This introductory paper to the special issue of Fusion Science and Technology commemorates early research on fusion conducted at Los Alamos (the singular entity denoted Los Alamos Laboratory/Los Alamos Scientific Laboratory/Los Alamos National Laboratory at different times is designated "Los Alamos" in this paper) in support of the eventual H-bomb program. We survey the historical origins of the thermonuclear program, what was known of fusion reactions at the outbreak of the war, and the remarkable breakthroughs involving particularly the prospect of deuterium-tritium (DT) reactions conducted during the war, and we summarize the papers in this volume. Much of the nuclear fusion technical history presented herein has not been previously reported. Papers describe aspects of fusion science during these days, on shock hydrodynamics and on electron-radiation coupling, and on nuclear physics including the discoveries of resonances in both the DT cross section and in the lithium tritium-breeding cross section. Three papers follow our colleague Mark Paris's finding Arthur Ruhlig's 1938 paper on the first observation of DT fusion: one on how it influenced subsequent Manhattan Project research, another on a modern calculation of that historic experiment, and a third that has repeated the experiment using modern experimental capabilities. Other papers discuss how the first H-bomb test, Ivy Mike, led to the discovery of the new elements einsteinium and fermium and how the DT fusion processes played a key role in our universe's development after the Big Bang. We also present a paper that analyzes the pioneering Cambridge University 1934 experiment by Marcus Oliphant, Paul Harteck, and Ernest Rutherford where deuterium-deuterium fusion was first observed and that describes how Ernest Lawrence missed identifying fusion in 1933. Finally, we present a summary of early concepts for controlled fusion energy that grew out of wartime discussions at Los Alamos. The papers show how J. Robert Oppenheimer played a leading technical role in the early developments of the H-bomb, before his later opposition-our first paper in this issue addresses the U.S. Department of Energy's 2022 vacation of the earlier 1954 decision to revoke his security clearance.

Neutron cross sections of the stable lithium isotopes 6Li and 7Li were of interest in the 1940s and 1950s in part because of their reactions, which form tritium using moderated neutrons on 6Li and higher-energy neutrons on either isotope. Lithium remains of interest today for use as a blanket and shielding material in fusion reactors, where it can be used to breed tritium for a self-sustaining fuel cycle. During the Manhattan Project, the resonance in the 6Li(n,t) reaction was discovered and later became important for enhancing tritium production for nuclear technologies. The dominant natural isotope 7Li was and remains of interest because of the expense of enriching 6Li. It has been oft reported that the 1954 Castle Bravo nuclear test had a yield twice as large as expected because the nuclear explosive device designers had not properly accounted for the benefits from the 7Li isotope in the fuel; we note that this explanation is false.

In order to benchmark methods used to calculate reaction-in-flight fusion reactions in inertial confinement fusion and address issues related to the first claimed observation of d(t,n)alpha reactions in 1938, secondary d(t,n)alpha reactions have been observed following d(d,p)t reactions in deuterium gas. A pulsed 200-nA, 2.2-MeV deuterium beam from the Triangle Universities Nuclear Laboratory FN tandem accelerator was injected into a cylindrical multiatmosphere deuterium gas target. The incident beam traversed along the target cylinder's 3-cm symmetry axis after its passage through a Havar entrance foil. Two different Havar foil thicknesses were used to obtain 1.5- and 0.6-MeV deuteron beams entering the deuterium cell. The cylinder's radius was 2 cm to allow for d(d,p)t tritons emitted perpendicular to the beam to range out in the deuterium gas. The neutron emission from the cell was observed via its time of flight to a liquid scintillator placed at various angles to the beam direction, at a distance of 243 cm. Pulse-shape-discrimination techniques were used to separate neutron and gamma-ray signals seen in the liquid scintillator. The observed probability of similar to 2 x 10(-4) for inducing secondary d(t,n)alpha fusion in the gas cell per d(d,p)t reaction is consistent with theoretical expectations.

