Gamma rays detectors
BGO crystals
from Novosibirsk
1) Comparative characteristics of detectors
2) Scintillation detectors
3) Semiconductor detectors
4) Crystal diffraction spectrometers
Plastic
Počet
HPGe
NaI(Tl) detector
for satellite
Fermi
NaI(Tl)
CZT
Energie [keV]
Comparison of natural background spectra detected by different types
of detectors (taken from presentation of ORTEC company)
HPGE detectors of satellite INTEGRAL
Comparative characteristics of detectors
Sensitivity – capability to produce measurable signal for given type of particle
and energy.
Depends on: 1) cross-section of ionization reactions, photon reactions, ...
2) detector mass, its construction
3) detector noise
4) thickness and type of material surrounding sensitive detector volume
Response – relation between particle energy and detector output (total charge or
current amplitude of pulse).
Response function F(E,E´) - spectrum S(E´) of monoenergetic beam is observed
by detector as complicated function. Usually near to Gauss function with
tail to lower energies. Measured distribution of pulse amplitude P(E):
P( E )   S ( E ) F ( E , E )dE 
E – energy at measured spectrum, E´- initial energy
Time response – time of detector signal creation
Pulse form – detector signal shape
leading edge, declining
(even more components)
fast component, slow component
Death time – time needed for creation and analysis of detector signal.
1) detector is not sensitive
2) Detector is always sensitive – „pile-up“ is created –
amplitude superposition
Assumption: death time τ is constant:
Case 1 (death time is not extended):
Real number of particles: NS = mT = k + mkτ
m – real count rate, T – time of measurement,
k – number of registered cases
Real count rate:
m
Dead time and its influence
k
T
k
1    
T 
Case 2 (death time is extended):
Distribution of intervals t between signal arrival: P(t )  m  e
then probability that t > τ:
 mt

P(t   )  m  e mt dt  e  m

and relation between registration number k and real count rate m is: k  m Te m
Detection efficiency – ratio between number of detected particles and number of particles
emitted by source – absolute efficiency. It consists of intrinsic efficiency εVNI and
geometrical efficiency (acceptance) εGEO : ε = εVNI·εGEO
Standard – line 1332 keV of 60Co
It is determined also relatively – detector comparably to standard (NaI(Tl) with
sizes 7.627.62 cm) in given geometry ( - distance 25 cm) εNaI = 0,12 %
Ratio between peak and Compton background – for gamma ray detectors – ratio
between maximal amplitude in peak 1332 keV and mean value in the region
1040 – 1096 keV
Energy resolution – the smallest distinguishable energy difference ΔE between two
near energies. Monoenergetic beam → ideally δ-function – practically peak with
finite width (mostly Gauss shape). Resolution is presented in the form of full
width at half maximum – FWHM). Relative resolution ΔE/E in [%] is also used.
differences from Gauss shape are given by:
FWTM – width in 1/10 of high
FWFM – width 1/50 of high
Gauss: FWTM/FWHM = 1.82
FWFM/FWHM = 2.38
Also other distributions, asymmetries,
electrostatic spectrometer – Lorentz shape
FWHM – energy resolution:
(It is valid for scintillation, semiconductor, gas detectors)
Number of created charge carriers, photons …:
N
E
eS
where eS is mean energy needed for creation of charge carrier or photon
Ionization and deexcitation – Poisson distribution → standard deviation:
Relation between FWHM and σ for Gauss shape:
 N
FWHM = 2.35 ·σ
Detector absorbing only part of energy:
Deposited energy E freely fluctuate → Poisson distribution is valid:
E  FWHM  2.35 N  eS  2.35
E
 es  2.35 E  eS
eS
Detector absorbing total energy (photon detectors):
Deposited energy is fixed finite value → Poisson is not valid, correction introduces Fano:
  FN
Relative energy resolution:
where F – Fano correction
R
F  eS
E
1
 2.35
~
E
E
E
E  FWHM  2.35 F  E  eS
Comparison of absolute and relative resolution for scintillation and semiconductor detectors
FWHM value is influenced by another factors: absorption of charge carriers, photons
properties of electronic
….
In the case of independent contributions: (ΔE)2 = (ΔETN)2 + (ΔEPN)2 + (ΔEELEK)2 + …
Time resolution – the smallest resolvable time difference – definition similar to energy
resolution
Space resolution – the smallest resolvable space difference – definition similar to previous
Tolerance to radiation damages – irradiation → damages, crystal
lattice defects, bugs
less sensitive – liquid and gas detectors
more sensitive – scintillation and mainly semiconductor detectors
Gauss shape
before irradiation
Shape after irradiation
Detectors work in strong radiation field
During experiments on accelerators
Illustration of downgrade of HPGE detector
of INTEGRAL satellite after irradiation
(A.Thevenina report)
Sometime gradual regeneration is possible, HPGe detector is possible to regenerate
after warming
Scintillation detectors
Scintillation detector: 1) Scintillator
2) Photomultiplier + magnetic shielding (or photodiode)
3) Base
Ionization radiation passage → excitation of atoms a molecules
deexcitation → energy → light production - luminescence
Information: 1) Energy
2) Time – they are fast
3) Particle identification from pulse shape
Fluorescency – fast energy conversion to light 10-8s
Phosphorescency - delayed energy conversion to light μs – days – longer λ
Properties of photomultipliers, photodiodes,
avalanche photodiodes – see literature
Discharge has exponential behavior:
One-component
Binary:
N  N0e
N  A e
τR – fast component,
Požadavky na scintilator:


