Ultra-II Cherenkov Telescopes: Physics Goals & Design Considerations

1. A CONCEPT FOR CHERENKOV TELESCOPES FOR ULTRA-II • Florian Goebel, Anton Kabelschacht, Eckart Lorenz

2. SUMMARY PHYSICS GOALS AND PROSPECTS • Study of the spectra of galactic point sources as well as extended galactic sources up to around 100 TeV • Details of spectral cut-off parameters of these sources • Spectral shapes of low redshift AGNs with high sensitivity • All-sky monitoring with high sensitivity, alerts for the low threshold array and quick switchover to dedicated sources in case of flaring sources • Study of diffuse gamma-radiation from the galactic plane • Study of possible isotropic gamma-emission from Topological Defects • Study of the chemical composition between 1012-1015 • Search for fine structures in the general CR spectrum • Long-term studies of some flaring sources • Possibly a search for quark-gluon plasma (needs theoretic input) • Extension of GLAST studies • No studies of GRBs • May be: use of the array as an air fluorescent detector in the 1016-1018 eV range. Needs some bright ideas for the trigger, attractive for small groups • Coincidence studies with large VHE neutrino detectors for some dedicated source candidates. • Crazy idea : study of high atmosphere small discharges (Elfs etc) see EUSO proposal. • Search of extraterrestrial intelligence by using the array as an all-sky monitor for optical signals (proposal of ManeL Martinez)

3. BASIC DESIGN CONSIDERATIONS • IT IS IMPOSSIBLE TO BUILD AN OPTIMAL DETECTOR THAT SPANS 4 ORDERS OF • MAGNITUDE IN ENERGY DUE TO THE STEEP POWER LAW OF FLUXES • ONE NEEDS TWO DETECTOR CONFIGURATIONS FOR THE CTA TO SPAN FROM • ≈ 10 GEV TO ≈ 100 TEV • LEA (≈ 10GEV - > 1 TEV) + ULTRA II < (1 TEV - >100 TEV) • ULTA II (ULTRA LARGE TELESCOPE ARRAY) 100 TELESCOPES SPREAD OVER 1-2 km2 • EACH TELESCOPE WITH ≈ 18 m2 MIRROR AREA • SENSITIVITY ≈ 10 x HIGHER THAN PAST ARRAYS • LOWER THRESHOLD LIMIT : ≈ 400 GEV IN ZENITH POSITION BUT HIGH g/h SEPARATION • POWER AT ≈ 1 TEV • UPPER OPERATION LIMIT ≈ 200 TEV • TELESCOPE CONFIGURATION: BLEND OF HEGRA TYPE CONSTRUCTION (LOWER PART) • AND MAGIC CONSTRUCTION (UPPER PART)

4. SOME SPECIAL ISSUES • THE TECHNICAL CONSIDERATIONS ARE NOT VERY CHALLENGING COMPARED • TO THE LEA PART • THE DOMINANT COST IS DRIVEN BY THE CAMERA -> USE OF CLASSICAL PMTS • -> USE OF 25 mm HEMISPHERICAL PMTS (CONSERVATIVE, LOW RISK, PLENTY • OF EXPERIENCE, NO NEED TO MAXIMIZE QE, PANEQUE LACQUER OK, • PMTS MATURE, G-APDS STILL IN EARLY DEVELOPMENT PHASE) • INSTEAD OF INVESTING IN IMPROVING QE BY FANCY WORK- • -> INCREASE MIRROR AREA-> 18m2 (> 2x HEGRA IACT AREA) • NEARLY ENTIRELY TO BE CONSTRUCTED BY INDUSTRY: • ALL WORK CAPACITY NEEDED FOR HESS II, MAGIC II AND LEA • AIM FOR CONSTRUCTION TIME ≈ 4 YEARS • INSTALL A TELESCOPE BY 4-5 PEOPLE IN < 1 WEEK

5. SOME TECHNICAL ISSUES • FOUNDATION: a la HEGRA : SIMPLE CONCRETE BLOCK 3 x3x1 m3 + thin working platform • UNDERCARRIAGE: LIKE FOR HEGRA CTS WITH CRANE BALL BARING • PLUS ROTATING WORKING PLATFORM • UPPER STRUCTURE LIKE MAGIC SPACE FRAME- ALUMINIUM TUBES • BUT ONLY 2 LAYER SPACE FRAME USING A TETRAEDER AS BASIC ELEMENT • DRIVE MOTORS: STEPPING MOTORS LIKE FOR HEGRA • CAMERA SUPPORT MAST: GOTHIC ARC (PREFORMED I-BEAM) HOLD BY • PRESTRESSED STEEL CABLES • MIRRORS: HEXAGONAL, MADE FROM HIGHGLY REFLECTIVE AL-ANOD PLATES • SUPPORTED BY HEXCELL SANDWICH (a la CURRENT PADOVA CONSTRUCTION) • NO NEED FOR DIAMOND MACHINING • HIGH WEATHER RESISTANCE DUE TO MULTILAYER COATING • POORER FOCUSSING THAN IN MAGIC BUT OK BECAUSE OF 0.25° PIXELS • ALTERNATIVE MIRROR PRODUCTION : REPLICA METHOD - WUERZBURG • ALTERNATIVE MIRROR PRODUCTION: THIN ALUMINIZED GLASS FOILS BACKED BY • HEXCELL SANDWICH • NO ACTIVE MIRROR CONTROL BUT AUTOMATIC MIRROR ADJUSTMENT EVERY • 1-2 MONTH

