1. New developments in photodetection
3rd Beaune conference
New developments in photodetection
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Study of Multi Anodes Photomultipliers
for the PreShower read out
of the LHCb experiment
Olivier Deschamps
On behalf of the LPC CLERMONT-FERRAND LHCb group
O.Deschamps
MAPMT Study

2. The PreShower is of major importance for the first trigger level
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The Large Hadron Collider beauty experiment
High energy Physics detector aiming at understanding the origin of the matter/anti-matter asymmetries in the universe.
LHCb
20 meters long spectrometer on the Large Hadron Collider at CERN
First data expected for 2007
Efficient trigger able to cope
with the 40 MHz LHC beam frequency
The PreShower is of major importance for the first trigger level
O.Deschamps
MAPMT Study

3. About hundred 64-Anodes PMTs perform the Read Out of the
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The LHCb PreShower
One centimeter thick lead sheet as radiator
in front of a 6000 cells scintillator wall.
About hundred 64-Anodes PMTs
perform the Read Out of the
6000channels.
Cells are grouped by block of 64, forming a preshower module.
Scintillation light is extracted from the plastic cells with
1mm thick helicoidal Wavelength Shifting Fibres.
Both WLS fibres ends are connected to long clear fibres.
Each cell is read out through one channeof a 64-Anodes PMT.
O.Deschamps
MAPMT Study

4. A test bench has been developed for the MultiAnode PMT studies
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The PreShower Read Out
8x8 matrix of fibres connects
the 64 preshower channels to the
64 Anodes of a PhotoMultiplier Tube
ASIC specially designed
Perfect 40MHz integrator
more details in
the poster session I
H7546 from Hamamatsu
A test bench has been developed
for the MultiAnode PMT studies
O.Deschamps
MAPMT Study

5. Purposes of the test bench Description of the experimental setup
Outline
Purposes of the test bench
Description of the experimental setup
Protocol – stability checks
Results :
linearity measurements
gain measurements
anode uniformity measurements
cross-talk features
Conclusions and perspectives
O.Deschamps
MAPMT Study

6. Purposes Of The Test Bench
FEATURES ADRESSED IN THIS TALK
Linearity:
The PreShower has to detect Minimim Ionizing Particles (20-30 photoelectrons)
and to measure electromagnetic shower energy deposit (up to 100 MIPs)
A 10 bits dynamics is required for PreShower.
The photomultiplier response required to be linear over the whole range
Anode uniformity of the response:
Same supply voltage for the 64 channels.
The gains must (will) be adjusted by dedicated load resistors on the very front-end card.
However non-uniformity must be in the larger ratio (1:3).
Uniformity of the response within one channel:
this information may serve the adequate design of the connectic geometry fibre/PMT.
Cross-talk between the PMT channels:
a large signal on one channel should not induce fake signal for adjacent channels. The cross-talk must be of the order of 1 %
O.Deschamps
MAPMT Study

7. Experimental Setup - Global Sketch
O.Deschamps
MAPMT Study

8. mimics the PreShower signal satisfactorily
Experimental Setup – the Light Sytem (1)
FILTER
LED
2.5 cm
OPTICAL QUARTZ FIBRE
µ-LENS
OPTICAL COUPLER
BALL LENS
3 mm
MonoAnode PMT as reference
Studied MultiAnodes PMT
Light signal shape
Test Bench Signal
(5ns Diode Voltage Pulse)
PreShower signal
from cosmics
mimics the PreShower signal satisfactorily
O.Deschamps
MAPMT Study

9. Experimental Setup – the Light Sytem (2)
Fibre Active Core 200 µm
THE LIGHT SYSTEM DESIGN
O.Deschamps
MAPMT Study

10. Experimental Setup – the Light Sytem (3)
CHARACTERISTICS OF THE FILTER
Linear in optical density up to D=3
D = log (1/T)
Light attenuation up to a factor 103
The filter displacement is motorised
The step resolution is about 20 µm.
90 % of the dynamics is covered in 1/3 of the whole distance.
A lot of points are available at very small light yield.
O.Deschamps
MAPMT Study

