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Plastic. HPGe. Po čet. NaI(Tl). CZT. Energie [keV]. BGO crystals from Novosibirsk. Gamma rays detectors. NaI(Tl) detector for satellite Fermi. 1) Comparative characteristics of detectors 2) Scintillation detectors 3) Semiconductor detectors 4) Crystal diffraction spectrometers.

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gamma rays detectors

Plastic

HPGe

Počet

NaI(Tl)

CZT

Energie [keV]

BGO crystals

from Novosibirsk

Gamma rays detectors

NaI(Tl) detector

for satellite

Fermi

1) Comparative characteristics of detectors

2) Scintillation detectors

3) Semiconductor detectors

4) Crystal diffraction spectrometers

Comparison of natural background spectra detected by different types

of detectors (taken from presentation of ORTEC company)

HPGE detectors of satellite INTEGRAL

slide2

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 functionF(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):

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

slide3

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 countrate, T – time of measurement,

k – number of registered cases

Dead time and its influence

Real count rate:

Case 2 (death time is extended):

Distribution of intervals t between signal arrival:

then probability that t > τ:

and relation between registration number k and real count rate m is:

slide4

Detection efficiency – ratio between number of detected particles and number of particles

emitted by source – absolute efficiency. It consists of intrinsic efficiency εVNIand

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

slide5

FWHM – energy resolution:

(It is valid for scintillation, semiconductor, gas detectors)

Number of created charge carriers, photons …:

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: FWHM = 2.35 ·σ

Detector absorbing only part of energy:

Deposited energy E freely fluctuate → Poisson distribution is valid:

Detector absorbing total energy (photon detectors):

Deposited energy is fixed finite value → Poisson is not valid, correction introduces Fano:

where F – Fano correction

Relative energy resolution:

slide6

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

slide7

Gauss shape

before irradiation

Shape after irradiation

Tolerance to radiation damages– irradiation → damages, crystal

lattice defects, bugs

less sensitive – liquid and gas detectors

more sensitive – scintillation and mainly semiconductor detectors

Illustration of downgrade of HPGE detector

of INTEGRAL satellite after irradiation

(A.Thevenina report)

Detectors work in strong radiation field

During experiments on accelerators

Sometime gradual regeneration is possible, HPGe detector is possible to regenerate

after warming

slide8

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

slide9

Discharge has exponential behavior:

One-component

Binary:

τR – fast component, τP – slow component

Example of signal shape of

binary scintillator

Požadavky na scintilator:

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

slide10

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 fastτ ~ 2 ns,

NE111:τleading edge= 0.2 nsand 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, PbWO4very useful for high energy gamma

BaF2 very fast (fast component), two components

ρ[g/cm3]eS[eV]τ[ns]

Anthracene ~0,8 60 30

Plastic (NE111) ~1.2 100 1.7

NaI3.67 25 230

BGO 7.13 300 300

BaF2 4.89 125 0.6 (fast c.)

600 (slow k.)

Limiting theoretical resolution,

without inclusion of influence of

electronic and charge carrier trapping

Fano coefficient is for scintillators F ~ 1

slide11

TAPS and ALICE photomaterials

BaF2 crystals of photon spectrometer TAPS

ultraviolet components λ=220nm and λ=310 nm

Crystal PbWO4 of high energy photon

spectrometer ofproject ALICE,

blue λ= 420 nm andgreen λ= 480-520 nm

slide12

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

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

WWW pages of W. Westmaier

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

slide13

Basic semiconductor properties:

forT=77 KSi Ge

Z 14 32

Atomic mass 28.09 72.60

Density ρ [g/cm3] 2.33 5.33

Energy gap [eV] 1.1 0.7

Electron mobility μe[ 104cm2/Vs] 2.1 3.6

Hole mobility μd[104cm2/Vs] 1.1 4.2

eS [eV] 3.76 2.96

Fano coefficient F ~ 0.09 ~ 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

Position sensitive HPGe segmented

detectors are developed by LLNL

(Californian University) its WWW

Technical details – see recommended literature

slide14

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

Peak shape: FWTM/FWHM ~2.0 (Gauss 1.82)

FWFM/FWHM 2.65 – 3.00 (Gauss 2.38)

ΔEΣ2 = ΔETN2 + ΔEELEK2 + ΔEPN2

ΔETN – intrinsic uncertainty (carrier

creation)

ΔEELEK – uncertainty given by electronic

ΔEPN – uncertainty given by electron and

hole recombination and capture

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

Limiting theoretical resolution,

without inclusion of influence of

electronic

Energy measurement accuracy up to order eV

Massive practical usage → many commercially

produced types and models

slide15

Source

Collimator

φZ

φC

φK

ΘB

Crystal

lattice

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

ΘB – Bragg angle

Angular FWHM φ of intensity afterwards is

(for small values of all angles in radians)

φ2 ≈ φZ2 + φK2 + φC2

Detector

Different crystal geometries: Plane crystals

Curved crystals

Different configuration: with one crystal Θ = ΘB

with two crystals Θ = 2ΘB

R – angular resolution

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