S tudy of Multi Anodes Photomultipliers for the PreShower read out of the LHCb experiment

O.Deschamps MAPMT Study 1

Study of Multi Anodes Photomultipliers

for the PreShower read out

of the LHCb experiment

3rd Beaune conference New developments in photodetection

Olivier DeschampsOn behalf of the LPC CLERMONT-FERRAND LHCb group


LHCb

The Large Hadron Collider beauty experiment

High energy Physics detector aiming at understanding the origin of the matter/anti-matter asymmetries in the universe.

20 meters long spectrometer on the Large Hadron Collider at CERNFirst data expected for 2007

Efficient trigger able to copewith the 40 MHz LHC beam frequency

The PreShower is of major importance for the first trigger level


One centimeter thick lead sheet as radiator

in front of a 6000 cells scintillator wall.

The LHCb PreShower

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.

About hundred 64-Anodes PMTs

perform the Read Out of the

6000channels.


The PreShower Read Out

A test bench has been developedfor the MultiAnode PMT studies

H7546 from Hamamatsu

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


Outline

Purposes of the test bench

Description of the experimental setup

Protocol – stability checks

Results :1. linearity measurements2. gain measurements3. anode uniformity measurements 4. cross-talk features

Conclusions and perspectives


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 %

Purposes Of The Test Bench

FEATURES ADRESSED IN THIS TALK


Experimental Setup - Global Sketch


Experimental Setup – the Light Sytem (1)

FILTERLED

2.5 cm

OPTICAL QUARTZ FIBREµ-LENS

OPTICAL COUPLERBALL LENS

3 mm

MonoAnode PMT as reference

Studied MultiAnodes PMT

Test Bench Signal (5ns Diode Voltage Pulse)

mimics the PreShower signal satisfactorily

PreShower signal from cosmics

Light signal shape


THE LIGHT SYSTEM DESIGN Fibre Active Core 200 µm



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.



Experimental Setup – PM Support System

DESIGN OF THE PM TABLE

The PMT moveswith respect to the optical fibre positionthanks to a x/y-translation-motorised table .The step resolution is again about 20 µm.

Meant to allow fine structure tests of PMT window.


Experimental Setup – Monitoring and Acquisition

MONITORING BY LABVIEW:

PM HV Supply

PM displacements in (x,y)

Optical filter displacements

READ OUT:

12 bits CHARGE ADC Lecroy

Acquisition Monitored With Labview

HAND MONITORING :

The diode voltage

The pedestal mode

Result from the mean and sigma of gaussian fit to the charge distribution from the PM response.Number of photo-electrons as

MEASUREMENTS :

)σ/(σ)MM(N 2P

2Q

2PQγe


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

Protocol stability checks


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.



CONVERSION CHARGE-CURRENT : suppose a triangular signal shape and measure the time half width.


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.



No effect of a saturating channel on others.

SATURATION IS A LOCAL PHENOMENON

Channel 1 firedReference PM

METHOD :

Make use of the optical Cross-Talkbetween channels.

Fibre is 1mm far from the PM window

EFFECT OF THE SATURATION OF ONE

CHANNEL ON THE OTHERS


RESULTS (2) – the Gain Measurement

Nice fit of the power law Q=aVb for the PM64 up to HV = 900V (limit of a linear response).

METHOD : make vary the supply voltage of the PMT by step of 25V

GeNQaVQ

e

b


RESULTS (3) – the anode uniformity

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) FIBRE CONNECTION

METHOD : measure the geometry of the PM window Fibre displacements of 200µm steps (by-product : the uniformity of the PM response within one channel).


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



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.

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

RESULTS (3) –anode uniformity within a pixel


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



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

RESULTS (4) – Cross-Talk


Conclusions

the MultiAnodes PMT test bench is in successful operation

First results addressed:

1. gain measurements 2. anode uniformity measurements3. linearity measurements 4. cross-talk features

MA64 PMT corresponds so far to the PreShower requirements


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

Perspectives


Acknowlegements


Protocol test : light yield stability check

The light is found stable within ± 1%

Light yield variation (9 hours run)

Q)/QQ(L/L i

Gaussianly distributed 6% fluctuation

Number Of Photoelectrons:

)/()MM(N 2P

2Q

2PQe

Reference monoanode PMT R5900 Atlas-Tilecal-like are used


Protocol test :pedestal stability check

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.

METHOD : pedestal measurement from the diode trigger, varying the HV and the operation time.


Protocol test :optical coupling stability

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.

)QQ/(QR 211

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.

THE OPTICAL COUPLING 2 reference R5900 PMTs


Results – The Linearity

CROSS-CHECK : THE SHAPE DISTORTION

IMAX = 0,8 mA5% deviation


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.


Gain measurements and calibration

Short and long term drift

Dark current

Measurements with few photo-electrons

Everything we did not think about yet

Purposes Of The Test Bench (2)

FEATURES NOT ADRESSED IN THIS TALK

Documents

S tudy of Multi Anodes Photomultipliers for the PreShower read out of the LHCb experiment