General Information

Individual Study Projects We have a few more slots to fill

Muon Lifetime Experiment We can use the experiment until April 18 Monday, April 8 we will go over the setup and take a look at the equipment in Smith Lab We have time for three groups:

Group 1 Setup Tuesday, April 9; Finish Friday morning (4/12) Group 2 Setup Friday, April 12; Finish Monday morning (4/18) Group 3 Setup Monday April 15; Finish on April 18

Today’s Agenda Postponed: Interaction of Particles with Matter Characteristics of a particle detector Scintillators

Wednesday: Signals and Electronics

Functional Components of a Detector Decay scheme of 137Cs

Functional Components of a Detector

Characteristics• Resolution

• Efficiency

• Sensitivity

• Deadtime

Energy ResolutionResolution = E/E

The width arises because of fluctuations in the number of ionizations or excitations produced.

If w is the energy needed to produce an ionization or excitation, one would expect,on average

N = E/w Poisson process (ie variance = mean)

= ÷N

If we take the resolution as the full width half maximum (FWHM) of the distributionwe get

R = 2.35 ÷N /N = 2.35 ÷(w/E)(the factor 2.35 relates the standard deviation of a Gaussian to its FWHM)

Function of energy deposited; improves with higher energy Better resolution for smaller w (e.g. silicon detectors)

Fano Factor f Poisson statistics can’t be applied if all energy is absorbed. Fano found that the variance in this case is

= F÷N

F<1 for gases, semi-conductors -> greatly improves resolution

Efficiency and Deadtime Absolute Efficiency

Function of geometry and the probability of interaction in the detector:

tot = (events detected) / (events emitted by source)

tot = int geometry

Intrinsic Efficiency Fraction of events actually hitting the detector that are registered

int = (events detected) / (events impinging on detector)

Deadtime Some detectors require some time to process an event and might not be

sensitive for new events during this time. If the count rate is sufficiently low this effect can be corrected.

Rtrue = true rateRmeasured = measured rate = detector deadtime

Z2 electrons, q=‐e0

W. Riegler, Particle Detectors

Interaction with the atomic electrons. The incoming particle looses energy and the atoms are excited or ionized.

Interaction with the atomic nucleus. The particle is deflected (scattered) resulting in multiple scattering of the particle in the material. During these scattering events a Bremsstrahlung photons can be emitted.

In case the particle’s velocity is larger than the velocity of light in the medium, the resulting EM shockwave manifests itself as Cherenkov Radiation. When the particle crosses the boundary between two media, there is a probability of the order of 1% to produce an X ray photon, called Transition radiation.

Electromagnetic Interaction of Particles with Matter

M, q=Z1 e0

Basic EM Interactions

e+ / e-

IonizationdE/dx ~ 1/2, z2

BremsstrahlungdE/dx ~ 1/m2, z4

Photoelectric effect

Compton effect

Pair production

E

E

dE/d

x

E

dE/d

x

E

E

Interaction of Particles with Matter

Any device that is to detect a particle must interact with it in some way almost …

In many experiments neutrinos are measured by missing transverse momentum.

E.g. e+e- collider. ptotal = 0, If Σ pi of all collision products is ≠0 neutrino escaped.

Claus Grupen, Particle Detectors, Cambridge University Press, Cambridge 1996 (455 pp. ISBN 0-521-55216-8) 8W. Riegler/CERN

Creation of the Signal

4/2/20129

Charged particles traversing matter leave excited atoms, electron-ion pairs (gases) or electrons-hole pairs (solids) behind.

Excitation:The photons emitted by the excited atoms in transparent materials can be detected with photon detectors like photomultipliers or semiconductor photon detectors.

Ionization:By applying an electric field in the detector volume, the ionization electrons and ions are moving, which induces signals on metal electrodes. These signals are then read out by appropriate readout electronics.

Detectors based on registration of excited Atoms Scintillators

Detectors based on Registration of excited Atoms Scintillators

Emission of photons of by excited Atoms, typically UV to visible light.

a) Observed in Noble Gases (even liquid !)

b) Inorganic Crystals Substances with largest light yield. Used for precision measurement of

energetic Photons. Used in Nuclear Medicine.

c) Polycyclic Hydrocarbons (Naphtalen, Anthrazen, organic Scintillators) Most important category. Large scale industrial production, mechanically

and chemically quite robust. Characteristic are one or two decay times of the light emission.

Typical light yield of scintillators:

Energy (visible photons) few of the total energy loss. e.g. 1 cm plastic scintillator, 1, dE/dx=1.5 MeV, ~15 keV in photons; i.e. ~ 15 000 photons produced. Only a fraction of which will be detected

Scintillation DetectorScintillation

Two material types: Inorganic and organic scintillators

high light output lower light outputbut slow but fast

p h o t o d e t e c t o r

Energy deposition by ionizing particle production of scintillation light

(luminescense)

Scintillators are multi purpose detectorsCalorimetryTime of flight measurementTracking detector (fibers)Trigger counterVeto counter…..

