# Presentation: 227021

## The Geiger-APDa novel photon detector and its application in astrophysics experiments and positron emission tomography

- A. Nepomuk Otte
- MPI für Physik, Munich / Humboldt Universität, Berlin

## Outline

- why new photon detectors for experiments in astroparticle physics
- the G-APD and some of its characteristics
- application of G-APD in:
- positron emission tomography (PET)
- air Cherenkov telescopes

- Many future experiments will use
-  >> 100,000 photon detectors

-  robust and stable
-  easy to calibrate
-  blue sensitive
-  low cost (+ low peripheral costs)
-  compact
-  low power consumption
-  ...
-  highest possible photon detection efficiency

- Astroparticle experiments that will use this photon detector

- Requirements to be fulfilled by the photon detector candidate:

## Cosmic Ray Physics from Space

**http://www.euso-mission.org/**

-  Highest energy cosmic rays > 1020 eV
-  GZK mechanism
-  sources of CR
-  ...

**Notes:**
very faint signals 100 phe for 10^19 eV event
neutrino physics

## Ground based Gamma Ray Astrophysics 

- http://wwwmagic.mppmu.mpg.de/

- Gamma Ray induces electromagnetic cascade

- relativistic particle shower in atmosphere

- Cherenkov light

- fast light flash (nanoseconds)
- 100 photons per m2 (1 TeV Gamma Ray)

- MAGIC: world largest air Cherenkov telescope

## VHE gamma-ray sources: status ICRC 2007

- Rowell

- 71 known sources

- very successful above 100GeV

- factor of 6 increase within 4 years

**Notes:**
12 sources when I started
now 71 sources known

## pushing to lower energies

- entering an unexplored energy window between 10 GeV and 100 GeV

-  extragalactic background light studies
-  gamma ray bursts
-  dark matter
-  tests of quantum gravity
-  pulsars
-  ...

- requires:

-  larger light collectors
-  high efficiency photon detectors

- currently used: classical photomultiplier tubes with ~20% QE

## The G-APD

- a promising photon detector concept invented in Russia in the 80’s

*P. Buzhan et al. *
*http://www.slac-stanford.edu/pubs/icfa/fall01.html*

-  sensors with ~60% efficiency become available
-  internal gain ~105 -106
-  compact and robust
-   ...

- advantages

- disadvantages

-  small sizes (<5x5mm2)
-  optical crosstalk (10%)
-  high dark count rate (~MHz)
-  ...

- Otte et al., IEEE TNS. 53 (2006) 636.
- SNIC-2006-0018, Apr 2006

- 3x3 mm2 G-APD

-  of
- semiconductor photon detectors
- with
- internal amplification

## Working modes of avalanche photodiodes

- Bias: (10%-20%) ***ABOVE*** breakdown voltage
- Geiger-mode: it’s a ***BINARY*** device!!
- Count rate limited
- Gain:  **“***infinite***”**** **!!

- Bias: slightly  ***BELOW***  breakdown
- Linear-mode: it’s an ***AMPLIFIER***
- Gain: limited < 300 (1000)
- High temperature/bias dependence
- No single photo electron resolution

- Geiger Mode

- Linear/Proportional Mode

## Advantages of APDs in Geiger Modeor Single Photon Avalanche Diodes (SPADs)

-  Large standardized output signal
- high immunity against pickup
-  High sensitivity for single photons
-  Excellent timing even for single photo electrons (<<1ns)
-  Good temperature stability
-  Low sensitivity to bias voltage drifts
-  Devices operate in general < 100 V
-  Complete insensitive to magnetic fields
-  No nuclear counter effect (due to standardized output)

- The principal disadvantage for many applications:

- It is a binary device

- One knows: There was at least one electron/hole initiating the breakdown

- but not
- how many of them

- solved in the G-APD concept

- Basic unit in a G-APD is a Single Photon Avalanche Diode (SPAD)

- Breakdown in SPAD is quenched by individual polysilicon resistor (passive quenching)

- from B. Dolgoshein (ICFA 2001)
- http://www.slac.stanford.edu/pubs/icfa/

## The G-APD

- typically 100...2000 small SPADs / mm2

- All SPADs connected in parallel

- Only one common signal line

- small signal replacement circuit

- quenching resistor

- SPAD

## SiPM output is the analog sum of all SPADs

- Well defined output signal per SPAD  multi pixel resolution

## Dynamic Range

- Dynamic range naturally limited by number of available SPADs

- working condition:
- Number of photo electrons < SPAD cells

- From probability considerations:

-  20% deviation from linearity if
-     50% of cells respond

## Photon Detection Efficiency (PDE)orEffective Quantum Efficiency

- Photon Detection Efficiency (PDE)orEffective Quantum Efficiency

- Most important parameter of a photon detector!!