11. Experimental Setup – PM Support System
DESIGN OF THE PM TABLE
The PMT moveswith respect to the optical fibre position
thanks to a x/y-translation-motorised table .
The step resolution is again about 20 µm.
Meant to allow fine structure tests of PMT window.
O.Deschamps
MAPMT Study

12. Experimental Setup – Monitoring and Acquisition
MONITORING BY LABVIEW:
PM HV Supply
PM displacements in (x,y)
Optical filter displacements
HAND MONITORING :
The diode voltage
The pedestal mode
READ OUT:
12 bits CHARGE ADC Lecroy
Acquisition Monitored With Labview
MEASUREMENTS :
Result from the mean and sigma of gaussian fit
to the charge distribution from the PM response.
Number of photo-electrons as
O.Deschamps
MAPMT Study

13. Protocol stability checks
Light yield stability
Reference monoanode PMT R5900 Atlas-Tilecal-like is used
The light is found stable within ± 1%
Thenumber of photon-electrons is gaussianly distributed with sigma at 6%
Pedestal stability
Pedestal spread is found to be around one ADC channel.
Stable along the operation time within 0.1 ADC channels
Stable with High Voltage variations within 0.2 ADC channels.
Optical coupling stability
The light spot size on the filter is 3mm wide and logarithmically distributed
Two reference monoanode PMTs are used to chec the coupling ratio stability
Stable within few per mil for 90% of the dynamics.
Linearity measurements with a reference PMT possible in this range
O.Deschamps
MAPMT Study

14. RESULTS (1) – the Linearity
METHOD : make vary the attenuation – 50 measurements. Suppose the PMT is linear in the region of very high attenuations; fit of a straigth line to the response and compute the deviation to the expected linearity.
O.Deschamps
MAPMT Study

15. RESULTS (1) – the Linearity
CONVERSION CHARGE-CURRENT : suppose a triangular signal shape and measure the time half width.
O.Deschamps
MAPMT Study

16. RESULTS (1)– the Linearity
CONCLUSIONS :
There are several ways to express the linearity of the PMT response.
The MA64 PMT is linear (within 5%) up to 100 photoelectrons at 850 V.
The MA64 PMT is linear (within 5%) up to 2000 photoelectrons at 650 V (expected working voltage).
Convenient way to compare with the electronics board design : a deviation to the linearity at the 5 % level is observed for a maximal current at the anode of about 1 mA.
Covers satisfactorily the whole dynamics range.
O.Deschamps
MAPMT Study

17. RESULTS (1) – the Linearity
Reference PM
Channel 1 fired
EFFECT OF THE SATURATION OF ONE CHANNEL
ON THE OTHERS
METHOD :
Make use of the optical Cross-Talk
between channels.
Fibre is 1mm far from the PM window
No effect of a saturating channel on others.
SATURATION IS A LOCAL PHENOMENON
O.Deschamps
MAPMT Study

18. RESULTS (2) – the Gain Measurement
METHOD : make vary the supply voltage of the PMT by step of 25V
Nice fit of the power law Q=aVb for the PM64 up to HV = 900V 
(limit of a linear response).
O.Deschamps
MAPMT Study

19. RESULTS (3) – the anode uniformity
METHOD :
measure the geometry of the PM window
Fibre displacements of 200µm steps
(by-product : the uniformity of the PM response within one channel).
The pixels are set regularly
similar answer between pixels
double structure; dissymmetry x/y (this image of the PMT window is consistent with the physical view)
 FIBRE CONNECTION
border effects dissymmetric
(negligible in x, huge in y)
O.Deschamps
MAPMT Study

20. RESULTS (3) – the anode uniformity
As an illustration of the test bench facility, gain uniformity measurements for the 64 channels of the Flat Panel PMT from HAMAMATSU studied in LPC from the purpose of medical imaging
Dispersion less than a factor 3
12 channels of the M64 APMT
O.Deschamps
MAPMT Study