RequirementsHigh efficiencyTransparent (to its own radiation)Spectral range <-> PhotodetectorsFast

Rise time (ns) Decay time (ns – s)

LHC bunchcrossing 25ns LEP bunchcrossing 25s

Organic (‘Plastic’) Scintillators

Low Light Yield Fast: 1-3ns

Inorganic (Crystal) Scintillators

Large Light Yield Slow: few 100ns

Detectors based on Registration of excited Atoms Scintillators

Inorganic scintillatorsInorganic crystalline scintillators (NaI, CsI, BaF2...)Three effects: exitons (bound electron hole pairs), defects, activators (e.g. Tl)dE/dx per scintillator photon for electrons: 25 (NaI) – 300 (BGO)

c o n d u c t i o n b a n d

v a l e n c e b a n d

E gt r a p s

a c t i v a t i o nc e n t r e s( i m p u r i t i e s )

lum

ines

cens

e

quen

chin

gh o l e

e l e c t r o n

s c i n t i l l a t i o n( 2 0 0 - 6 0 0 n m )

exci

tatio

n

e x c i t o nb a n d

often 2 time constants:• fast recombination (ns-s) from activation centre• delayed recombination (phosphorescence, 100 s)

Due to the high density and high Z inorganic scintillator are well suited for detection of charged particles, but also of .2-3 orders of magnitude slower than organic scintillators (Exception: CsF (5 ns))

Inorganic ScintillatorsLight output of inorganic crystals shows some temperature dependence

Practically no temperature dependence in organic scintillators (-60 to +20 degrees C)

Liquid noble gases (LAr, LXe, LKr)

A

A +

A 2 *

A 2+

A

A

e -

io n iz a t io n

c o l l is io nw ith g .s .a to m s

e x c ite d m o le c u le

io n iz e dm o le c u le

d e -e x c ita t io n a n d d is s o c ia t io n

U V 1 3 0 n m (A r )1 5 0 n m (K r )1 7 5 n m (X e )

A *e x c ita t io n

A 2*

re c o m b in a t io n

also here one finds 2 time constants: few ns and 100-1000 ns, but same wavelength.

PbWO4

(From Harshaw catalog)

Inorganic ScintillatorsProperties of some inorganic scintillators -> Take a look at the detector section of the PDG book

4 104

1.1 104

1.4104

6.5 103

8.2 103

Photons/MeV

PbWO4 8.28 1.82 440, 530 0.01 100

LAr 1.4 1.295) 120-170 0.005 / 0.860LKr 2.41 1.405) 120-170LXe 3.06 1.605) 120-170 4 104

5) at 170 nm

0.002 / 0.0850.003 / 0.022

PbWO4 ingot and final polished CMS ECAL scintillator crystal from Bogoroditsk Techno-Chemical Plant (Russia).

2. Organic scintillators: Monocrystals or liquids or plastic solutions

Monocrystals: naphtalene, anthracene, p-terphenyl….Liquid and plastic scintillatorsThey consist normally of a solvent + secondary (and tertiary) fluors as wavelength shifters. Fast energy transfer via non-radiative dipole-dipole interactions (Förster transfer). shift emission to longer wavelengths longer absorption length and efficient read-out device

Organic scintillators

Scintillation is based on the 2 electrons of the C-C bonds.

Emitted light is in the UV range.

Absorption and Emission Stokes Shift

If emission and absorption occur at the same wavelengths, most emitted photons would be absorbed within a short distance resulting in poor light output.Since excitation goes to higher vibrational states in the S1 band, whereas decay goes from the base S1 state, the emission spectrum is shifted to lower energies (longer wavelengths).

Organic scintillators (backup)

Some widely used solvents and solutes

After mixing the components together plastic scintillators are produced by a complex polymerization method. Some inorganic scintillators are dissolved in PMMA and polymerized (plexiglas).

s o lv e n t s e c o n d a r yf lu o r

t e r t ia r yf lu o r

L iq u ids c i n t i l l a to r s

B e n z e n eT o lu e n eX y le n e

p - t e r p h e n y lD P OP B D

P O P O PB B OB P O

P la s t ics c i n t i l l a to r s

P o ly v i n y lb e n z e n eP o ly v i n y l to lu e n eP o ly s t y r e n e

p - t e r p h e n y lD P OP B D

P O P O PT B PB B OD P S

Schematic representationof wave length shiftingprinciple

(C. Zorn, Instrumentation In High Energy Physics, World Scientific,1992)

yield/NaI

0.5

Properties of Organic Scintillators

Organic scintillators have low Z (H,C). Low detection efficiency (practically only Compton effect). But high neutron detection efficiency via (n,p) reactions.

