Transcript Slide 1

Detector Design Principles
Radiation Detectors
2
Ionization Detectors
• Ionization chambers (gas and
solid-state)
• Proportional counters
• Avalanche counters
• Geiger-Müller counters
• Cloud/bubble chambers
• Track detectors
Scintillation Detectors
• Phosphorescence counters
• Fluorescence counters
(inorganic solid
scintillators, organic solid
and scintillators)
• Čherenkov counters
Associated Techniques
•
•
•
•
Photo sensors and multipliers
Charged-coupled devices
Electronic pulse shape analysis
Processing/acquisition electronics
W. Udo Schröder, 2007
Primary Ionization Track (Gases)
incoming particle
ionization track 
ion/e- pairs
Minimum-ionizing particles
Different counting gases
Helium
3
GAS (STP)
Radiation Detectors
e-
I+
≈Linear
for DE«E
W. Udo Schröder, 2007
Argon
Xenon
CH4
DME
dE/dx ( keV /cm )
0.32
2.4
6.7
1.5
3.9
<n> (ion pairs/cm)
6
25
44
16
55
Statistical ionization process: Poisson statistics
Detection efficiency e depends on average number
<n> of ion pairs
e  1e
DE  <n>
(Sauli. IEEE+NSS 2002)
 n
GAS (STP)
Helium
Argon
Higher e for slower particles
thickness
e
1 mm
45
2 mm
70
1 mm
91.8
2 mm
99.3
Radiation Detectors
4
Effective Ionization Energies
Mean energy per
ion pair larger
than IP because
of excitations
Large organic molecules have low-lying excited rotational states 
excitation without ionization through collisions  =“quenching” additives
W. Udo Schröder, 2007
Driven Charge Transport in Gases
Electric field E = DU/Dx separates +/- charges (q=ne+, e-)
P(x)
Charge Diffusion in Electric Field
t0
5
x
P(x)
Radiation Detectors
t1 >t0
x
E
dN

dx
 ( x  w  t )2 
 exp 

4D  t 

4Dt
N0
e
E   drift velocity
2m
   v mean time between collisions
w

D
kT
e
 :
w
mobility
E
x
P(x)
t2 >t1
x
W. Udo Schröder, 2007
Cycle: acceleration – scattering/ionization
Drift (w) and diffusion (D) depend on field
strength E and gas pressure p (or r).
w  w(E p);
D  D(E p)
Ion Mobility
GAS
ION
6
Ar
Ar+
CH4
CH4+
Ar+CH4 80+20 CH4+
µ+ (cm2 V-1 s+1) @STP
1.51
2.26
1.61
Ion mobility + = w+/E
Independent of field,
for given gas at p,T=const.
Typical ion drift velocities
(Ar+CH4 counters):
Radiation Detectors
w+ ~ (10-2 – 10-5) cm/s
W. Udo Schröder, 2007
 slow!
E. McDaniel and E. Mason
The mobility and diffusion of
ions in gases (Wiley 1973)
Signal Generation in Ionization Counters
Primary ionization: Gases I  20-30 eV/IP, Si: I  3.6 eV/IP
d
Capacitance C
7
-
x
Energy loss De: n= nI =ne= De/I number
of primary ion pairs n at x0, t0
x0
Force: Fe = -eU0/d = -FI
Energy content of capacitor C:
+
R
Radiation Detectors
Ge: I  3.0 eV/IP
Cs
0
DU(t)
Signal
C 2
U0  U 2  t    CU0 DU  t 

2
2) W  t   ne Fe  xe  t   x0  + nI FI  x I t   x0 
1) W  t  
+
U0
W. Udo Schröder, 2007
neU0
 x I  t   xe  t  
d
w
1) + 2)
DU  t  
W t 
CU0

+
t  t
 t0 
ne  +
w  t   w   t    t  t0 

Cd 
Time-Dependent Signal Shape
Total signal: e & I components
Drift velocities
(w+>0, w-<0)
De
w + t   w  t   t  t0 
DU t  
Cd
Radiation Detectors
8
w + t 
De
C
10 3 w  t 
Both components
measure De and
depend on position of
primary ion pairs
DU(t)
x0 = w-(te-t0)
De x0
C d
For fast counting
use only electron
component.
t0
W. Udo Schröder, 2007
te~s
tI~ms
t
Amplification Counters
Single-wire gas
counter
signal
gas
9
C
+
Radiation Detectors
-
W. Udo Schröder, 2007
counter
gas
R
-
U0
+
Proportional Counter
gas
10
Rc
+
-
counter
gas
Anode Wire
Radiation Detectors
C
Voltage U0  (300-500) V
R
-
U0
+
RA
W. Udo Schröder, 2007
signal
Anode wire: small radius
RA  50 m or less
Field at r from wire
RI
E (r ) 
E(RI)=UI
RI
U0
1

