Event grade can be used to discriminate between X-ray and cosmic ray events

In general, X-ray events split into simpler/smaller shapes (single, singly-split) ✓

Cosmic ray events are more complex

ASCA code grades are one example

On-board grade filtering can further reduce telemetry

Grade filtering can improve spectral resolution - split events are noisier than singles

Grade 7 - everything else

✓

✓

✓

✓

✓


ACIS X-ray/Particle Discrimination

Blobs/streaks - charged particles. Small dots - X-ray events.


121

CCD Quantum Efficiency

Transmission through dead layers(channel stops, gates, oxide layers)

T = i e-i ti

Absorption in depleted region

A = 1 - e-

Sid

QE = (1 - e-Sid) i e-i ti is linear absorption coefficient

t thickness of dead layerd depletion depth


122

Optical Blocking Filters

CCDs sensitive to optical photons Cause noise and pulse-height calibration issues Filter materials usually plastic and aluminum

Imager Filter: Al/Polyimide/Al:120/200/40 nm

Spectrometer Filter:Al/Polyimide/Al:100/200/300 nm


123

Filter Transmission

Bare CCD

CCD+flter

At low energies (< 0.5 keV), > 50% reduction in efficiency

CCD only

CCD+filter


124

Photoelectric interaction of a single X-ray photon with a Si atom produces “free” electrons:

Spectral resolution depends on CCD readout noise and physics of secondary ionization:

CCD characteristics that maximize spectral resolution: Good charge collection and transfer efficiencies at very low signal

levels Low readout and dark-current noise (low operating temperature) High readout rate (requires trade-off vs. noise)

CCD X-ray Spectroscopy: The Basic Idea

Ne EX w (w 3.7 eV/e )

e2 F Ne (F 0.12; not a Poisson process)

FWHM (eV) 2.35 w e2 read

2


ASCA SIS (1993): 4 e- RMSChandra ACIS (1999): 2 e- RMS


126

Spectral Redistribution Function

Input spectrum has three spectral lines: Mn-L (0.7 keV), Mn-K (5.9 keV), and Mn-K (6.4 keV)

Instrument produces Si-K fluorescence and escape peaks, low-energy features Off nominal features ~2% of total


From C. Grant, X-ray Astronomy School 2007 (Prigozhin et al., 1999, Nuclear Instruments and Methods)

128

Low-Energy Detection Efficiency

Many astrophysically interesting problems require good low-energy (< 1 keV) efficiency (pulsars, ISM absorption, SNR, …)

Low energy X-rays are lost to absorption in gate structures and filter

Solutions: Thinned gates, open gates (XMM EPIC-MOS, Swift) Back-illumination (Chandra ACIS, XMM EPIC-PN,

Suzaku XIS)


Chandra ACIS Focal Plane

The Advanced CCD Imaging Spectrometer (ACIS) contains 10 planar, 1024 x 1024 pixel CCDs ; four arranged in a 2x2 array (ACIS-I ) used for imaging, and six arranged in a 1x6 array (ACIS-S ) used either for imaging or as a grating readout. Two CCDs are back-illuminated (BI ) and eight are front-illuminated (FI ).


130

XMM-Newton EPIC-MOS

The MOS EEV CCD22 is a three-phase frame transfer device on high resistivity epitaxial silicon with an open-electrode structure; it has a useful quantum efficiency in the energy range 0.2 to 10 keV. The low energy response of the conventional front illuminated CCD is poor below ~700 eV because of absorption in the electrode structure. For EPIC MOS, one of the three electrodes has been enlarged to occupy a greater fraction of each pixel, and holes have been etched through this enlarged electrode to the gate oxide. This gives an "open" fraction of the total pixel area of 40%; this region has a high transmission for very soft X-rays that would have otherwise be absorbed in the electrodes. In the etched areas, the surface potential is pinned to the substrate potential by means of "pinning implant". High energy efficiency is defined by the resistivity of the epitaxial silicon (around 400 Ohm-cm). The epitaxial layer is 80 microns thick (p-type). The actual mean depletion of the flight CCDs is between 35 to 40 microns: the open phase region is not fully depleted. Image and caption taken from http://xmm.vilspa.esa.es/external/xmm_user_support/documentation/technical/EPIC


131

The field of view of the EPIC MOS 1 cameras: The camera detector co-ordinate frames are noted. 7 CCD’s each 10.9 x 10.9 arcmin

