Transcript PMTs & APDs, Components Of An "Optical" SXR

PMTs & APDs, Components Of An "Optical" SXR
Diagnostics For MFE Plasmas
Luis F. Delgado-Aparicio V.
The Plasma Spectroscopy Group
The Johns Hopkins University
([email protected])
Previous Talks At The Seminar
• 1st year seminar:
A Bit Of Plasma Physics And Fusion.
• 2nd year seminar (Fall):
USXR - SXR Diagnostics For Magnetically Confined Fusion
Plasmas.
How do we built these
diagnostics?
What kind of signals do we obtain?
Where and how do we use it?
What kind of data we get?
• 2nd year seminar (Spring) and paper:
PMTs & APDs, Components Of An "Optical" SXR
Diagnostics For MFE Plasmas
In What Fields Of P&A are PMTs and APDs Useful?
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UV/Visible/IR Spectrophotometers
Atomic Absorption Spectrophotometers
Photoelectric Emission Spectrophotometer
Fluorescence Spectrophotometer
Raman Spectrophotometer
Liquid or gas Chromatography
X-ray Diffractometer & Fluorescence Analyzer
Electron Microscope
2.
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Mass Spectroscopy & Solid Surface
Analysis
3.
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Cell Sorter
Flurometer
Biotechnology
Environmental Monitoring
Dust Counter
Turbidimeter
NOx and SOx meters
4.
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Solid Surface Analysis with narrow beam of e-,
ions, visible light and X-Rays
3.
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Spectroscopy
Gamma Camera
Positron CT
Liquid ScintillationCounter
In-Vitro Assay
X-Ray Phototimers
5.
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Radiation Measurement
Area Monitors
Survey Meters
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Medical Applications
Photography and Printing
Color Scanner
In What Fields Of P&A are PMTs and APDs Useful?
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High Energy Physics
Hodoscope (look at note).
TOF Counters.
Cherenkov Counters.
Calorimeters.
Neutrino Experiments.
Neutrino and Proton Decay Experiment.
Air Shower Counter.
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Astrophysics
Measurement of X-Rays from Outer Space.
Measurement of Scattered Light from Fixed
Stars and Interstellar Dust.
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Laser
Laser Radar.
Fluorescence Lifetime Measurement.
10.
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Plasma
Plasma measurements: Te, ne.
Thompson Scattering.
Doppler Shifting effects.
Impurity transport.
Impurities and ion control.
X-Rays analysis.
Collisional, LIF and normal modes on
MSE.
Plasma Transport & Turbulence !!!
Note: PMTs are coupled to the ends of long,
thin plastic scintillators arranged orthogonally in
two layers. They measure the time and position
at which charged particles pass through the
scintillators!
Remember the “Optical” SXR system ?
Benefits!
Description
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Compactness & portability (size
issue).
UHV fiber optic window.
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Optically thin to neutron
bombardment.
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Multi-clad, hi numerical aperture
(NA  1) fiber optics.
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 5 - 10% of the cost of the solid
state diode system.
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PMT!
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Electronics far away of the
machine (tokamak).
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Current amplifiers (106 - 1011
V/A).
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Conservation of the initial photon
statistics along the detection chain
due to scintillation properties.
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Scintillator.
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APD!
Remember the components of the SXR Array?
1.
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6.
A scintillator with an appropriate
conversion efficiency (CE) response
(previous graphs).
0.3 m Ti and 0.5 - 5 m Be foil to
filter out visible light and some
USXR!
Columnar (crystal growth) CsI:Tl film
( 30 m thickness) deposited on a
2mm fiber optic plate (FOP, NA  1).
40 mm in diameter,  1/3” thickness
UHV fiber optic window (FOW)
mounted in a 6” flange.
Hi throughput, multi-clad, nonscintillating, 1.5 m long fiber optics
(NA  0.7 T  40%).
Femto current pre-amplifiers (104 1011 V/A).
7.
Bi-alkali (Sb-Rb-Cs, Sb-K-Cs), multianode (16 channels), low cross talk,
high gain ( 106) Hamamatsu photomultiplier tube (PMT). Allow photon
counting experiments. Magnetic
shielding is necessary!
8.
Advanced Photonix’s large area
avalanche photodiode (APD). One
channel, 5 mm in diameter,
modified, hi quantum efficiency (
90%), high internal gain ( 300),
internally amplified, cooled (-20 to 0
oC) module.
PMT vs. APD!!! (price & performance)
What is a PMT (Photomultiplier Tube)?
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Is one of the most versatile and “efficient”
devices in the market with high sensitivity
and ultra-fast response to low incident
light levels and single photon counting!
• It mainly consist of a photoemissive
cathode (photocathode) followed by focusing
electrodes, an electron multiplier section and
an anode (electron - current collector).
PMT Construction Characteristics
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Multiplication scheme
Emission of photoelectrons into the
i.
vacuum (P  10-4 Pa  10-6 Torr).
Electrostatic focusing of the e- towards ii.
the electron multiplier stage.
Electron secondary emission at each
collision with the dynodes.
The multiplied e- are collected by the
iii.
anode  output signal.
The PMT provides:
1. High sensitivity
2. Exceptional low noise
3. Fast time response for photons
from UV (300 nm) to NIR (800 nm)
Hamamatsu H6568-10
44 multi-anode array, metal package,
head-on type PMT.
The e- multiplier has n = 12 dynodes. It
is a metal channel type  unique ability
to deliver a high speed response due to
narrow space between dynodes. Less effect
from an external magnetic field!!!
The bialkali photocathode (A = 18.118.1
mm2) and it is made out of Sb-Rb-Cs & SbK-Cs. Favorable blue-green sensitivity for
scintillator flashes from NaI:Tl (CsI:Tl).
PMT Optic, EM & Thermal Characteristics (I)
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Cathode Radiant Sensitivity (SC)
SC 
I
P
incident

