PMTs & APDs, Components Of An "Optical" SXR Diagnostics For MFE Plasmas

Luis F. Delgado-Aparicio V.The Plasma Spectroscopy Group

The Johns Hopkins University(delgapa@pha.jhu.edu)

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.

• 2nd year seminar (Spring) and paper:

PMTs & APDs, Components Of An "Optical" SXR Diagnostics For MFE Plasmas

How do we built these How do we built these diagnostics?diagnostics?

What kind of signals do we obtain?What kind of signals do we obtain?

Where and how do we use it?Where and how do we use it?

What kind of data we get?What kind of data we get?

In What Fields Of P&A are PMTs and APDs Useful?

1. Spectroscopy• 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. Mass Spectroscopy & Solid Surface Analysis

• Solid Surface Analysis with narrow beam of e-, ions, visible light and X-Rays

3. Biotechnology• Cell Sorter• Flurometer

3. Environmental Monitoring• Dust Counter

• Turbidimeter

• NOx and SOx meters

4. Medical Applications• Gamma Camera

• Positron CT

• Liquid ScintillationCounter

• In-Vitro Assay

• X-Ray Phototimers

5. Radiation Measurement• Area Monitors

• Survey Meters

6. Photography and Printing• Color Scanner

In What Fields Of P&A are PMTs and APDs Useful?

7. High Energy Physics• Hodoscope (look at note).• TOF Counters.• Cherenkov Counters.• Calorimeters.• Neutrino Experiments.• Neutrino and Proton Decay Experiment.• Air Shower Counter.

8. Astrophysics• Measurement of X-Rays from Outer Space.• Measurement of Scattered Light from Fixed

Stars and Interstellar Dust.

9. Laser• Laser Radar.• Fluorescence Lifetime Measurement.

10. 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 intwo layers. They measure the time and positionat which charged particles pass through thescintillators!

Remember the “Optical” SXR system ?

Description

• Scintillator.

• UHV fiber optic window.

• Multi-clad, hi numerical aperture (NA 1) fiber optics.

• PMTPMT!

• Current amplifiers (106 - 1011 V/A).

• APDAPD!

Benefits!

• Compactness & portability (size issue).

• Optically thin to neutron bombardment.

5 - 10% of the cost of the solid state diode system.

• Electronics far away of the machine (tokamak).

• Conservation of the initial photon statistics along the detection chain due to scintillation properties.

Remember the components of the SXR Array?

1. A scintillator with an appropriate conversion efficiency (CE) response (previous graphs).

2. 0.3 m Ti and 0.5 - 5 m Be foil to filter out visible light and some USXR!

3. Columnar (crystal growth) CsI:Tl film ( 30 m thickness) deposited on a 2mm fiber optic plate (FOP, NA 1).

4. 40 mm in diameter, 1/3” thickness UHV fiber optic window (FOW) mounted in a 6” flange.

5. Hi throughput, multi-clad, non-scintillating, 1.5 m long fiber optics (NA 0.7 T 40%).

6. Femto current pre-amplifiers (104 - 1011 V/A).

PMT vs. APD!!! (price & performance)

7. Bi-alkali (Sb-Rb-Cs, Sb-K-Cs), multi-anode (16 channels), low cross talk, high gain ( 106) Hamamatsu photo-multiplier 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.

What is a PMT (Photomultiplier Tube)?

• 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 CharacteristicsMultiplication scheme

• Emission of photoelectrons into the vacuum (P 10-4 Pa 10-6 Torr).

• Electrostatic focusing of the e- towards the electron multiplier stage.

• Electron secondary emission at each collision with the dynodes.

• The multiplied e- are collected by the anode output signal.

The PMT provides:1. High sensitivity2. Exceptional low noise3. Fast time response for photons from UV (300 nm) to NIR (800 nm)

Hamamatsu H6568-10i. 44 multi-anode array, metal

package, head-on type PMT.ii. 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!!!

iii. The bialkali photocathode (A = 18.118.1 mm2) and it is made out of Sb-Rb-Cs & Sb-K-Cs. Favorable blue-green sensitivity for scintillator flashes from NaI:Tl (CsI:Tl).

