A cloud chamber investigation of photonuclear reactions in light nuclei

This thesis describes a series of experiments in which a number of photonuclear reactions in light nuclei were studied by passing the X-ray beam from a 23 Kev electron synchrotron through a Wilson cloud chamber. These experiments, which were carried out between 1950 and 1955, were among the first to use this method. Consequently the thesis deals with the results on these reactions and with the Information obtained on cloud chamber technique. The first chapter of the thesis contains a comprehensive review of the results of photonuclear experiments prior to this investigation. This review shows that most of these results were for (y,n) reactions and that there was a need for further results on other reactions. It is noted that a study of photonuclear reactions in light nuclei would probably be of greatest value and that the cloud chamber method should provide more comprehensive Information than other methods. The experiments chosen studied the He4(y,p) reaction, a number of photonuclear reactions in N14 and the (Y,a), (y,2a) and (y,ap) reactions in Ne20. The information obtained on the He4(y,p) reaction was limited since the peak energy of the synchrotron was only 3 Mev above the threshold for the reaction. However this experiment showed that the He4(Y,P) cross section In this energy region Is approximately one-tenth of the value deduced from the first measurement of the cross section for the Inverse reaction, H3(p,Y)He4. This lower cross section has been confirmed by subsequent measurements of both the He4(Y,p) and H3(p,y) cross sections. In the measurements on N14 the (Y,P)f (Y,pn), (Y,a)f (Y,2a) and (Y,ap) reactions were studied. The range distribution of protons of energy up to 3 Mev shows sharp well-defined peaks at energies of 0.51, 1.63 and 2.92 Mev. These energies correspond closely to the energies of the resonances reported for the C13(p,Y)N14 (ground state) reaction. For these resonances the ratios of the cross sections for the (Y,P) and (p,y) reactions are In good agreement with the predictions of the principle of detailed balancing. At higher energies the following measurements were mades: (1) the (Y,P) process was Investigated by measuring the range distribution of the C13 nuclei, (11) both (Y,P) and (Y,pn) processes were studied by measuring the numbers of events at three different synchrotron peak energies. These measurements showed that both the (Y,p) and (Y,pn) cross sections are small at energies up to 15 Mev and large In the neighbourhood of 20 Mev. The estimated values of the integrated cross sections up to 23 Mev for the (Y,P) and the (Y,Pn) reactions are both approximately 0.02 Mev-barn. The (Y,a), (Y,2a) and (Y,ap) cross sections are each about 0.1 millibarn. In addition this experiment shows that the cross sections for N14(Y,n) disintegrations leaving N13 in the levels at 2.36, (3.51 and 3.56)t 6.91 and 7.42 Mev could be determined from an analysis of (y,pn) events. Although the conditions of the present experiment did not permit a full analysis of these eventst, upper limits were obtained for the numbers of (Y,n) disintegrations leaving N13 the 2.36, (3.51 and 3.56) and 6.91 Mev levels. The following results were obtained for the (Y,A), (y,2a) and (y,ap) reactions in Ne20. For a large proportion of the events the total energy of the a-particle and recoil is lees than 5 Mev. If these low energy events are attributed to the Ne20(y,a)O16 ground state) reaction then the cross section for this reaction, integrated up to 10 Mev, Is approximately 0.3 Mev-millibarn. For quantum energies in the vicinity of 20 Mev, the average cross sections are: (Y,a) reaction, less than 0.1 millibarn (Y,2a) reaction, approximately 0.5 millibarn and (y,ap) reaction, approximately 1 milllbarn. If the (y,2a) and (y,ap) stars are assumed to be due to (y,a) and (y,p) disintegrations leaving 016 and F19 in highly excited states then, an analysis of these stars shows that (1) a number of 016 and F19 levels are Involved and (11) for at least one-third of the (Y,2a) stars the Initial (Y,a) disintegration must leave 016 in excited states below the first level with an isotopic spin of one. Since the number of stars available for analysis was limited, it is not possible to identify any 016 or F19 levels and so confirm the above assumption. These results are compatible with, although they do not prove, the predictions of the isotopic spin selection rules for (Y,A) reactions in Ne20. In obtaining these results it was shown that the cloud chamber method gives very accurate results for the energy region in which the energies of the protons and a-particles can be determined from range measurements and further that the tracks of the recoil nuclei can be measured with a total error which is only slightly greater than the unavoidable error due to range straggling and multiple scattering. The thesis contains a description of the method developed to measure the lengths and directions of these tracks which were from 0.3 to 6 mm long. The potential of the cloud chamber method is discussed in a concluding chapter which lists a number of experiments for which its use is either necessary or desirable. In particular it is noted that an investigation of star-producing reactions could be of considerable value