Studies in the technique of short time measurement and its application to nuclear isomerism

Currie, William M (1963) Studies in the technique of short time measurement and its application to nuclear isomerism. PhD thesis, University of Glasgow.

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The work reported in this thesis was concerned with the use of a fast coincidence measuring system, i.e. an apparatus using scintillation counters for the measurement of short time intervals between nuclear radiations. The results deal partly with the technique itself and partly with its application to the study of nuclear isomerism. It has been recognised for a long time now that there are certain inherent limitations to the measurements which can be made with a fast coincidence system. In the first place there is a lower limit to the resolving time which can be obtained and in the second place almost all fast coincidence systems generate by virtue of their detection processes energy dependent, instrumental time delays. This means that the apparent time of an event is influenced by the energy of the radiation. These two aspects of instrumentation are of great practical importance whore very short times are being measured . Chapters 2 and 3 are concerned with these problems of instrumentation, chapter 2 with tile development and theory of the subject and chapter 3 with a precise experimental study of instrumental time delays and resolution. The results are very satisfactory. They demonstrate the actual performance of a system, provide illustrative curves and numbers, and indicate the manner in which any other system may be quickly assessed and its limitations ascertained. In addition the measurements provide pleasing confirmation of the latest theoretical work which has been done on the subject and reveal the inadequacy of some of the earlier treatments. The rest of the thesis is concerned mainly with the measurement of isomeric lifetimes, four new results being reported and discussed. The background to the subject is considered in chapter 1 where the various features of V-ray emission from nuclei are reviewed in simple terms and the relationship between these electromagnetic phenomena and the shell and collective models of the nucleus is described. the value of the measurements lies principally in their bearing on these models, and while they provide only a small addition to the already substantial data on the subject they are not insignificant. Three of the measurements have yielded results which do not accord with the general tendency of enhanced rates for E2 transition probabilities, thus emphasising the need for more detailed knowledge of nuclear states, particularly where there is mixing of collective, and single particle excitations. The first lifetime measured was in semi-magic Ce140. It is an E2 lifetime, 17 times longer than the single proton estimate. This was at first found rather surprisiag, lees anether E2 lifetime in Ce140 had already been determined as 17 tines shorter than this estimate. As explained in chapter 4, the result is not quite so surprising whet mor careful consideration is given to the results for other nuclei. This investigation suggested there might be some ether retarded E2 transitions in nuclei similar to Ce140, that is to say in other semi-magic nuclei which seemed to exhibit mixed excitation modes. An immediate choice for further study was Ba138, and Sn120, with Z; = 50 rather than N= 82, was also selected for experiment. This work is described in chapter 5. In both cases successful measurements were made, the result for Ba138 being very similar to that for Ce140, and the Sn120 result not only giving a lifetime but also correcting as earlier misinterpretation of the decay scheme. In chapter 6 the measurement of a mixed E2 and M1 transition rate in v49 is reported. This work was carried out as part of a plan for the investigation of many isomeric levels sources prepared on the H.T, set by 14 MeV neutron activation. The work on Ba138 and Sn120 was also carried out by this means but only after the efficacy of the procedure had been demonstrated in other cases. Finally, in chapter 7 (and part of the introduction) an there is a discussion of an Aunsuccessful attempt to observe the electric quadrupole perturbation in the delayed angular correlation between the 133 and the 482 keV Y-rays of Ta181 This was the initial project undertaken by the author. It took up the greater part of 3 1/2 years without yielding any positive results. The reason for this lies simply in the difficulty of the experiment, which was tackled in several ways which will be the subject of still further study.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: P I Dee
Keywords: Nuclear physics and radiation
Date of Award: 1963
Depositing User: Enlighten Team
Unique ID: glathesis:1963-73606
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 14 Jun 2019 08:56
Last Modified: 14 Jun 2019 08:56

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