6. SOME TECHNICAL ISSUES, II • CAMERA : 5° Ø, 0.25° PIXEL SIZE -> ALMOST MAGIC LAYOUT INNER SECTION • FOR f = 7 m • -> COPY OF MAGIC I PRINTED CIRCUIT • PMTS 6 STAGE PMTS (GAIN≥ 105) + TRANSIMPEDANCE PREAMP • BANDWITH CAN BE LOWER THAN FOR MAGIC PMTs • DYNAMIC RANGE OF PREAMP ≈ 500 SUFFICIENT • ET SAYS THAT PRICE FOR 33000 PMTS CAN BE € 100/PMT IF NO UV GLAS NEEDED • ET CAN BUILD CAMERA PRINT CIRCUIT, TEST AND ASSEMBLE, HT + PREAMPS • SHORT COAX CABLES (RG174) FROM CAMERA TO READOUT ELECTRONICS • LOCATED IN SPACE FRAME • CAMERA WINDOW: UV TRANSMITTING PLEXIGLASS • LIGHT CATCHERS a la MAGIC, LINED WITH POSSIBLY DIELECTRIC MIRROR FOIL • HT: IN CAMERA, NEW COMPACT VERSION • DIGITIZER: SWITCHED CAPACITOR ARRAY (DOMINO CHIP) ACTING BOTH AS • DELAY AND F-ADC (500 MHZ, HIGHER FREQ. NOT NEEDED). MAIN PROBLEM • CURRENT READOUT MUCH TOO EXPENSIVE -> CUSTOM IC • TRIGGER: RATHER SIMPLE: 2 FOLD NEXT NEIGHBOR FOR EACH TELESCOPE • + 2 FOLD COINCIDENCES BETWEEN 1,2,… OF THE 6 NEXT NEIGHBOR TELESCOPES • WILL START READOUT OF DOMINO (TRIGGER RATE 25-40 hz) • OTHER PHYSICS (FLUORESCENCE DETECTORS MIGHT NEED MORE COMPLEX • LOGIC) • USE OF DIGITAL SIGNALS VIA OPTICAL FIBERS WHENEVER POSSIBLE

7. SUMMARY TELESCOPE PARAMETERS • 1. mirror area: ≈ 18 m2 • 2. Mirror layout: see Fig 1 • 3. Focal distance 7 m (f/D ≈ 1.4) • 4. Number of mirror elements: 18 • 5. Mirror profile: quasispherical, Davis-Cotton • 6. Operation range: Azimuth: 350°, Declination + 100°-> -75° • 7. Camera diameter ≈ 5 ° • 8. Pixel size 0.25° • 9. Nr of pixels ≈ 330 • 10. Photon sensors: PMTs, 1’’ Ø, hemispherical, 6 dynodes, max gain 105 • 11. Max slewing speed: 60°/min (fast slewing not needed) • 12. Drive motors: stepping motors with planetary gears of small backlash • Angular measurements: by 13 or 14 bits absolute shaft encoders read out by CAN bus or equivalent bus • Trigger: each telescope: two next neighbour pixels. Between telescopes: wide gate coincidence triggering coinciding telescopes. Under normal conditions at least 2 telescopes should trigger in coincidence. Alternatively, for all sky monitor observations, telescopes trigger autonomously. For air fluorescence studies a special trigger is needed • 15. Pulse digitizing: 1024 deep switched capacitor array running at 500 MHz, ≈ 10 bit dyn. range. • Location of readout electronics: in small containment mounted in mirror dish and single fiber optical connection to central electronics counting house. • 17 High reliable and self-protecting electronics for near-remote operation • Clock: central GPS controlled clock with fiber optics fan-out to each telescope. Relative clock • time precision ≈ 1 nsec.

