FIRST DETECTION OF AIR SHOWER CHERENKOV LIGHT BY

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 DEALS WITH A DEVELOPMENT FOR IMPROVING DETECTORS FOR GROUND-BASED, VERY HIGH ENERGY (VHE) g ASTRONOMY • GROUND-BASED VHE g 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 g (+ CR) AIR SHOWERS • A BIG EXPERIMENTAL CHALLENGE: HOW TO DISCRIMINATE gs 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

KIFUNE PLOT 44 SOURCES (13 AGNs) 2006) Mkn 180 PG 1553 NOT ALL SOURCES IN INNER GALACTIC PLANE SHOWN ALL SOURCES HAVE SPECTRA EXTENDING ABOVE 1 TEV, RARELY SPECTRA EXTEND ABOVE 10 TEV (CRAB->80 GEV)MANY AGNS HAVE A SOFT SPECTRUM

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 Ge. V -> 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? ? ? 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 PHOTOSENSOR WITH HIGH GAIN AND SINGLE ELECTRON RESPONSE) PROMIZE TO HAVE A HIGHER PDE -> MOTIVATION TO TEST ALREADY PROTOTYPES

THE GEIGER MODE-APD G-APD vs PMTS ADVANTAGES HIGHER PDE 60 -80% POSSIBLE, NOW 30 -50 % LOW OPERATION VOLRAGE: 30 -100 V 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/mm 2 OPTICAL CROSSTALK TEMPERATURE DRIFT OF GAIN/PDE LONG SIGNAL DECAY TIME/CELL RECOVERY LIMITATION DYNAMIC RANGE

THE VERY FIRST TEST, FALL 2006 JUST A TRY TO SEE IF ONE CAN AT ALL DETECT CHERENKOV LIGHT FROM AIR SHOWERS DETECTOR: 4 GROUPS OF 4 G-APDS(originally to measure optical pulsation of the CRAB pulsar by MAGIC) G-APDs: SSPM_0606 BG 4 MM from PHOTONIQUE, area 4. 4 mm 2 each, peak sensitivity at 580 nm, n-on-p structure, ≈ 4 -6 Mhz noise rate/pixel For some studies cooled by a Peltier cooler 4 x 4 G-apds SINGLE PHE SIGNAL TESTBOX INSTALLED ON SIDE OF MACIC CAMERA FIRST EVER DETECTED CHERENKOVSIGNAL FROM UNIT 2 AND 3 SIGNL FROM UNIT 2 DELAYED BY 4 NSEC TRIGGER BY UNIT 1 AND 2 HORIZONTAL GRID: 50 nsec VERTICAL GRID: 0. 1 m. V

THE SECOND TEST(winter 2007) USE OF 4 G-APDS FROM HAMAMATSU, TYPE MPPC AREA: 3 x 3 mm 2, cells: 50 x 50 m 2 , bias 70 V, noise rate ≈ 200 k. Hz/ mm 2, , light collection enhanced by simple cone to 6 x 6 mm 2 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 Cherenkov light Airobicc concept disc from air shower The 4 G-apds With light catchers TRIGGER PMTS, AREA ENHANCED BY SIMPLE LIGHT CATCHERS TRIG. THRESHOLD ≈ 10 -15 phe COINC. WIDTH: 5 nsec

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 e. V ≈ 50 % OF TRIGGERS: DECENT SIGNALS IN G-APDS A TYPICAL EVENT HORIZONTAL SCALE: 50 nsec VERTICAL SCALE: 200 m. V 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 2 WINTER 2007 PLANAR MIRROR VIEWING ZENITH 2. FOCUSSING MIRROR FOCUSSING ON TEST SETUP DRIFT MODE OBSERVING CR AIR SHOWERS FROM THE ZENITH group of four MPPCs placed in the focal plane of a solar light concentrator at the Paul-Scherrer-Institut. PMTs MPPCs planar mirror 120 m² parabolic mirror 8. 5 m Ø f/D = 0. 5 only 15 m² mirror area used because of limited acceptance

SOME EVENT EXAMPLES(recorded with a 2 Ghz F-ADC system) PMTS G-APDS SUMMED SIGNAL OF 16 PIXELS. THIS CORRESPONDS TO ABOUT A NORMAL MAGIC PMT PIXEL AREA (there was some timing jitter between the different events)

TEST 4: USING 4 G-APDS (HAMAMATSU MPPC) MOUNTED ON MAGIC SUMMER 2007 TRIGGER BY MAGIC CAMERA EVENTS RECORDED BY MAGIC DAQ (SIGNALS BY OPT. FIBERS TO COUNTING HOUSE, DIGITIZATIONBY NEW 2 GHZ MULTIPLEXER F-ADC SYSTEM MAGIC Pixel Size 4 MPPC-33 -050 C from Hamamatsu: sensor size: 3 x 3 mm 2 single cell size: 50 x 50µm 2 nominal bias: 70. 4 V dark rate at nominal bias: ~2 MHz gain at nominal bias: 7. 5*105 crosstalk at nominal bias: 10% Array of 4 MPPCs: light catchers with factor 4 concentration; 6 x 6 mm 2 onto 3 x 3 mm 2 6 x 6 mm 2 HOLLOW CONE VM 2000 FOIL LINED WITH UV DIELECTRIC REFLECTOR FOIL G-APD, 3 x 3 mm 2 UV TRANSMITTING PLEXI-CONE peak photon detection efficiency 55%, need confirmation

3. Test Installation of 4 MPPC in front Of the MAGIC camera Trigger by air shower C-light Comparison of signal in neighbor Pmt 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 2 phe 4 phe 1 phe MPPCs 70 phe 35 phe 15 phe PMTs

Shower Signals: G-APD vs PMT event selection: two PMTs next to Si. PMs with more than 15 photoelectrons in each tube counts ~300 events from ~30 min data signals are correlated on average a larger signal in Si. PMs

ratio of signals Si. PM / (scaled) PMT event by event 100% efficiency assumed for the light catcher in front of the MPPCs on average 1. 6 times more light detected with MPPCs (crosstalk corrected) in reality higher due to non perfect light concentrator

CALIBRATION (IMPORTANT BECAUSE OF TEMPERATURE DRIFT) BY LOW LIGHT LEVEL LED PULSER Pedestal UV-LEDs 375 nm single phe-resolution degraded due to light from night sky background and readout chain 1 phe 2 phe 3 phe … easy calibration

PARAMETERS OF OPTICAL ELEMENTS FOR COMPARISON OF DIFFERENT LIGHT SENSORS

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

POSSIBLE FURTHER IMPROVEMENTS i) ii) iii) A further increase in PDE over the entire spectral range between 290 -700 nm, i. e. , a widening of the peak PDE range. Alternatively an enhancement of the UV sensitivity by wavelength shifters. Larger G-APDs of 5 x 5 or 10 x 10 mm 2 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 vi) special differential amplifier allows to obtain narrow pulses for the Hamamatsu G-AP vi) A reduction in optical crosstalk of well below 5%. vii) A further reduction of the intrinsic noise to below 100 k. Hz/mm 2 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 CT 3 TELESCOPE IN LA PALMA READOUT 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 3 x 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