1. Light mixer proposal Arjan Heering, Anton Karneyeu
HCAL Phase 1 Upgrades Meeting / 28 Apr 2016

2. The problem
Original design: fiber attached directly to the surface of the input window
SiPM operates best (linearity of response) if the light distributed uniformly over the input window
Solution: introduce light mixer
Light input fibers
SiPM
SiPM input window

3. Setup LED pulse is 25 ns APD for light reference, connected to QADC
SiPM connected to QIE11 readout
LED
APD
4 input channels: Y11 -> 4 meter clear fiber -> ODU -> MIXER -> SiPM

4. Configurations Connected fibers: 1 fiber 4 fibers Mixer diameter:
2.5 mm
2.6 mm
2.7 mm
2.8 mm
Mixer thickness:
No mixer
1 mm
1.6 mm

5. All measurements / RAW

6. SiPM Gain calculation
Low-light runs: resolution is enough to resolve single electron (pixel) peaks Average gain over all channels / runs: 52 fC / pixel

7. SiPM charge -> pixels

8. Light yield efficiency, %
Diam\Thickness
1fib 1 mm
4fib 1 mm
1 fib 1.6 mm
4 fib 1.6 mm
2.5 85 77 75
2.6 78 76
2.7 88 83 73
2.8 70 61 65
Normalized to the baseline (no mixer)
Extracted from low-light region (linear behavior)
=> Diameter 2.6 mm / thickness 1.6 mm looks best

9. APD Gain calculation
At low light SiMP expected to demonstrate linear behavior
Fit 5 low-light points with line, the slope gives qadc counts to effective photo- electrons conversion

10. APD qadc counts -> effective photo electrons
APD effective p.e. = Input Nph * PDE * mixer_efficiency
From TB: 15k p.e. ~ 300 GeV

11. Linear correction factor
Mixer improves linearity
Thickness 1.6 mm and 1 mm perform similar

12. Linearity of mixer t = 1 mm / d = 2.6 mm

13. Conclusions
Mixer thickness 1 mm, diameter 2.6 mm

14. Backup

15. Correction factor (vs. real light)

16. Correction factor (vs. real light) of the mixer t = 1 mm / d = 2.6 mm

21. Shunt factor 11.5 (design)

22. Shunt factor 10.9