21. RESULTS (3) –anode uniformity within a pixel
METHOD : displace the PMT by step of 100 µm and measure the response.
 FIBER AT 200 µm FROM PMT WINDOW
FIBER AT 1000 µm FROM PMT WINDOW 
Evidence of a double structure - intuitively dictated by the first look at the PM window - but strongly dissymetric. When the fibre is moved back, light is averaged and there is a useful surface large in X, thin in Y.
O.Deschamps
MAPMT Study

22. RESULTS (3) – the anode uniformity
CONCLUSIONS
The measured pixels are set regularly
The maximum of the PMT answer is dissymmetric in x and y
 may serve the design of the FIBRE CONNECTION
There are border effects again dissymmetric
 may serve the design of the FIBRE CONNECTION
Variation from one channel to the other (less than 3) within the electronics gain correction requirements so far.
Non-uniformities occur in the multiplication chain
O.Deschamps
MAPMT Study

23. RESULTS (4) – Cross-Talk
METHOD :
the fibre is 100µm far from the PM window
fire a central pixel and measure the 8 neighbours charge
result is normalised to the central value
two channels measured
Basically NO INTRINSIC CROSS-TALK between the PMT channels
O.Deschamps
MAPMT Study

24. Conclusions the MultiAnodes PMT test bench is in successful operation
First results addressed:
gain measurements
anode uniformity measurements
linearity measurements
cross-talk features
MA64 PMT corresponds so far to the PreShower requirements
O.Deschamps
MAPMT Study

25. More details in a dedicated poster :
Perspectives
Perform short and long term stability tests.
In particular check the aging behaviour that could be crucial in the concern of the LHCb hostile environment.
Test other candidates :
XP1700 (Photonis),
M16 et M64 (six dynode stages) HAMAMATSU
Multi-elements APD array
Perform the acquisition with the LHCb PreShower full electronics chain developped.
More details in a dedicated poster :
« ReadOut system for the LHCb PreShower », LPC/LHCb, Session I
O.Deschamps
MAPMT Study

26. Acknowlegements
O.Deschamps
MAPMT Study

27. Number Of Photoelectrons:
Protocol test : light yield stability check
Reference monoanode PMT R5900 Atlas-Tilecal-like are used
The light is found stable within ± 1%
Light yield variation (9 hours run)
Number Of Photoelectrons:
Gaussianly distributed  6% fluctuation
O.Deschamps
MAPMT Study

28. Protocol test :pedestal stability check
METHOD : pedestal measurement from the diode trigger, varying the HV and the operation time.
Pedestal spread is found to be around or less than one ADC channel.
Pedestal mean value is stable along the operation time within 0.1 ADC channels
Pedestal mean value is stable with HV variation (500V to 900V) within 0.2 ADC channels.
O.Deschamps
MAPMT Study

29. Protocol test :optical coupling stability
2 reference R5900 PMTs
THE OPTICAL COUPLING
The light spot size on the filter is 3mm wide.
The filtered light is then dissymetric
and logarithmically distributed.
It is likely that the optical coupling is sensitive to the opening angle of the light from the fibre. If the initial spread is conserved, the coupling ratio R could be erratic.
Significant effect is observed in the region of transition between full transparency and active part of the filter. Flat answer within few per mil for 90% of the dynamics. Linearity measurements with a reference PMT possible in this range.
O.Deschamps
MAPMT Study

30. Results – The Linearity
CROSS-CHECK : THE SHAPE DISTORTION
IMAX = 0,8 mA
5% deviation
O.Deschamps
MAPMT Study

31. Results – Anode Uniformity
is the non-uniformity related to the photocathode or the multiplication system ?
CONCLUSION : no correlation between Npe and the response. Also supported by the gain studies.
O.Deschamps
MAPMT Study

32. Purposes Of The Test Bench (2)
FEATURES NOT ADRESSED IN THIS TALK
Gain measurements and calibration
Short and long term drift
Dark current
Measurements with few photo-electrons
Everything we did not think about yet
O.Deschamps
MAPMT Study