Detectors for X-ray astronomy
48
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
Adapted from Hill, Urbino 08
"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
Adapted from Hill, Urbino 08
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
Adapted from Hill, Urbino 08
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
Adapted from Hill, Urbino 08
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
Adapted from Hill, Urbino 08
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)
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.
Courtesy of S. Tulloch
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.
Courtesy of S. Tulloch
Structure of a CCD
76
RAIN (PHOTONS)
BUCKETS (PIXELS)
VERTICALCONVEYORBELTS(CCD COLUMNS)
HORIZONTALCONVEYOR BELT
(SERIAL REGISTER)
MEASURING CYLINDER(OUTPUT AMPLIFIER)
Courtesy of S. Tulloch
CCD Analogy
77
Exposure finished, buckets now contain samples of rain.
Courtesy of S. Tulloch
78
Conveyor belt starts turning and transfers buckets. Rain collected on the vertical conveyoris tipped into buckets on the horizontal conveyor.
Courtesy of S. Tulloch
79
Vertical conveyor stops. Horizontal conveyor starts up and tips each bucket in turn intothe measuring cylinder .
Courtesy of S. Tulloch
80
`
After each bucket has been measured, the measuring cylinderis emptied , ready for the next bucket load.
Courtesy of S. Tulloch
81 Courtesy of S. Tulloch
82 Courtesy of S. Tulloch
83 Courtesy of S. Tulloch
84 Courtesy of S. Tulloch
85 Courtesy of S. Tulloch
86 Courtesy of S. Tulloch
87
A new set of empty buckets is set up on the horizontal conveyor and the process is repeated.
Courtesy of S. Tulloch
88 Courtesy of S. Tulloch
89 Courtesy of S. Tulloch
90 Courtesy of S. Tulloch
91 Courtesy of S. Tulloch
92 Courtesy of S. Tulloch
93 Courtesy of S. Tulloch
94 Courtesy of S. Tulloch
95 Courtesy of S. Tulloch
96 Courtesy of S. Tulloch
97 Courtesy of S. Tulloch
98 Courtesy of S. Tulloch
99 Courtesy of S. Tulloch
100 Courtesy of S. Tulloch
101 Courtesy of S. Tulloch
102 Courtesy of S. Tulloch
103 Courtesy of S. Tulloch
104 Courtesy of S. Tulloch
105
Eventually all the buckets have been measured, the CCD has been read out.
Courtesy of S. Tulloch
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
Courtesy of S. Tulloch
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
Courtesy of S. Tulloch
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
Courtesy of S. Tulloch
Charge Transfer in a CCD 2.
109
1
2
3
+5V
0V
-5V
+5V
0V
-5V
+5V
0V
-5V
1
2
3
Courtesy of S. Tulloch
Charge Transfer in a CCD 3.
110
1
2
3
+5V
0V
-5V
+5V
0V
-5V
+5V
0V
-5V
1
2
3
Courtesy of S. Tulloch
Charge Transfer in a CCD 4.
111
1
2
3
+5V
0V
-5V
+5V
0V
-5V
+5V
0V
-5V
1
2
3
Courtesy of S. Tulloch
Charge Transfer in a CCD 5.
112
1
2
3
+5V
0V
-5V
+5V
0V
-5V
+5V
0V
-5V
1
2
3
Courtesy of S. Tulloch
Charge Transfer in a CCD 6.
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.
Courtesy of S. Tulloch
Charge Transfer in a CCD 7.
114
1
2
3
+5V
0V
-5V
+5V
0V
-5V
+5V
0V
-5V
1
2
3
Courtesy of S. Tulloch
Charge Transfer in a CCD 8.
115
Photoelectric Absorption in Silicon
Depth of active region in typical X-ray CCD
Best X-ray sensitivity
From C. Grant, X-ray Astronomy School 2007
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
From C. Grant, X-ray Astronomy School 2007
From C. Grant, X-ray Astronomy School 2007
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.
From C. Grant, X-ray Astronomy School 2007
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
✓
✓
✓
✓
✓
From C. Grant, X-ray Astronomy School 2007
ACIS X-ray/Particle Discrimination
Blobs/streaks - charged particles. Small dots - X-ray events.
From C. Grant, X-ray Astronomy School 2007
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
From C. Grant, X-ray Astronomy School 2007
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
From C. Grant, X-ray Astronomy School 2007
123
Filter Transmission
Bare CCD
CCD+flter
At low energies (< 0.5 keV), > 50% reduction in efficiency
CCD only
CCD+filter
From C. Grant, X-ray Astronomy School 2007
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
From C. Grant, X-ray Astronomy School 2007
ASCA SIS (1993): 4 e- RMSChandra ACIS (1999): 2 e- RMS
From C. Grant, X-ray Astronomy School 2007
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
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)
From C. Grant, X-ray Astronomy School 2007
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 ).
From C. Grant, X-ray Astronomy School 2007
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
From C. Grant, X-ray Astronomy School 2007
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
From C. Grant, X-ray Astronomy School 2007
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
From C. Grant, X-ray Astronomy School 2007
Photon Pileup
Energy
Cou
nts
/sec
Bright
Brightest
Brighter
Monochromatic Source Thermal Spectrum
PSF Distortion(From Chandra Proposer’s Observatory Guide)
From C. Grant, X-ray Astronomy School 2007
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
From C. Grant, X-ray Astronomy School 2007
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)
From C. Grant, X-ray Astronomy School 2007
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)
From C. Grant, X-ray Astronomy School 2007
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
143
144
145
146
147
148
149
150
151
152
153
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)
162
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
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