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Title: Measurement of the γ + 4He total photoabsorption cross-section using a gas-scintillator active target
Author: Al Jebali, Ramsey
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 2013
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A large number of experiments have been performed in the past 60 years in an attempt to understand the near threshold photodisintegration of 4He. Available experimental data are inconsistent and do not provide reliable guidance for theoretical calculations for the total and partial cross-sections in the energy regime near breakup threshold. Even with the most recent experimental work done on the subject, the situation still has not been fully clarified. This thesis reports a measurement of the total cross-section for photodisintegration of 4He below pi-production threshold, carried out in 2009 at the up-graded tagged photon facility at MAX-lab in Lund, Sweden, in collaboration with the Photonuclear group of Lund University. The aim of this measurement is to provide a reliable and precise set of data so that the accuracy of theoretical models can be judged reliably. The experiment was performed using a Helium Gas-Scintillator Active Target (HGSAT), built and developed at the University of Glasgow. The helium target acts also as a detector of the 4He photodisintegration charged products. A photon beam, energy tagged in the range 11-68 MeV, was directed towards the HGSAT, which operated at a pressure of 2MPa, at room temperature. The resultant scintillation is collected and detected by a number of photomultiplier tubes (PMT) mounted on the HGSAT. 12 external neutron detectors and two 10" sodium iodide (NaI) detectors were used during the experiment to distinguish 4He(γ,n)3He and 4He(γ,γ')4He events, although these events have not been analysed here. The tagged photon coincidence signal was separated from random background using two methods: background filtering and fitting procedure. The two methods produced similar results. A Geant4-based Monte-Carlo simulation was developed to derive the HGSAT detection efficiency. Trigger thresholds needed in the simulation were estimated by evaluation and matching the normalised yield to previously measured γ+4He cross-sections above 40 MeV where more consistency in the dataset is observed, hence the results presented in this thesis are preliminary and will remain so until the absolute detection efficiency is determined. The preliminary total γ+4He cross-section obtained in this work peaks at ~2.85mb at a photon energy (Eγ) of ~27MeV, falling to ~1mb at Eγ = 60MeV. The measured cross-section is compared with previous data and recent theoretical calculations made using the Lorentz Integral Transform (LIT) technique. The present cross-section is already in reasonable agreement with the theoretical calculations and a number of previous experimental data. Future work to reduce systematic uncertainties will include analysis of the pulse height response of the HGSAT and further lower intensity experimental runs.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Q Science (General) ; QC Physics