FIRST DETECTION OF AIR SHOWER CHERENKOV LIGHT

BY GEIGERMODE-AVALANCHE PHOTODIODES

E. Lorenz, MPI f Physics, Munich and ETH Zurich

WORK DONE BY : A. Biland, I. Britvitch, E. Lorenz, N. OTTE, F. Pauss, D. Renker, U. Rösler ETH- Zürich, PSI-Villigen, MPI-Munich

OVERVIEW•MOTIVATION

•THE TESTS

•AUXILIARY ISSUES

•THE NEXT PLAN

•CONCLUSIONS

MOTIVATION OF THIS STUDY

THIS STUDY DEALS WITH A DEVELOPMENT FOR IMPROVING DETECTORSFOR GROUND-BASED, VERY HIGH ENERGY (VHE) ASTRONOMY

•GROUND-BASED VHE ASTRONOMY IS A SECTION OF ASTROPARTICLE PHYSICS

•FIELD WAS OPENED IN 1989 (WHIPPLE, CRAB DETECTION) AND IS NOW VERY PRODUCTIVE (> 70 SOURCES DISCOVERED)

•OBSERVATIONS ARE CARRIED OUT BY SO-CALLED IMAGING ATMOSPHERIC CHERENKOV TELESCOPES (IACT) DETECTING THE CHERENKOV LIGHT FROM (+ CR) AIR SHOWERS

•A BIG EXPERIMENTAL CHALLENGE: HOW TO DISCRIMINATE s AGAINST THE HUGE CHARGED COSMIC RAY BACKGROUND

•THERE ARE QUITE A FEW OBSERVATIONS (RAPID FLARING...) WHICH CANNOT BE CARRIED OUT BY SATELLITE BORNE DETECTORS (TOO SMALL AREA)

•WE NEED BETTER DETECTORS -> HIGHER SENSITIVITY. LOWER THRESHOLD

Mkn180PG1553

44 SOURCES(13 AGNs)

NOT ALL SOURCES IN INNER GALACTIC PLANE SHOWN

KIFUNE PLOT

ALL SOURCES HAVE SPECTRA EXTENDING ABOVE 1 TEV, RARELY SPECTRA EXTEND ABOVE 10 TEV (CRAB->80 GEV)MANY AGNS HAVE A SOFT SPECTRUM

2006)2006)

SOURCE DISCOVERY SITUATION AUGUST 2007>70 SOURCES (NEARLY DOUBLED SINCE AUGUST 2006)

FROM G. ROWELL

WE WANT BETTER DETECTORS

LOWER THRESHOLD TO EXPLORE ENERGY REGION BETWEEN 20 AND 100 GeV-> NEED TO DETECT MORE PHOTONS ->NEED HIGHER QE/PDE LARGER MIRRORS

HIGHER SENSITIVITY:AIM FOR DETECTING 0.001 OF CRAB FLUX-> NEED MORE COLLECTION AREA -> MORE TELESCOPES -> NEED TO DETECT MORE PHOTONS ->NEED HIGHER QE/PDE AND/OR LARGER MIRRORS

LARGER MIRRORS??? CURRENTLY : MAGIC 17 m Ø, 64 tons HESS II 27 m Ø, > 500 tons COSTS BECOME AN ISSUE OPTICAL LIMITATIONS FOR LARGE Ø MIRRORS

PDE: PHOTON DETECTION EFFICIENCY = QUANTUM EFFICIENCY x CONVERSION INTO A DETECTABLE PHOTOELECTRON BETTER QUANTITY THAN USING QE FOR PHOTON DETECTORS

WE NEED PHOTOSENSORS WITH HIGHER PDE !

COMMENT 1: GOOD PMTS HAVE A PDE 10-20% LOWER THAN QE

COMMENT 2: THE MEAN QE IN MODERN IACTS IS ONLY <QE> ≈ 12-15% BETWEEN 300-600 nm

COMMENT 3: WHAT COUNTS IS NOT THE PEAK QE BUT THE INTEGRALBETWEEN 300-600 nm OF THE CHERENKOV SPECTRUM FOLDED BY THE SPECTRAL QE/PDE CURVE

WE CANNOT INCREASE MUCH MORE THE MIRROR AREA -> WE NEED SENSORS WITH A HIGHER AND BROADER PDE

GEIGER MODE AVALANCHE PHOTODIODES ( A SOLID STATE PHOTO- SENSOR WITH HIGH GAIN AND SINGLE ELECTRON RESPONSE) PROMIZE

TO HAVE A HIGHER PDE -> MOTIVATION TO TEST ALREADY PROTOTYPES

THE GEIGER MODE-APDG-APD vs PMTSADVANTAGES HIGHER PDE 60-80% POSSIBLE, NOW 30-50 % LOW OPERATION VOLRAGE: 30-100V TYP VERY COMPACT, FLEXIBLE GEOMETRY HIGH SER (CALIBRATION EASIER!) VERY ROBUST (NOT DAMAED BY DAYLIGHT WHEN UNDER BIAS INSENSITIVE TO MAGNETIC FIELDS EVENTUALLY LOW COST(NOT NOW) VERY FAST SIGNAL RISETIME