ln( RC RA ) r
Avalanche RI RA, several mean
free paths needed
e- q+
Pulse height mainly due to
positive ions (q+)
Pulse Shape
t
event 1
Pulse shape : time t , wire length L
DU (t ) 
t0  e /  CU 0 , mobility   wdrift / E
DU
e  dielectric constant
event 4
DU
event 4
event 2
event 1
Radiation Detectors
11
event 2
C
DU
W. Udo Schröder, 2007
t
q
 ln(1 + )
t0
4e L
R
t
long decay time of pulse 
pulse pile up, summary information
differentiate electronically,
RC-circuitry in shaping amplifier,
individual information
for each event (= incoming particle)
Bragg-Curve Sampling Counters
Sampling Ion Chamber with divided anode
Radiation Detectors
12
DE1 DE2 Eresidual Anodes
isobutane
50T
DE
W. Udo Schröder, 2007
Sample Bragg
energy-loss curve
at different points
along the particle
trajectory
improves particle
x identification.
DE (channels)
ICs have excellent
resolution in E, Z,
A of charged
particles but are
“slow” detectors.
Gas IC need very
stable HV and gas
handling systems.
Energy resolution
Radiation Detectors
13
IC Performance
 e2  F nip  F
F<1 Fano factor
Eresidual (channels)
W. Udo Schröder, 2007
De
I
Solid-State IC
+
+
-
n
14
i
p
Solids have larger density  higher stopping
power dE/dx  more ion pairs, better
resolution, smaller detectors (also more
damage and little regeneration 
max accumulated dose ~ 1023 particles
c
+
Semiconductor n-, p-, i- types
DU(t)
Band structure of solids:
Si, Ge, GaAs,.. (for e-, lcp, g, HI)
Radiation Detectors
R
E
U0
e-
Capacitance Si :
2.2

C 
3.7
Conduction
EF
h+
rnU0 pF mm2
-
r pU0 pF mm2
Bias voltage U0 creates
charge-depleted zone
W. Udo Schröder, 2007
Valence
+
Ionization lifts e- up
to conduction band
 free charge
carriers, produce
DU(t).
Particles and Holes in Semi-Conductors
e
Conduction Band
eC 0
eF
15
Fermion statistics:
 2m 2 3 V

ne  e   
e   fe  e  V  volume
2 3
 2

 2m 2 3 V

nh  e   
e   fh  e  ne  nh !!
2 3
 2

e F  e C  e G 2   e G 2 for e C : 0
eeG
eV
h+
Valence Band
Occupation numbers f

 e + eG 2 
e : fe  e   1 + exp 

 kT


1

 e  eF
fe  e   1 + exp 
 kT

Radiation Detectors


 e + e G 2  
h : fh  e   1 + exp 

kT



W. Udo Schröder, 2007
1
 e + eG 2 


exp
  kT

kT 25meV e G


1
+
Small gaps eG (Ge) 
large thermal currents.
Reduce by cooling.



