Gamma ray spectroscopy using a scintillator detector

Résumé du document

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.

Extraits

[...] To do this we transferred the datas to the buffer using the program. Figures 2 and 3 show the calibrated spectra of 22Na and 137Cs. Figure Calibrated spetrum of 22Na, which gives the counts in function of the energy. Figure Calibrated spetrum of 137Cs, which gives the counts in function of the energy. II.2 The Background We also get a spectrum of the background to see if it was relevant compare to the spectra of the sources. To get this, we accumulated a spectrum without putting any source in the detector. [...]

[...] -Photo-Electric effect: A γ photon interacts with an electron, ejecting this electron from the atom. The kinetic energy of the photon-electron pair equals the energy of the incident photon minus the binding energy. The Photo-Electric effect converts a photon signal into an electrical signal where the free electrons are collected as a current. -Compton effect: A γ photon loses enough energy to an electron to eject this electron from the atom. Then a new photon of lower energy is emitted. [...]

[...] 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. [...]

[...] 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. Those photons don't withdraw the photoelectric effect and go through the photomultiplier where they are sent back in the opposite direction. There is one photopeak at 662 keV on the 137Cs spectrum. Caesium137 β- -decay, that is to say it emits an electron. This produces an excited nucleus 137Ba, that emits the photon at 662 keV by the γ-decay. [...]

[...] If we know the activity of a standard source S1, we can find the activity of another source U1 with the formula A(U1) = ( - ) * A(S1) IV.4 Error on the activity The relative error of the activity is given by the formula: ( ΔA / A ) = ( ( Δ∑U1 + ( Δt / t + ( ΔG / G + ( Δεp / εp + (Δf / f ) The error on the activity of each peak is described in Figure 10. Figure10: Error on the activity computed for each peak. These errors could have been minimized by using a better detector, with a better resolution. V. Conclusions I have learnt from this experiment how to analyse a spectrum of a radioactive source. I also computed the resolution of the spectrometer and the acitivity of a gamma emitter. As we saw, these results can be improved by better detectors. [...]