Gamma rays detectors 1) Comparative characteristics of detectors 2) Scintillation detectors 3)...

$: Gamma rays detectors 1) Comparative characteristics of detectors 2) Scintillation detectors 3) Semiconductor detectors 4) Crystal diffraction spectrometers.$
Gamma rays detectors

1) Comparative characteristics of detectors

2) Scintillation detectors

3) Semiconductor detectors

4) Crystal diffraction spectrometers

HPGE detectors of satellite INTEGRAL

Energie [keV]

Poč

et HPGe

Plastic

NaI(Tl)

CZT

Comparison of natural background spectra detected by different types of detectors (taken from presentation of ORTEC company)

BGO crystals from Novosibirsk

NaI(Tl) detectorfor satelliteFermi

Comparative characteristics of detectors

Sensitivity ndash 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 ndash relation between particle energy and detector output (total charge or current amplitude of pulse)

Response function F(EEacute) - spectrum S(Eacute) 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 ndash energy at measured spectrum Eacute- initial energy

Time response ndash time of detector signal creation

Pulse form ndash detector signal shape leading edge declining (even more components) fast component slow component

EdEEFESEP )()()(

1) detector is not sensitive2) Detector is always sensitive ndash bdquopile-upldquo is created ndash amplitude superposition

Death time ndash time needed for creation and analysis of detector signal

Real number of particles NS = mT = k + mkτ

m ndash real count rate T ndash time of measurementk ndash number of registered cases

Case 1 (death time is not extended)

Real count rate

TkTk

m1

Case 2 (death time is extended)

Assumption death time τ is constant

Distribution of intervals t between signal arrivalmtemtP )(

then probability that t gt τ

mmt edtemtP )(

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

Dead time and its influence

Detection efficiency ndash ratio between number of detected particles and number of particles emitted by source ndash absolute efficiency It consists of intrinsic efficiency εVNI and geometrical efficiency (acceptance) εGEO ε = εVNIεGEO Standard ndash line 1332 keV of 60Co It is determined also relatively ndash detector comparably to standard (NaI(Tl) with sizes 762762 cm) in given geometry ( - distance 25 cm) εNaI = 012

Ratio between peak and Compton background ndash for gamma ray detectors ndash ratio between maximal amplitude in peak 1332 keV and mean value in the region 1040 ndash 1096 keV Energy resolution ndash the smallest distinguishable energy difference ΔE between two near energies Monoenergetic beam rarr ideally δ-function ndash practically peak with finite width (mostly Gauss shape) Resolution is presented in the form of full width at half maximum ndash FWHM) Relative resolution ΔEE in [] is also used differences from Gauss shape are given by FWTM ndash width in 110 of high FWFM ndash width 150 of high Gauss FWTMFWHM = 182 FWFMFWHM = 238 Also other distributions asymmetries electrostatic spectrometer ndash Lorentz shape

Detector absorbing only part of energy

Detector absorbing total energy (photon detectors)

Ionization and deexcitation ndash Poisson distribution rarr standard deviation

Number of created charge carriers photons hellip

(It is valid for scintillation semiconductor gas detectors)

Se

EN

N

where eS is mean energy needed for creation of charge carrier or photon

Relation between FWHM and σ for Gauss shape FWHM = 235 σ

FWHM ndash energy resolution

Deposited energy E freely fluctuate rarr Poisson distribution is valid

Deposited energy is fixed finite value rarr Poisson is not valid correction introduces Fano

where F ndash Fano correction

SsS

S eEee

EeNFWHME 352352352

NF SeEFFWHME 352

Relative energy resolution EE

eF

E

ER S 1

~352

Time resolution ndash the smallest resolvable time difference ndash definition similar to energy resolution

Space resolution ndash the smallest resolvable space difference ndash definition similar to previous

FWHM value is influenced by another factors absorption of charge carriers photons properties of electronic hellipIn the case of independent contributions (ΔE)2 = (ΔETN)2 + (ΔEPN)2 + (ΔEELEK)2 + hellip