We describe the advancing knowledge of fusion cross sections from 1934 through the development of the first thermonuclear tests fielded by Los Alamos (the singular entity denoted Los Alamos Laboratory/Los Alamos Scientific Laboratory/Los Alamos National Laboratory at different times is designated "Los Alamos" in this paper) in the Pacific in 1951-1952; this technical history has not been previously documented. We compare these nuclear reaction cross sections to the current state of their knowledge as codified in the Evaluated Nuclear Data File (ENDF) databases, focusing on the Big Five reactions: 3H$${(d,n)<^>4}$$(d,n)4He, 3He$${(d,p)<^>4}$$(d,p)4He, 2H$${(d,n)<^>3}$$(d,n)3He, 2H$${(d,p)<^>3}$$(d,p)3H, and 3H$${(t,2n)<^>4}$$(t,2n)4He. At Oppenheimer's July 1942 University of California, Berkeley, "galaxy of luminaries" conference, Konopinski suggested that the cross section for 3H$${(d,n)<^>4}$$(d,n)4He "DT" could be large, and although Teller described this as an "inspired guess," we provide evidence instead suggesting that Konopinski knew of a 1938 measurement by Ruhlig that secondary DT reactions were "exceedingly probable." Bethe's direction that the DT cross section should be measured at Purdue University (Purdue) in 1943 led to the remarkable and unexpected finding that the DT cross section exceeds deuteron-deuteron (DD) by a factor of 100. This was a game-changing result, making Teller's dream, i.e., the terrestrial production of fusion energy, feasible. Eyewitness accounts are transcribed from the earliest discoveries of the large magnitude of the resonant DT cross section. A description is given of the Manhattan Project's early 1942-1944 DD measurements at the University of Chicago, the 1943 DT measurements at Purdue, and the subsequent 1945-1946 DD and DT measurements at Los Alamos. The Los Alamos experiments, led by Bretscher, were the first to extend to very low incident ion center-of-mass energies in the 6- to 50-keV range needed in applications and the first to identify, characterize, and document the 3/2+ "Bretscher state" responsible for the resonance-enhanced DT cross section. The early measurements were based on thick-target experiments that required a knowledge of hydrogen-isotope stopping powers, much of which was informed by 1930s German studies. We end with the high-accuracy APSST (named for Arnold, Phillips, Sawyer, Stovall, and Tuck) measurements at Los Alamos, 1951-1952. The very first 1942-1946 measurements were accurate to about 50% or somewhat better, but by the early 1950s, the cross sections were determined much more accurately, to within a few percent of our best values today, which come from R-matrix Energy Dependent Analysis (EDA) code analyses of the data, most notably the very accurate 1980s-1990 Los Alamos DT and DD fusion data from Jarmie and Brown. We show that Fermi, in his 1945 Los Alamos lectures, anticipated the S-factor (for the DT cross section), which is a concept widely used later in nuclear astrophysics. To this long abstract, we add a final tidbit: Marshall Holloway, a coauthor on the first-ever 1943 DT cross-section measurement at Purdue, went on to lead the engineering and fabrication of the first H-bomb test, Ivy Mike.

While studying d(d,n)3(H)e fusion in 1938, Ruhlig observed protons with energies larger than 15 MeV. Ruhlig suggested that these high-energy protons were generated by tritium-on-deuterium fusion neutrons scattering protons out of a thin cellophane foil placed inside a cloud chamber. This led Ruhlig to hypothesize that he was observing secondary (in-flight) tritium-on-deuterium fusions and conclude that the d(t,n) reaction "must be an exceedingly probable one." This was the first attempt to quantify the probability of d(t,n) fusion, using the similar to 1-MeV tritons generated by d(d,p)t fusion. This caused some Manhattan Project scientists to suggest that the d(t,n) cross sections are significantly higher than those for deuteron-on-deuterium fusion and led to the first measurement of d(3He,p) and d(t,n) cross sections in 1943. Here, we have used modern cross sections and stopping powers to estimate the expected numbers of high-energy protons associated with in-flight d(t,n) reactions in Ruhlig's experiment. Our estimate is four orders of magnitude lower than Ruhlig's observed rate. However, the number of high-energy protons in Ruhlig's experiment can be obtained via simulation if the protons are assumed to have been emitted by secondary in-flight d(similar to He,p) reactions, with various plausible assumptions about the experimental geometry and target-backing thickness. Our calculations demonstrate that quantitative information about the fusion of A = 3 ions with deuterium could have been obtained via experiments similar to Ruhlig's well in advance of the advent of 3He ion and triton beams in 1943. This opportunity seems to have been missed.