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
phot oel ectric
 e 
e N e t
    QE
N p h t hc 
nm  QE

C
S
1242
Anode Radiant Sensitivity (SA)
S  PI
A

I phot oel ectric

 G  


 Pincident 
incident
A node
SA 
Hamamatsu’s H6865-10 (HV = 800 V)
i.
QE  5% (for  = 550 - 560 nm)
ii. Sc  22.33 mA/W
iii. SA  2.23104 A/W
If I assume 10 pW of
visible light power we will
have 2.2310-7 A
 nm  QE  G
1242
Final Test Sheet
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Tungsten Filament Lamp (2856 K)
HV = 800 V
SC = 66.66 mA/W
SA = 6.86  104 A/W
 G=KVn  1.03  106
5.
Anode Dark Current = 0.47 nA
Gamp = 107 V/A signal = 2.23 V
PMT Optic, EM & Thermal Characteristics (II)
What voltage (Gain) should
I use to avoid burning out the
photocathode?
350
1.E+04
300
250
200
1.E+03
150
100
Current amplifier at
107
50
V/A
1.E+02
0
300
400
500
600
Voltage (V)
Sampling rate: 400 kHz
0.7
0.75
0.8
Anode Radiant Sensitivity (A/W)
Assuming a 1 - 5 nW of visible
light power per channel, we
should work with < 500 Volts!
450
400
Gain (G=SA/SC)
Take a look at the PMT’s
specs (maximum ratings)
 625 nA per channel
1.E+05
PMT Optic, EM & Thermal Characteristics (III)
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Thermoionic emission
Ionization of residual gases
Glass scintillation
Field emission when V  Vmax
Linear function of the supply voltage.
Remember G = KVn ? V should be
very stable and provide minimum ripple!
RECOMMENDATIONS:
Measure Id after 30 min storage in dark!
Cooling the photocathode especially in
application such as photon counting!
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iii.
Dark Current (Id)
Uniformity & Cross Talk
Important for multianode PMT!
Minimum normalized anode uniformity
of 57 %.
Maximum anode cross talk of 0.5%
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Magnetic Field Shielding
Important factor of loss of Gain (G < 0).
If not properly aligned and shielded the
photoelectrons will deflect from their original
trajectories.
The head on types (i.e. H6865-10) tend to be
more adversely influenced.
H
H
out
in

