Lecture 1—Basic Principles of Scintillation Unit III: Non-imaging Scintillation Detectors. Lecture 1—Basic Principles of Scintillation CLRS 321 Nuclear Medicine Physics and Instrumentation I

Objectives Define scintillation Describe the structure & purpose of a NaI (Tl) crystal and the crystal’s proportional response to deposited gamma energy Describe the components of a photomultiplier tube and their function Recognize factors affecting scintillation detector efficiency Explain the concept of peak broadening

Scintillation Detector = Energy Transfer Incident Photon Energy Visible Light Energy Electrical Energy

NaI (Tl) Crystal (hermetically sealed in reflective material because it is hygroscopic) Visible light Gamma Photon Paul Christian, Donald Bernier, James Langan, Nuclear Medicine and Pet: Technology and Techniques, 5th Ed. (St. Louis: Mosby 2004) p 53.

For each keV of gamma photon energy absorbed, 20-30 visible light photons are released by the crystal. Therefore, a 140 keV photon will cause about 3000 visible light photons to be released from the crystal Visible light produced is 325 to 550 nm wavelength 140 keV Gamma Photon

Important Magic to Remember The higher the gamma photon energy : The more visible photons created The visible light emitted from the NaI (Tl) crystal is PROPORTIONAL to the incident energy of the gamma photon.

The Scintillation Process in NaI(Tl) Figure 2-1 Band structure and scintillation process in thallium-activated sodium iodide. The three steps in the scintillation process are: a) some of the free electrons created by a gamma ray interaction have enough energy to jump across the forbidden gap to the conduction band; b) once in the conduction band, these electrons migrate to the activation center excited-state orbitals, and the holes in the valence band to the activation center ground-state orbitals; c) the transition of an electron from an activation center excited-state orbital to an activation center ground-state orbital produces a scintillation photon. © 2010 Jones and Bartlett Publishers, LLC

1 thallium atom is in place of about 1/1000 sodium atoms http://www.webelements.com/webelements/compounds/media/Na/I1Na1-7681825.jpg

Timing after a gamma interaction: Scintillation peak in about 30 nsec and about 2/3 of light emitted after 230 nsec At lower rates of interaction (low count rate), a scintillation event typically ends before the next Hence scintillation detectors operate in pulse mode

Photomultiplier Tube http://www.kolumbus.fi/michael.fletcher/pmt_1.jpg

Crystal/PMT Interface Simon Cherry, James Sorenson, & Michael Phelps, Physics in Nuclear Medicine, 3d Ed., (Philadelphia: Saunders (Elsevier) 2003), pg. 101.

K2CsSb or Na2CsSb Photocathode NaI (Tl) Crystal Gamma Photon Visible Light Emitted electrons from photocathode Optical Window (transparent material)

Proportionality Maintained Note: This is the least efficient phase of the transfer For every 3 – 5 visible light photons reaching the photocathode, 1 electron is emitted

Photomultiplier Tube (PMT) Construction Focusing Grid – Guides in Electrons Dynodes – Usually positively charged photoemissive coated electrodes with increasing voltages Anode – end positively charged High voltage power supply – needed to increase incrementally the potential difference between dynodes

Increased voltages between dynodes : means increased KE of electrons between dynodes Increased KE of electrons mean that more electrons are knocked off at each dynode Dynodes apply a factor of 108 to the initially released several hundreds of electrons resulting in trillions of electrons arriving at the anode. (Still small – 1 Amp = 1 C/s; 1 C = 6.3 X 1018 electrons)

Bottom Line—Charge is Proportional to Incident Photon Energy

Inefficiencies from the transfer of energy The proportionality of the system is approximate and contains random statistical error Peak broadening Higher energy gamma increases “information carriers” resulting in narrower peak

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