دراسة امتصاص الإشعاع (دراسة معامل الامتصاص الخطي والكتلي لأشعة جاما )

Abstract

Multichannel analyzer (MCA) provides an array of counting channels, or scalars, and systematically stores incoming pulses in these channels according to some prescribed criterion. The criterion defines a mode of operation. MCA can scan a whole energy range and record the number of pulses they count in each of the channels. A full width at half maximum (FWHM) is an expression of the extent of a function, given by the difference between the two extreme values of the independent variable at which the dependent variable is equal to half of its maximum value. Gamma rays are photons (quanta of light) and have no electric charge and no rest mass. Therefore, the interaction of gamma rays with matter is weak. There are 3 mechanisms that are important from the point of view of radiation protection such as (i) photoelectric effect (ii) Compton scattering (iii) pair production. This work will focus to photoelectric effect. The idea of the experiment is when the photons emitted from the source and strike the detector,The PMT consists of a photocathode, a focusing electrode, and 10 or more dynodes that multiply the number of electrons striking at each dynode. A chain of resistors typically located in a plug-in tube base assembly biases the anode and dynodes. Complete assemblies including the scintillator and PMT are available. The properties of a scintillation material required to produce a good detector are transparency, availability in large size, and large light output proportional to gamma-ray energy. Sodium iodide [NaI(Tl)] crystals are commonly used. The high Z of iodine in NaI(Tl) crystals result in high efficiency for gamma-ray detection. Resolution for a 2inch diameter by 2 inch length crystal. The light decay time constant for a NaI(Tl) crystal is about 0.23 µs. Typical charge-sensitive preamplifiers translate this into an output voltage pulse with a risetime of about 0.5 µs. Fast coincidence measurements cannot achieve the very short resolving times that are possible for plastic scintillators, especially at low gamma-ray energies. So a good signal will appeared from the MCA represented by peaks we take the bigger one to get the results . And before we start the details of our experiment we’d like to going into some properties of gamma ray and its interactions with matter according to the probability of its interactions and the attenuation of its intensity during the interaction with different types and thicknesses of materials, as we used the (AM-241) source of gamma.