ne2
ne
 2m 2 3 V
 ne nh  
 2 2 3

rms
 e 
 exp   G 
 2kT 
2




 e 
e exp   G 
 kT 
 conductivity at T
Radiation Detectors
e- Potential Si Bloc
16
Semiconductor Junctions and Barriers
eo-+ o+ +o
Donor Acceptor
ions
n
p
Need detector with no free carriers.
Si: i-type (intrinsic), n-type, p-type by
diffusing Li, e- donor (P, Sb, As), or
acceptor ions into Si.
h+
o +o +o + +
+
-
- -o -o -o - o - o - o
o+ o+ +
o +
o +o +o + +
-
- -o -o -o - o - o - o
o+ o+ +
o +
o +o +o + +
-
- -o -o -o - o - o - o
space charge
o o o o o o
o o o o o o
d
Similar: Homogeneous n(p)type Si with reverse bias U0 also
creates carrier-free space dn,p:
up to 1mm possible.
W. Udo Schröder, 2007
Trick: Increase effective gap 
Junctions diffuse donors and acceptors
into Si bloc from different ends.
Diffusion at interface  e-/h+
annihilation  space charge
Contact Potential and zone depleted
of free charge carriers
Depletion zone can be increased by
applying “reverse bias” potential
dn, p  3.3  105 rn, pU0  m
rn, p
20 k  cm, U0  500V
 d  70  m
Surface Barrier Detectors
17
EF
Metal
Junction
CB Semi
conductor
Different Fermi energies adjust to on contact.
Thin metal film on Si surface produces space
charge, an effective barrier (contact potential)
and depleted zone free of carriers. Apply reverse
bias to increase depletion depth.
VB
Insulation
Metal film
Radiation Detectors
Insulating
Mount
Silicon wafer
depleted
dead layer
Ground
+Bias
Front: Au Back: Al
evaporated electrodes
W. Udo Schröder, 2007
Possible:
depletion depth ~ 300
dead layer dd  1
V ~ 0.5V/
Over-bias reduces dd
Metal case
Connector
ORTEC
HI detector
Charge Collection Efficiency
High ionization density at low electric fields: Edeposit > Eapp
Lower apparent energy due to charge recombination, trapping.
Low ionization density (or high electric fields): Edeposit  Eapp
Typical charge collection times: t ~ (10-30)ns
Pulse height defect
18
Moulton et al.
EPhD : Edeposit  Eapp
Fit :


a( Z , A)
EPhD Edeposit  10 b(Z , A)  Edeposit
a( Z )  2.230  10
5 2
Z + 0.5682
Radiation Detectors
b( Z )  14.25 / Z + 0.0825
W. Udo Schröder, 2007
a( A)  3.486  10
6 2
A + 0.5728
b( A)  28.40 / A + 0.0381
Affects charge collection
time  signal rise time.
Exploit for A, Z identification
Si-Strip Detectors
Radiation Detectors
19
Typically (300-500) thick.
Fully depleted, thin dead layer.
Annular:
16 bins (“strips”) in polar (q) ,
4 in azimuth (f) (Micron Ltd.)
Rectangular with 7 strips
W. Udo Schröder, 2007
Ge gray Detectors
Ge detectors for g-rays use p-i-n Ge junctions.
Because of small gap EG, cool to -77oC (LN2)
Radiation Detectors
20
Ge Cryostate (Canberra)
W. Udo Schröder, 2007
Ge cryostate geometries (Canberra)
Properties of Ge Detectors: Energy Resolution
Superior energy resolution,
compared to NaI
Radiation Detectors
21
DEg ~ 0.5keV @ Eg =100keV
W. Udo Schröder, 2007
Size=dependent mall detection
efficiencies of Ge detectors 
10% solution: bundle in 4arrays GammaSphere,Greta
EuroGam, Tessa,…
Townsend Gas Avalanche Amplification
_
22
Radiation
Nonlinear
Region
U0
M
IC Region
Radiation Detectors
d
+
U0~kV/cm
I
U0
Amplification M
W. Udo Schröder, 2007
M
n
1

i(t )dt ; nip  primary IP

nip
nip
 :  nM  d 1.Townsend coefficient
Avalanche Formation
Townsend Coefficient
Electron-ion pairs through
gas ionization
dn    n  dx
n( x)  n0  e  x for   const
Electrons in outer shells are more readily
removed, ionization energies are smaller
for heavier elements.
n( x)  n0  exp
  ( x)dx
x
0
Parallel Plate Counters: t-Resolution
cathode sensitive
layer
d~1/
24
eanode +
Charges produced at
different positions along
the particle track are
differently amplified.
 non-linearity nip(DE)
W. Udo Schröder, 2007
ff
ff
U
p
PPAC
+
PPAC
Radiation Detectors
R
PPACs for good time
resolution, U(p,f)f
Sparking and Spark Counters
/p
g
25
Different cathode
materials
Impact ionization
Probability g
+
d
-
Radiation Detectors
Amplification by
impact ionization
n
e d
M 
n0 1  g  e d  1
Prevent spark by reducing for ions:
collisions with large organic molecules 
quenching additives, self-quenching gases
W. Udo Schröder, 2007
Sparking :g  e d  1
p (101  103 ) Torr
26
Radiation Detectors
W. Udo Schröder, 2007
Avalanche Quenching
A. Sharma and F. Sauli, Nucl. Instr. and Meth. A334(1993)420
Radiation Detectors
27
in Argon
Reduce and
energy of ions by
collisions with
complex organic
molecules (CH4, …).
Excitation of
rotations and
vibrations already
at low ion energies
Organic vapors = “self quenching”
CH4
W. Udo Schröder, 2007
Multi-Wire Proportional Counters
Magic Gas: Ar(75%), isobutane (24.5%), freon (0.5%)
HV:kV/cm
Important
for detection of high-energy
particles, beam profile,..
(Charpak 1968-80)
28
Equipotential Lines
dac
Radiation Detectors
Anode Wires
s
Anode
Wires
Cathode
Wire
Planes
Field at ( x, y ) (0, 0)
C
  2
2 
V ( x, y )  U 0
ln  4  sin
x + sinh
4e  
s
s
2e
Capacitance C 
; d ac
 d ac s  ln( d s)
W. Udo Schröder, 2007