PMT Optic, EM & Thermal Characteristics (I)• Cathode Radiant Sensitivity (SC)

• Anode Radiant Sensitivity (SA)

Final Test Sheet1. Tungsten Filament Lamp (2856 K)

2. HV = 800 V

3. SC = 66.66 mA/W

4. SA = 6.86 104 A/W

G=KVn 1.03 106

5. Anode Dark Current = 0.47 nA

CS = photoelectricIincidentP

=e eN Δt

pN hν Δt=

ehc

⎛ ⎝ ⎜

⎞ ⎠ ⎟ λ ⋅QE

∴ CS ≈λ nm( )⋅QE

1242

AS = AnodeIincidentP

=G⋅ photoelectricIincidentP

⎛

⎝ ⎜ ⎜

⎞

⎠ ⎟ ⎟

∴ AS ≈λ nm( )⋅QE⋅G

1242

Hamamatsu’s H6865-10 (HV = 800 V)i. QE 5% (for = 550 - 560 nm)

ii. Sc 22.33 mA/W

iii. SA 2.23104 A/W

If I assume 10 pW ofvisible light power we will

have 2.2310-7 A

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?

Take a look at the PMT’sspecs (maximum ratings) 625 nA per channel

Assuming a 1 - 5 nW of visible light power per channel, we should work with < 500 Volts!

Current amplifier at 107 V/A

Sampling rate: 400 kHz

1.E+02

1.E+03

1.E+04

1.E+05

300 400 500 600

Voltage (V)

Gain (G=SA/SC)

0

50

100

150

200

250

300

350

400

450

Anode Radiant Sensitivity (A/W)

PMT Optic, EM & Thermal Characteristics (III)

• 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.

•

: magnetic permeability 500000

t: thickness 0.05”

D: inner diameter of the shield case 2.24”

• 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!!!

• Dark Current (Id)

1. Thermoionic emission

2. Ionization of residual gases

3. Glass scintillation

4. Field emission when V Vmax

5. Linear function of the supply voltage. Remember G = KVn ? V should be very stable and provide minimum ripple!

6. RECOMMENDATIONS:

i. Measure Id after 30 min storage in dark!

ii. Cooling the photocathode especially in application such as photon counting!

• Uniformity & Cross Talki. Important for multianode PMT!

ii. Minimum normalized anode uniformity of 57 %.

iii. Maximum anode cross talk of 0.5%

outHinH

=3μt4D

≈8370

PMT’Drift - Life and Time Response Characteristics

• Time Response• 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.

• 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.

• Anode Pulse Rise Time.- Is the time the signal rises from 10% to 90% of the pulse’s peak amplitude

• For the H-6568-10, the APRT and TTS are 0.83 and 0.3 ns!!!

• Drift - Life Time1. The change in the anode output current

for small (long) periods of time.

2. It can be in the order of 5 - 50% for 1 - 104 h.

3. The major concern is caused by the damage to the last dynode due to heavy e- bombardment.

4. DO NOT EXCEED THE MAXIMUM RATINGS (625 nA per channel)!!!

5. If stability (i.e. photon counting) is a prime consideration, do not exceed 62.5 nA!

Before APDs, PIN Diodes• “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 - 2m (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)

• For the regions of interest the QE is fairly linear!

• 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).

• Insensitive to external magnetic fields!

• Responsivity: 0.05 - 0.3 A/W

• Successfully used in the European SOHO and American SNOE missions!

• Used at the Ring Accelerator Experiment (RACE) at LLNL and several fusion related laboratories around the world.

QE≈ photonE (eV)

3.7

What is an APD (Avalanche Photodiode) ?

• 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 e- producing 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: 11105 V/W.

• Built in current amplifier (104 V/A)!

1. Cutoff 600 kHz2. Sensitivity 11107 V/W.3. Built in current amplifier (106 V/A)!

With custom made modifications!

We still need an increase of one/two orders of magnitude! Reduce TTEC or change the amplifier!

Preliminary PMT Results (CDX-U)

DA = Diode ArrayOA = Optical ArraySGI = 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!