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Photofission of U235 and U238 Using a Compton Scattering Monochromator James Edison Hall Iowa State University

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Photofission of U235 and U238 Using a Compton Scattering Monochromator James Edison Hall Iowa State University Iowa State University Capstones, Theses and Retrospective Theses and Dissertations Dissertations 1970 Photofission of U235 and U238 using a Compton scattering monochromator James Edison Hall Iowa State University Follow this and additional works at: https://lib.dr.iastate.edu/rtd Part of the Nuclear Commons Recommended Citation Hall, James Edison, "Photofission of U235 and U238 using a Compton scattering monochromator " (1970). Retrospective Theses and Dissertations. 4230. https://lib.dr.iastate.edu/rtd/4230 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]. 70-25,787 HALL, James Edison, 1942- 0'5c: 900 PHOTOFISSION OF U AND U USING A COMPTON SCATTERING MONOCHROMATOR. Iowa State University, Ph.D., 1970 Physics, nuclear University Microfilms, A XEROX Company, Ann Arbor, Michigan THIS DISSERTATION HAS BEEN MICROFILMED EXACTLY AS RECETVFn PHOTOFISSION OF AND USING A COMPTON SCATTERING MONOCHROMATOR by James Edison Haï 1 An Abstract of A Dissertation Submitted to the Graduate Faculty in Partial Fulfillment of The Requirements for the Degree of DOCTOR OF PHILOSOPHY Approved Signature was redacted for privacy. f Major Work Signature was redacted for privacy. or Department Signature was redacted for privacy. Iowa State University Of Science and Technology Ames, Iowa 1970 TABLE OF CONTENTS Page I. INTRODUCTION I A. Theory 1 B. Purpose 8 C. Literature Survey 9 II. GAMMA RAY SOURCE 12 A. Introduction 12 1. Discrete energy sources 12 2. Variable energy sources 13 3. Comparison of source intensities 16 B. Compton Scattering Facility 18 1. Neutron-capture source 18 2. Scattering platd 24 3. Target chamber and shielding 31 4. Direct (n,y) beam 35 5. Scattered beam 45 a. Energy spectrum 45 b. Intensity 50 Ml. EXPERIMENTAL DETAILS 66 A. Fragment detection apparatus 66 B. Targets 67 C. Calibration 70 IV. EXPERIMENTAL RESULTS 74 A. Relative Yield from 74 B. Relative Yield from U^^^ 75 C. Absolute Yield from U^^® 77 V. INTERPRETATION AND CONCLUSIONS 80 A. Photofission Yield Equation 80 B. Present Approach 81 C. Interpretation 83 D. Concluding Remarks 86 VI. LITERATURE CITED 88 VII. ACKNOWLEDGMENTS 9I 1 I. INTRODUCTION A. Theory A striking characteristic of the very heavy nuclei (Z s 90) is their ability to undergo fission whenever they are excited with sufficient energy to overcome a fission energy barrier. The mechanism by which fission occurs at different excitation energies is in many ways very similar. First, the stably deformed target nucleus is excited by a nuclear reaction to an excitation energy E, forming a compound nucleus. This is a complex state in which the excitation energy is distributed among all the degrees of freedom of the nucleus, including the surface deformation. As the process moves towards fission, an increasing amount of the energy becomes potential energy of deformation and the nucleus enters what is called the transition state. At some finite distortion of the transition state nucleus, known as the saddle- point deformation, the increase in energy due to nuclear forces (surface tension) is equal to the decrease in the Coulomb energy. If the nucleus is slightly elongated beyond the saddle shape, the Coulomb forces drive the nucleus to fission. However, if the deformation is slightly reduced, the surface forces predominate and the nucleus reverts back to its original shape. The lowest energy required to reach the saddle-point deformation is defined as the fission barrier. Bohr suggested in I955 that a nucleus possessing this energy is in an almost "cold" state of internal excitation since the energy is expended as deformation energy (1). 2 He proposed that the low-lying levels at the saddle point ought to resemble the spectra near the ground states of stably deformed nuclei. This is shown in Fig. 1 for an even-even nucleus. The low-lying states of an even-even transition state nucleus are predicted to be collective levels, since intrinsic states are not expected until sufficient energy is available for breaking a nucléon pair (about 1.2 MeV). Bohr also postulated that the lowest group of levels should be rotational levels built on the 1^ = 0+, K = 0 level, where I is the angular momentum, rr is the parity, and K is the projection of the angular momentum onto the nuclear symmetry axis. The energies of the rotational states relative to the "ground state" are given by E = ^ ^ 1(1+1) where I is the moment of inertia of the system about an axis perpen­ dicular to the symmetry axis. Since