1. Ultra fast SF57 based SAC M. Raggi Sapienza Università di Roma
PADME collaboration Meeting
LNF 17/1/2017

2. PADME Small angle calorimeter
Need to measure photons from ~100 MeV
No need for high light yield material 0.1-2p.e./MeV more than enough
Need to cope with very high rate several ~10 e+ per 100ns
Avoid scintillation mechanism if possible (t too long)
Need a good time resolution ~200 ps
Need very fast photosensors with low transit time spread
Need to be radiation tolerant (oder 1Gy per 1013 e+ on target)
Mauro Raggi - Sapienza Università di Roma

3. PADME SF57 based SAC Inefficiency for electrons at BTF
Very fast Cherenkov signal
Few100MHz rate capability
Low light yield expected
Need ~0.5 p.e./MeV
Very good time resolution ~200ps
Need very fast photosensors with low transit time spread
Inefficiency for electrons at BTF
NA62 (from OPAL) lead glass (Schott SF57)
Mauro Raggi - Sapienza Università di Roma

4. The PMT Hamamatsu R9880U-110 Compact ultra fast High Gain PMT
Diameter 16 mm only 8 mm sensitive area
Only 0.57 ns rise time and 0.2 ns transit time spread
Typical gain 2x106
Mauro Raggi - Sapienza Università di Roma

5. Read out board CAEN V1742 CAEN V1742 Sampling frequency 5Gs/s
12bits for 1V dynamic
1024 samples (200ns)
Mauro Raggi - Sapienza Università di Roma

6. First Small Angle Calorimeter test
R9880U-110
First tests during November calorimeter beam-time
Just one lead-glass bar, 20×20×200 mm3,
Wrapped in Teflon no optical coupling
Hamamatsu R9880U-110, operated at 950V (G~1.5x106)
Readout with CAEN V1742 digitizer set to 5 GS/s
Mauro Raggi - Sapienza Università di Roma

7. SF57+R9880-U110 Very short signals!
3 peaks in ≈5 ns
Black tape at end
1.5 ns pulse
Black tape
20 ns
Light reflections?
n = , speed of light≈16 cm/ns, 40 cm=2.5 ns
Number of events with multiple peaks reduced by rotating the crystal by ≈ 60° wrt the beam direction
Much better results by placing black tape absorber on the crystal front face.
Technology choice SF57+R9880U-110 seem ok!
Mauro Raggi - Sapienza Università di Roma

8. Long beam pulses 150 ns 150 ns 200 ps 200 ps
Mauro Raggi - Sapienza Università di Roma

9. Automatic peak fitting
Signal wdt = 3.5 samples
Means 3.5*0.2 = 700ps
Very short signals!!
200 ps
Root macro able to identify multiple peaks and measure the position (time)
Can be used to measure the double pulse resolution of the detector.
Integral of fit function to compute charge under development.
Mauro Raggi - Sapienza Università di Roma

10. Multi peak event Two real peak identified and position measured 200 ps
Mauro Raggi - Sapienza Università di Roma

11. Signal amplitude and time
Mauro Raggi - Sapienza Università di Roma

12. Npeaks and TDiff
Minimum Tdiff 2.5ns means that we can distinguish peaks 2.5 ns apart!
Mauro Raggi - Sapienza Università di Roma

13. Run 490 high multiplicity long bunch
200 ps
Run 490 was a long pulse high multiplicity run
Mauro Raggi - Sapienza Università di Roma

14. Amplitude and time Several overlapping particles
Signal up to 140 ns but distribution not very flat.
SAC can be used to monitor beam bunch structure
Mauro Raggi - Sapienza Università di Roma

15. 200 MeV run 494 (one peak charge)
No Black tape
Spectrum fitted with a Landau distribution
MPV = 5.2 pC
Q=5.2 pC= eNpeG ⇒ Npe=Q/(eG)= 5.2E-12/(1.6E-19*1.5E6)~21.6 p.e.
Order 0.1 p.e./MeV of incident energy (200MeV) to be corrected with MC for deposited energy
Mauro Raggi - Sapienza Università di Roma

16. 200 MeV run 495 (one peak charge)
Black tape
Spectrum fitted with a Landau distribution
MPV = 4 pC
Q=4pC= eNpeG ⇒ Npe=Q/(eG)= 4E-12/(1.6E-19*1.5E6)~17.2 p.e.
Order pe/MeV of incident energy (200MeV) to be corrected with MC for deposited energy
Mauro Raggi - Sapienza Università di Roma

17. Energy deposit simulation 200 MeV
Simulated single crystal of SF57
20x20x200 mm3
Incident electrons energy 200MeV
Radius of the beam spot 3mm.
Energy deposit ~ 130 MeV
Fraction of deposit ~65%
Renormalizing LY of the SF57
Run 494 = 0.1/ = 0.154
Run 495 = 0.083/0.65 = 0.127
Mauro Raggi - Sapienza Università di Roma

18. Improving current setup
20mm
Surface coverage of the PMT 9880-U110
0.4x0.4 x p = 0.5 cm2 on 2x2=4cm2 just 12.5% coverage
Does fast PMT with large photocathode area exist?
Can we use light guides (Winston cones?)
No optical coupling crystal-PMT
Deep air gap due to borders in the PMT.
Can use silicon glue to get better optical coupling
No proper mechanical support
PMT gain ~1.5x106 can be raised up to ~3x106
30x30 mm2 with 2 PMT each same ratio
20mm
30 mm
30 mm
Mauro Raggi - Sapienza Università di Roma

19. Improving crystal LY SF57
Can we found a glass with better transparency in the UV region?
SF57 seems to have a bad fall down at ~400 nm
Important gain is expected in the light yield due to better matching with Cherenkov spectrum
Mauro Raggi - Sapienza Università di Roma

20. Lead-fluoride PbF2 vs SF57
Density
7.77
5.51 X0
0.93
1.54
Moliere radius
2.12
2.61
Interaction Length (l)
22.1
20.6
l/X0
23.65
13.3 n
1.8
Higher density, more compact showers, better l/X0 ratio. Better transparency down to ~250 nm
10x more radiation hard wrt SF57
SF57
arXiv: v2
Mauro Raggi - Sapienza Università di Roma

21. Radiation damage
Radiation damage 60Co: 1) PbF2 after 200Gy of 60Co 2) Lucite after 200Gy of 60Co 3) SF5 after 200Gy of 60Co
PbF2 damage after high dose
No effect up to 100Gy
Serious damage at 1KGy
~1Gy is the expected dose at PADME
arXiv: v2
Mauro Raggi - Sapienza Università di Roma

22. Results obtained with PbF2
Mauro Raggi - Sapienza Università di Roma

23. Conclusions
Cherenkov radiator coupled with a R9880-U100 PMT can provide extremely fast signal
RMS~600ps have been measured with electrons at BTF
Reasonably high light yield ~0.15 p.e. MeV can be reached
Even higher performance ~1p.e. MeV can be achieved using PbF2
Interesting solution to be explored PbF2
10x higher radiation hardness wrt SF57
4-5x higher light yield
More compact calorimeter (X0 only 0.93 cm)
Two samples of PbF2 received from mu2e are ready to be tested
Cherenkov radiators maybe a suitable technology for PAMDE SAC detector
Mauro Raggi - Sapienza Università di Roma