闪烁探测器

Scintillation Detector

周荣 Rong ZHOU

四川大学 核科学与工程技术学院

College of Nuclear Science & Engineering, SCU

zhourong@scu.edu.cn

Outline

• Configuration and Principle

• Scintillator

• Photomultiplier

PMT

MCP

PD / APD

SiPM

• Scintillation Detector and Spectrometer

CONFIGURATION AND PRINCIPLE

OF SCINTILLATION DETECTOR

Fluorescence Caused by

Radiation

Configuration of a Scintillation

Detector

Operating Principle

Radiation injects, Molecules are excited or ionized,

Molecules de-excite, Scintillation photons are emitted.

Scintillation photons are collected by cathode of PMT,

Electrons are emitted

Electrons fly into dynode and more electrons are emitted and fly into next

dynode, which is electrons multiplication

electric signal is output from anode.

Scintillator: radiation to photon PMT: photon to electric signal

SCINTILLATOR

Various Scintillator

• Inorganic scintillator crystal(doped): NaI(Tl), CsI(Tl), ZnS(Ag),

LaBr3:Ce3+, LaCl3:Ce3+,...

glass: LiO2·2SiO2(Ce),...

pure crystal: BGO, LSO, LYSO, BaF2, ......

• Organic scintillator organic crystal scintillator

organic liquid scintillator

organic plastic scintillator

• Gas scintillator Ar, Xe

Characteristics of Scintillator

• Emission spectrum

• Luminous efficiency & light yield

• Decay time

Emission Spectrum

Emission spectrum of scintillator is continuous. Emission spectrum of scintillator should match absorption spectrum of PMT when develop a detector.

Luminous efficiency

%100E

EC

ph

np

luminous

efficiency energy lost in

scintillator of

incident particles

total energy of

output light

For example: in NaI(Tl), for β particles, Cnp=13%, for α particles, Cnp=2.6%

Light Yield

E

nY

ph

ph energy lost in

scintillator of

incident particles

total number of

scintillation photons

vh

C

Evh

E

E

nY

npphph

ph 1

For example: in NaI(Tl), for β particles of 1MeV, hv≈3eV,

1-4 MeV 103.43

13.0

eVYph

Decay time

• excitation process takes 10-11~10-9 second

• de-excitation and light output obey

exponential decrease.

t

entn

0

Decay time

0

00 ndtennt

ph

tphe

ntn

Fast and Slow decay

sf

t

s

st

f

f

sf en

en

tntntn

The ratio of nf and ns has connection with incident particles, which could be used to discriminate particle category.

NaI(Tl) Crystal

• High light yield

• High Z and ρ(3.67g/cm3)

• Hygroscopic

CsI(Tl)

• Expansive

• High Z and ρ(4.51g/cm3)

• Low light yield

• Tractable

Other Scintillation

• ZnS(Ag): translucent, for α counts

measurement

• BGO:High Z and ρ

• BaF2: fast decay time(0.6ns)

• Liquid scintillator: fast decay time(2.4ns)

• Plastic scintillator: fast decay time(1~3ns)

Scintillator Table

A Good Scintillator

• Suitable for detected particles

• match PMT well

• high stopping power

• high light yield

• fast decay time

• wide linear region of energy reponse

Collection of light

• Reflecting layer

• light coupling agent

• light guide

PHOTOMULTIPLIER

Photomultiplier

• Photomultiplier Tube(PMT)

• Micro channel plate

• (Avalanche) Photodiode (PD/APD)

• Silicon Photomultiplier(SiPM)

PMT

Vacuum Shell

Dynode Anode

track of electrons incident

photons

Focusing Electrode

photocathode

Various type of PMT

Focusing type VS non-focusing

type

Focusing PMT

• Fast response time

• Time measurement

• Line or ring structure

Non-focusing PMT

• Large gain

• Eenergy measurement

• Shutter or box-grate

structure

Response Spectrum

Luminous Sensitivity

• Cathode luminous

sensitivity

• Anode luminous

sensitivity

Fi

S kk Lm/A

current of cathode

light flux

Fi

S AA Lm/A

current of aode

light flux

Gain

dynodefirst by the gathered electrons ofNumber

anodeby collected electrons ofNumber M

86 10~10n

K

A

K

A gi

i

S

SM

electron transport efficiency between dynodes

Dark Current and Noise

• Thermal emission

• Ionization or excitation of residual gas

• Point discharge or current leakage

Parameter of Noise and

Dark Current

• Noise energy equivalent

Een=Ven*Eγ/Vγ

• Dark current of anode

its average value of noise pulses, usually 10-6

~ 10-10 A

Ven: discrimination voltage at 50 cps

Rise Time and Transit Time

• Rise time

The time it takes that PMT output current increase from 10% to 90% of maximum.

• Transit time

The average time it takes that one electron from cathode to anode.

Stability

• short-time stability

start to measure 30 minutes after power on.

• long-time stability

has connection with temperature, material

and technology.

Usage of PMT

• keep out of light

• supply correct H.V.

• suitable divider resistance

• add parallel capacitance between last

dynodes and anodes.

Signal Output of PMT

Ia Ik

Ia

Equivalent output circuit

入R//RR L0

eCapacitanc dDistribute

CCC 入'

0

eMTYEQ ph

Charge Output

• e: unit charge

• M: multiplication factor of PMT

• T: efficiency from photons to electrons collected by the first dynode.