132

XMM-Newton EPIC-PN

The PN-CCDs are back-illuminated. In the event of an X-ray interaction with the silicon atoms, electrons and holes are generated in numbers proportional to the energy of the incident photon. The average energy required to form an electron-hole pair is 3.7 eV at -90° C. The strong electric fields in the pn-CCD detector separate the electrons and holes before they recombine. Signal charges (in our case electrons), are drifted to the potential minimum and stored under the transfer registers. The positively charged holes move to the negatively biased back side, where they are 'absorbed'. The electrons, captured in the potential wells 10 microns below the surface can be transferred towards the readout nodes upon command, conserving the local charge distribution patterns from the ionization process. Each CCD line is terminated by a readout amplifier. The picture shows the twelve chips mounted and the connections to the integrated preamplifiers.Image and caption from http://xmm.vilspa.esa.es/external/xmm_user_support/documentation/technical/EPIC/index.shtml#2.2


133

CCD’s Onboard XMM-Newton

The field of view of the EPIC pn camera; The EPIC prime boresight is marked with a small box. 12 CCD’s each 13.6 x 4.4 arcmin.

134

CapellaROSAT PSPC ( ~ 1-2)

135

CapellaASCA SIS0 ( ~ 10-20)

136

Photon Pileup If two or more photons interact within a few

pixels of each other before the image is readout, the event finding algorithm may regard them as a single event

Increased amplitude Reduction of detected events Spectral hardening of continuum Distortion of PSF

Correcting for pile-up is complicated Best to set up observation to minimize pileup


Photon Pileup

Energy

Cou

nts

/sec

Bright

Brightest

Brighter

Monochromatic Source Thermal Spectrum

PSF Distortion(From Chandra Proposer’s Observatory Guide)


Readout Streak/Out-of-time Events

Photons that interact while imaging array is transferring Assigned incorrect row/chip y value

Events may have poor initial calibration

Can be modeled & removed Streak events have higher time resolution, no pileup

Transfer Direction

3C273 - Quasar with jet


139

CCD Operating Modes

X-ray CCDs operated in photon-counting mode Spectroscopy requires ≤ 1 photon interaction per pixel

per frametime Minimum frametime limited by readout rate Tradeoff between increasing readout rate and noise For ACIS, 100 kHz readout 3.2 s frametime Frametime can be reduced by reading out subarrays or

by continuous parallel clocking (1D imaging)

1.5 s 0.8 s 0.4 s (CXC Proposer’s Observatory Guide)


Charge Transfer Inefficiency

X-ray events lose charge to charge trapping sites.

Leads to: Position dependent gain Spectral resolution degradation Position dependent QE

Caused by radiation damage or manufacturing defects

Depends on: Density of charge trapping

sites Charge trap capture and re-

emission properties (temperature)

Occupancy of charge traps (particle background)


Al K

Ti K

Mn K

141

Hot Pixels, Flickering Pixels

Radiation damage or manufacturing defects can cause pixels to have anomalously high dark current

Can regularly exceed event threshold and cause spurious events

Extreme cases may be removed onboard, otherwise filtered in data analysis

Strongly correlated with temperature More important for ASCA (–60C) and Suzaku (–90C) than

ACIS (–120C) Unstable defects cause flickering pixels

Lower frequency, more difficult to detect and remove

142

De Plaa et al. (2006)

Cal source Cal source

Fixed pattern noise

154

Microcalorimeter

ASTRO-H SXS: Silicon bolometers

155

ASTRO-H Soft X-ray Spectrometer

156

Transition Edge Sensors

e.g., Mo/Au

e.g., Bi/Cu

Athena X-IFU

157

Frequency Domain Multiplexing

Athena baseline

Time Domain Multiplexing

158

Event Grade

159

X-IFU Performance

160

http://hyperphysics.phy-astr.gsu.edu/hbase/Solids/squid.htmlhttp://hyperphysics.phy-astr.gsu.edu/hbase/Solids/coop.html

Threshold for SQUID: 10-14 TB field of heart: 10-10 TB field of brain: 10-13 T

161

≈1.4 eV (T=100 mK)

Onboard ASTRO-E, ASTRO-E2/Suzaku, ASTRO-H/Hitomi (SXS)

163

Athena X-ray Integral Field Unit

Launch foreseen in 2028!

