The article seeks to find out the average energy that is required for every scintillation photon as well as the energy resolutions present in crystals of sodium iodide that is activated with thallium and cesium iodide that is doped with thallium for gamma rays. Through this information, that author can comprehend the pathways that radiation energy to luminescence takes in scintillators. This information is important if one is to draw a comparison between scintillation yields from various scintillators. As such, this article adduces that the use of quantities that are related to scintillation efficiency is appropriate in understanding these pathways. Consequently, the article uses the average energy that is required to emit one scintillation photon (Knoll 136).
The average energies in gamma rays for the two crystals were measured using a photomultiplier tube that acted as a vacuum photodiode. When the crystal is connected to a photomultiplier tube, it gives forth an output signal. The amplitude of the signal was given using the following formula: - S=Nsp X Fcol X Fdyn X Fgan
S=the amplitude of the signal
Nsp= total number of scintillation photos that were produced by the crystal
Fcol= collection efficiency for the photons produced at the photomultiplier tube photocathode
Fcov= conversion efficiency for photons to electrons at the photomultiplier tube photocathode
Fdyn= collection efficiency for photoelectrons found at the first dynode of the photomultiplier tube photocathode
Fgan=total gain of the successive dynode chain (Martin 201).
Fig.1. Block diagram of experimental system
The number of photoelectrons was measured by coupling a crystal to the photomultiplier tube of equal diameter. Optical coupling fluid of equal refractive index as the material used to make the window is used to fill the interface between the photomultiplier tube and the crystal. Teflon tapes are used to cover the sides of the interface. The combination of the crystal and the photomultiplier tube is enclosed in a lead shield. The crystal is then irradiated with gamma rays sourced from sodium, cesium and cobalt. This generates photons that are mopped by the photocathode of the photomultiplier tube and finally converted into photoelectrons. The photon collection in the crystal is then calculated.
Two Monte Carlo codes SPC4 and EGS4 are used to obtain the collection efficiency for the photons produced at the photomultiplier tube photocathode. EGS4 counts the secondary electrons that are generated by the gamma rays hitting a crystal. SPC4 is used to trace the transport of every photon from the crystals from the minute they are generated to their termination (Bakshi & Ajay 75). The quantum efficiency and reflectance of the photomultiplier tube photocathode is also measured. This is for proper evaluation of the quantum efficiency and reflectance at the photocathode of the photomultiplier tube. This was done in order to determine the incidence of photons at any angle because the implement is initially designed to measure photons at an incident angle. To calculate the average energies required for the production of one scintillation photon, the total number of photoelectrons measures using many different combinations of the crystals and the photomultiplier tube (L'Annunziata 158).
The procedure was carried for both crystals of sodium iodide that is activated with thallium and cesium iodide that is doped with thallium. The average energies that are required to emit one scintillation photon when 662 keV gamma rays was used in crystals of sodium iodide that is activated with thallium was found to be 15.0 and 13.3 for cesium iodide that is doped with thallium (Sasaki, Tawara, Saito, & Shibamura 6427).
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Sasaki, S., Tawara, H., Saito, K., Miyajima, M., & Shibamura, Eido. Average Enerrgies Required per Scintillation Photon and Energy Resolutions in NaI(TI) and CsI (TI) Crystals for Gamma Rays. Japanese Journal of Applied Physics. 45(8A) 2006:6420- 6430. Print.