Monte Carlo Studies for a Future Instrument. Stephen Fegan Vladimir Vassiliev UCLA

Transcription

1 Monte Carlo Studies for a Future Instrument Stephen Fegan Vladimir Vassiliev UCLA

2 Approach to simulations CORSIKA 6200, 6500, 6502 w/bernlöhr Simulation of response of a single cell of an infinite array of telescopes Hexagonal layout, 3500m elevation Separation between telescopes m Perfect telescope optics Diameter of telescopes 5-15m Scale simulations to 1km 2, neglect boundary Trigger studies Pixelation and reconstruction of gamma-rays

3 Sample Design Distance From Center Of Array [m] Array telescopes 2. 8 hexagonal rings m separation Telescope and Detector 1. ø10m equivalent 2. QE = 0.25 (Bialkali) 3. 15º field of view Facts and Figures 1. Outer radius: 640m 2. Single cell area: 5543m 2 3. Total area: 1.06km 2 Distance From Center Of Array [m]

4 Cell effect Distance [m] Infinite Array Of Telescopes m ASL R Cherenk = 85m 2. D Scopes = 80m Geometry Dictates That 1. Impact point of every shower is in some cell 2. B Max = 47m 3. At least 3 telescopes contained in Cherenkov light pool Distance [m]

5 Cell effect Some Photoelectron threshold density [PE/m 2 ] m m m 2 Distance from shower core [m] PE density after: 1. Atmosphere 2. Mirror reflection 3. Photocathode VERITAS-like Most of effective area comes from distant showers. Eff. area is strong function of energy. Cell Geometry Only interested in photons in within cell a distance of approx. L/ 3 from telescopes.

6 Trigger Efficiency vs Pixel Size I (Central Telescope) Central Telescope Trigger Efficiency El: 3.5 km QE: 1.0, D=7m QE: 0.5, D=10m QE: 0.5, D=7m QE: 0.25, D=10m Parameters: E γ =42 GeV FoV=15 o Rnsb=1kHz Optimum trigger sensor pixel size is 0.07 o -0.3 o Weak dependence on QE, D, El Trigger Pixel Size [degree]

7 Trigger Efficiency vs Pixel Size II (Full Array) Array Trigger Efficiency p=0.05 o p=0.08 o p=0.10 o p=0.13 o p=0.16 o p=0.20 o Array Trigger: Three telescopes above operational threshold Array Parameters: Elevation: 3.5 km QE: 0.25 Reflector: 10 m FoV: 15 o Photon Energy [GeV]

8 Performance vs. spacing (vary #, fix A) Cost $$$ 91 tel/km 2 61 tel/km 2 37 tel/km 2 (Ø=7m, 1kHz sustainable NSB trigger rate per telescope)

9 Performance vs. spacing (vary A, fix #) 37 tel/km 2 61 tel/km 2 91 tel/km tel/km 2 (Ø=7m, 1kHz sustainable NSB trigger rate per telescope)

10 Performance vs. spacing (Ø=7m, 1kHz sustainable NSB trigger rate per telescope)

11 P.R.E. vs mirror area for 1km 2 array Cost $$$ Array configurations each with 1km 2 fiducial area. Hexagonal rings: 3 x 213m = 37 scope 4 x 160m = 61 scope 5 x 128m = 91 scope 6 x 106m = 127 scope 7 x 91m = 169 scope 8 x 80m = 217 scope Scope configuration: Ø = 5, 7, 10, 15m Ratio of mirror diameter to telescope separation, D/L [1]

12 Trigger Telescope Multiplicity Average Number of Telescopes in Trigger El=4.5km, QE: 1.0, D=7m El=4.5km, QE: 0.5, D=10m El=3.5km, QE: 1.0, D=7m El=3.5km, QE: 0.5, D=10m QE: 0.5 El: 4.5 km, D=7m QE: 0.25 El: 4.5 km, D=10m QE: 0.5 El: 3.5 km, D=7m QE: 0.25 El: 3.5 km, D=10m 40 GeV γ triggers 7 telescopes Photon Energy [GeV]

13 Cleaning Sample Event Photon direction [deg] Photon direction [deg] Event 1 (42 GeV) Photon direction [deg] Event 2 (42 GeV)

14 Cleaning Voronoi Diagram Photon direction [deg] Photon direction [deg] Photon direction [deg] Event 1 (42 GeV) Event 2 (42 GeV)

15 Cleaning P.E. Separation Scales Diff. density [Arbitrary] γ: 21 GeV NSB: 150 γ/deg 2 γ: 42 GeV NSB: 150 γ/deg 2 P.E. separation [deg] QE: 0.25 Reflector Diameter: 10m Elevation: 3.5 km Trigger pixel size: o Voronoi Diagram P.E.-P.E. separation scales in Image: o o γ: 100 GeV NSB: 150 γ/deg 2

16 Cleaning Sample Event Single 42 GeV event View from 4 telescopes Optimal cleaning (from consideration of angular reconstruction) keeps only photons near core Multiple cleaning schemes may be appropriate. Shower axis Shape cut Energy estimate

17 Performance of pixelated cameras dx Cleaning based on number of photons in neighborhood of each pixel. Cleaning scheme defined by two parameters: R radius of cleaning circle N number of photons Optimized for pixel size, dx, to give best reconstruction of gamma-rays. Almost degenerate: density

18 Sensitivity vs detector pixel size 40 GeV 100 GeV Simulations: 40, 100 GeV γ-rays with CORSIKA 7m telescopes 80m separation Perfect optics Pixellated cameras Analysis: 3D reconstruction Optimized cleaning based on density of PEs in image Results: 8 pixels 30-40% lower sensitivity 2-4 optimal

19 Lessons Cell Effect : operation in 40 GeV range possible with mid-sized telescopes Optimum trigger pixel size is ~0.1º (6 ) Optimum image pixel size is ~0.03º (2 ) Hard cleaning required to optimize reconstruction of shower axis Angular resolution of 6 (3 ) possible for 40GeV (100GeV) photons with 80m separation

20 What sensitivity might be achievable? Thought experiment: 1. 1km 2 effective area at all energies ang. resolution at all energies Background: 1. Diffuse background extrapolated from EGRET. 2. Cosmic electrons % proton flux from π 0 interactions. 4. Higher fraction of proton events assuming increasing rejection with energy.