t
R
t
R
 Be

t
P
τP – slow component
Example of signal shape of
binary scintillator
1) High efficiency of excitation energy conversion to fluorescent light
2) Conversion should be linear
3) Transparency for fluorescence light (light emission should be in different
range than light absorption
4) Fluorescent spectrum should be compatible with photomultipliers
5) Short decay constant
6) It should have good optical properties and easily machinable
7) Index of refraction should be near to n = 1.5 (glass) – good crossing passage
of light to photomultiplier
Organic scintillators: 1) Organic crystals – anthracene, stilbene
2) Liquid organic scintillators very resistive against radiation
damage, measured radioactive substance can be part of detector
3) Plastic scintillators – very fast τ ~ 2 ns,
NE111: τleading edge = 0.2 ns and τ = 1.7 ns
lower Z → small σ for photoeffect, Compton scattering dominates, addition of heavy
element admixture (Pb) → increasing of photopeak, decreasing of light output
Inorganic scintillators: are slower, higher Z → more suitable for gamma radiation,
CsI(Tl), NaI(Tl) (is hygroscopic), BGO (Bi4Ge3O12), BaF2,PbWO4
BGO, BaF2, PbWO4 very useful for high energy gamma
BaF2 very fast (fast component), two components
ρ [g/cm3]
Anthracene
~0,8
Plastic (NE111) ~1.2
NaI
3.67
BGO
7.13
BaF2
4.89
eS [eV]
60
100
25
300
125
τ [ns]
30
1.7
230
300
0.6 (fast c.)
600 (slow k.)
Fano coefficient is for scintillators F ~ 1
Limiting theoretical resolution,
without inclusion of influence of
electronic and charge carrier trapping
Crystal PbWO4 of high energy photon
spectrometer of project ALICE,
blue λ= 420 nm and green λ= 480-520 nm
TAPS and ALICE photo materials
BaF2 crystals of photon spectrometer TAPS
ultraviolet components λ=220nm and λ=310 nm
Semiconductor detectors
Very common: HPGe (earlier Ge(Li)) – need liquid nitrogen cooling
Si – for low energy range
Newer and up to now more special: CdTe, HgI2, CdZnTe (CZT) – up to now for
lower energies, cooling is not necessary, eS ~ 4.4 eV
Ge, Si – four valence electrons – electron release (its transposition from valence
to conduction band) → creation of hole and free electron
WWW pages of W. Westmaier
Impurity with 3 valence electrons – electron recipient →
→ hole predominance → semiconductor of p type
Impurity with 5 valence electrons – electron donor →
→ predominance of electrons → semiconductor of n type
Ge(Li) detector – 1012 impurity atoms per cm3
HPGe – 109 impurity atoms per cm3
Prevention of thermal production of electron-hole pairs
→ temperature 77 K
Capture and recombination on dislocations and impurities
HPGe detector placed inside
Shielding lead box
Basic semiconductor properties:
for T=77 K
Z
Atomic mass
Density ρ [g/cm3]
Energy gap [eV]
Electron mobility μe[ 104cm2/Vs]
Hole mobility μd [104cm2/Vs]
eS [eV]
Fano coefficient F
Si
14
28.09
2.33
1.1
2.1
1.1
3.76
~ 0.09
Ge
32
72.60
5.33
0.7
3.6
4.2
2.96
~ 0.06
ve = μe·E
vd = μd·E
Voltage on detector more than 1000 V
Small pulses → necessity of preamplifier:
detector → premaplifier → amplifier → ADC
→ analyzer, computer
Technical details – see recommended literature
Position sensitive HPGe segmented
detectors are developed by LLNL
(Californian University) its WWW
Parameters for 60Co line with energy 1332 keV
Relative efficiency
To the standard (NaI(Tl)):
10 – 70 %
(εNaI = 0.12 % εGEO ~ 0.58 % εVNI ~ 20 % )
peak/compton:
1:30 až 1:60
Resolution: FWHM 1.7 – 2.3
ΔEΣ2 = ΔETN2 + ΔEELEK2 + ΔEPN2
ΔETN – intrinsic uncertainty (carrier
creation)
ΔEELEK – uncertainty given by electronic
ΔEPN – uncertainty given by electron and
hole recombination and capture
Peak shape: FWTM/FWHM ~2.0
(Gauss 1.82)
FWFM/FWHM 2.65 – 3.00 (Gauss 2.38)
Low energies – Si and thin HPGe detectors,
beryllium window
High energies - HPGe with large volume,
aluminum window
longer (6 μs) or shorter (2 - 4 μs) time constant
of amplifier
Energy measurement accuracy up to order eV
Massive practical usage → many commercially
produced types and models
Limiting theoretical resolution,
without inclusion of influence of
electronic
ETN  FWHM  2.35 F  E  eS
Crystal diffraction spectrometers
Consist of 1) crystal lamina (quartz crystal, calcite)
2) detector of X- and gamma rays
Characteristic angles influenced on line width
φZ – angle of source visibility from crystal
φK – angle of collimator visibility from
source
φC – angle of diffraction line FWHM
Source
ΘB – Bragg angle
Collimator
φZ
Angular FWHM φ of intensity afterwards is
(for small values of all angles in radians)
φC
φK
Detector
ΘB
φ2 ≈ φZ2 + φK2 + φC2
Different crystal geometries: Plane crystals
Curved crystals
Different configuration: with one crystal Θ = ΘB
with two crystals Θ = 2ΘB
Crystal
lattice
E



   R
E


R – angular resolution
E
  const 
~ const  E
E
Example of measurement accuracy: 169Yb → 169Tm line 63 keV – E = 63.12080(16) keV
Necessity to include influence of nucleus reflection during photon emission and accuracy
of energy standard determination