Electron detectors and spectrometers
Necessity of detection in the wide energy range:
Atom physics meV - eV
Auger electrons eV – 100 keV
Beta and gamma decays keV – MeV
Particle decays to e+e-, pair production MeV – 10 GeV
Detectors or combination of magnetic and electric fields
and detectors are used
1) Gas detectors
2) Channeltrons
3) Semiconductor detectors
4) Electrostatic spectrometers
5) Magnetic spectrometers
6) Di-lepton spectrometers
7) Cherenkov detectors
Big spectrometer of „orange“ type
(application center of Karlsruhe Institute
and Technical University at Darmstadt)
Gas filled detectors
Efficiency near to 100 %
1) Geiger- Müler counters: work in the range of discharge (IV)
2) Proportional detectors: work in the range of proportionality
(III) (amplification ~ 107)
3) Ionization chambers: without amplification → weak output
signal (II)
Widely used in previous period, semiconductor detectors are used presently
Position sensitive:
1) Multi-wire proportional chambers – anode sensitive wires are placed between two
cathode plates (signal is taken from anodes)
2) Drift chambers – drift of charge from ionization to anode, typical drift velocities
~ 5 cm/μs, it is possible determine from time distance
3) Time projection chambers – cylinder filled by gas end cap by multi-wire
chambers, placed in homogenous magnetic fields, make
possible 3D measurements
Wide application in high energy electron and positron spectrometry
Channeltron
Usage for energies 0,01 – 30 keV
Channel from glass or ceramic
Semiconductor surface layer
Amplification ~
107
Polovodičová
vrstva
Elektroda
Small sensitivity to gamma detection
Primární
elektron
Sekundární
elektrony
Possibility of configuration to
chaneltron plates – millions of miniature
electron amplifiers working independently
Výstupní
Skleněná
stěna kanálku elektrony
Channeltron scheme
Amplification ~ 104 Two in cascade ~ 107
Distance of channels:
8 – 30 μm
Small sensitivity to
magnetic field
Death time ~ 10 ns
Channeltrons of BURLE Company
Účinnost [%]
Position sensitive:
Energie [eV]
Dependency of detection efficiency on energy
Semiconductor detectors
Intensive exploitation of semiconductor silicon detectors
Energy resolution ~ 0,9 – 1,9 keV for energy 100 – 1000 keV
Lower energies – very thin window is important → the smallest possible absorption
Usage of magnetic transporter – magnetic field transports electrons
to place with smaller background
Position sensitive detectors:
1) Silicon Strip Detectors – thin (1 μm) aluminum strips are on silicon wafer
(thickness of 300 μm) and under them p+ implantation (boron)
- operate as separate electrodes
2) Silicon Pixel Detectors – structure of single cells
3) Silicon Drift Detectors – set of electrodes, charge
afterwards drifts in electric field, one from
coordinates is determined from drift time
SDD detector of experiment ALICE
Electrostatic and magnetic spectrometers
Motion of charged particle in electric and magnetic fields:


FE  e  E
1) Electric field - acting force:

 
FM  e  v  B
2) Magnetic field – acting force:


B  v
If
hold valid
F  ma  m
v
2
and then
 evB
r
p
p
v
1  
c
FWHM
We determine relation EKIN = f(Br) (
me
m 
1
mc
2
v
2
c
2
E KIN  mc
2
 mec
2
 E KIN
E KIN
 m e c  E KIN 
m
e
c
2

2
e
2
 Br 
2
c
2
 E KIN

 mec  1 


2

Br
):
2
 e 


 m c    Br
 e 

2

 1


R 
p

p
  Br

Br
  ( Br )
where was taken :
2
v 
2 2
2 2
2 4
2 2
2 4
2 4
2
2 4
 m c  1  2   m e c  m c  m v  m e c  m c  ( eBr )  m e c
c 

2
  Br
2
Resolution of magnetic spectrometer is given by momentum resolution:
Resolution of electrostatic by energy resolution: R 

me
m 
where m – relativistic mass of electron:
p  mv  eBr 
FWHM
  E KIN
We determine relation between momentum and energy resolutions:
E KIN 
p c  me c
2
2
2
dE KIN
and then:
pc
 m e c  dE KIN 
4
2
p c  me c
2
2 2
p c

