Properties of the 3" EMI 9822QKB PMTs

The page under construction.

There are 240 CsI crystals in the calorimeter ball, of which 200 require 3" photomultiplier tubes (PMT). Our trigger efficiency and background correction requirements place the following demands on PMT gain and linearity:

Discussion of PMT non-linearity

We have adopted a Bill Stephens modification of the EMI recommended high voltage divider. This divider minimizes the so-called "super" or "over"-linearity exhibited by many PMTs well below the onset of saturation.

The figure below shows the high voltage divider currrently used in our linearity tests described below.

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The next figure shows gain as a function of voltage for two versions of the 9822 PMT.

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Below is shown the high rate FET base currently under production for the pibeta experiment.

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Discussion of PMT rate-dependent gain shifts

Ideally, the gain of a PMT is constant. In practice, the gain depends on a number of external factors such as temperature and magnetic field, as well as the average anode current arising from the photon flux at the cathode. This latter effect can be attributed to several causes: Our high voltage divider is designed to minimize contributions from the first type of gain shift. To reduce the dynode dependent effect we have chosen the SbCs dynode offered by EMI.

Estimate of Michel background rate

The following parameters define the problem: Thus, the resulting ADC sensitivity is 1.4 pC/MeV or 11 pC/MeV at the PMT. The gain necessary to achieve this depends on the photoelectron/MeV efficiency of each CsI crystal/PMT combination, e.g- 150 p.e./MeV requires a gain of 6.8E5. In any case, there will exist a Michel background of 4 kHz in each CsI crystal with an average charge/pulse of 500 pC, or 2 microamps.

Test setup used to measure rate dependence of gain

We use a two LED technique to measure the rate-induced gain shift. A reference LED, shown in the diagram below, is pulsed at 10 Hz and the gain of the PMT adjusted to produce a charge/pulse of 780 pC, corresponding to the charge expected from a 70 MeV pibeta photon. A background LED is pulsed at a computer adjustable rate from 25 Hz to 18000 Hz, with a charge/pulse approximately 1/2 of the reference LED. An ADC is used to digitize the PMT charge resulting from the reference LED.


A comparison of the gain shift as a function of background LED rate for BeCu and SbCs dynodes is shown below.

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Finally the gain shift at 5 kHz has been measured for all pibeta 9822QKB PMTs currently in house at UVa and PSI. A comparison of BeCu and SbCs dynodes is shown below.

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Database of UVA rate effect measurements:

BeCu DynodesSbCs Dynodes
 Ser.No.        R
 Ser.No.        R

R is the ratio Q(5 kHz)/Q(25 Hz), where Q is the charge measured for the reference LED discussed above., 15. Feb 1995