164

NGC 1275 with Hitomi/SXS(and Suzaku XIS)

Superconducting Tunnel Junction

Josephson junction

Two films of superconducting metal (e.g., Nb, Ta, Hf) separated by a thin insulating layer

Incoming photon perturbs equilibrium of junction. If bias voltage is applied across STJ with parallel magnetic field to suppress the Josephson current, electric charge can be measured, and is proportional to energy of photon

Operated below the superconductor's critical temperature (typically below 1 K)

Good also for optical, UV; high count rates

Courtesy, LLNL, UC

E 10 100 eV (FWHM)Courtesy, LLNL, UC

Detectors for gamma-ray astronomy

170

gamma-ray interaction with matter

Photoelectric effect

hEe = h- Eb

E

dNdE

h

h‘

h

e-

E

dNdE

h

Compton continuum

Com

pton

edge

= 0° =180°

he-

e+

E

dNdE

h

2moc2

Compton effect

Pair production

Photopeak; Eb≈0

Compton edge

Double escape peak

(also single esc. peak)

P. v. Ballmoos, CESR

171

gamma-Ray Spectra

e-e-

e+

photoeffect

Comptoneffect

pair production

escape 511 keV

511 keV

escape Compton scattered

E

dNdE

h

Compton continuum

Com

pton

edg

eh2moc2

phot

opea

k

multiple Compton scattering

E

dNdE

h

Compton continuum

Com

pton

edg

eh2moc2

phot

opea

k

h2

moc

2si

ngle

esc

ape

multiple Compton scattering

doub

le e

scap

eh

moc

2

• Compton edge

Adapted from P. v. Ballmoos

Ee h h

h ( ) h1 2h m0c

2

Ee ( ) h 2h /m0c

2

1 2h m0c2

172

10-2

10-1

100

101

0.1 1 10 100 103 104

mass attenuation for NaIat

tenu

atio

n [c

m2 /g

m N

aI]

E [MeV]

pairCompton

photoel.

Three interactions from gamma-rays with matterFro

m v

on

Ba

llmo

os

Z5

ZZ2

I I0ex , photo Compton pair

173Evans (1955)

174

174

• I conversion : Gamma-rays do not interact with matter unless they undergo a “catastrophic” interaction. In all cases of practical interest, Gamma-rays are detected by their production of secondary electrons.

• II ionization of detector medium by fast electrons → creation of large number of charge carriers

• III collection (reconversion) of detector signal, amplify current and converted by an ADC

Detecting Gamma-rays

matter

h

e-

X or gamma-ray photon mean free path ≈ 10-1 m

fast electronmean free path ≈ 10-3 m

P.v. Ballmoos

175

Gamma-ray detectors

• Gas-filled detectors• Ionization chambers• Proportional counters• Geiger counters• Scintillators• Scintillators• Photomultipliers• Spectral resolution : scintillators vs semiconductors• Semiconductors • Narrow gap / low temperature semiconductors• Wide bandgap / high temperature semiconductors• Examples• Phonon detectors • Ultra-low temperature detectors for gamma-ray

detection ?

P. v. Ballmoos

176

P. v. Ballmoos

Background

cosmic gamma ray background cosmic rays

trapped protons

177

Scintillators

• Developed to counter the low efficiency of PC

• Incident photon excites the scintillation material which emits photons in proportion to the photon energy (visible or UV)

• This fluorescent light is collected by photomultipliers or photodiodes

• Three main properties: scintillation efficiency, decay times of induced luminescence, and stopping power (related to Z value)

• Two classes of materials: inorganic crystals and organic liquids

178

Inorganic scintillators

• High scintillation efficiency and high-Z value• Crystalline insulators (empty conduction band)• Activators (impurities), e.g., Tl in NaI (COMPTEL) or CsI (IBIS-

PICsIT) crystals, used to enhance efficiency and shift to visible• High efficiency (20%) vs pure crystals with natural impurities (e.g.,

Bi4Ge3O12=BGO with 2%; Integral SPI ACS, but high Z)

Knoedlseder (2003)

179

Organic scintillators• Weak intermolecular interactions: excited states of individual molecules

(indep. of state of material, i.e., gaseous, solid and liquid, or incorporated into plastics)

• Aromatic compounds (planar molecules made of benzenoid rings: e.g., anthracene=C14H10; stylbene=C14H12)

• Interactions with heavy or slow particles (p or n) suppresses fast fluorescence leading to pronounced tail of light pulse

• Often used as particle anticoincidence shields, rarely as spectrometers (low scintillation efficiency and low Z-values)

Decay with ≈ps to lowest vibrational state (S10)

2-30ns

spin-forbidden

10-4 – 1 s

NB: repopulation of S10 from T10 possible, leading to slow fluorescence with ≈s

Knoedlseder (2003)

180

Photomultipliers

Gain of 107-109

From Bruker-AXS

181

Semi-conductor detectors

• See introduction on semiconductors in previous slides (conduction/valence bands)

• Gamma-ray interaction produces secondary electrons; they produce electron-hole pairs in the conduction/valence bands

• Electric field separates the pairs before recombination, drifting electrons to anode and holes to cathode. Charge is collected, which is proportional to energy deposited in detector

