The aim of the experiment is to learn about the basic techniques of gamma ray energy spectroscopy and the interpretation of a spectrum, using a NaI(TI) detector. We obtained the spectra of two gamma sources using a spectrometer and the Maestro program. We also computed the energy resolution of the spectrometer and the absolute activity of a gamma emitter using different methods. Radioactive nuclei decay into other particles: α-decay, β-decay, and γ-decay, where α particles are 4He nuclei, β particles are electrons or positrons, and γ particles are photons. Here we will study γ-rays decay. It occurs when the nucleus is too high in energy, so it goes to a lower energy state and emits a gamma particle, that is to say a photon. The number of protons and neutrons stay the same, on the other hand α and β decay. γ rays can easily penetrate most materials, that's why they are so dangerous to manipulate. However, they can easily be emitted from the source and be detected.
[...] We used the formula Eγs = (Eγ * 0.511 0.511 + Eγ cosθ)) where Eγs is the energy of the scattered photon in MeV, θ is the scattering angle for the scattered gamma, and Eγ is the incident gamma energy in MeV. Then we got the value of the Compton edge which equals Eγ - Eγs . Analysis of the 22Na spectrum: There are two phoptopeaks on the 22Na spectrum. The first at 511keV and the second at 1276 keV. [...]
[...] III.3 The absolute Activity The activity of a gamma emitter is given by the formula: Activity = ( - ( t*G*εp*f ) where is the sum under the photopeak, b is the background component of the spectrum, t is the live time in seconds, εp is the intrinsic peak efficiency, f is the decay fraction of the unknown activity which is the fraction of total disintegrations in which the gamma is emitted, and G is the area of the detector. [...]
[...] -Then, we searched a more accurate value of the FWHM using a fitting routine to fit a Gaussian. First we used the ChnConv program to convert a Maestro format into an ASCII file. Then we made graphs, in excel, of the three peaks we have (two for 22Na and one for 137Cs), using the values of energy we imported from MAESTRO. Finally we searched the best approximation of the parameters of a Gaussian equation to fit the graphs of the different peaks. [...]
[...] The first one is the photopeak at 511 keV, corresponding to the emission of a positron, that is to say the β+-decay of 22Na. The second peak, at 1274 keV, is due to the γ-decay of the excited nucleus. There are also the Compton edges situated just before the peaks. We can see them as a decrease of energy just before the peaks. And finally there is the backscatter peak at 190 keV, which is the backscattering of the photons of the γ-decay in the photomultiplier. [...]
[...] The programme gave us the value of the FWHM, and we obtained similar values using a Gaussian fitting routine. We could do this because of the Gaussian shape of the peaks, due to the photons distribution. The resolution we found give us an idea of how powerful is the detector, that is to say how well it can detect every particle. IV.3 The absolute Activity The peak efficiency represents the number of counts falling in the peak area over the total number of counts hitting the detector. [...]
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