The Advanced GAmma Tracking Array (AGATA) project is an European collabora- tion project that is aiming to construct a next generation gamma-ray spectrometer for nuclear-structure applications. The final array will consist of 180 highly-segmented HPGe detectors mounted into 60 triple clusters forming a full 4π shell. AGATA will utilise pulse-shape analysis and gamma-ray tracking to achieve high photopeak efficiency and peak-to-total. Central to the AGATA spectrometer is the Nouvelle Ac- quisition temps-Réel Version 1.2 Avec Linux (NARVAL) data-acquisition system that will perform pulse-shape analysis and gamma-ray tracking calculations in real time (online). The system is comprised of separate blocks of actions, called actors. Each one containing a self-sufficient piece of code designed for a specific action. AGATA data can be downloaded and replayed through an offline version of NARVAL, known as the NARVAL-emulator. With a new format and techniques there is need to define new methodology to analyse the data. Discussed in the first part of this work will be processing of AGATA data and how by utilising the actors of the NARVAL-emulator different output formats can be chosen. In the next part of this work, the Compton polarimetry capability of an AGATA triple cluster will be investigated by utilising pulse-shape analysis and the detector’s granularity. The basic design of a Compton polarimeter consists of two detectors; one acting as a scatterer and the second as a absorber. The same principles apply to an AGATA triple cluster with each sector act- ing as the scatterer and absorber. The pulse-shape analysis algorithm identifies the positions and energies of the interaction points. Data was obtained from the heavy- ion fusion-evaporation of 110Pd (32S,4n) 138Sm at 135 MeV was used to investigate the Compton polarimetry capability of an AGATA triple cluster. This work utilised the phenomenon that the scattering cross-section for gamma rays is larger in the direction perpendicular to its electric field vector than parallel. This is described by the differ- ential Compton scattering cross-section for polarised gamma rays. A useful event to determine linear polarisation occurs if the incident gamma ray is Compton scattered from the first interaction and then fully absorbed at the second interaction. The sum of the energies deposited in the two interactions gives the energy while the azimuthal scattering angle contains the linear polarisation information. To determine the linear polarisation of a gamma ray, a polarisation sensitivity and asymmetry measurement can be made. Such measurements were made in this work using gamma rays from 138Sm of known linear polarisations. The polarisation sensitivity method utilised the incident and scattered gamma-ray energy, in addition to the azimuthal scattering an- gle information produced from the pulse-shape analysis algorithm. The experimental polarisation sensitivities ranged from 5.0±0.3% at 139.7 keV to 16.4±0.1% at 1287 keV. A asymmetry measurement was carried by determining the number of gamma rays that scattered perpendicular and parallel to the reaction plane. A positive asym- metry would indicate an electric transition and a negative a magnetic one. It was found that there was partial consistency with expectation. From the asymmetry and polarisation sensitivity measurement an experimental linear polarisation for each gamma ray was determined. The result should be in the range −1≤P(θγ)≤+1 with the sign propagating through from the asymmetry measurement. Finally a coinci- dence efficiency and figure of merit was measured and are given as 32.6±1.0% and 3.9±0.2%, respectively at 346.9 keV. Overall it was concluded that there was limited Compton polarimetry capability and proposed reasons for this were discussed.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:564223 |
| Date | January 2011 |
| Creators | Filmer, Fay |
| Contributors | Boston, Andrew; Nolan, Paul |
| Publisher | University of Liverpool |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://livrepository.liverpool.ac.uk/8353/ |
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