DISATVANTAGES STILL PROTOTYPES LIMITED AREA HIGH DARK COUNT RATE (0.1-1 MHZ/mm2

OPTICAL CROSSTALK TEMPERATURE DRIFT OF GAIN/PDE LONG SIGNAL DECAY TIME/CELL RECOVERY LIMITATION DYNAMIC RANGE

THE VERY FIRST TEST, FALL 2006JUST A TRY TO SEE IF ONE CAN AT ALL DETECT CHERENKOV LIGHT FROM AIR SHOWERSDETECTOR: 4 GROUPS OF 4 G-APDS(originally to measure optical pulsation of the CRAB pulsar by MAGIC)G-APDs:

SSPM_0606BG4MM from PHOTONIQUE, area 4.4 mm2 each, peak sensitivity at 580nm, n-on-p structure, ≈ 4-6 Mhz noise rate/pixelFor some studies cooled by a Peltier cooler

4 x 4 G-apds

TESTBOXINSTALLED ON SIDE OFMACIC CAMERA

FIRST EVER DETECTED CHERENKOVSIGNALSIGNAL FROM UNIT 2 AND 3SIGNL FROM UNIT 2 DELAYED BY 4 NSECTRIGGER BY UNIT 1 AND 2HORIZONTAL GRID: 50 nsecVERTICAL GRID: 0.1 mV

SINGLE PHESIGNAL

THE SECOND TEST(winter 2007)USE OF 4 G-APDS FROM HAMAMATSU, TYPE MPPCAREA: 3x3 mm2, cells: 50x50 2 , bias 70 V, noise rate ≈ 200 kHz/ mm2,, light collection enhanced by simple cone to 6x6 mm2 area

SET-UP VIEWING DIRECTLY THE NIGHTSKY (LIKE A MINIATURE AIROBBICS SET-UP)TRIGGER BY A COINCIDENCE OF TWO OPEN PMTS VIEWING THE NIGHT SKY

Airobicc concept Cherenkov lightdisc from airshower

The 4 G-apdsWith light catchers

TRIGGER PMTS, AREA ENHANCED BY SIMPLELIGHT CATCHERSTRIG. THRESHOLD ≈ 10-15 pheCOINC. WIDTH: 5nsec

TEST NEAR PSI: HIGH NIGHST SKY BACKGROUND LIGHT FROM NEARBY ZÜRICH, VILIGEN DAY OF KYRILL TRIGGER RATE ≈ 1 TRIGGER/ 5 MIN (AS PREDICTED FROM AIROBICC) THRESHOLD ≈ 1015 eV ≈ 50 % OF TRIGGERS: DECENT SIGNALS IN G-APDS

A TYPICAL EVENTHORIZONTAL SCALE: 50 nsecVERTICAL SCALE: 200 mV

NOTE HIGH NIGHT SKY BACKGROUND LEVEL

TEST WITH A LIGHT PULSER AND SOME BACKGROUND LIGHT

TEST 3 ON THE PSI SOLAR TEST FACILITY USING SAME SET-UP AS IN TEST 2WINTER 2007PLANAR MIRROR VIEWING ZENITH2. FOCUSSING MIRROR FOCUSSING ON TEST SETUP DRIFT MODE OBSERVING CR AIR SHOWERS FROM THE ZENITH

placed in the focal plane of a solar light concentrator at thePaul-Scherrer-Institut.

planar mirror 120 m²

parabolic mirror 8.5 m Ø

only 15 m² mirror area used because of limited acceptance

PMTs

MPPCs

f/D = 0.5

group of four MPPCs

SUMMED SIGNAL OF 16 PIXELS.THIS CORRESPONDS TO ABOUT A NORMAL MAGIC PMT PIXEL AREA(there was some timing jitter between the different events)

SOME EVENT EXAMPLES(recorded with a 2 Ghz F-ADC system)

PMTS

G-APDS

TEST 4: USING 4 G-APDS (HAMAMATSU MPPC) MOUNTED ON MAGICSUMMER 2007TRIGGER BY MAGIC CAMERAEVENTS RECORDED BY MAGIC DAQ (SIGNALS BY OPT. FIBERS TO COUNTING HOUSE, DIGITIZATIONBY NEW 2 GHZ MULTIPLEXER F-ADC SYSTEM

