Gamma Ray Spectroscopy

AbstractThe abstract goes here.


Gamma ray spectroscopy is a useful tool for analysing the isotopic composition of a radioactive material. The energy of photons in gamma-ray spectroscopy are typically on the order of 10-1000keV, introducing a collection of interactions not observed in traditional optical spectroscopy. These interactions include the degree of penetration through a medium of a specific composition, lead for example, and the energetic dependency on such interactions.

Gamma-ray sources in most cases are the result of a parent nucleus decaying via alpha particle emission, resulting in an daughter nucleus which is in an excited state. The nucleus transitions to the ground state configuration and releases the energy difference as a photon. The shell model of the nucleus best explains this effect, whereby a nucleon is found in a higher shell upon decay and transitions to the lowest unoccupied vacancy. ({Front Matter}) Figure 1 outlines this process.

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Other sources of gamma-rays are also possible, such as from astronomical events (Rossi 1964)(Chupp 1976), although they are not relevant to this study as the focus is from sources within the nucleus.

As the gamma-rays are not charged particles, they cannot be detected directly, although interactions with matter can produce measurable effects such as the Photoelectric Effect and Compton Scattering. Measuring the intensity of gamma-ray emissions from a source can be completed using the apparatus shown in Figure 2. This setup involves placing a source under a photomultiplier tube (PMT) with a scintillating crystal. Gamma-rays from the source enter the scintillator, where an interaction causes an electron to be ejected into the PMT. The electron is accelerated through several dynodes to a collector plate, with each successive dynode producing greater numbers of electrons of a specific energy. The resultant voltage/current pulse recorded at the detector is proportional to the energy of the incident gamma-ray.

The pulses are shaped through an amplifier, where the height of each peak correlates to the energy of the gamma-ray. This requires the amplifier output to be analysed in steps or small windows of energy. Use of a discriminator circuit can be used to create the range of observed energy. The step is referred to as a channel for which a given energy (pulse height) can be recorded. The range of energy ’viewed’ in a channel can be varied by changing the PMT voltage or the gain of the amplifier, defined as delta E. Therefore, sweeping over a range of channels will detect a range of gamma-ray energies from the sample. This can be accomplished using a multi-channel analyser (MCA). Single photon counting is obtained using this method, hence values are reported as numbers of counts per channel. This obtains a spectrum of energy values which must be calibrated using known values of spectrum features.

Using the above described method, the resolution of the setup depends on several factors such as the voltage of the PMT, number of channels used, amplifier gain and type of scintillator crystal used. Here within the resolution is assessed using the variables which give the greatest control: the PMT voltage and amplifier gain.