In 1939, Niels Bohr and John Wheeler formulated a theory of neutron-induced nuclear fission based on the hypothesis of the compound nucleus (Bohr and Wheeler 1939). The “Bohr hypothesis” is at the heart of every current theoretical fission model; it states that the decay of a compound nucleus for a given excitation energy, spin, and parity is independent of its formation (Bohr 1936). The formation of the compound nucleus is long enough (approximately 10-16 seconds, which is 107 times longer than the time a nucleon needs to traverse the nucleus) for the nucleus to “forget” the details of its origin. Although this hypothesis is widely accepted by theorists and experimentalists, its implications (especially concerning fission product yields) have never been experimentally verified precisely via direct comparison of fission observables. Currently, the Bohr hypothesis is verified to about 20 percent accuracy in nuclear reactions such as (t,pf), (3He,df), and (3He,tf) (Britt et al. 1968, Cramer and Britt 1970, Britt and Cramer 1970, Younes and Britt 2003a, 2003b). We performed direct experimental validation of the practical consequences of the Bohr hypothesis during induced nuclear fission. We compared the fission product yields of the same 240Pu compound nucleus produced by two different reactions: (i) n+239Pu and (ii) gamma+240Pu. The two Pu isotopes provided a valuable set of nuclides for use in investigating the influence of the spin and parity of the compound nucleus on the detailed fission product yields distribution, including shell and pairing effects and fission dynamics. The only difference between these two reactions was a small mismatch in the spin and parity distributions of the 240Pu compound nucleus. High-precision measurements of the 240Pu fission product yields using monoenergetic photons produced at the High Intensity Gamma-Ray Source facility were performed using a gamma-ray beam energy of 11.2 MeV, creating the same compound nucleus and excitation energy as neutron-induced fission of 239Pu at an incident neutron energy of 4.5 MeV.
Nuclear fission, the most pronounced collective nuclear-structure phenomenon, relates the fission product yields (FPYs) and their kinetic energies to the potential energy of the fission system (Vandenbosch and Huizenga 1973). The evolution of the fission system (from the initial particle impact through intermediate saddle points to scission and finally to the configuration of separated fission fragments) is governed by multi-dimensional potential-energy surfaces and the shell structure of the fragments. At low excitation energies, shell and pairing effects in nuclei influence both the mass and energy distribution of the fission fragments; therefore, nuclear structure details are required to describe experimental fission data. Models based on different phenomenological and microscopic approaches are used to describe the complexity of the fission process and are assessed by how well they describe the various experimental observables (Wagemans 1991). One of these observables is the dependence of the FPYs on excitation energy. The magnitude and the slope of the energy dependencies of the FPYs have significant impacts on basic and applied physics. For example, r-process nucleosynthesis cannot be fully understood without a precise knowledge of the fission properties of very neutron-rich isotopes of the heaviest elements, which are presently not accessible to direct measurements (Iliadis 2015). Precise knowledge of FPYs is critical to nuclear reactor operation, nondestructive nuclear fuel investigation, accelerator-driven systems, and decay heat (Chadwick et al. 2010, Laurec et al. 2010, MacInnes et al. 2011, Nethaway et al. 1977). In addition, information about the FPYs has a fundamental impact on basic fission studies and the antineutrino spectrum (Hayes et al. 2014). The accuracy of purely theoretical predictions is insufficient for quantitative predictions of the energy dependence of FPYs on the precision that present nuclear applications require (Schmidt 2008). However, it is very difficult to make quantitative predictions about the FPYs, especially within a microscopic approach based on time-dependent Hartree-Fock calculations, where the experimental magnitude of the energy dependence is comparable to (or even smaller than) the uncertainties of the theoretically calculated FPYs (Schunck et al. 2015, Randrup and Moller 2013).
We performed the first-ever experiment to identify the practical consequences of the Bohr hypothesis in the context of induced nuclear fission. We measured the FPYs from the 240Pu (gamma,f) reaction at the High Intensity Gamma-Ray Source (HIGS) facility at Triangle Universities Nuclear Laboratory (TUNL) (Weller et al. 2009) and compared the results to the FPYs obtained from the 239Pu(n,f) reactions that our group recently measured at the TUNL Tandem Van de Graaff accelerator (Bhatia et al. 2014 and 2015, Gooden et al. 2016, Tonchev et al. 2017). Both reactions (see figure) produced a 240Pu compound nucleus with the same excitation energy since the incident energies of the neutron and photon beams were carefully chosen to account for the 6.5 MeV difference in binding energy between 239Pu and 240Pu. The only difference was the different spin-parity distributions from n + 239Pu and Jπ = 1- from gamma + 240Pu, which is widely believed to be inconsequential at high excitation energies.
High-precision FPY measurements for 240Pu(gamma,f) were performed at the HIGS facility, using the successful methodologies established during our studies of neutron-induced fission of 239Pu at TUNL by our multi-laboratory Lawrence Livermore National Laboratory-Las Alamos National Laboratory-TUNL collaboration (Bhatia et al. 2014 and 2015, Gooden et al. 2016, Tonchev et al. 2017). This is an important observation because neutron-induced fission measurements on radioactive targets, such as 237U(n,f)238U, 232U(n,f), 238Pu(n,f), and 231Th(n,f), could instead be replaced by photofission measurements on stable targets, such as 238U(gamma,f)238U, 233U(gamma,f), 239Pu(gamma,f), and 232Th(gamma,f), respectively. We demonstrated this capability for the first time by comparing the possible relationship between the FPYs data sets from two fission reactions: 239Pu(n,f) and 240Pu(gamma,f).