E KIN
dp
p c  me c
2
E KIN
2
2
2
4
2
2
dp
4
E KIN  2 E KIN m e c dp
2

p
wanted relation between resolutions:
2

E KIN E KIN  m e c
dp
p

d ( Br )
2

Br

2

E KIN  m e c
p
dE KIN
E KIN
2

mec
 1 
2
p 
E KIN  m e c
E KIN  2 m e c dp
2
2

mec
 1 
2

E KIN  m e c

 dp

 p

 d ( Br )

 Br

For nonrelativistic case:
E KIN 
dE KIN
E KIN
p
2
2m e

1
 dE KIN 
p
2
dp
2p
dp 
2m e
 2
p
dp
me
dp
E KIN m e p
p
Agreement with nonrelativistic limit (EKIN << mec2)
For ultrarelativistic case:
E KIN  E  pc 
dE KIN
E KIN

dp
p
Agreement with ultrarelativistic limit (EKIN >>
mec2)
Relation between energy and
momentum resolution
Basic characteristics of electron spectrometers
1) Range of measured energies: 0,01 – 1000 keV
2) Already mentioned resolution R:
8·10-8 – 10-1
3) Solid angle to which detected electrons are emitted Ω: 0,0001 – 20 % ze 4π
4) Sizes of source or irradiated target σ: ~ 0,5 mm2 – 200 cm2
5) Transmission T – part of monoenergetic electron beam, which will reach detector
6) Total luminosity L = T·σ : 10-7 – 10-1 cm2
7) Electron-optical quality: T/R or L/R
8) Intensity of used magnetic fields B: 0,0001 – ~3 T
Very important for source preparation– exclusion of electron energy losses in source
material.
Electrostatic spectrometers
Usage up to energies 50 keV (too high voltage is necessary for higher energy
and it is also problem with relativistic corrections)
Magnetic fields – focuse electrons to measuring place, using slits makes
possible selection of momentum (energy)
Electric fields – produce potential barrier, which transmits only electrons,
which energy is higher then given threshold
Integral method of measurement – during every measurement (given decelerating
potential)
Differential method of measurement - motion at magnetic field
determines only definite energy range
Single channel method of measurement → big accent
on time stability and continuous calibration
Used detectors:
1) Channeltrons – suitable for low energies ~ keV
Microchannel plate – position sensitive
2) Silicon detector – can measured also energy
drift and pixel detectors – position sensitive
Electrostatic spectrometer
ESA 12 (NPI ASCR Řež)
Magnetic spectrometers
Magnetic field is used for determination of electron momentum (energy)
Resolution: R = Δp/p = 10-3 ÷ 10-2
Many types were used during time:
1) Plane spectrometers – field has plane symmetry
2) Lens spectrometers – field has axial symmetry
Plane and lens type of
magnetic spectrometers
„Orange“ and „mini-orange“ type of spectrometer
(Magnets are splitted to sectors – mostly six sectors placed around axis)
source
lead
absorber
beam
detector
Spectrometer of miniorange type (University Bon)
Transmission [%]
Compact device, magnets produce homogenous field – changes of magnet configuration
make possible change of transmission maximum energy (by this also spectrometer efficiency)
Energy [keV]
Magnetic transporter and silicon detector
On beam measurement → intensive background of gamma photon and other particles
Magnetic field is used for transport of electron outside place with background,
electron energy is determined by silicon detector
„soft transition between magnetic spectrometers and transporters“
Usage
1) toroidal magnetic field:
→ motion on cycloid
2) magnetic field of solenoid: Bz = B, Bx = By = 0 → motion on spiral
Efficiency of system is given by transmission of transport system and also by
efficiency of detector
Some spectrometers of „orange“ and „miniorange“ types can be used also as
transporters
High-energy physics – dilepton spectrometer
Study of particle decay to e+ e- or μ+ μ- channels, production of such pairs through
virtual photons → necessity of spectrometer of leptons with high energy
Spectrometer composition:
Necessary for momentum determination and identification of positive and
negative particles:
1) Very intensive magnet (often superconductive)
2) Position sensitive detectors ahead magnet and under magnet
(multiwire proportional chambers, Cherenkov detectors)
Improvement particle identification (suppression of hadron background):
3) Detectors discriminating hadron and electromagnetic showers
4) Detectors measuring time of flight
Scheme of di-lepton spectrometer NA50 and its wire chambers
Usage of Cherenkov radiation detectors
Experiment CERES:
Experiment HADES: