Detectors in Nuclear and Particle Physics Prof. Dr. Johanna Stachel

Detectors in Nuclear and Particle Physics Prof. Dr. Johanna Stachel
Detectors in Nuclear and Particle Physics
Prof. Dr. Johanna Stachel
Department of Physics und Astronomy
University of Heidelberg
June 9, 2015
J. Stachel (Physics University Heidelberg)
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June 9, 2015
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5. Scintillation counters
5 Scintillation counters
Scintillators
Photon detection
Photomultiplier
Photodiodes
Propagation of light
Applications of scintillation detectors
J. Stachel (Physics University Heidelberg)
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Scintillation counters
5. Scintillation counters
detection of radiation by means of scintillation is among oldest methods of particle
detection
historical example: particle impinging on ZnS screen → emission of light flash
Principle of scintillation counter:
dE /dx is converted into visible light and transmitted to an optical receiver
sensitivity of human eye quite good: 15 photons in the correct wavelength range
within ∆t = 0.1 s noticeable by human
scintillators make multipurpose detectors; can be used in calorimetry, time-of-flight
measurement, tracking detectors, trigger or veto counters
Scintillating materials:
inorganic crystals
organic crystals
polymers (plastic scintillators)
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Scintillation counters
Scintillators
5.1 Scintillators
Inorganic crystals: crystal (electric insulator) doped with activator (color center) e.g. NaI(Tl)
energy loss can promote electron into
conduction band → freely movable in
crystal
conduction band
e- in conduction band
exciton band
activatorlevels
valence band
J. Stachel (Physics University Heidelberg)
exciton
also possible: electron does not reach the
conduction band; in this case it remains
electrostatically bound to the hole → ≡
‘exciton’
exciton moves freely through crystal →
transition back into valence band under
light emission inefficient process
hole in valence band
Detectorphysics
doping with activator (energy levels in
band gap) to which energy is transferred
→ photon emission can be much more
likely
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Scintillation counters
Scintillators
Inorganic crystals
exciton + activator A → A∗ → A + photon
or A + lattice vibration
typical decay time of signal: ns - µs depending on material
example:
NaI(Tl)
λmax
=
τ
X0
=
=
410 nm ∼
= 3 eV
0.23 µs
2.6 cm
quality of scintillator: light yield εsc ≡ fraction of energy loss going into photons
example:
for NaI(Tl)
εsc
J. Stachel (Physics University Heidelberg)
38000 photons with 3 eV per MeV energy loss (deposit in scint.)
3.8 · 104 · 3 eV
∼
= 11.3%
=
6
10 eV
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← good
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Scintillation counters
Scintillators
characteristics of different inorganic crystals
type
λmax [nm] τ [µs] photons per X0 [cm]
MeV
NaI(Tl)
CsI(Tl)
BGO (bismuth germanate)
BaF2 slow component
BaF2 fast component
CeF3
PbWO4
410
565
480
310
220
330
430
0.23
1.0
0.35
0.62
0.0007
0.03
0.01
38000
52000
2800
6300
2000
5000
100
2.6
1.9
1.1
2.1
2.1
1.7
0.9
advantages of inorganic crystals:
• high light yield
• high density →
good energy resolution for compact detector
disadvantage:
• complicated crystal growth
J. Stachel (Physics University Heidelberg)
→
$$$ several US$ per cm3
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Scintillation counters
Scintillators
application in large particle physics experiments
BaBar (SLAC):
6580 CsI(Tl) crystals
depth 17 X0
total 5.9 m3
readout Si photodiode (gain = 1)
noise 0.15 MeV
dynamic range 104
always need to consider: match of spectral
distribution of light emission, absorption
and sensitivity of photosensor
CMS (LHC):
76150 PbWO4 crystals
26 X0
total 11 m3
read-out APD (gain = 50)
noise 30 MeV
dynamic range 105
PMT sensitivity
Bialkali PMT
PbWO4 : fast, small radiation length,
good radiation hardness compared to other
scintillators, but comparatively few photons
(order of 10 photoelectrons per MeV)
J. Stachel (Physics University Heidelberg)
300
Detectorphysics
BGO
NaI
350
400
450
λ [nm]
500
Spectral intensity
typical spectral distributions:
550
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Scintillation counters
Scintillators
Organic crystals
scintillation is based on the delocalized π electrons of aromatic rings
(see below)
λmax [nm] τ [ns] light yield
rel. to NaI
naphthalene
anthracene
J. Stachel (Physics University Heidelberg)
348
96
12%
440
30
50%
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Scintillation counters
Scintillators
Plastic scintillators
polymer + scintillator + possibly wavelength shifter or liquid + scintillator + wavelength shifter
Polymers (transparent)
H
polystyrene
C
C
H
H
lucite (plexiglas)
polyvinyltoluene
Liquid (transparent): benzene, toluene, mineral oil
λmax [nm] τ [ns] εsc
Scintillators
p-Terphenyl
5
360
1
25%
N
N
PBD
440
O
low light yield: in plastic scintillator typically 1 photon per 100 eV energy loss
low radiation length X0 = 40 − 50 cm, fast decay time (order of) ns, cheap, easy to shape
- typically also high neutron detection efficiency via (n,p) reactions
J. Stachel (Physics University Heidelberg)
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Scintillation counters
Scintillators
primary fluorescent structure
agent
λmax decay time light yield
emission
[ns]
rel. to NaI
[nm]
naphtalene
348
96
0.12
anthracene
440
30
0.5
p-terphenyl
440
5
0.25
360
1.2
420
1.6
420
1.2
N
N
PBD
O
wavelength shifter
N
POPOP
CH3
bis-MSB
N
O
O
CH=CH
CH=CH
CH3
what does wavelength shifter do?
it absorbs primary scintillation light and reemits at longer wavelength
→ good transparency for emitted light
adapts wave length to spectral sensitivity of photosensor
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Intensity [arb. units]
Scintillation counters
primary
scintillation
Scintillators
wavelength
shifter
1.0
different
BBQ Emission
wavelength
BBQ Absorption
shifter
PBD Emission
POPOP Emission
0.5
0
200
300
400
500
600
700
Wavelength λ [nm]
emission spectra of primary fluorescent substance (PBD) and of two different wavelength shifters
(BBQ and POPOP)
and absorption spectrum a wavelength shifter
J. Stachel (Physics University Heidelberg)
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Scintillation counters
Scintillators
principle of operation of organic scintillator: aromatic molecules with delocalized π-electrons
valence electrons pairwise in π states, level scheme splits into singlet and triplet states
Triplet states
S2
T1
S1
T0
3- 4 eV
γ
S0
Singlet states
fluorescence
10-8 s
phosphorescence
10-3 s
excitation of π electrons
energy absorption → S1∗ , S2∗ → S1
radiationless on time scale 10−14 s
fluorescence: S1 → S0
ionization of π electrons followed by recombination populates T states
phosphorescence T → S0
excitation of σ-electrons → thermal deexcitation, radiationless, collisions and phonons
other ionization → radiation damage
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Scintillation counters
Scintillators
material transparent for radiation with Eγ < S10 − S00
Stokes shift due to Franck-Condon principle
typical
Absorption
Emission
λ
electronic excited state
E
ground state
A
F
excitation on time scale 10−14 s
typical vibration time scale 10−12 s
typical S1 lifetime 10−8 s
excitation into higher vibrational state
deexcitation from lowest vibrational state
nuclear distance
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Scintillation counters
primary fluorescent
good light yield
absorption spectrum
needs to be matched to
excited states in base
material
in base material energy deposit
→ excitation
generally bad light yield
transfer of excitation to primary
fluorescent
base material A
Scintillators
primary fluorescent
agent B
depending on material,
a secondary fluorescent
(wavelength shifter) is
introduced to separate
emission and absorption
spectrum (transparency)
secondary fluorescent
agent C
wave length shifter
excitation
SIA
EIA
γA
S0A
J. Stachel (Physics University Heidelberg)
SIB
EIB
S0B
SIC
γB
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EIC
γC
S0C
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Scintillation counters
Scintillators
Scintillating gases
- many gases exhibit some degree of scintillation
λmax [nm] γ/4.7 MeV α
N2
He
Ar
390
390
250
800
1100
1100
contributes in gas detector to electric discharge
careful in Cherenkov detectors!
Pierre Auger Observatory for cosmic ray induced air showers: employs water Cherenkov detectors
and fluorescence detectors to observe UV fluorescence light emitted by atmospheric nitrogen (up
to 4W at maximum of cascade)
- liquid noble gases: lAr, lXe, lKr also scintillate
in UV (120-170 nm), good light yield (40 000 photons per MeV),
fast (0.003 and 0.022 µs)
usage in (sampling) calorimeters
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Scintillation counters
Scintillators
5.2 Photon detection
5.2.1 Photomultiplier
i) photo effect in photocathode:
γ + atom → atom+ + e −
Te = hν − W
W : work function, in metals 3 − 4 eV,
bad! comparable to energy of scintillation photon
⇒ specially developed alloys (bialkali, multialkali) with W = 1.5 − 2 eV
figure of merit: quantum yield
Q=
#photoelectrons ∼
= 10 − 30%
#photons
Threshold of some photosensitive materials
TMAE,CsI
UV
visible
100
4.9
250
multialkali
bialkali
TEA
12.3
GaAs ...
E (eV)
3.1
2.24
1.76
400
550
700
λ (nm)
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Scintillation counters
Scintillators
typical spectral sensitivity
cut-off at small wavelength: glass window can be replaced by quartz, extending range to smaller
wavelengths (see e.g. fast component of light of BaF2 )
quantum efficiency η [%]
25
bialkali
20
15
10
5
0
300
350
400
450
500
wavelength λ [nm]
550
spectral sensitivity (quantum efficiency) of a bialkali (SbKCs)
photocathode as a function of the wavelength
also used:
- SbRbCs
- SbCs
- SbNa2 KCs (multialkali)
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Scintillation counters
photocathode
focussing
electrode
Scintillators
vacuumtight
glass tube
dynodes
anode
voltage divider
- U0
working principle of a photomultiplier electrode system mounted in an evacuated glass tube
photomultiplier usually surrounded by a mu-metal cylinder (high permeability material) to shield
against stray magnetic fields (e.g. the magnetic field of the earth)
ii) multiplication of photoelectrons by dynodes
- electrons are accelerated towards dynode
- knock out further electrons in dynode
# leaving e −
secondary emission coefficient δ =
# incident e −
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Scintillation counters
typically
# dynodes
δ = 2 − 10
n = 8 − 15
Scintillators
G = δ n = 106 − 108
δ dependent on dynode potential difference:
δ = k · UD
G = a0 (kUD )n
operational voltage UB = nUD
a0 : collection efficiency between cathode and first dynode
dynodes connected via resistive divider chain
dG
dUD
dUB
=n
=n
G
UD
UB
Limitations in energy measurement
linearity of PMT: at high dynode current possibly saturation by space charge effects
IA ∝ nγ for 3 orders of magnitude possible
photoelectron statistics for mean number of photoelectrons ne given
by Poisson distribution
nen exp (−ne )
Pn (ne ) =
n!
with good PMT, observation of single photoelectrons possible
photoelectron statistics for a given energy loss dE /dx respectively Eγ defined by
ne =
dE
photons
×
× light collection efficiency × quantum efficiency
dx
MeV
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Scintillation counters
Scintillators
e.g. in NaI for 10 MeV incident photon:
ne
=
10 MeV ×
ne
ne
=
0.8%
√
38000
× 0.2 × 0.25 = 15000
MeV
fluctuations of secondary electron emission at mean multiplication factor δ (again Poisson)
δ n exp(−δ)
Pn (δ) =
(n!)
