The two-photon decay in nuclei has been formally theorized for many years and attempted to be measured on numerous occasions. The special case of a 0 + → 0 + transition in nuclei was examined for certain isotopes, and a branching ratio for the two-photon decay determined for each isotopes. Measurements of the branching ratio in nuclei other than this special case had so far proven unsuccessful. Motivated to find the two-photon branching ratio in a case where the transition competes with the single photon transition, we study the 11/2 − 137 Ba isomer. The experiment was performed at the Technische Universität Darmstadt using the available LaBr 3 scintillation detectors.
We first study the absorption of various gamma energies by lead and compare the resulting values to a GEANT4 simulation. With an ideal value for lead shield thickness, the experimental setup is built in order to obtain a high two-photon count rate, while suppressing direct Compton scattering between detector pairs and suppressing other background interference. In order to suppress the background, plastic scintillators were placed atop the experimental setup. To treat the daunting level of random coincidences measured with this setup, fine energy and time gates were placed on the processed events in order to limit observation to the region of interest.
Throughout the experiment, three different detector pair angles were successfully examined: 72 ◦ , 120 ◦ , and 144 ◦ . With these three angles a partial representation of the angular distribution of the two-photon decay is observed. The branching ratios were measured to be 1.56(23)·10 −6 , 0.55(22)·10 −6 , and 0.70(18)·10 −6 for the angles of 72 ◦ , 120 ◦ , and 144 ◦ respectively, with the values of 72 ◦ and 144 ◦ recorded in Ref.[1]. This experiment therefore shows it is possible to obtain a value for the two-photon branching ratio in the
11/2 − excited state of 137 Ba . A precise determination of this value, and for that of other nuclei, might contribute to solve current fundamental open problems such as restricting the parameters of the equation of state, or accurately determining neutron skin thickness.
| Identifer | oai:union.ndltd.org:USASK/oai:ecommons.usask.ca:10388/ETD-2014-11-1884 |
| Date | 2014 November 1900 |
| Contributors | Rangacharyulu, Chary |
| Source Sets | University of Saskatchewan Library |
| Language | English |
| Detected Language | English |
| Type | text, thesis |
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