• Yph: light yield

• E: energy loss of incident particles in scintillator

EQ

Current Output

• photon number emitted by scintillator in

unit time:

• electron number collected by the first

dynode in unit time

tphe

ntn

tph

e eTn

tn

decay time of

scintillator

etArea eM

)(tp

2et

1et

3et

eMt

FWHM

The moment that one photoelectron

achieves the first dynode

The moment that each multiplied

electron achieves anode

current pulse caused by

single photoelectron

tdtpe

TntI

t ttph

0

)(

tph

e eTn

tn

Current Pulse of One

Scintillation

etteMtp if

tdtteMe

TntI

t

e

ttph

0

)()(

then

ett

ett

tI

t

et

Voltage Pulse

t

CRtCRt

tdetIC

etV

0

/

0

/

00

00

dt

tdVC

R

tVtI

)()(0

0

/)( tphe

eMTntI

eMTnQ ph

//

00

00

0

00

)()( tCRt ee

CR

CR

C

QtV

00CR

tCR

t

eeC

QtV 00

0

//

00

00

0

00

)()( tCRt ee

CR

CR

C

QtV

00CR

0000

0

CRtt

eeCR

C

QtV

45

25/00 CR

5.2/00 CR

400 CR

4000 CR

40000 CR

= 250ns，C0=1pF R0：10K，100K，1M，10M，100M

5.2/00 CR

00CR

200 CR

400 CR

=250ns，R0=100K， C0：1pF，2.5pF，5pF，10pF

Output Fluctuate of PMT

MTnMnn pheA

ph

nn

vph

12 Poisson

distribution

Tnn phe Tnn phe

ne

112

Mnn eA

222 1111

M

e

Mee

nnnnA

Mnn eA

1

1

nM

1

111.......

11111

1

1

1

2

111

2

nM

22 11

M

e

nnA

1

11

1

1

Tnph

1

11

1

1

Tn ph

h

h

E

E

Anh 36.236.2

1

11

136.2

1

Tnph

phph

n

TTph

nnn

vvTn

vph

A

1)1(

1

11

12

22

1

2

Micro Channel Plate

Avalanche Photodiode（APD）

APD is a kind of diode the PN junction of which is light sensitive.

Various APD device

• gain 102~103，sensitive

for temperature

• small volume

• suitable for MRI/PET

imaging system（NOT

infected by magnetic

field）

Silicon Photomultiplier——

SiPM

• Proposed by Russian in 1990's，a lot of

APD operating in Geiger mode are

collected in parallel with a common anode.

Advantage of SiPM

• High gain（105~106， as

large as PMT）

• Low operating voltage

（<100V）

• Low power consumption

• tiny volume

• high efficiency

• not sensitive for magnetic

field

• easy to form large array

For each PAD unit

occupies only hundreds

of μm2，it could contian

thousands of APD unit in

1mm2 aera！

SCINTILLATION SPECTRAMETER

OF SINGLE CRYSTAL

Configuration of Scintillation

Spectrometer

Scintillator Pre-

amplifier PMT Divider

H.V. L.V.

Linear

Amplifier

MCA

SCA

Scaler

Pulse Amplitude Spectrum

Energy Spectrum

E

dE

dN

Small Size Crystal (<1cm)

ePhotoelectric absorption

e

escaped scattered photon

Single Compton scattering e

eescaped photons produced by annihilation pair

production

E

dE

dN

E

dE

dN

2

02 cmhv

2

02 cmhv hv

hv

photoelectric peek (total energy peak)

Compton

plateau

Compton

edge

Compton

plateau double escape peak

2

02 cmhv

photoelectric peek

(total energy peak)

Infinite Size Crystal

photoelectric

absorption e

Compton

scattering

ee

Pair production

e

e

eeE

dE

dN

hv

total-energy peak

Middle Size Crystal

e

photoelectric absorption

e

escaped photons after multiple scattering

Compton

scattering

ee

single escaped photon produced by annihilation

Pair production

e

e

e

2

02 cmhv

2

02 cmhv

hv

hv

photoelectirc peak (total-energy peak)

multiple Compton

scattering

E

dE

dN

E

dE

dN

2

0cmhv

single escape peak

2

02 cmhv

photoelectric peak (total-energy peak)

multiple

Compton

scattering

double escape peak

E

dE

dN

③

MeV511.0

②

annihilation

peak

Backscattering

peak

①

characteristic

X-ray peak

characteristic X-ray

photoelectric absorption

Compton scatter

annihilation photon ③

①

②

MeV2.0

Example of Measured

Energy Spectrum

Performance of Scintillation

Spectrometer

• Response Function

pulse amplitude spectrum of a certain

monoenergetic γ ray

• Energy resolution

%100b

a

1

11

136.2

1

en

photoelectrons'number collected by D1 in one

scintillation

multiplication factor of D1

h

h

E

E

Anh 36.236.2

1

1136.2

1E

se

sn

ELet

Average energy needed to produce one photoelectron collected by D1。

Discussion

• ne=nph·T, nph↑η↓

• δ1↑, η↓

• The stability of H.V.

• Channel width

nbaVM 0

7nb 0

07V

V

M

M

2

28.01h

channel width

FWHM

Performance of Scintillation

Spectrometer • Energy linearity

light yield varies with energy of electron.

• Detection efficiency

size(thickness) and Z,ρ of crystal

• Resolving time

RC constant of output circuit and decay time of crystal

• Time lag and Time resolution

electron transit time and its dispersing

• Stability

determined by PMT's stability

0EChGE

Homework

• 1,2,3,4,5,6,8,9,10