Comparison of absolute and relative resolution for scintillation and semiconductor detectors

Illustration of downgrade of HPGE detector of INTEGRAL satellite after irradiation (AThevenina report)

Tolerance to radiation damages ndash irradiation rarr damages crystal lattice defects bugs less sensitive ndash liquid and gas detectors more sensitive ndash scintillation and mainly semiconductor detectors

Shape after irradiation

Gauss shape before irradiation

Detectors work in strong radiation field During experiments on accelerators

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 rarr excitation of atoms a moleculesdeexcitation rarr energy rarr light production - luminescence

Information 1) Energy 2) Time ndash they are fast 3) Particle identification from pulse shape

Fluorescency ndash fast energy conversion to light 10-8sPhosphorescency - delayed energy conversion to light μs ndash days ndash longer λ

Properties of photomultipliers photodiodesavalanche photodiodes ndash see literature

Discharge has exponential behavior

R

t

eNN

0

PR

tt

eBeAN

One-component

Binary

τR ndash fast component τP ndash slow component

Požadavky na scintilator

1) High efficiency of excitation energy conversion to fluorescent light2) Conversion should be linear3) Transparency for fluorescence light (light emission should be in different range than light absorption4) Fluorescent spectrum should be compatible with photomultipliers5) Short decay constant 6) It should have good optical properties and easily machinable7) Index of refraction should be near to n = 15 (glass) ndash good crossing passage of light to photomultiplier

Example of signal shape of binary scintillator

ρ [gcm3] eS [eV] τ [ns]Anthracene ~08 60 30Plastic (NE111) ~12 100 17NaI 367 25 230BGO 713 300 300BaF2 489 125 06 (fast c) 600 (slow k)

Organic scintillators 1) Organic crystals ndash anthracene stilbene 2) Liquid organic scintillators very resistive against radiation damage measured radioactive substance can be part of detector 3) Plastic scintillators ndash very fast τ ~ 2 ns NE111 τleading edge = 02 ns and τ = 17 ns lower Z rarr small σ for photoeffect Compton scattering dominates addition of heavy element admixture (Pb) rarr increasing of photopeak decreasing of light output

Inorganic scintillators are slower higher Z rarr more suitable for gamma radiation CsI(Tl) NaI(Tl) (is hygroscopic) BGO (Bi4Ge3O12) BaF2PbWO4

BGO BaF2 PbWO4 very useful for high energy gamma BaF2 very fast (fast component) two components

Fano coefficient is for scintillators F ~ 1Limiting theoretical resolution without inclusion of influence of electronic and charge carrier trapping

Crystal PbWO4 of high energy photon spectrometer of project ALICEblue λ= 420 nm and green λ= 480-520 nm

BaF2 crystals of photon spectrometer TAPS

ultraviolet components λ=220nm and λ=310 nm

TA

PS

an

d A

LIC

E p

hot

o m

ater

ials

Semiconductor detectors

Very common HPGe (earlier Ge(Li)) ndash need liquid nitrogen cooling Si ndash for low energy range

Newer and up to now more special CdTe HgI2 CdZnTe (CZT) ndash up to now for lower energies cooling is not necessary eS ~ 44 eV

Ge Si ndash four valence electrons ndash electron release (its transposition from valence to conduction band) rarr creation of hole and free electron

Impurity with 3 valence electrons ndash electron recipient rarr rarr hole predominance rarr semiconductor of p typeImpurity with 5 valence electrons ndash electron donor rarr rarr predominance of electrons rarr semiconductor of n type

Ge(Li) detector ndash 1012 impurity atoms per cm3

HPGe ndash 109 impurity atoms per cm3

Prevention of thermal production of electron-hole pairs rarr temperature 77 K

Capture and recombination on dislocations and impuritiesHPGe detector placed inside Shielding lead box