for Poisson with mean hni = δ
variance
contribution to resolution
σn2 = hni = δ
σn
1
= √
hni
δ
N stages of dynodes which each amplify by factor δ:
σn 2
1
1
1
1 − δ −N ∼ 1
=
+ 2 + ... + n =
=
hni
δ
δ
δ
δ−1
δ−1
σn
1
= √
dominated by first stage
hni
δ−1
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Scintillation counters
Scintillators
Pulse shape:
I
U(t)
ideal current source with parallel resistance R and capacitance C
light incident with decay time of scintillator τsc
Nγ = N0 exp (−t/τsc )
anode current
Gne e
exp (−t/τsc ) = I0 exp (−t/τsc )
τsc
Z
Q=
I dt = I0 τsc = Gne e
I (t) =
I (t) =
→ voltage signal
U(t)
dU(t)
+C
R
dt
(with U(t = 0) = 0)
Q ·R
U(t) =
τ − τsc
J. Stachel (Physics University Heidelberg)
t
t
exp −
− exp −
τ
τsc
Detectorphysics
τ = RC
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Scintillation counters
Scintillators
2 possible realizations (limiting cases) optimized for i) pulse height or ii) timing:
i) RC τsc
Q
t
t
U(t) =
exp −
− exp −
C
τ
τsc
 Q
t


1 − exp −
τ t

C
τ
sc
=
t
Q


t τsc
 exp −
C
τ
rising edge of pulse characterized by τsc linear in t
pulse length characterized by τ = RC
Umax ∼
= Q/C ∝ Nγ
ii) RC τsc
U(t)
=
=
energy measurement
t
τ Q
t
exp −
− exp −
τsc C
τsc
τ

Q
t
τ


1 − exp −
t τsc

τsc C
τ
Q
t
τ



exp −
tτ
τsc C
τsc
rising edge of pulse given by small RC , again linear in t
decay of pulse given by τsc
sensitivity to Q/C weakened by small RC
J. Stachel (Physics University Heidelberg)
→
Detectorphysics
→
time measurement
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Scintillation counters
Scintillators
time resolution given by:
- rise time of signal (order 1 − 2 ns)
- transit time in photomultiplier (order 30 − 50 ns)
respectively, variations in transit time (order 0.1 ns for good PMT)
transit time variations via
- path length differences cathode - first dynode
∆t ∼
= 1 ns
5 ns
for cathode ∅ 10 cm
∅ 50 cm
hence spherical arrangement for very large PMTs
(e.g. 20” in Superkamiokande)
- energy spread of photoelectrons when they leave the photocathode
timing difference for photoelectron accelerated from rest
(Te = 0) relative to one with Te
√
2mTe
∆t =
eE
therefore maximize potential difference between cathode and
first dynode, e.g.
Te = 1 eV
E = 200 V/cm
J. Stachel (Physics University Heidelberg)
→
∆t = 0.17 ns
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Scintillation counters
Scintillators
strong reduction of pathlength difference:
“micro channel plate”
arrangement of 104 − 107 parallel channels
(glass tubes)
of 10 − 50 µm diameter, 5 − 10 mm length
electric field inside by applying voltage to one end
(∼ 1000 V) and coated inside with resistive layer
acting as a continuous dynode
realization: holes in lead glass plate
G = 105 − 106
∆t = 0.1 ns
further advantage: can be operated inside
magnetic field
difficulty: positive ions created by collisions with
rest gas inside channel must be prevented from
reaching photo cathode (otherwise death of MCP)
→ extremely thin (5 − 10 nm) Al window between
channel plate and photocathode
J. Stachel (Physics University Heidelberg)
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Scintillation counters
Scintillators
characteristics for several commercially available
PMTs and microchannel plates
Amperex
XP 2020
RCA
8854
amplification
> 3 · 107 3.5 · 108
2200
2500
HV anode-cathode (V)
microchannel voltage (V)
rise time τR (ns)
1.5
3.2
transit time τT (ns)
28
70
0.51
1.55
transit time variation τS , one PE
transit time variation τS0 , many PEs
0.12
number of PEs for transit time τS0 meas. 2500
quantum yield (%)
26
27
photocathode diameter (mm)
44
114
Cu Be GaP/BeO
dynode material
J. Stachel (Physics University Heidelberg)
Detectorphysics
Hamamatsu ITT Hamamatsu
R 647-01 F 4129 R 1564U
> 106
1000
2
31.5
1.2
0.40
100
28
9
1.6 · 106
5 · 105
2500
0.35
2.5
0.20
0.10
800
20
18
3400
0.27
0.58
0.09
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Scintillation counters
Scintillators
time resolution influenced by transit time variation and dimensions of scintillator
(timing variation of light collection):
300
long scintillators
2 m length
2 - 5 cm thickness
20 - 40 cm width