- limiting factors:

-  Fraction of sensitive area (20% - 80%)

-  Intrinsic quantum efficiency

-  Surface reflection losses

-  Probability for Geiger breakdown
-    (depends on electric field)

- W.Oldham, P.Samuelson, P.Antognetti, IEEE Trans. ED (1972)

- In total: Currently claimed best PDE values are ~60%

-  SPAD recovery time (passive quenching)

-  Active volume / absorption length

## Measurement of the Photon Detection Efficiency

- PDE measurements are not an easy task
- optical crosstalk
- dependency on bias voltage

- often a photomultiplier with unknown photoelectron collection efficiency is used as reference

-  Overestimation of the PDE

## A method to measure the PDE

- Otte et al., NIM A **567 **360–363, 2006

- use calibrated PiN-diode as reference
- use integrating sphere with two exit ports (splitting ratio of several thousand)
- flash PiN-diode and G-APD with pulsed monochromatic light source

## Problems:

- Optical Crosstalk
- High Dark Count Rate

-  SPADs not only detect photons
-    they also emit photons during
-    breakdown

- Emission microscopy picture of a prototype SiPM

- Optical Crosstalk

-  Optical crosstalk
- Artificial increase in signal
-  **E**xcess **N**oise **F**actor of SiPM

- Photons can trigger additional cells

- Sketch from Cova et al. NIST 2003
- Workshop on single photon detectors

- can be quite significant

- Using optical crosstalk to learn more about the photons emitted in avalanches

## Light Emission in Avalanches

- measured spectra do not show similar behavior
- emission mechanisms not well known
- very few absolute measurements

- W. J. Kindt

## optical crosstalk spectrum from dark noise

- Try to reproduce this distribution with Monte Carlo simulations

## SiSi: The SiPM Simulator

- *Elisabeth ”Sisi” von Wittelsbach was the empress consort of Emperor Franz Joseph of Austria. She was born 1837 in Munich, Bavaria and murdered 1898 in Geneva, Switzerland

## SiSi

**SiSi**

- SiSi is an “almost” complete simulator of a SiPM

- photoelectrons in non-depleted bulk are subject to simple diffusion model;
- lifetime of electrons is a free parameter

- simulation of avalanche photons:
- black body radiation with free parameters:
-  **temperature**
** ****intensity**
- isotropic emission

**Notes:**
For this purpose I developed SiSi
full geometrical simulation of a SiPM

## Example of a good match

- residuals

- Residuals can be explained by dark counts which are not simulated in SiSi

- no unique set of model parameters (Temperature and Intensity of photon spectrum)

## Characteristics of Photons that cause Optical Crosstalk

- Intensity (1.15 ... 1.40 eV):
-    ~3*10-5 photons / electron
-     (systematic uncertainty of ~2)

- FWHM: ~0.2 eV

- Peak: ~1.26eV

- 2 photon spectra that reproduce the measured crosstalk distributions

- energy distribution of photons causing optical crosstalk

- Crosstalk is only caused by photons within a narrow range of energies

## Reason for narrow range of photon energies

- strong dependence of absorption lengths on photon energy

- absorption in same cell

- not absorbed in G-APD

- Two possible applications for G-APDs:

- positron emission tomography (PET)
- air Cherenkov telescopes

## Positron Emission Tomography (PET)

- Basic principle

_*PET*_ : image distribution of a radio-labeled tracer inside the body

_*Tracer Molecules*_ : Labeled by positron emitting isotopes
- ( 11C, 13N, 15O, 18F)

## The Reconstruction Problem

- what you are looking for!

- This is...

## G-APDs in PET: the first studies

- Advantage: very compact, no sophisticated amplifier needed, ...

- Otte, et al. NIM A 545 (2005)

- direct coupling of SiPM to crystal
- no cooling
- Factor 4 area miss match  between SiPM and crystal

- wrapped crystals

- G-APDs

- signal readout

## G-APDs in PET: the first studies

- Advantage: very compact, no sophisticated amplifier needed, ...

