We are concerned about the energy resolution and the photopeak efficiency of a different scintillation detectors ; to be specific we are testing the Nal and LaBr crystals . We used Na22 as a gamma source .For LaBr we had a 15.81% Energy Resolution , Nal-1 has 35.75% , Nal-2 has 27.30% .
The crystals LaBr and Nal was tasted using a gamma rays source (Na22) and a photomultiplier that produces an electrical signal when gamma ray is interacting with the crystal inside of it ; that signal undergoes an amplification process and in the end the signal is converted to a digital one using a multichannel analyzer connected directly to the computer ; and from the computer we can see and analyze the gamma spectrum .
Gamma-Ray Interactions :
There are three types of interaction mechanism for gamma rays : photoelectric absorption, Compton scattering , and pair production .All the three lead to a partial or complete transfer of Gamma energy to electron energy .
Gamma-Ray Detection :
A high energy Gamma ray enters a scintillator crystal producing a flash of low energy photons .The photons are directed to the photosensitive surface (photo cathode) of a photomultiplier where they eject electrons . These electrons are collected and amplified to have a current pulse , which is converted to a voltage pulse . The amplitude of the voltage pulses is related to the energy of the electrons freed by Gamma ray .Here we studied these pulses using a multichannel analyzer where the pulses are sorts according to hight with the number of counts in a separated channels , to give an energy distribution of the fast electrons (Gamma spectrum) .Figure 1. shows a typical MCA gamma spectrum .
Scintillation Detectors :
First of all we should define what a scintillation material is . A scintillation martial is a martial which exhibits flashes of lights (scintillations) when interacting with ionizing radiation . An Ideal Scintillation material has the following properties :
1- It should convert the kinetic energy of charged particles into a detectable light with a high scintillation efficiency .
2- This conversion should be linear .
3- The medium should be transparent to wavelength of its own emission for good light collection
4- The decay time of the induced luminescence should be short so that fast signal pulses can be generated .
5-The material should be of good optical quality and subject to manufacture in sizes large enough to be of interest as a practical detector .
6- Its index of refraction should be near that of glass (~1.5) to permit efficient coupling of the scintillation light to a photomultiplier tube .
Note that no material meets all these criteria .
Photomultiplier tubes :
Figure 2. Photomultiplier tube .
A photomultiplier tube is a vacuum tube consisting of an input window, a photo cathode, focusing electrodes, an electron multiplier and an anode usually sealed into an evacuated glass tube . A photo cathode converts the incident light photons to a low energy electrons . Because we have a small number of photoelectrons to form a convenient electric signal , we use an electron multiplier to increase the number of electrons . The charge signal is collected at the anode .
Multichannel Analyzer :
A multichannel analyzer is a succession of single-channel analyzers whose windows are all the same width and are arranged sequentially, in order of increasing energy .Thus by plotting the count-rate of each SCA versus its mean energy window setting ,a spectrum of count-rate versus energy will result .Multichannel analyzer operates in two modes : pulse height analyzer (PHA) mode, and multichannel scaler (MCS) mode. In PHA mode, the input pulses are sorted according to the amplitude, and in MCS mode they are sorted according to their arriving time .
It is the ability of a detector to distinguish between a two closely different energies ; it is defined as the Full width of the Photopeak at half maximum (FWHM) divided by the number of Peak channel ,and multiplied by 100. For Gaussian Distribution :
where s is the standard deviation .
Energy Resolution = FWHM x 100 .1
Number of Peak Channel
Detector Efficiency :
etot = total number of detected photons in the Photopeak .2
total number of photons emmited by the source
a.Checking The Signal :
Befoe we start taking the data , we should check the modules we are using ,that every thing works as it should be , and that the max ratings of every device are not exceced . We used an oscilliscope to read the output signal for every module .
1- Chicking if there is a signal (a nagitve signal ) coming out of the photomultilpier ; by connecting the anode output to an oscilliscope .
2- The fast amp has a gain = 10 ; so the output singnal should be 10 times bigger than the input signal .
3- The Amplfier inverts the input signal and changes its amplitude up to a maximum gain = 640.
4- For the Low Threshold Discrimnator the threshold values can be programmed in a range from -1 mV to -255 mV with
-1 mV steps ; and the width is programmable from 6 ns to 106 ns .
5- The Gate and Delay generator we are using accepts either polarity of input logic pulse,and provides a delay of up to 110 ï¿½ï¿½s .
b. Energy Resolution and Photopeak
1- LaBr Crystal :
-N22 Spectrum using LaBr as a detector :
-N22 Photopeak ; The fitting function is Gaussian function with 3 parameters ‘ f(x) = p0*exp(-0.5*((x-p1)/p2)2)) ‘ :
-Calculting the Energy Resolution for LaBr :
From formula .1 :
Energy Resolution = 15.81%
-Calculating the Photopeak Efficiency for LaBr :
From formula .2 :
Photopeak Efficiency =