Assume that we need to produce signals of 100 pe’s

  This is larger than the current average design goal for each scint.


Photoelectrons:  The number that we should use to compare pulses generated by flashers is photoelectrons.  Photo multiplier tubes, pmt, will run at different voltages and can have very different gains.  So measuring just the voltage seen by a pmt of a flash is not sufficient. We need to know how much light was produced. The amount of light in a pulse (number of photons) will determine how a tube’s voltage or gain should be adjusted so as to see the signal. If, for example, a large light flash (large number of photoelectrons) is generated I can lower the voltage on the pmt. This lowers the pmt gain. The light flash will produce a fixed amount of light but I can adjust the pmt so that it has a one volt signal in response to the flash. If the light flash is weak (small number of photons) I can increase the voltage of the pmt so that the response of the tube is again 1 volt. So the size of the signal in volts is not sufficient to characterize the flash.


How do you measure the brightness? The amount of light (number of photons) is not directly measured by a pmt. What is measured is the signal generated by only those photons that create an electron at the photocathode. Typically 20% of the photons produce an electron.  The number of photons is then approximately 5 times the number of photoelectrons. Common practice is to quote a light flash in terms of the number of photoelectrons it should generate on reasonable pmt.

We will use photoelectrons to measure the brightness of the flash and we will need to adjust our flashers so that they produce about the same amount of light at the photo multiplier tube as a decay positron does when it passes through a MULAN scintillator. Therefore we need to know the typical signal produced by a positron in photoelectrons.


We can easily generate single photoelectrons and measure them on the oscilloscope. The relevant signal characteristic is the area of the subsequent pmt signal. Measuring several single photoelectron signals we can find their average area, SPE-Area. If we divide any signal’s area by the average single photoelectron area, SPE-Area, we will determine the number of photoelectrons for our signal. This is the standard we will use in discussing our signals.


70 pes  Design goals presented at conference call May 29, 2003 (Chitwood)


60 pe’s  Presented at the February collaboration someone reported that roughly 60 pe’s were seen during the summer 2002 run. This was an unofficial estimate.


150 inner 300 outer pes  Presented at the March 200 meeting. Very early detector designs.


30-60 pes  Presented at the May 2001 meeting. Analysis of the 2000 data based on width.


40 pes  Presented at the May 2001 meeting. Detector studies in the lab.