3t
 8370
4D
: magnetic permeability  500000
t: thickness  0.05”
D: inner diameter of the shield case  2.24”
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The shielding should cover as long as half of
the PMT’s diameter on both sides.
BNSTX  1 kG  extra shielding (1/4” iron)
Problem of installation in NSTX!!!
PMT’Drift - Life and Time Response Characteristics
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Drift - Life Time
The change in the anode output current
for small (long) periods of time.
It can be in the order of 5 - 50% for 1 104 h.
The major concern is caused by the
damage to the last dynode due to heavy ebombardment.
DO NOT EXCEED THE MAXIMUM
RATINGS (625 nA per channel)!!!
If stability (i.e. photon counting) is a
prime consideration, do not exceed 62.5
nA!
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Time Response
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Electron Transit Time (ETT).- Defined as
the interval between of a delta function-type
light pulse at the photocathode and the
peaking time of the anode output.
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The fluctuation of the ETTs between
individual light pulses is called the Transit
Time Spread (TTS) and it is defined as the
FWHM of the frequency distribution of the
ETTs at a single photoelectron event.
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Anode Pulse Rise Time.- Is the time the
signal rises from 10% to 90% of the pulse’s
peak amplitude
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For the H-6568-10, the APRT and TTS are
0.83 and 0.3 ns!!!
Before APDs, PIN Diodes
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“Mature”, low cost, high quantum
efficiency, compact and reliable!
Fabricated from semiconductors,
Silicon (Si) and/or Gallium Arsenide
(GaAs).
 ranges: 250 - 1100 nm (Si) and 800
nm - 2m (GaAs).
Production of a single pair of charge
carriers, electrons (e-) and holes (h).
AIM: Collect the photon induced
charge carriers as signals before they
recombine  Natural Gain: G = 1
XUV photons: about 3.7 eV required
to generate an e- - hole pair.
Collection of charge: possible due to a
“pn” or “pin” diode junction structure.
p-type semiconductor material: doped
to produce an excess of holes.
n-type semiconductor material: have
an excess of e-.
•Junction: Steep concentration of gradients causing the
e- to diffuse into the p-layer and the holes to diffuse into
the n-layer.
•This ambipolar-type diffusion in the “depletion
region” region result in an electric field often referred
as the “internal bias”.
•The voltage/current measured is linear with the
incident light flux.
•ONE LIMITATION: Lack of internal Gain.
EXAMPLE: Silicon p-n Photodiodes (AXUV Series)
E
QE 
photon
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For the regions of interest the QE is fairly
linear!
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The only QE lost is due to the front (3 to 7
nm) silicon dioxide window at
wavelengths for which absorption and
reflection are not negligible (8 - 100 eV).
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Insensitive to external magnetic fields!
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Responsivity: 0.05 - 0.3 A/W
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Successfully used in the European SOHO
and American SNOE missions!
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Used at the Ring Accelerator Experiment
(RACE) at LLNL and several fusion
related laboratories around the world.
(eV)
3.7
What is an APD (Avalanche Photodiode) ?
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The avalanche photodiode combines the
benefits of both the PIN diodes and the PMT:
high quantum efficiency and internal gain!
  0.3 - 16 mm
Gains up to 50 - 350. Higher = ?
Previously used in telecommunications,
scintillation detection, PET, experimental low
and high energy physics and nuclear
medicine.
High reverse external bias voltage (1 - 2 kV)
creates a strong field that accelerates the eproducing secondary e- by impact ionization
 50 < G < 1000!
The majority of APDs have a QE  70 - 90%,
G  102 and Id  2 - 200 nA.
We will work with the Beveled-edge LAAPD
modules from Advanced Photonix.
What about the Advance Photonix197-70-74-591 APD?
•  = 5mm active area diameter
• Blue enhanced large area cooled
APD
• For   555 nm, QE  80% and
the responsivity  110 A/W (for
G = 300, V = 1.7 - 2.0 kV).
• Id  6 -18 nA
• For   555 nm the rise time is
in the order of 10 -15 ns.
What about the APD 197-70-74-661 OEM module?
• Built in the previous LAAPD.
• Built in TEC, TTEC  0 oC.
• Sensitivity at 1MHz &   555 nm:
11105 V/W.
• Built in current amplifier (104 V/A)!
With custom made modifications!
1. Cutoff  600 kHz
2. Sensitivity  11107 V/W.
3. Built in current amplifier (106 V/A)!
We still need an increase of one/two
orders of magnitude!
 Reduce TTEC or change the amplifier!
Preliminary PMT Results (CDX-U)
DA = Diode Array
OA = Optical Array
SGI = Supersonic Gas
Injector
Present & Future Work (Lab & NSTX)
• CsI:Tl time response study at low and high energies (USXR-SXR). Is the
crystal nature of the deposition giving us one and only one time response
characteristic?
• Correlation with JHU re-entrant SXR array in NSTX!
• Neutron bombardment comparison between OA and DA!
• Comparison between PMT and APD!
• Do some physics (MHD, De & e)!
• PUBLISH OR PERISH!