time resolution σt
ps
200
rise time ~ 300 ps
rise time ~ 600 ps
100
short scintillators
1 cm
computed
0
500
1000
ps
PM transit time variation
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Scintillation counters
Scintillators
time variations by different light paths in scintillator:
I
without
filter
6
" near "
10 cm
4
affect both time resolution and pulse height
typical attenuation length about 1 m
attenuation mostly at short wavelengths
with filter
2
⇒ use of yellow filter reduces dependency
0
400
440
480
[nm]
520
430 nm
1.8 m
I
15 cm
without
filter
6
5 mm
4
" far "
170 cm
with filter
2
also: read-out of long scintillator at both ends
reduces both timing variations and spatial
dependence of pulse height
J. Stachel (Physics University Heidelberg)
Detectorphysics
0
400
440
480
520
[nm]
430 nm
amplitude distribution with and without
yellow filter in front of cathode
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Scintillation counters
Scintillators
Photomultipliers in magnetic field
B-field disturbs focusing of photoelectrons and secondary electrons
typical kinetic energies T ≤ 200 eV
in region of dynodes: B ≤ 10−4 T needed
typical magnitude of effect: B = 0 → 0.15 · 10−4 T means IA →
1
I
2 A
solution: small fields can be shielded by so-called µ-metal
~ and B
~ parallel)
use of mesh-type dynodes (E
use of channel plate or photodiodes
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Scintillation counters
Scintillators
5.2.2 Photodiodes
normal photodiode: PIN type gain = 1,
i.e. each photoelectron contributes 1 e to final signal (see chapter 4)
avalanche photodiode (APD): typical gain = 30 − 50 (CMS EMCal)
amplification of photocurrent through avalanche multiplication of carriers in the junction region
(high reverse bias voltage, 100-200 V)
U
E
p ++
p
SiO2 window
photon conversion
acceleration
n
gain
intrinsic, only electron drift
i
n ++
Al
x
0
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Scintillation counters
Propagation of light
5.3 Propagation of light
in scintillator itself:
• absorption Nγ = N0 exp(−x/L)
with L: absorption length
• reflection at the edge, total reflection for
θ > θtot = arcsin(n0 /ns )
in typical scintillator n ∼
= 1.4, θtot ∼
= 45◦
light guide
- the light exiting the scintillator on one end (rectangular cross section) needs to be
guided to PMT (normally round cross section) ⇒ ‘fish tail’ shape
photomultiplier
scintillator
J. Stachel (Physics University Heidelberg)
light guide
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Scintillation counters
Propagation of light
Light guide
Liouville theorem is valid also for guiding light:
∆x · ∆θx = const.
i.e. product of width and divergence is constant
for guiding light ∆θ = const,
∆x must not decrease, otherwise loss of light,
so keep area constant
curvature should only be weak to maintain
total reflection for photons captured once
(adiabatic light guide)
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Scintillation counters
Propagation of light
Wavelength shifter
when enough light: can use 2nd wavelength shifter, e.g. along edge of scintillator plate,
wavelength shifter rod absorbs light leaving scintillator and reemits isotropically at (typically)
green wavelength, small part (5 − 10%) is guided to PMT
advantage: can achieve very long attenuation length this way, correction small
BBQ
green
PM
UV emission
air gap
blue (λmax = 420 nm)
400
300
200
50
100
150
length of wavelength shifter [cm]
light absorption in 3 mm thick BBQ
wavelength shifter rod:
better uniformity of light collection by
giving up shorter wavelength component
(yellow filter)
scintillator
J. Stachel (Physics University Heidelberg)
without filter
with filter
500
0
POPOP
ionizing particle
relative light intensity
600
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Scintillation counters
Applications of scintillation detectors
5.4 Applications of scintillation detectors
time-of-flight measurement, 2 scintillation counters (read-out on both ends) at large enough
distance
precise photon energy: crystal calorimeter
sampling calorimeter for photons and hadrons: alternating layers of absorber (Fe, U, . . .)
