PMT_Energy_Measurements - Energy Measurement Using a...

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1 Energy Measurement Using a Photomultiplier Tube (PMT) Scintillator detectors consisting of scintillating material coupled to a photomultiplier tube are widely used for precision energy measurements of photons, electrons and hadrons. An example of large plastic scintillator–PM arrays used by the ATLAS collaboration at the LHC to measure the energy of very high energy hadrons was given in Lecture 2 (see posted slides). In this week’s lab you will learn about pulse-height measurements and how to use pulse- height spectra to measure the energy of gamma rays emitted by different radioactive sources. You will study the relationship between high-voltage bias and the output of a photomultiplier tube, and compare the spectra made with plastic and sodium-iodide scintillator detectors. Prior to coming to lab you should open and study the three links Multichannel Analyzer (MCA), Light Pulser and Radioactive Sources found under Laboratory Apparatus Information on my web page. You also should calculate the energy value of the Compton edge of the 133 Ba, 137 Cs and 60 Co sources before coming to the lab. 2.1 Photomultiplier Response To explore the gain of a photomultiplier tube we use a light pulser to simulate the light output of the scintillator. The light pulser provides a light pulse of short duration at an intensity that depends on the applied voltage. Using this device we can explore the response of the phototube (PMT gain) as a function of the light intensity striking the photocathode. In effect we are simulating the light output of a scintillator. The light pulser is a simple instrument in which a capacitor is charged and then discharged through an LED using a Field Effect Transistor (FET) as a switch. The light pulser requires a DC voltage roughly in the +25 to +40 volts range and a positive trigger input pulse of 2- 4 volts. The first step is to make the pulser give appropriate light pulses. Connect the trigger output of a square pulse generator (i.e. the HP 8011A standalone or BNC 8010 NIM pulser) to the external trigger input of the scope and the signal output to the channel 1 input of the scope. Terminate the signal with 50 . Adjust the pulser to make pulses of width 30-40 nanoseconds, height of about 4 volts, and a pulse period (interval between pulses) of 20 to 30 microseconds.
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2 For this experiment you will use the 10 stage Electron Tubes 9266KB photomultiplier tubes available in the laboratory. This tube has a maximum rated voltage of 1000 volts. For our application we will operate the tubes in the 750-800 V range. If saturation effects seem to be present reduce the light level of the pulser, which can be done by either narrowing the pulse width or reducing the pulser power supply voltage. One should always look at the PM output pulses on the scope as the voltage is raised for the first time to check for light leaks or abnormal behavior. Connect the pulser supply and trigger voltages. With the voltage set at about 25-30 volts
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PMT_Energy_Measurements - Energy Measurement Using a...

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