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Photofission Cross Sections of 232Th and 236U from Threshold to 8 Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations 1972 Photofission cross sections of 232Th nda 236U from threshold to 8 MeV Michael Vincent Yester Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/rtd Part of the Nuclear Commons Recommended Citation Yester, Michael Vincent, "Photofission cross sections of 232Th nda 236U from threshold to 8 MeV " (1972). Retrospective Theses and Dissertations. 6137. https://lib.dr.iastate.edu/rtd/6137 This Dissertation is brought to you for free and open access by the Iowa State University Capstones, Theses and Dissertations at Iowa State University Digital Repository. It has been accepted for inclusion in Retrospective Theses and Dissertations by an authorized administrator of Iowa State University Digital Repository. For more information, please contact [email protected]. INFORMATION TO USERS This dissertation was produced from a microfilm copy of the original document. While the most advanced technological means to photograph and reproduce this document have been used, the quality is heavily dependent upon the quality of the original submitted. The following explanation of techniques is provided to help you understand markings or patterns which may appear on this reproduction. 1. The sign or "target" for pages apparently lacking from the document photographed is "Missing Page(s)". If it was possible to obtain the missing page(s) or section, they are spliced into the film along with adjacent pages. This may have necessitated cutting thru an image and duplicating adjacent pages to insure you complete continuity. 2. When an image on the film is obliterated with a large round black mark, it is an indication that the photographer suspected that the copy may have moved during exposure and thus cause a blurred image. 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University Microfilms 300 North Zeeb Road Ann Arbor, Michigan 48106 A Xerox Education Company I 73-9502 YESTER, Michael Vincent, 1944- ___ PHOTOFISSION CROSS SECTIONS OF AND U FRCM THRESHOLD TO 8 MeV. Iowa State University, Ph.D., 1972 Physics, nuclear University Microfilms, A Company, Ann Arbor. Michigan THIS DISSERTATION HAS BEEN MICROFILMED EXACTLY AS RECEIVED. Photofission cross sections of zszTh and 2 from threshold to 8 MeV by Michael Vincent Yester A Dissertation Submitted to the Graduate Faculty in Partial Fulfillment The Requirements of the Degree of DOCTOR OF PHILOSOPHY Department: Physics Major Subject: Nuclear Physics Approved: Signature was redacted for privacy. In cfafi^qfe^ ifit Major Wcrk Signature was redacted for privacy. For the Major Department Signature was redacted for privacy. duate College Iowa State University Ames, Iowa 1972 PLEASE NOTE: Some pages may have indistinct print. Filmed as received. University Microfilms, A Xerox Education Company ii TABLE OF CONTENTS Page I. INTRODUCTION 1 II. THEORY 6 III. COHPTON SCATTERING FACILITY A. Technical Aspects B. Photon Beam Energy Spectrum 20 C. Beam Monitor 30 D. Beam Flux 31 IV. PHOTOFISSION HEASOREMENTS 35 A. Experimental Apparatus 35 1. Targets 35 2. Vacuum chamber 35 3. Detection system 37 a. Detectors 37 b. Counting system 38 c. Detection efficiency 41 B. Experimental Procedure 43 C. Photofission Yields 44 1. 232Th yield 44 2. 23&U yield 46 V. DATA ANALYSIS 49 A. Least Structure Analysis 49 B. Present Application of Least Structure Analysis 53 C. Test and Particular Aspects of Analysis Procedure 55 iii Page VI. PHOTOFISSION CROSS SECTIONS 59 A. 232Th Photofission Cross Section 59 B. 23*0 Photofission Cross Section 66 VII. DISCUSSION OF RESULTS AND CONCLUSIONS 69 A. 232Th 69 1. Comparison with other measurements 69 2. Interpretations 72 B. 23&U 73 1. Comparison with other measurements 73 2. Interpretations 74 C. Comparison of 232xh and 236Q Results 76 D. Comparison of 23sn and 2 3 6u Results 78 E. Suggestions For Future Work. 80 F. Concluding Remarks 81 VIII. BIBLIOGRAPHY 83 IX. âCKNGSLEDGSfîSÎÎTS 87 X. APPENDIX A: ANALYTICAL LINE SHAPES FOB GE(LI) SPECTRA 88 XI. APPENDIX B: DERIVATION OF LEAST STRUCTURE SOLUTION 91 1 I. INTRODUCTION Within the last ten years, new discoveries have greatly altered our concepts of the fission process. Isomeric levels with half-lives ranqinq from 1.5 nsec to 1U.0 msec have been observed in which the only known mode of decay is fission, and subbarrier fission resonances have been observed in reac­ tion studies. These observations have been explained by the­ oretical calculations that predict