• Energy required to generate electron-hole pair is =(14/5)Eg+c, where 0.5≤c≤ eV

• Common detectors made of Ge (e.g., Integral SPI; Eg=0.74 eV, =2.98 eV), Si (=3.61 eV), CdTe (Integral ISGRI; Eg=1.6eV, =4.43 eV)

182

scintillator vs. semiconductor

• Scintillation eff. ~ 12% => 120 keV ( V/UV) Energy to form e-/hole pair : Eeh ≈ 3 eV

• Vis. photon energy ~ 3eV => 40'000 V/UV ph Nsem

≈ 106/3eV ≈ 300’000 charge

carriers

• on photocathode => 20'000 photons Fsem

≈ 0.06-0.14 (Fano factor)

• quantum eff. QE ≈ 20% => 4’000 photo-e- (Nsci)

• R = 0.42 (Nsci

/Fsci

)1/2 ≈ 25 R = 0.42 (Nsem

/Fsem

)1/2 ≈ 500

scintilator e.g NaI

ban

d g

ap

conduction band

valence band

Eg > 5 eVhvis

h = 1 MeV

e

e

e

hvis

Eeh 20 eV =>

5.104 e-/hole pairs

PMT

h = 1 MeV

PA

solid state detector e.g. HP Germanium

Coldfnger (80 K)

+HV

R

conduction band

valence band

Eg 1 eV

e

eh+ e-

P. v. Ballmoos

183

Comparison : scintillator / semiconductor spectra

Knoll, 1989

P. v. Ballmoos

184

For gamma-rays with energies of ~ 1 – 10 MeV, direct scintillation or solid state detection becomes inefficient. Photons interact with matter mainly through Compton scattering

Have the gamma-ray undergo Compton scattering event in an upper detector layer (1); determine direction of motion and energy of the down-scattered photon in a second, lower detector layer (2).

Need to also measure energy and direction of the recoil electron in layer 1 to uniquely determine gamma-ray direction.

Pair production telescope

For E>30 MeV

Trajectories are measured from the particle tracking detectors, while the detector at the bottom (e.g., calorimeter or scintillator) will give the energy.

The particles trajectories of the electron-positron pair are similar to the photon trajectory for E >> 2mec

2.

Adapted from J. Paul (2000), GDR PCHE

Theoretically for E > 2mec2 (i.e., E >

1,022 MeV), but in practice for E>30 MeV

186Fermi/GLAST LAT, 20 MeV-300 GeV

TeV astronomy

Science with E>30 GeV

188

Tülün Ergin Graduate School Workshop "Physics at TESLA" October 2-5, 2001

TeV astronomyAir Showers: Secondaries or higher order particles created, when theprimary CRs hit the Earth‘satmosphere.

– Photon-induced air showers: • Pure electromagnetic in nature • Only electrons, positrons and

photons– Hadron-induced air showers:

• Hadronic cascades• Decays of pions, hadrons• Muons and neutrinos produced

in decays• Later electromagnetic

cascades

Tülün Ergin Graduate School Workshop "Physics at TESLA" October 2-5, 2001

Ground Based Atmospheric Cherenkov Detectors

• Measure the Cherenkov radiation, which is produced by the passage of air showers through the atmoshere, with a photon detector.

• Atmospheric Cherenkov Telescopes (ACT) are used

– Huge detection areas (up to 0.1 km2 or more)

– High detecting rates at TeV energies

• But high background ! (hadronic CRs)

Particle speed exceeds light speed in medium:

Vs>Cs

cos ct nct

1n

F 12 blue radiation

Spectrum dominated by blue photons

191

Ground-Based Atmospheric Cherenkov Detectors

• There are differences between the Cherenkov light emission produced by photon-induced air showers and hadron-induced air showers

How can this difference be measured ?

Take an image of the shower!

192

Atmospheric Cherenkov Imaging

Cherenkov Imaging gives the ability to distinguish compact images of gamma-ray showers from more irregular images from hadronic showers

Arrival directions also determined with high accuracy

_

Atm

osp

heri

c h

eig

ht

20 km

e+e_

5o

p

To record the Cherenkov light images of gamma-ray initiated air showers, a large camera, consisting of an array of PMTs in the focal plane of a large optical reflector, is used.

193

Hadron Gamma ray

Muon Ring Sky Noise

Types of images seen by atmosphericCherenkov camera

194

~ 10 km shower

5 nsec

gamma-ray

100m Seff>3x104 m2 at 1 TeV

in principle also at 10 GeV

Seff = 1m2 at 1 GeV

Crab flux: F(>E)=5x10-4 (E/1TeV)-1.5 ph/m2 hr

gamma-ray telescopes – “fast” detectors

From Aharonian (TeVPA 2008)

30 GeV – 30 TeV

detectors for x-ray astronomy

Documents