1. BTF microbunching structure with Micro-Channel Plate PMT
Francesco Iacoangeli, Gianluca Cavoto
INFN – Roma

2. Introduction
A prototype for the Cherenkov detector for proton Flux Measurement (CpFM) of UA9 experiments was tested in October at the Frascati BTF.
See M.Garattini’s talk.
The aim of test beam was to measure the performance of optical coupling and the detection efficiency of a CpFM prototype.
Some different setups were studied. The angular scan and intensity scan were run so as to find the best configuration for final CpFM of UA9.
During system’s characterization the structure of microbunching appears on the waveforms

3. Detector
The CpFM prototype is made up of a fused silica radiator and a Micro-Channel Plate photomultiplier (MCP) coupled together (directly or through a fiber bundle).
Fibers/MCP adapter
Fiber Bundle
Cerenkov Radiator
MCP hamamatsu R3809U Series
Holder with thin window

4. Cerenkov radiator
The used Cerenkov radiator has a geometrical coupling not optimized
The Cerenkov detected light per electron depends on details of light reflection and diffusion in the radiator and at the radiator-PMT interface
Radiator surface
21 mm
MCP Area
11mm

5. Photo detector
Micro Channel Plate Hamamatsu PMT with Fast Time response:
Pros:
-Typical Rise Time: 150ps
-Typical Fall Time: 350ps
FWHM < 50ps
Low noise
Cons:
The effective photocathode area is a circle with diameter 11mm
 Only a part of Cerenkov radiator is coupled (11mm x 21mm rectangular section)

6. Readout electronics
Oscilloscope Lecroy WaveRunner 6 Zi :
Bandwidth : 1Ghz
Resolution: 8-bit
Sample rate: 10 Gsample/s
The bandwidth and the sample rate of oscilloscope isn’t optimal to see the microbunching structure

7. Hand Platform Rotation Stage
Test Setup
BTF Calo
(Pb Glass Cerenkov)
Čerenkov
Radiator
Oscilloscope
MCP-PMT Signal
Ethernet
Beam window
Control Room PC
Hand Platform Rotation Stage

8. Typical Waveform 5 e- event
Volt
5 e- event
100ps/div
- Due to fast time response of MCP-PMT, Waveforms can show the peak of each micro bunch.
- The measured charge of each peak should be proportional to number of electrons contained in the micro-bunch

9. Given the type of waveform the total charge of the signal is more significant than its amplitude
Unfortunately in the tested configuration the charge of a single particle event is significantly fluctuating
Nevertheless we tried to understand if it’s possible to use the CpFM to study the microbunching structure of BTF beam

10. Charge distribution Ne=1.5 Ne=1.5
Charge was calculated on a 40ns window centered on pulse minimum
Events with zero electron was suppressed
The plot of charge for nominal multiplicity of 1.5 e- shows the CpFM (MPC plot) isn’t good to discriminate single electron events

11. Charge resolution Ne=1,5 Ne=1,5 RMS/mean=0,16 RMS/mean=0,57
We considered the events selected by the first peak of CALO charge spectrum in order to estimate the resolution on single particle regime of CpFM
RMS/mean ratio value shows a big variance for CpFM with single electron events.

12. Efficiency
Efficiency=1- (MPC pedestal events) −(Calo pedestal events) Events –(Calo pedestal events) = 0,79
Ne=1,5
Ne=1,5
Overflow Events=1377
Overflow Events=3329
- pedestal values of charge for CALO and MPC were cut
The ratio of efficiency of CpFM , normalized on CALO no-zero e- events is: 79%

13. Waveform persistence CpFM - Ne=35e- events CpFM zoomed
- The persistence of waveform show that the CpFM prototype can discriminate the distribution of particles within the 10ns long bunch
CALO
- We found a two-peak shape of distribution that need to be explained (maybe due to gun RF ripple ?)

14. Time measurements MCP threshold CALO threshold Single Electron Event
The thresholds to perform time measurements were selected depending on minimum observed signal in single particle regime but to reject background
The values of threshold were fixed for all multiplicity of beam.

15. Time distribution
The delay between Trigger (LINAC NIM trigger signal) point out the time distribution of particles on the 10 ns bunch.
Ne=1,5
10 ns
We don’t know the cause of the 2 distinct distributions on CALO delay histogram,.
Ne=1,5
10 ns

16. Time resolution
- Time resolution measurements were performed with multi-particles events (Ne=236) so as to have at least 1 particles in the first microbunch.
RMS=266ps
Ne=236
RMS=455ps
10 ns
In this case too, we don’t know the cause of the 2 distinct distributions
20 ns
We measure the delay from trigger edge (LINAC NIM Timing signal) of the first particle of each event
The CpFM take out a distribution similar to the CALO’s one but by far less light

17. Conclusions
The CpFM prototype was not optimized to perform time measurements and readout electronics bandwidth should be upgraded for fast timing signal, nevertheless we can recognize the microbunching beam structure
Efficiency and resolution are quite affected by fluctuation on light detection: a suitable geometric coupling can decrease the ligth loss
Performing a new data-taking, with a better geometrical match and fast timing readout electronics, will be good to improve the measurement the microbunching structure
Eventually this system could be integrated as a BTF beam monitor detector.

18. SPARE

19. BTF Setup Diamond LAL Cerenkov INFN Cerenkov e- Beam BTF Remote
Control Table
BTF Calorimeter

20. Geometrical efficiency and optical coupling
Bundle of fiber spot
Lower light collection efficiency due to geometry:
MCP-PMT area:
π(11mm/2)2=95 mm2
Fibers cable area:
25π*(0.6mm/2)2=7.07mm2
Geometrical efficency ratio:
7/95 = ~7 %
10 mm
20 mm
75 mm ± 5 mm
Electron’s beam
Remaining efficiency losses due to
optical coupling and fibers attenuation = 7/3.3 = ~ 50 %

21. Direct contact VS Fibers
47˚
(normalized to electron path length into the radiator)
Charge per Electron
Radiator/beam angle
With fibers bundle the light detected per electron are 30 times less

22. Radiator with fibers bundle
Charge per Electron
(normalized to electron path length into the radiator)
47°
Radiator/beam angle

23. Fiber’s bundles Bandle 1: Bandle 2:
Fibers: Fibertech
Type AS600/660 UVST
Coating 780 μm Silicone
Jacket 940 μm Tefzel
0.6 dB/m at 350 nm
We tested 2 different bundles of same fibers
Bandle 1:
Homemade
Fibers hand-cut by diamond wheel
lapped with glass sand papers with increasingly grit ( )
Length: ~ 13 cm
Bandle 2:
Manufactured by Fibernet
lapped with industrial lapping process
Length: ~ 47cm
1 fiber cracked on manufacturing 

24. Time precision
The separated plot of each distribution show a RMS of 266ps for the first and of 455ps for the second.
This is promising to repeating the measure with more performing readout electronics

25. Time distribution 1e-
Ne=1
10 ns
measure window
10 ns