Detector Rates and Lifetime

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As the rates close to the photon beam line can be substantial it is necessary to investigate the consequences on the performance and lifetime of the scintillator itself and its readout devices.

Rate Estimates

The estimated rates in a given paddle depend on the upstream detector geometry and target material. Some estimates can be found in GlueX-DB 1471. In figure 7 on page 9 of this document you find the estimated rates for all paddles with the half sized paddles on the side (41 - 44). The rates for the central paddles are expected to be rather large up to 12 MHz! These are calculations with 6cm wide paddles of 1 inch thickness. Therefore splitting the central paddles in half is a necessity to cut down the rates to a more acceptable level.

PMT specifications

According to the specifications for the Hamamatsu PMT H9779 (8-stage) the maximum operating current is 0.1 mA. Elton estimated the signal amplitude based on this maximum current assuming a gain of 5*10^5 and 50ns signal base line. This results in an amplitude of 50mV. From bench tests with an LED on an H10534 PMT assembly with a 10-stage H9779 PMT and operating voltage of 1900V resulting in a gain of 4.1*10^6 and a measured 270 photo electrons per LED pulse a signal amplitude of 1.45V was recorded. The signal base line was about 20ns. Assuming a triangular signal shape the total charge of the signal is 0.5 * 1.45V * 20*10^-9s / 50Ohm = 2.9*10^-10 C. At a rate of 3MHz this would correspond to a current of 0.9 mA!

Rate and Current calculations

The basis of this estimate is GlueX-Doc 1471 figure 6 and 7. The following is an energy spectrum of a tof paddle for the expected electromagnetic background.
Tof adc thresh zero.gif Tof adc thresh 20.gif
The peak around 50 represents the photons that convert at the entrance to the TOF paddle or before thereby looking like a MIP particle. The shape of the peak is a typical Landau distribution. The mean of this peak (ADC=54) is approximately the mean energy loss of MIPs in plastic scintillator (2MeV/cm) and amounts to about 5MeV.
At zero threshold the mean of the full distribution is about 32. And corresponds to a rate of about 16MHz for the innermost paddle according to figure 8 of GlueX-Doc 1471. In order to estimate the current in the PMT for such a situation several assumptions need to be made:

  • Signal base line 25ns
  • Signal shape triangular
  • Signal amplitude for mean MIPs (ADC=54) 1000mV
  • Load 50 Ohms
  • Rage at zero threshold 16MHz

Here it is assumed that the HV of the PMT is set such that the mean energy loss of MIPs results in a 1Volt signal amplitude. The mean charge for a zero threshold is then given as
Q = 32./54 * 1/50Ohms * 25ns * 1/2 = 0.148 *10^-9
with a rate of 16MHz this results in a current of
I = Q*16MHz = 0.148 *10^-9 * 16*10^6 = 2.37 mA
This is a factor of about 24 higher than the maximum recommended current for the PMT of 0.1mA!

Reduce Rates and Current

For the innermost paddle where a full background rate of about 16MHz is expected at zero threshold, it is unavoidable to reduce the size of the paddles to reduce these very high rates. Reducing the width of the paddle by two will roughly cut the rate in half. Therefore such paddles will see a maximum rate of about 8MHz. At such a rate the current will still be far to high (factor 12) and it is necessary to reduce the current by reducing the signal amplitude. This can be achieved by reducing the gain. This means the operating high voltage is considerably smaller and it is important to see weather the voltage divider is still adequate for this lower HV. It is important to maintain a sufficiently large voltage drop between the photo cathode and the first dynode to ensure good timing. So at low operating voltage the choice of resistors may need to be reconsidered. This may also be true with respect to the last dynode.
In the situation at hand the mean signal amplitude for MIPs will need to be below 100mV and an amplification of the signal of about 5 to 10 has to be applied preferentially at the location of the divider. Such an amplification could be part of the base.
According to figure 7 in GlueX-Doc 1471 such a designed is probably required for the PMTs of the first 3 full paddles close to the beam line with the first paddle being split in half to make two paddles with 30mm width each. The four half paddles with only one PMT each should also have bases in this manner. The remaining paddles of the TOF see sufficiently low rates and can be operated with standard PMT bases.


Monte Carlo results

Monte Carlo data has been generated similar to GlueX-Doc 1471 to further investigate the issue of rates in the TOF detector. First we look at the relative rates between paddles by looking at the paddles hit distribution shown in the following plot:
Paddle hits.gif
Paddle number 20 is the long paddle (two sided readout) closest to the beam hole. The rate from electromagnetic background is 16MHz at high luminosity according to GlueX-Doc 1471. A first step in reducing this rate in this paddle is by splitting it in half make it two paddles of 3cm width each. What is also easy to see is that at the location of the fourth paddle the rate is about a factor of 10 less. At the location of paddle 15 (sixth from the center) the rade is reduced by a factor of 15.
The second important characteristic to know about this electromagnetic background is the energy distribution created in the TOF paddles. This is shown in the following figure:
Paddle embackgrd adc.gif
Paddle 20 is the closed paddle to the beam hole. The first number in each line represents the total integral of hits in each histogram. The second number is the mean energy deposition in the histogram.