at the saddle point the nucleus is much more deformed, X will be much larger than for the stably 2 deformed nucleus, fi /2I has been calculated from liquid drop theory O "3 O (2) to be about 2 keV for U at the saddle point, whereas it has a value of about 7 keV for the compound nucleus. Thus, the low-lying rotational levels in the transition state nucleus are predicted to be more closely spaced than in the compound nucleus. We should mention at this point that although the constancy of I and M (projection of ! on a space fixed Z axis) is guaranteed by the law of conservation of angular momentum, there is no such restriction on K. In going from the original compound nucleus to the transition 3 }l=2+,3+,4+.5+-. K=2 )l= i~ 2-,3-4—,K=I = 3-,5—; K= 0 = 0+,2+,4+,-', K=0 TRANSITION STATE NUCLEUS 1I = 2+,4+,6+, K=2 J I=2+,4+,6+, "; K = 0 }I = 1-, 3-, 5—•. K = 0 I=0+, 2+,4+,6+, 8+' ; K=0 Ej-Eo" 71(I + !) ksV COMPOUND NUCLEUS 1 L_ DEFORMATION Fig. 1. Bohr's "fission channels" for the transition state of an even-even nucleus 4 state, the nucleus suffers many changes in shape and redistributes its energy and angular momentum in many ways. The K-values of the transition state nucleus are therefore unrelated to the initial K-values. The transition state then corresponds to a collection of channels, each having its own K-value, which are accessible from a given state of the compound nucleus specified by E, I, M, and parity. Once at or past the saddle point the nucleus has a highly deformed shape which is considered to be axially symmetric in the sense that K remains a good quantum number. The assumption that K is conserved upon entry into a fission channel is supported by angular distribution measurements. It is useful to examine the predictions of what other bands of levels should be present in the transition state. Above the rotational band a sequence of collective vibrations is expected. A K = 0, 1^ = 1 ^ 3,5,... band is expected and attributed to a pear-shaped vibration in which material sloshes back and forth from one end of the deformed nucleus to the other. Also expected are low-lying levels built on a bending vibration like the bending vibration of a linear molecule. The bending vibrational channel will couple with the collective rotation to give a band of states with K = 1 and I^ = 1", 2", 3", 4", . The energy spectra of normal even-even heavy nuclei contain a level designated as "y-vibrationa1" which involves a periodic collective deformation about axial symmetry. Collective rotations can be super­ imposed on the y-vibration giving a band with K = 2 and I^ = 2^, 3^, 4^, . The band built on the ^-vibrational level, which is well 5 known for normal heavy even-even nuclei, should not have a counterpart in the transition state since it corresponds to collective motion along the axis of symmetry. This is the principal feature of the act of fission. One could also consider the multiphonon states of these various modes which would produce bands at higher energies. The low-lying levels in the transition state for an odd A nucleus will be very different from those in an even-even nucleus. In an odd A nucleus the lowest transition states are states of intrinsic excitation of a single nucléon, and their associated rotational bands with I = K, K + I, K + 2, . and both parities. These bands of intrinsic excitations should be closely spaced (approximately every 250 keV). Thus Bohr's fission channel theory can be summarized as follows. The compound nucleus state has two constants of motion, total angular momentum I and parity. As the deformation of the nucleus in this state increases, the nucleus makes transitions from one energy surface to another until it reaches the saddle point. By this time a large part of the energy of the system has been absorbed in deformation and, for low excitation energies, the nucleus is "cold". It can be regarded as being in one of a few available transition states which have the same total angular momentum and parity as the compound-nucleus state. The available transition states in low energy fission are also limited by the available energy. In the last few years there have been many attempts to calculate the potential energy surface of the nucleus as a function of deformation. Strutinsky has introduced a model where he incorporates shell effects 6 into the well-known liquid drop model (3, 4). In this, the shell- correction term, as a function of deformation, is calculated directly from summed Nilsson single-particle energies of the deformed shell- model potential. Strutinsky finds that the minima in the shell- correction term occur at deformations where a gap is left in the structure of unfilled single particle levels immediately above the Fermi energy.
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