- Otte, et al. NIM A 545 (2005)

- direct coupling of SiPM to crystal
- no cooling
- Factor 4 area miss match  between SiPM and crystal

-  Energy resolution 22% FWHM
-    on 22Na coincidence spectrum

-  Time resolution 1.5 nsec FWHM

- Things have quite improved since then

- first ever
- measurement

## Result of measurements with MW-3 (3x3 mm2) Geiger- mode APDs from Dubna (Z. Sadygov) + LYSO crystals (2x2x10 mm3)  

- Alexey Stoykov,
- Dieter Renker (PSI)

- Energy Resolution:
- 12% FWHM

- Time Resolution:
- 540ps

- (limited by crystal)

- MRS diode used

- (2006)

## Imaging Air Cherenkov Technique

## Figure of Merit

- Cherenkov spectrum folded with photon detector response

- Cherenkov spectrum on ground

- photon detector response

## Application of G-APDs in air Cherenkov telescopes

- Cherenkov spectrum folded with photondetector response

- _Figure of Merit_

- MPPC from Hamamatsu with highest PDE
- (from data sheet MPPC
- with 100x100 μm2 cells)

- photomultiplier tubes

- factor >2 increase in sensitivity compared to bialkali PMTs

- can hope for

## Test on La Palma with MAGIC

- 4 MPPC-33-050C from Hamamatsu:
- 	sensor size: 3x3mm2
- 	single cell size: 50x50μm2
- 	nominal bias: 70.4V
- 	dark rate at nominal bias: ~2MHz
- 	gain at nominal bias: 7.5*105
- 	crosstalk at nominal bias: 10%

- peak photon detection efficiency 55%

- needs to be confirmed

- Array of 4 MPPCs:
- 	light catchers with factor 4 concentration; 6x6mm2 onto 3x3mm2

- MAGIC Pixel Size

- array mounted next to the MAGIC camera for 3 nights for fine tuning and tests

- array not removed or protected during day
- it was raining for one day!

- G-APDs signals recorded by the
- MAGIC DAQ with each trigger

- Array mounted onto the MAGIC camera entrance window for two nights

## Light recorded from Calibration Runs

- UV-LEDs 375nm

- Pedestal

- 1 phe

- 2 phe

- 3phe

- ...

- single phe-resolution degraded because of light from night sky

- easy calibration

- some recorded showers 

-  reduced systematics

- location of MPPC array

- 1 phe

- MPPCs

- PMTs

- 2 phe

- 4 phe

- 1 phe

- 70 phe

- 35 phe

- 35 phe

- 15 phe

## Slide 47

## Shower Signals: MPPC vs PMT

- event selection:
- 	two PMTs next to MPPCs with more than 15 photoelectrons in each tube

- signals are correlated

**counts**

- ~300 events from ~30 min data

- on average MPPC record a larger signal

- ratio of signals
-    MPPC/ (scaled) PMT
- event by event

-  on average 1.6 times more light detected with MPPCs (crosstalk corrected)

- 100% efficiency assumed for the light catcher in front of the MPPCs

- in reality higher due to non perfect light concentrator

## Summary

**The silicon photomultiplier is a real breakthrough in photon detection!!**

- CMOS like technology  prospects for cheap mass production <10$ per mm2

- It can not be damaged by exposure to strong source of light

- Offers high internal amplification (>105)

- Fast timing (<nsec)

- No aging

- Low power consumption (1...100μW/mm2)

- High photon detection efficiency (>60%)

- ...

## Summary

- In PET G-APDs already outperform other photon detectors

- + they allow to build very compact and robust scanners insensitive to magnetic fields  combination of MRT and PET

- PMTs in Cherenkov telescopes could be replaced by G-APDs

- at least 1.6 times more light recorded
- G-APD intrinsic noise is 10 times lower than the “noise” coming from the night sky
- optical crosstalk is an issue for IACTs and should be reduced to below 5%

## The End

## Ongoing Development:SiPM exploiting Backillumination

- Ongoing Development:SiPM exploiting Backillumination

- predicted characteristics:
- PDE > 80%
- Single photo electron time jitter ~ nsec
- Cooling is mandatory

- By the Semiconductor Laboratory
- affiliated to the MPIs for Physics and Extraterrestrial Physics

-  test structures of novel avalanche structure will be finished next month
-  After successful evaluation  prototypes end 2007

- Crosstalk problem can be a showstopper!!
- 	will be evaluated by dedicated structures
- 	small cell capacitance is of advantage