y 

s
Field strength close
to anode wires:
d
V(x,y)  1/r
Position-Sensitive Semiconductor Detectors
Gerber et al., IEEE TNS24,182(1977)
Double-sided x/y matrix
detector, resistive readout.
xn
Q1  Q 
Lx
29
R
y
R
Radiation Detectors
Q
x
R
R
~2000 cm, 300 U0160V
W. Udo Schröder, 2007
ym
Q3  Q 
Ly
(Lx  xn )
Q2  Q 
Lx
Q4  Q 
(Ly  ym )
Ly
Q1 + Q2  Q3 + Q4  Q  DE
Frisch Grid Ion Chambers
30
x
cathode
d
x0
particle
Radiation Detectors
dFG
0
Anode/FG signals out
W. Udo Schröder, 2007
Suppress position dependence
of signal amplitude by
shielding charge-collecting
electrode from primary
ionization track.
Insert wire mesh (Frisch grid)
at position xFG held constant
potential UFG.
e- produce signal only when
inside sensitive anode-FG
volume, ions are not “seen”.

De
DU  t  
w  t  t  tFG 
CdFG
not x dependent.
x-dependence used in “drift
chambers”.
Electronics: Charge Transport in Capacitors
Simple charge motion, no secondary ionization/amplification
q+
31
U
conducting
electrodes
Charges q+ moving
between parallel
conducting plates of a
capacitor induce tdependent negative images
q+ on each plate.
Radiation Detectors
q+
W. Udo Schröder, 2007
t Connected to circuitry,
q+
+
R e
Electronics
current of e- from negative
electrode is proportional to
charge q+.
Electron Transport
Multiple scattering/acceleration produces effective spectrum P(e)
 calculate effective  and :
2 e
  P e  
1

w 
E e 
d
e
D

  e   v  e   P  e  de



3m
e  v  e  
3
v e   2e m
32
Simulations
Radiation Detectors
w- ~ 103 w+
W. Udo Schröder, 2007
Electron Transport:
Frost et al., PR 127(1962)1621
V. Palladino et al., NIM 128(1975)323
G. Shultz et al., NIM 151(1978)413
S. Biagi, NIM A283(1989)716
http://consult.cern.ch/writeup/garfield/examples/gas/trans2000.html#elec
Radiation Detectors
33
Stability and Resolution
W. Udo Schröder, 2007
• Anisotropic diffusion in electric field (Dperp >Dpar).
• Electron capture by electro+negative gases,
reduces energy resolution
• T dependence of drift: Dw/w  DT/T ~ 10-3
• p dependence of drift: Dw/w  Dp/p ~ 10-3-10-2
• Increasing E fields
 charge multiplication/secondary+ ionization
 loss of resolution and linearity
 Townsend avalanches
Free Charge Transport in Matter
P(x)
1D Diffusion equation  P(x)=(1/N0)dN/dx
t0
34
x
x
P(x)
Radiation Detectors
t1 >t0
x
P(x)

x2 


 exp 
 Gaussian
4Dt

 4D  t 

N0
:
rms
x 2   x  2Dt
1
D   v 
3
D diffusion coefficient,
<v> mean speed
 mean free path
Thermal velocities :
t2 >t1
x
W. Udo Schröder, 2007
dN

dx
v 
8kT

m
D(ion)
8
3
D(e  )
v2
Maxwell+Boltzmann
velocity distribution
Small ion mobility
-
Solid-State Gamma Detectors
V
Radiation Detectors
35
+
W. Udo Schröder, 2007