Our research supports NNSA and Laboratory goals in stockpile stewardship. Our research also enhances the Laboratory’s core competencies in nuclear, chemical, and isotopic science and technology. Our study of methods of obtaining precision FPY measurements on several high-impact isotopes extends our fundamental knowledge of nuclear fission by providing accurate benchmark data for theoretical models to assess the predictability of the Laboratory’s simulations from the two most important incident particles inducing fission: neutrons and photons. It will also add the capability to simulate photofission reactions to the Laboratory’s FREYA computer-simulation code.
We successfully performed the first experiment to identify the practical consequences of the Bohr hypothesis in the context of induced nuclear fission. In order to compare FPY data from two different nuclear reactions, it was imperative that both mechanisms were studied under the same experimental conditions. 11.2 MeV photons at the HIGS facility were used to induce fission of 240Pu, creating a compound nucleus with the same excitation energy as 4.5 MeV neutron-induced fission of 239Pu. By comparing FPYs from both reactions, we investigated the role of the entrance channel, as well as the spin and parity of the compound nuclear system during the fission process. The combination of these effects was shown to be small, even at excitation energies a few MeV above the fission barrier. Our study demonstrated a new method for determining the FPYs for unstable isotopes using photofission as a prototype reaction. As an example, direct reactions such as 237U(n,f)238U are very difficult to perform, while indirect reactions like 238U(gamma,f)238U are comparatively easy to measure. Our novel approach can be applied to a much larger class of fission reactions.
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——— . 2015. "Exploratory Study of Fission Product Yields of Neutron-Induced Fission of 235U, 238U, and 239Pu at 8.9 MeV." Physical Review C 91(6): 064604. doi: 10.1103/PhysRevC.91.064604.
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Tonchev, A. P., et al. 2017. "Energy Evolution of the Fission-Product Yields from Neutron-Induced Fission of 235U, 238U, and 239Pu: An Unexpected Observation." Fission and Properties of Neutron-Rich Nuclei" (ICFN6), Sanibel Island, FL, November 2016. LLNL-PROC-717402.
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Younes, W., and H. C. Britt. 2003a. "Neutron-Induced Fission Cross Sections Simulated from (t,pf) Results." Physical Review C 67(2): 024610. doi: 10.1103/PhysRevC.67.024610.
——— . 2003b. "Simulated Neutron-Induced Fission Cross Sections for Various Pu, U, and Th Isotopes." Physical Review C 68(3): 034610. doi: 10.1103/PhysRevC.68.034610.
Bhike, M., et al. 2017. "Exploratory Study of Fission Product Yield Determination from Photofission of 239Pu at 11 MeV with Monoenergetic Photons." Physical Review C 95: 024608. LLNL-JRNL-698599.
Krishichayan, et al. 2017. "Photofission Cross-Section Ratio Measurement of 235U/238U Using Monoenergetic Photons in the Energy Range of 9.0–16.6 MeV." Nuclear Instruments and Methods in Physics Research Section A 854(11): 40–44. LLNL-JRNL-737088.
Silano, J. A. 2017a. "Modernizing the Fission Basis: Short Lived Fission Product Yield Measurements in 235, 238U and 239Pu." FIESTA Fission School and Workshop, Santa Fe, NM, September 2017. LLNL-PRES-738667.
——— . 2017b. "Short Lived Fission Product Yield Measurements in 235, 238U and 239Pu." FIESTA Fission School & Workshop, Santa Fe, NM, September 2017. LLNL-POST-738557.
——— . 2017c. "Modernizing the Fission Basis: Short Lived Fission Product Yield Measurements in 235, 238U and 239Pu." Fall Meeting of the APS Division of Nuclear Physics Pittsburg, Pennsylvania, September 2017. LLNL-PRES-740400.
——— . 2018a. "Photofission Activities at HIGS." Workshop on Future Uses and Enhancements of the fission TPC, Livermore, CA, June 2018. LLNL-PRES-748801.
——— . 2018b. "Testing the Bohr Hypothesis: Photofission of 240Pu." LLNL Postdoctoral Poster Symposium, Livermore, CA, August 2018. LLNL-POST-752644.
——— . 2018c. "Validating the Bohr Hypothesis: Measuring the Energy Evolution of Fission-Product Yields from Photon-Induced Fission of 240Pu." Nuclear Photonics, Brasov, Romania, June 2018. LLNL-PRES-753382.
——— . 2018d. "Validating the Bohr Hypothesis: Measuring the Energy Evolution of Fission-Product Yields from Photon-Induced Fission of 240Pu." 5th Joint Meeting of the APS Division of Nuclear Physics and the Physical Society of Japan, Waikoloa, Hawaii, October 2018. LLNL-PRES-759530.
Tonchev, A. P. 2016a. "Modernizing the Fission Basis." Fall Meeting of the APS Division of Nuclear Physics, Vancouver, Canada, October 2016. LLNL-PRES-704201.
——— . 2016b. "Fission Product Yields Measurements using Quasi-Monoenergetic Neutrons." Invited talk at the 6th International Conference on Fission and Properties of Neutron-Rich Nuclei (ICFN6), Sanibel Island, FÒ. LLNL-PRES-752939.
——— . 2017a. "Connecting Nuclear Structure to Steller Astrophysics." Invited talk at the Institute for Nuclear Theory (INT), Seattle, WA, March 2017. LLNL-PRES-740761.
——— . 2017b. "Connecting Nuclear Structure to Steller Astrophysics." Invited talk at the 16th International Symposium on Capture Gamma-Ray Spectroscopy and Related Topics, CGS16, Shanghai, China, September 2017. LLNL-PRES-740761.
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