Basic Instrumentation - Part I of III

1. Scintillation Device
1. Collimator - will be discussed later
2. Aluminum (Al) shell and NaI(Tl) crystal
1. Al shell cover the crystal by three sides, leaving one end exposed
2. Little to no protection
3. Can shield alpha and beta
4. Has reflective properties and helps in the scintillation process
5. It is air tight where no moisture gets, is hygroscopic and hermetically sealed
3. Crystal is NaI contaminated with Tl - NaI(Tl)
1. Hygroscopic
2. Tl improves the scintillation process
3. Scintillation



1. Gamma rays are absorbed by the crystal (Compton and Photoelectric)
2. Excitation of outer shell election causes election to move into a higher orbit
3. Gamma ray returns to its normal orbit, it scintillates releasing energy in the form of light

Important - The entire process of recording gamma event must be proportional - From scintillation to the end produce (ex. counts)



4. Photomultiplier Tube (PM Tube)
1. Photocathode coverts light into electrons
2. The greater the light, the more electrons are released because of the increase in the energy gamma
3. Via the high voltage (HV) in the system, the electrons move towards the first dynode
4. For Each dynode that is the electrons encountered, they double in the amount and move on to the next dynode
5. Each PM tube has between 10 to 12 dynodes
5. As the amount of electrons increase a pulse height is created
   1. Pulse height
   1. As the electrons move through the system proportionally the pulse height continues to increase
   2. Once light has been converted to electrons the purpose of our imaging system is to magnify the pulse height which initially starts with the PMT
   2. Pre-Amplifier (Pre-Amp)
   1. PMT(s) are hard wired the rest of the imaging/counting system
   2. Impedance is applied which will slightly reduce the pulse height in order to prevent noise in the system
   3. It does not amplify the pulse height
   3. HV and Amplifier
   1. HV allows the pulse height to move through the system (stated earlier)
   2. Pulse height will be amplified as much as 8000 times

    

   4. Lower Level and Upper Level Discriminators (LLD and ULD) or Pulse Height Analyzer (see above graph)
   1. LLD and ULD sets a window in which the pulse will be recorded or rejected
   2. Any pulse that is below the LLD is rejected
   3. Any pulse that is above the ULD is rejected
   4. Only the pulse height that is between the LLD and ULD are allowed to continue through the system
   5. An electron pulse that falls within the LLD and ULD is recorded
   6. Diagram shows a window set at 20% around 140 keV gamma
   7. Calculate the LLD and ULD settings for a 140 keV gamma with a 20% window

   Step 1 - 20% / 2 = 10%

   Step 2 - Convert 10% to 0.1

   Step 3 - 140 * 0.1 = 14 keV

   Step 4 - (140 + 14 =) 154 keV is the ULD setting and (140 - 14 =) 126 keV is the LLD

   
   5. Reading the gamma event
   1. In the diagram used in this lecture a scaler reads the gamma event as a count
   2. Counts can be set to collect per second - cps
   3. Counts can be set to collect per minute - cpm
   4. Gamma camera records the gamma event on the location of a x/y axis to determine its location (next lecture)