WW

W p

ages

of

W W

estm

aier

for T=77 K Si GeZ 14 32Atomic mass 2809 7260Density ρ [gcm3] 233 533Energy gap [eV] 11 07Electron mobility μe[ 104cm2Vs] 21 36Hole mobility μd [104cm2Vs] 11 42eS [eV] 376 296Fano coefficient F ~ 009 ~ 006

Basic semiconductor properties

Position sensitive HPGe segmenteddetectors are developed by LLNL (Californian University) its WWW

ve = μeE vd = μdE

Voltage on detector more than 1000 V

Small pulses rarr necessity of preamplifier

detector rarr premaplifier rarr amplifier rarr ADC rarr analyzer computer

Technical details ndash see recommended literature

Parameters for 60Co line with energy 1332 keV

Relative efficiency To the standard (NaI(Tl)) 10 ndash 70 (εNaI = 012 εGEO ~ 058 εVNI ~ 20 )

peakcompton 130 až 160

Resolution FWHM 17 ndash 23

Peak shape FWTMFWHM ~20 (Gauss 182) FWFMFWHM 265 ndash 300 (Gauss 238)

Energy measurement accuracy up to order eV

ΔEΣ2 = ΔETN

2 + ΔEELEK2 + ΔEPN

2

Low energies ndash Si and thin HPGe detectors beryllium windowHigh energies - HPGe with large volume aluminum window

longer (6 μs) or shorter (2 - 4 μs) time constantof amplifier

ΔETN ndash intrinsic uncertainty (carrier creation)ΔEELEK ndash uncertainty given by electronicΔEPN ndash uncertainty given by electron and hole recombination and capture

Massive practical usage rarr many commercially produced types and models STN eEFFWHME 352

Limiting theoretical resolution without inclusion of influence of electronic

Crystal diffraction spectrometersConsist of 1) crystal lamina (quartz crystal calcite) 2) detector of X- and gamma rays

Different crystal geometries Plane crystals Curved crystals

Different configuration with one crystal Θ = ΘB

with two crystals Θ = 2ΘB

Characteristic angles influenced on line width

φZ ndash angle of source visibility from crystalφK ndash angle of collimator visibility from sourceφC ndash angle of diffraction line FWHM

ΘB ndash Bragg angleAngular FWHM φ of intensity afterwards is(for small values of all angles in radians)

φ2 asymp φZ2 + φK

2 + φC2

Example of measurement accuracy 169Yb rarr 169Tm line 63 keV ndash E = 6312080(16) keVNecessity to include influence of nucleus reflection during photon emission and accuracy

of energy standard determination

Detector

Crystallattice

Source Collimator

φK

φZ φC

ΘB

RE

E

EconstE

Econst

~

R ndash angular resolution

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Comparative characteristics of detectors

Sensitivity ndash 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 ndash relation between particle energy and detector output (total charge or current amplitude of pulse)

Response function F(EEacute) - spectrum S(Eacute) 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 ndash energy at measured spectrum Eacute- initial energy

Time response ndash time of detector signal creation

Pulse form ndash detector signal shape leading edge declining (even more components) fast component slow component