and scintillator with wavelength shifter rods and PMTs
scintillating fibre hodoscope: layers of fibres, diameter order 1 mm or less, precision
tracking, fast vertexing
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Scintillation counters
Sampling calorimeter
Applications of scintillation detectors
(see Chapters 8/9)
to photo detector
typically enough light available and uniformity of
response and linearity more important
light emerging from end of scintillator sheet
absorbed by external wavelength shifter rod and
reemitted isotropically
scintillator
optical fiber
in machined
groove
wavelength shifter rods can be replaced
by wavelength shifting scintillating
fibers embedded into scintillator sheet
or directly into absorber
air gap essential for total internal reflection
only a few % of energy loss in light
photomultiplier
wavelength shifter/
lightguide
wavelength
shifter 2
absorber
scintillator
wavelength shifter 1
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Scintillation counters
Applications of scintillation detectors
Scintillating fibre hodoscopes
follow track of a charged particle in fine steps but not in gas detector
60 µm fibre in a fibre bundle covered with
cladding of lower n, single track resolution
few tens of µm
track in scintillating fibre array,
fibre diameter 1 mm
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Scintillation counters
Applications of scintillation detectors
Example: Scintillation fibre hodoscope COMPASS at CERN SPS
cover beam area of a 100 − 200 GeV muon beam, 108 Hz or 106 Hz per fiber channel
J. Bisplinghoff et al., NIM A490 (2002) 101
to provide enough photoelectrons 4 layers
of fibres of 1 mm diameter
fibres in each column joined to same PMT
pixel of a multianode PMT
→ 30 photoelectrons per muon
fibre configuration for scintillating fibre
hodoscope with 3 layers of fibers
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Scintillation counters
Applications of scintillation detectors
SCSF-78MJ scintillating fibers, 1.5 m attenuation length, active area about 10 × 10 cm2 ,
then light guides of clear fibers 1.5 m long (attenuation length 4 m) to PMT
high radiation tolerance (important for
beam hodoscope): 100 kGy (10 Mrad) lead
to only 15% reduction of signal.
light output of Kuraray SCSF-78MJ scintillating fibers after local irradiation (≈
100 kGy), as indicated by shaded vertical
bars
light attenuation of light guides (clear fibers
PSMJ, Kuraray Corp.), as measured before (solid squares) and after (open squares)
about 10 kGy of irradiation (more than 10
times what is expected for beam halo), homogeneously applied across the entirely of
their length.
attentuation length of lightguide drops from
4 m to 1.2 m
J. Stachel (Physics University Heidelberg)
Detectorphysics
June 9, 2015
37 / 304
Scintillation counters
Applications of scintillation detectors
30.0 ∓ 0.5
25.7 ∓ 0.5
’price’ for light-saving use of clear fibers:
an additional joint → glue
4 . 16
45 PITCH
glue not radiation hard (yellows)
→ needed to learn to ’fuse’ fibers
FILLED WITH
INSULATOR
Top View
H6568 MA-PMT: equipped with a common
photocathode followed by 16 metal channel dynodes
each with 12 stages of mesh type and a multi-anode
read-out. They are arranged as a 4 × 4 block (individual
effective photocathode pads with an area of 4 mm ×
4 mm each and a pitch distance of 4.5 mm (see figure).
0.8 MAX
30.0 ∓ 0.5
17.5
PHOTOCATHODE
45 ∓1
Hamamatsu 16-anode PMT was a breakthrough in gain
uniformity and cross talk
POM CASE
figure: layout and dimensions of the multi-channel photomultiplier tube H6568. The upper part shows the front
view of the cathode grid.
450
- hV
: RG-174/U (RED)
ANODE OUTPUT
: COAXIAL CABLE
ANODE INDICATION
Side View
J. Stachel (Physics University Heidelberg)
Detectorphysics
June 9, 2015
38 / 304
Scintillation counters
Applications of scintillation detectors
noise only 1/5 of single photoelectron
response (SER)
low cross talk (less than 5 %)
good gain uniformity (about 20 %)
voltage divider for dynodes needs to be
specifically designed to be stable at rates up
to 100 MHz
‘active base’ (use of transistors instead of
resistors for last stages) instead of simple
voltage divider, otherwise drop of signal
with rate due to large currents through last
dynodes leading to drop of interstage
voltage
achieved time resolution 330 ps
J. Stachel (Physics University Heidelberg)
Detectorphysics
June 9, 2015
39 / 304
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