a potential energy versus deformation function for heavy nuclei which has two maxima, rather than the previously accepted simple potential barrier with a single maximum. It also has been predicted that the second well, the well between the two maxima, could be quite deep (3 MeV) permitting the existence of discrete nuclear states in this well. In addition, it has been postulated that only certain quantum states (fission channels) are available to the nucleus at the saddle-point of fission, and, it is predicted that these states are widely separated in en­ ergy and represent simple types of motion of the nucleus. While the above ideas are now supported by considerable experimental evidence, little is known of the character of the states in the second well. An attempt has been made to detect gamma-ray emission that precedes isomeric fission, but such transitions in the second well have not been observed (1). The subbarrier fission resonances observed in high res­ olution studies with particle reactions usually are attribut- 2 ed to transitions from the compound nuclear states of the first veil to levels in the second well. Thus, the levels in the second well can fce studied in their role as intermediate states to fission from the compound nuclear states in the first well. However, the maximum transfer of angular momen­ tum can be quite large in particle-induced fission, and it is difficult to assign spins and parities to the compound nucle­ ar levels that are excited. Photofission experiments can provide valuable information in this respect. In photon ab­ sorption, electric-dipole excitation is dominant, although electric-quadrupole and magnetic-dipole excitations can also contribute. For even-even nuclei, however, collective Ml ex­ citations are forbidden eliminating excitations of 1+ states near threshold. Therefore, only 1- and 2+ states will con­ tribute to the photofission cross section near threshold. Hence; photofission experiments can provide a starting point for interpreting the reaction data and, in general, can fur­ nish tests of fission theories. The early photofission measurements were made using gam­ ma rays from the i9F(p,ay)i*0 reaction and bremsstrahlung as the photon sources. Hyde (2) presents a good discussion of the early experiments and a comparison of some of the data. To summarize the major results, evidence of a plateau at ap­ proximately 6.0 MeV was observed for 238u, z^zTh, and 236g, In the experiments performed with bremsstrahlunç, a differen­ 3 tial analysis technique was employed to obtain the cross sec­ tions from the experimental yield data, and the overall reso­ lution in the results was poor. The experiments using the gamma rays from the^^'F (p,a-y) reaction only provided data at 6.1%, 6.9, and 7.0 MeV. Thus, the possibility of struc­ ture in photofission cross sections has been an open ques­ tion. Recently, monochromatic gamma rays from thermal neutron capture have been used for photofission studies. Cross sec­ tions can be obtained only at various discrete energies, how­ ever, and the results are difficult to interpret as the ener­ gy spread of the gamma rays is of the order of the spacing between the levels in the compound nucleus. To circumvent the above problems, a Compton scattering facility was built by Knowles and Ahmed (3) to provide a variable-energy monochromatic photon beam, and photofission studies were initiated. A similar facility was constructed at the Ames Laboratory Research Reactor (4,5), and a series of photofission experiments was planned. Hall began this project (6), and Anderl (7) has made a companion set of meas­ urements to the present work. The choice of target nuclei for this work was based on experimental as well as theoretical concerns. The silicon surface-barrier detectors used as fission-fragment counters in this work cease to function properly after receiving a 4 total dose of IQii alpha particles. Consequently, only nuclides with extremely long half-lives for alpha-particle decay were considered as suitable targets. Since many exper­ imental studies of the photofission reaction in zszTh have been made, this isotope was selected to enable direct evalua­ tion of our experimental technique; whereas, 23*0 was select­ ed since the photofission cross section of this nuclide has been measured previously only at energies of 6.14 and 7.0 MeV (8) . From a theoretical viewpoint zs^Th and offer an in­ teresting comparison. Both are even-even so the spectrum of fission channels at the threshold should be similar.
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