Light Collection Loss of light

Through absorption by scintillator materialIf I and Io are the intensities and L is the attenuation length

The attenuation length is typically around 1 m (hence this effect is usually less important)

Through the scintillator boundaries Wrap scintillator in foil (diffuse reflection like a Teflon film) Optical grease to couple to photo detector

Photons are being reflected towards the ends of the scintillator.

A light guide brings the photons to the Photomultipliers where the photons are converted to an electrical signal.Efficiency depends on angle of total internal reflections and conservation of phase space (Liouville Theorem)

By segmentation one can arrive at spatial resolution.

Because of the excellent timing properties (<1ns) the arrival time, or time of flight, can be measured very accurately Trigger, Time of Flight.

Scintillator Photon DetectorLight Guide

Light Guides

Typical Geometries:

UV light enters the WLS materialLight is transformed into longer wavelengthTotal internal reflection inside the

WLS material ‘transport’ of the light to the photo detector

Wavelength Shifting Use a fiber embedded in the scintillator instead of

unwieldy light guides The fiber collects scintillation light, shifts it to longer

wavelength and “pipes” it to a photo detector Evades the Liouville Theorem because shifting to longer

wavelength “cools” the light (reduced phase space)Shifting from 450 nm to 500 nm corresponds to an energy shift of

0.28 eV Increased “packing factor”

∆ / .

Optical Fibers Optical Fibers

Minimize ncladding

Ideal: n = 1 (air), but impossible due to surface imperfections

Multi-clad fibers Improved aperture

Long(er) absorption length for visiblelight (> 10 m)

corepolystyrene

n=1.59

cladding(PMMA)n=1.49

typically <1 mm

typ. 25 m

light transport by total internal reflection

n1

n2

6.69arcsin1

2nn

%1.34

d

corepolystyrene

n=1.59

cladding(PMMA)n=1.49

25 m

fluorinated outer claddingn=1.42

25 m%3.54

d

Scintillating Fiber Tracker Scintillating plastic fibers Capillary fibers filled with liquid scintillator

Planar geometries(end cap)

Circular geometries(barrel)

a) axialb) circumferentialc) helical

(R.C. Ruchti, Annu. Rev. Nucl. Sci. 1996, 46,281)

Advantages: High geometrical flexibility Fine granularity Low mass Fast response (ns)

Fiber Tracking

Readout of photons in a cost effective way is rather challenging.

CERN WA84: Active (Fiber) Target

Photo DetectorsPurpose: Convert light into detectable electronics signalPrinciple: Use Photoelectric Effect to convert photons to photoelectronsStandard Requirement:

High sensitivity, usually expressed as Quantum efficiency Q.E. = Np.e./ Nphotons

Main types of photodetectors: Gas based devices (see RICH detectors) Vacuum based devices (Photomultiplier) Solid state detectors

Threshold of some photosensitive material

100 250 400 550 700

TEATMAE,CsI

bialkalimultialkali

GaAs ...

12.3 4.9 3.1 2.24 1.76E (eV)

visibleUV

(nm)

Photo Multiplier Tube Operation Principle

Photo emission from photocathode

Focusing, acceleration Secondary emission from

dynodes

Gain Dynode gain g= 3-50 Total gain

10 dynodes with g = 4M = 410 ~ 106

N

iigM

1

(Philips Photonic)

e-

photon

Photo Cathode 3-step process

Photo ionization of molecule Electron propagation through cathode Escape of electron into the vacuum

Quantum Efficiency

Most photo-cathodes are semiconductorsBand model:

The photon energy has to be sufficient to bridge the band gap Eg, but also to overcome the electron affinity EA, so that the electron can be released into the vacuum.

e-

glass

PC

e-

Semitransparent photocathode

Opaque photocathode

PC

subs

trate

Typical Quantum EfficienciesBialkali

SbK2CsSbRbCs

Multialkali SbNa2KCs

Solar blind CsTe

(cut by quartz window)

(Philips Photonic)

Q.E.

Transmission of variousPM windows

• Typical efficiency forphoton detection: < 20%

• For very good PMs: registration of single photons possible.

Energy Resolution of PMTsThe energy resolution is determined mainly by the fluctuation of the number of secondary electrons emitted from the dynodes.

Fluctuations are the largest when n is small -> first dynodeTypical dynode materials: BeO(Cs), Cs3Sb, MgO; negative electron materials such as GaP(Cs) –higher emission yield but more difficult to fabricate

Poisson distribution: !),(

menmnP

nm

nnn

nn 1

Relative fluctuation:

(Philips Photonic)

Single photons.Pulse height spectrum of a PMT with Cu-Be dynodes.

