Preflight PMT calibration: functionality tests

L. Arruda, J. Casaus, C. Díaz, J. Marín, C. Palomares

Z measurement error contributions statistical:

systematics from non-uniformities:

• radiator: n, thickness, clarity, ...

• @ detection level:

• PMT photocathode efficiency

• intrinsic variation: spread in the gains and QE

• global gain variation of PMTs (e.g. temperature variations)

• LG efficiency

• LG-PMT optical coupling

21 pepepe NN

22

0

2121 Z

NZ pe

CIN105 spe =(67.5+/-0.3)%syst=(0.94+/-0.03)%

• From the 2003 test beam data, we know that the effective uncertainty arising from radiator non uniformities is estimated in the 1-2% range. We will consider this as the upper acceptable limit for the remaining contributions to the final uncertainty.

The photocathode luminous sensitivity Sk=Ik/k(A/lum) (data sheet) is proportional to the quantum efficiency

Typical number of PMTs on a Cerenkov ring ~20

Cathode sensitivity spread ~6%

with <Sk> ~ 92.9 A/lum

Crude estimation

systematic effect detected signal (npe)

syst~ /npe ~ max/sqrt(20) = 6%/sqrt(20) ~ 1.5%

From Hamamatsu datasheet:

Photomultiplier quantum efficiency variation

Z measurement requirements

In order to reach the Z accurate measurement (sytematics <1%), precise knowledge (<6% level) of single Unit Cell photo-detection efficiency and gains are required The intrinsic spread in PMT gains and QE (even more when additional

elements are considered, i.e. LG, glue...) imply the need of a well defined calibration and monitoring strategy at different stages of the RICH assembly

IDEA: Map on the different detection cells:

pedestals

gains

relative quantum efficiencies

Gain calibration procedure during RICH assembly

System used:A Unit Cell adapted LED system to perform single photoelectron calibrations on individual PMTs: a LED connected to 4 optical fibers each attached to a unit like the one in the scheme.

Calibration method● A widely used method to determine the gains consists in

measuring the signal recoiled in the anode at very low levels of light, where the great majority of sucesses detected in the PMT were generated by only one incident photon: single photoelectron method. The signal is what is called the single photon answer.

● A fit to the signal allows to determine the gain and the mean number of p.e.

Functionality test: 1st studies The system has been validated on a row with 8 PMTs (R7600-M16-00) coupled to

light guides used in the vibration and thermal tests in July 2005• Hamamatsu data available only for 6 out of 8 PMTs show a wide range to try

and find a linear correlation with the measurements HV at 800V

Preliminary functionality tests

POS. Gx5 (@800V) Cath. Sens. Cath. Blue Sens.

1 49.0 105.0 11.5

2 91.3 ---- ----

3 59.6 75.1 9.13

4 103.5 ---- ----

5 43.5 92.7 10.9

6 43.6 82.6 9.43

7 152.4 109.0 10.5

8 133.2 87.1 9.53

LV Final flight electronics for the acquisition Blue pulsed LED (430 nm) operating at 3.48 V with

filters to attenuate the light (3 blue filters to attenuate the light)

The whole setup is placed inside a black box (1140 X 890 X 720 mm, wall thickness 20 mm) with a top door and lateral door that allows an easier access to the devices.

Experimental setupfibers

PMT

PMTPMT

filters

PMT

Acquisition program in LabVIEW8.0 (VI – Virtual Instruments) that generate a binary file with the histograms for the 8*16 channels

Acquisition software

Front pannel

Previous tests and results• Feasibility Study

• The system has been studied in the same row in last July.• The system, located in a smaller light tight box, was flashed with a blue LED connected to a Unit cell that was used in each of the 8 PMTs.• Results• The PMT average number of photoelectrons (sum of 16 channels) is shown to repeat to a 2%, which fulfills the requirements.• The pixel to pixel comparison for the 8 PMTs shows a spread at the 5% level, consistent with no pixel level effects on the Unit Cell, i.e., PMT QE, LG & optical contact efficiencies are can be considered as PMT-wide quantities.

Efficiency variation: Q, optical contacts

PMT id 7PMT id 7

First, in order to confirm the black box light isolation and the stability of the electronics pedestals runs with 50000 events were regularly acquired .

Pedestals analysis

PMT 2

For the pedestal fit:

Geff= G(,1) + (1 - ) G(,2)

2eff

= 21

+ (1 - )22

Dark current!

Pedestals analysisDifferences to the first measurement

A variation in the mean value of the pedestal of 1 (~4 ADC channels) was observed between two days from 13h15 to 17h25. From the thermal cycles, this is expected to be compatible with a 10ºC variation.

The maximum increase of dark current was of ~10% in one channel but for the other PMTs no significant increase was observed.

Dark current is estimated as the fraction of events beyond 4 sigma of the second Gaussian

LED StabilityTo study the stability of the LED, two consecutive spe runs of 60000 events were acquired in the same conditions.

A bi-parametric model of the photomultiplier response was applyed wich allows to extract: gain, sigma gain, average npe

GAIN PM3 PM7

<npe>

PM1

Fit failure !!!Gain 5

LED StabilityRelative variations of gains and <npe>

PM1 PM3 PM7

The LED is stable for gains within 3% and for <npe> within 4%

Setup Stability

● To test the setup stability several tests were done:– Openning an closing of the front door simply to confirm

the enter of light– Unit cell adapted LED removed and replaced in the

same PMT– After any mechanical operation in the black box a re-

avaliation of gains and <npe> was done

Setup Stability

Pixels with a gain variation within (2-3)%

Pixels <npe> with a variation <5%

PMT quantum efficiency evaluation● A relative method is forseen for pixel efficiencies evaluation:

– We will select a PMT (the PMT with highest gain is a good candidate for a more precise <npe> evaluation from the fitting procedure) and using the same unit cell adapted LED system we will compare this <npe> with the <npe> evaluated in the PMT in study. This will be done pixel by pixel.

Future Tests during AMS integration● Proposed system:

– LED-fiber system to perform spe calibrations on the complete detection plane.

– 4 fiber will light the dector volume upwards from the detection plane being reflected in a metacrylate layer simulating the flight-foil. The reflected light uniformly illuminates the detector plane.

Future Tests during AMS integrationRelative radiant intensity Spatial distribution

Angular spectrum

Check the angular spectrum at the fiber exit of a blue led (430 nm, 16° 2θ1/2) using a rotating platform and a powermeter The spectrum is well fitted by a Gaussian distribution with σ = 16.4°

Future Tests during AMS integration• Use the RICH simulation to obtain the relative illumination of the 10800 channels in the detection plane

•Although each fiber mainly illuminates a RICH quadrant, the small overlap helps to uniform the illumination of the detection plane (few spikes with low photodection probability are found)

• Feasibility Study• More than 97% of the channels are illuminated with an average number of photoelectrons within a factor 5• Most of the badly illuminated pixels are actually sitting out of the mirror diameter

Future Tests during AMS integration

Conclusions• Preflight calibration is a key issue for the RICH in-flight performance regarding Z measurement. • Two different tools are proposed to match the calibration needs at different stages of the assembly and integration.• The Unit Cell adapted LED calibration has been validated on 8 PMTs. •The proposed LED-fiber system would provide prompt response to the calibration needs during the integration and, in particular, could fix the calibration constants.• The conceptual design has been validated by the simulation.• Magnetic field effect can be monitored.