4 MPPC-33-050C from Hamamatsu:

sensor size: 3x3mm2

single cell size: 50x50µm2

nominal bias: 70.4Vdark rate at nominal bias: ~2MHzgain at nominal bias: 7.5*105

crosstalk at nominal bias: 10%

peak photon detection efficiency 55%, need confirmation

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

MAGIC Pixel Size

HOLLOW CONE VM2000FOIL LINED WITH UV DIELECTRICREFLECTOR FOIL

UV TRANSMITTING PLEXI-CONE

6 x 6 mm2

G-APD, 3x3 mm2

3. TestInstallation of 4 MPPC in frontOf the MAGIC cameraTrigger by air shower C-lightComparison of signal in neighborPmt cells (9 cm**2)With 4 g-apd pixels (0.36 cm**2)Readout by 2 Ghz F-ADC

location of MPPC array

1 phe

MPPCs

PMTs

2 phe 4 phe 1 phe

70 phe 35 phe 35 phe 15 phe

Shower Signals:G-APD vs PMT

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

signals are correlated

cou

nts

~300 events from ~30 min data

on average a larger signal in SiPMs

ratio of signals SiPM / (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

UV-LEDs 375nm

Pedestal

1 phe

2 phe

3phe

…

single phe-resolution degraded due to light

from night sky background

and readout chain

easy calibration

CALIBRATION (IMPORTANT BECAUSE OF TEMPERATURE DRIFT)BY LOW LIGHT LEVEL LED PULSER

Zur Anzeige wird der QuickTime™ Dekompressor „TIFF (Unkomprimiert)“

benötigt.

PARAMETERS OF OPTICAL ELEMENTS FOR COMPARISON OFDIFFERENT LIGHT SENSORS

EVALUATION OF THE FIGURE OF MERIT OF DIFFERENTSENSORS (FOLDING OF C-SPECTRUM BY OPTICAL PARAMETERS AND THE PDE ( )

POSSIBLE FURTHER IMPROVEMENTS

i) A further increase in PDE over the entire spectral range between 290-700 nm, i.e., a widening of the peak PDE range.ii) Alternatively an enhancement of the UV sensitivity by wavelength shifters.iii) Larger G-APDs of 5x5 or 10x10 mm2 area (also of hexagonal geometry) without sacrificing the fast risetime.iv) A signal risetime around only 1 nsec.v) A fast fall time such as in the Photonique G-APDs, although a use of a clip-cable or a special differential amplifier allows to obtain narrow pulses for the Hamamatsu G-APDs.vi) A reduction in optical crosstalk of well below 5%.vii) A further reduction of the intrinsic noise to below 100 kHz/mm2 at 25°C. viii) Inclusion of micro-lenses or micro-light catchers matched to individual cells in order to overcome the losses due to the inert areas between cells.ix) Optical filters with high transmission between 300 and around 700 nm cutting off background light above 700 nm. The filter should be in optical contact with the plastic coating of the G-APDs.

THE NEXT PLAN

BUILD A 272 PIXEL (0.24°) PIXEL CAMERA AND TEST IT ON THE OLD HEGRA CT3TELESCOPE IN LA PALMAREADOUT BY 2 GHZ SWITCHED CAPACITOR ARRAY

CONCLUSIONS•ALL TESTS DEMONSTRATED THAT CHERENKOV LIGHT SIGNALS COULD BE DETECTED

•SINGLE PHOTOELECTRON DETECTION LEVEL REACHED

•NOISE RATE (OF MPPC) WELL BELOW NIGHT SKY LIGHT LEVEL-> NO COOLING

•SIGNALS WIDER THAT PMT SIGNALS, BUT CLIPPING POSSIBLE

•ALREAD HIGHER PDE 8> 1.5 x) WITH PRELIMINARY DEVICES THAN PMTS

•CALIBRATION NOT A PROBLEM

•POTENTIAL TO REACH 3x PDE COMPARED TO PMTS -> LARGE PHYSICS POTENTIAL

•MANY SMALL PROBLEMS STILL TO SOLVE

•PRICE STILL HIGH BUT PROSPECTS FOR LOW PRICES GOOD

WE ARE CLOSE TO BUILD THE FIRST REALISTIC IACT WITH A G-APD CAMERA

POSSIBLE SOLUTIONS TO OVERCOME LOSSES FROM THE DEAD AREA BETWEEN CELLS OF A G-APD

WORKS ONLY IF SOURCE HAS LIMITED ANGULAR EMISSION

CONE SOLUTION

LENS SOLUTION

THE PROBLEM IS THE ACTIVE AREA FRACTION. IF TOO SMALL, ONE CAN ONLY CONCENTRATE LIGHT FROM A SMALL SPACE ANGLE (LIOUVILL THEOREM)

NOW CCD CHIPS USE OFTEN MICROLENSES TO ACHIEVE 100% FILL FACTOR