EdEEFESEP )()()(






Real count rate

TkTk

m1





mmt edtemtP )(










Se

EN

N







SsS

S eEee

EeNFWHME 352352352

NF SeEFFWHME 352


eF

E

ER S 1

~352


















R

t

eNN

0

PR

tt

eBeAN

One-component

Binary













TA

PS

an

d A

LIC

E p

hot

o m

ater

ials










WW

W p

ages

of

W W

estm

aier




ve = μeE vd = μdE











ΔEΣ2 = ΔETN


2














2 + φC2



Detector

Crystallattice

Source Collimator

φK

φZ φC

ΘB

RE

E

EconstE

Econst

~


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Real count rate

TkTk

m1





mmt edtemtP )(










Se

EN

N







SsS

S eEee

EeNFWHME 352352352

NF SeEFFWHME 352


eF

E

ER S 1

~352


















R

t

eNN

0

PR

tt

eBeAN

One-component

Binary













TA

PS

an

d A

LIC

E p

hot

o m

ater

ials










WW

W p

ages

of

W W

estm

aier




ve = μeE vd = μdE











ΔEΣ2 = ΔETN


2














2 + φC2



Detector

Crystallattice

Source Collimator

φK

φZ φC

ΘB

RE

E

EconstE

Econst

~


Slide 1

Slide 2

Slide 3

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Se

EN

N







SsS

S eEee

EeNFWHME 352352352

NF SeEFFWHME 352


eF

E

ER S 1

~352


















R

t

eNN

0

PR

tt

eBeAN

One-component

Binary













TA

PS

an

d A

LIC

E p

hot

o m

ater

ials










WW

W p

ages

of

W W

estm

aier




ve = μeE vd = μdE











ΔEΣ2 = ΔETN


2














2 + φC2



Detector

Crystallattice

Source Collimator

φK

φZ φC

ΘB

RE

E

EconstE

Econst

~


Slide 1

Slide 2

Slide 3

Slide 4

Slide 5

Slide 6

Slide 7

Slide 8

Slide 9

Slide 10

Slide 11

Slide 12

Slide 13

Slide 14

Slide 15


















R

t

eNN

0

PR

tt

eBeAN

One-component

Binary













TA

PS

an

d A

LIC

E p

hot

o m

ater

ials










WW

W p

ages

of

W W

estm

aier




ve = μeE vd = μdE











ΔEΣ2 = ΔETN


2














2 + φC2



Detector

Crystallattice

Source Collimator

φK

φZ φC

ΘB

RE

E

EconstE

Econst

~


Slide 1

Slide 2

Slide 3

Slide 4

Slide 5

Slide 6

Slide 7

Slide 8

Slide 9

Slide 10

Slide 11

Slide 12

Slide 13

Slide 14

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TA

PS

an

d A

LIC

E p

hot

o m

ater

ials










WW

W p

ages

of

W W

estm

aier




ve = μeE vd = μdE











ΔEΣ2 = ΔETN


2














2 + φC2



Detector

Crystallattice

Source Collimator

φK

φZ φC

ΘB

RE

E

EconstE

Econst

~


Slide 1

Slide 2

Slide 3

Slide 4

Slide 5

Slide 6

Slide 7

Slide 8

Slide 9

Slide 10

Slide 11

Slide 12

Slide 13

Slide 14

Slide 15










WW

W p

ages

of

W W

estm

aier




ve = μeE vd = μdE











ΔEΣ2 = ΔETN


2














2 + φC2



Detector

Crystallattice

Source Collimator

φK

φZ φC

ΘB

RE

E

EconstE

Econst

~


Slide 1

Slide 2

Slide 3

Slide 4

Slide 5

Slide 6

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ve = μeE vd = μdE











ΔEΣ2 = ΔETN


2














2 + φC2



Detector

Crystallattice

Source Collimator

φK

φZ φC

ΘB

RE

E

EconstE

Econst

~


Slide 1

Slide 2

Slide 3

Slide 4

Slide 5

Slide 6

Slide 7

Slide 8

Slide 9

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Slide 11

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ΔEΣ2 = ΔETN


2














2 + φC2



Detector

Crystallattice

Source Collimator

φK

φZ φC

ΘB

RE

E

EconstE

Econst

~


Slide 1

Slide 2

Slide 3

Slide 4

Slide 5

Slide 6

Slide 7

Slide 8

Slide 9

Slide 10

Slide 11

Slide 12

Slide 13

Slide 14

Slide 15









2 + φC2



Detector

Crystallattice

Source Collimator

φK

φZ φC

ΘB

RE

E

EconstE

Econst

~


Slide 1

Slide 2

Slide 3

Slide 4

Slide 5

Slide 6

Slide 7

Slide 8

Slide 9

Slide 10

Slide 11

Slide 12

Slide 13

Slide 14

Slide 15

Gamma rays detectors 1) Comparative characteristics of detectors 2) Scintillation detectors 3)...

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Transcript of Gamma rays detectors 1) Comparative characteristics of detectors 2) Scintillation detectors 3)...