8. SUMMARY TELESCOPE PARAMETERS • COST ESTIMATE • 1. Cost per telescope: Total cost < 200 k€ •  Concrete foundation ≈ 5 k€ •  Mechanical structure: 25 k€ •  Motors, encoders, power: 10 k€ •  Mirrors: 10-20 k€ •  Camera +DAQ: 110 k€ •  Auxiliary equipment: 35 k€ • TOTAL COST OF ULTRA II • 25 M€ = 5M€ DEVELOPMENT COSTS+ 100 TELESCOPES • If you need finer pixels: either more money or fewer telescopes. • For 0.12°pixels: either 3x fewer telescopes or price 75 M€

9. A POSSIBLE DEVELOPMENT ROAD • A) MONTE CARLO STUDIES • CROSS CHECK CORRECTNES OF HADRONIC SHOWER PHYSICS • SIMULATION OF SENSITIVITY, TRIGGER PERFORMANCE • B)LIST OF TECHNICAL DEVELOPMENTS • HIGH PRIORITY DEVELOPMENTS • PROTOTYPE MIRROR DEVELOPMENTS • SPECIAL ASIC FOR DOMINO READOUT • TWOFOLD NEXT NEIGHBOR COINCIDENCE LOGIC • LOW POWER DISCRIMINATOR ASIC WITH EASY EXTERNAL CONTROL OF THREHOLD AND DELAY • MEDIUM PRIORITY • LOW POWER HT UNITS FOR PMTS • MECHANICAL DESIGN OF TELESCOPE STRUCTURE • ACTIVE MIRROR ADJUSTMENT • LIGHT CATCHER WITH DIELECTRIC REFLECTOR FOIL • PARALLEL STUDIES • HIGH RELIABILITY AND ROBUST OPERATION CONCEPT • REMOTE OPERATION CONCEPT • COST CUTTING STUDIES • QUICK INSTALLATION CONCEPT

10. Not useful, cutoff in UV

11. Multilayer Quartz-TiO2 Price of MIRO 4300 per panel of 1250x1250 mm, 0.5 mm 20 panels for developments. : 80 €/panel resp. mirror 2000 panels: 25 € /panel resp. mirror Note: 300G is not weather resistant

12. Mirror work at Wuerzburg

15. POSSIBLE MIRROR LAYOUT

16. LAYOUT MIRRORS (BLUE) AND TOP LAYER OF SPACE FRAME (BLACK) LOCATION OF ACTUATORS/FIX POINTS OF MIRROR PANELS ACTUATORS

17. LAYOUT MIRROR AND TOP LAYER SUPPORT FRAME CAMERA SUPPORT MAST MIRROR 108 cm CAMERA SUPPORT MAST 540 cm

18. BASIC SPACE FRAME ELEMENT MAGIC BASIC ELEMENT FOR ULTRA CONSIDERABLY STIFFER

19. PART OF TRIGGER LOGIC ≥2 PIXELS FIRING ≥2 PIXELS FIRING ACQUIRE ACQUIRE

20. POSSIBLE MODES OF OPERATION HIGHEST SENSITIVITY FOR SINGLE SOURCE SEARCH/STUDY: COMBINE LEA+ULTRA II AND FOCUS ONTO ONE SOURCE ‘ALL SKY MONITORING’: POINT ALL TELESCOPES TO DIFFERENT POINTS ON SKY COVER ≈ 0.5 STERAD BUT WITH LOW SENSITIVITY CAN ALSO BE USED AS A FLY’S EYE TYPE DETECTOR VARIANT: COMBINE 3(2) TELESCOPES FOR STEREO SUBCELLS AND POINT TO DIFFERENT POINTS ON THE SKY COVERS ≈ 0.15 STERAD, BUT WITH HIGHER SENSITIVITY (≈2-3) SPLIT ARRAY INTO TWO(2,3..) PARTS AND USE ONE PART FOR HIGH SENSITIVE SOURCE STUDIES WHILE USING OTHER (SMALLER) PART FOR LONG-TERM MONITORING OF DEDICATED FLARING SOURCES (AGNS) FOR VARIOUS STUDIES ….MONITORING TOGETHER WITH LARGE NEUTRINO DETECTORS…

21. CONCLUSIONS A 100 TELESCOPE ARRAY CAN BE BUILD WITHIN 4 YEARS (ASSUMES LARGE INDUSTRIAL SERIES PRODUCTION) MOST DEVELOPMENT CAPACITY NEEDED FOR LEA PART A COST OF 25 M € IS NOT UNREALISTIC THE BALANCE BETWEEN THE WISH FOR BETTER PERFORMANCE AND LIMITED BUDGET WILL BE BETWEEN THE NR OF TELESCOPES AND CAMERA PERFORMANCE RELATIVE CONSERVATIVE APPROACH POSSIBLE, NO CHALLENGING NEW AND UNPROVEN COMPONENTS NEEDED READOUT ELECTRONICS + TRIGGER VARIANTS MOST DEMANDING MC SIMULATIONS NOT MADE MC SIMULATIONS WILL MOST LIKELY GIVE GUIDANCE FOR SOME CRITICAL PARAMETERS SUCH AS TELESCOPE SPACING, PIXEL SIZE AND CAMERA FOV