Pulse height spectrum of a PMT with NEA dynodes.

coun

ts

coun

ts

1 p.e.

2 p.e.

3 p.e.

(H. Houtermanns, NIM 112 (1973) 121)

1 p.e.

More on resolutionTypical NaI(Tl) system (from H. Spieler)

511 keV gamma ray

25000 photons in scintillator

15000 photons at photocathode

3000 photoelectrons at first dynode

3x109 electrons at anode2 mA peak current

Resolution of energy measurement determined by statistical variance of produced signal quanta.

FWHMsmrEE %5..%2

30001

Dynode Configurations

position sensitive PMT’s

(Philips Photonics)

traditionalNew ‘micro-machined’

structures

PM’s are in general very sensitive to B-fields, even to earth field (30-60 T). -metal shielding required.

Many different dynode configurations have been developed to reduce size, or improve gain, uniformity over large photocathode diameters, transit time and transit time spread.

Microchannel PlatesContinuous Multiplier Structure

Channel Electron Multiplier

Microchannel PlateLead glass plateFast timingLow time dispersionImage Amplifier104-107 holesGain factors 103-104

What to expect – an exampleSome parameters for a typical plastic scintillation counter:

energy loss in plastic scintillator: 2MeV/cmscintillation efficiency of plastic: 1 photon/100 eVcollection efficiency (# photons reaching PMT): 0.1quantum efficiency of PMT 0.25

What size electrical signal can we get from a plastic scintillator 1 cm thick?A charged particle passing perpendicular through this counter:

deposits 2MeV which produces 2x104 ’s of which 2x103 ’s reach PMT which produce »500 photo-electrons

Assume the PMT and related electronics have the following properties:PMT gain = 106

500 photo-electrons produce 5x108 electrons or q = 8x10-11C

Assume charge is collected in 50nsec (5x10-8s)current = dq/dt = (8x10-11 coulombs)/(5x10-8s) = 1.6x10-3A

Assume this current goes through a 50 resistorV=IR=(50 )(1.6x10-3A)=80mV (big enough to see with Oscilloscope)

So a minimum ionizing particle produces an 80mV signal.

Efficiency of this counter

What is the efficiency of the counter? How often do we get no signal (zero photo electrons (PE))?

The prob. of getting n PE’s when on average <n> are expected is a Poisson process:

!)(

nennP

nn

The prob. of getting 0 photons is e-<n> =e-500 ~0. So this counter is »100% efficient.

Note: a counter that is 90% efficient has <n>=2.3 PE’s

Time dependence of emitted light Non-radiative transfer of energy from vibrational states to fluorescent state

Typical time: 0.2 – 0.4 ns

Decay of fluorescent stateTypical time: 1 – 3 ns

Rise with time constant r

Fall with time constant f

Total pulse shape

Note: the rise time is usually increased substantially by subsequent components in the system and variations in path length in large scintillators

rtetI /1

ftetI /

rf tto eeItI //

perabsorbedooo EENwithNQEgainI /

Operational Aspects of PMTs Voltage Divider

Electron multiplication at the dynodes depends on the potential between successive dynodes.

Potential distribution typically set by resistive divider

Typically. PMTs are operates at ~ 2kV 8-14 stages -> 100 – 150 V between dynodes

Typically larger for first stages to improve collection PMTs have (almost) linear gain until saturation sets in. NEA dynodes (GaP(Cs)) do not exhibit saturation

Linear response

Advanced PMTs Multi Anode PMT (Example: Hamamatsu R5900 series)

Flat Panel PMT (Hamamatsu)

Up to 8x8 channels. Size: 28x28 mm2. Active area 18x18 mm2 (41%). Bialkali PC: Q.E. = 20% at max = 400 nm. Gain 106

.

Gain uniformity and cross-talk used to be problematic, but recently much improved.

Excellent surface coverage (>90%)8 x 8 channels (4 x 4 mm2 / channel)Bialkali PC, Q 20%

Other Photon Detectors Photo Diodes Hybrid Photo Diodes Silicon Photomultiplier Visible Light Photo Counter

Gas Photo Multiplier

Liquid Noble Gases

Future Lecture

Cherenkov Detectors

Cryogenic Detectors

Scintillation Counter Plateau

Low voltage: very few countsWith increasing voltage (gain) the number of counts rises sharply once the signalPulses are above the discriminator thresholdRegeneration effects (after pulsing etc) at higher voltages

Scintillation counters are typically operated in the middle of the plateau

References used today Particle Detectors, CERN Summer Student Lecture 2008, W. Riegler Particle Detectors, CERN Summer Student Lecture 2003, C. Joram Radiation Detectors, H. Spieler Material from the books by Leo and Gruppen Particle Data Book