The term "Bretscher state" may not be as familiar as "Hoyle state," but its anthropic importance cannot be overstated. In Big Bang nucleosynthesis, the deuterium-tritium (DT) fusion reaction 3H $${(d,n)<^>4}$$(d,n)4He, enhanced by the 3/2+ resonance due to the Bretscher state, is responsible for $$ \sim 99$$similar to 99% of primordial 4He. While this fact has been known for decades, it has not been widely appreciated, and we recently proposed that its significance be commemorated by naming the 3/2+ state after Egon Bretscher, its discoverer. The importance of the resonant nature of the DT fusion reaction has been amplified by recent activities related to the production and use of terrestrial fusion including recent, net gain shots at the National Ignition Facility. Here, we aim to highlight the anthropic importance of the 4He-producing DT reaction that plays such a prominent role in models of nucleosynthetic processes occurring in the early universe. This primordial helium serves as a source for the subsequent creation of $$ \ge\!25$$>= 25% of the carbon, 12C and other heavier elements that comprise a substantial fraction of the human body. Further studies are required to determine a better characterization of the amount of 12C than this lower limit of 25%. Some scenarios of core stellar nucleosynthetic yield of 12C suggest that even higher percentages of carbon from primordial helium are possible.

This is a transcript of my talk for Nuclear Data for Science and Technology conference, ND2022. The talk is available on YouTube: www.youtube.com/watch?v=a_WYcTHco00 . This work is derived from a 2021 paper I wrote in the American Nuclear Society’s journal Nuclear Technology 207, S24 (2021). It was published in a special issue together with twenty-three other papers, for the 75 th Anniversary of the culmination of the Manhattan Project. It would be better to watch the video than read this transcript!

We describe the earliest measurements of the DT fusion cross section commissioned by the Manhattan Project, first at Purdue University in 1943 and then at Los Alamos 1945-6 and later, in 1951-2. The Los Alamos measurements led to the realization that a 3/2$^+$ resonance in the DT system enhances the fusion cross section by a factor of one hundred at energies relevant to applications. This was a transformational discovery, making the quest for terrestrial fusion energy possible. The earliest measurements were reasonably accurate given the technology of the time and the scarcity of tritium, and were quickly improved to provide cross section data accurate to just a few percent. We provide a previously-unappreciated insight: that DT fusion was first reported in Ruhlig's 1938 University of Michigan experiment and likely influenced Konopinski in 1942 to suggest its usefulness for thermonuclear technologies. We report on preliminary work to repeat the 1938 measurement, and our simulations of that experiment. We also present some work by Fermi, from his 1945 Los Alamos lectures, showing that he used the S-factor concept about a decade before it was introduced by nuclear astrophysicists.

We describe how the prompt fission neutron spectrum (PFNS) was determined for the Manhattan Project at Los Alamos. Early work before World War II at American and British universities is described, together with theoretical work by Feather at Cambridge and Bethe at Los Alamos. As the Manhattan Project was being planned in 1942, two experiments on natural uranium were commissioned that proved to be influential: 1) An integral experiment at Chicago by Christy and Manley that accurately determined the average PFNS spectrum energy, 2.2 ± 0.2 MeV; 2) Bloch and Staub’s Stanford cyclotron measurement of the PFNS spectrum, which obtained an average energy of 1.70 ± 0.34 MeV. These two papers, previously unavailable outside of Los Alamos, are reproduced in the Supplementary Appendix. From these data, at the beginning of the project in 1943 Serber estimated an average 235 U PFNS energy of 2 MeV, and indeed this agrees with today’s best estimate. The challenges facing the scientists involved both the availability of only very small samples of enriched uranium and plutonium targets, and fast neutron detection technologies. During the project, 235 U and 239 Pu PFNS were measured by Nicodemus and Staub. These also proved to be quite accurate and gave an average spectrum energy of 2 MeV for 235 U. [This is not reproduced in the Appendix because it was published after the war in Physical Review 89, 1288 (1953)]. New methods were developed to enable more accurate measurements, and this paper describes how the PFNS was determined surprisingly well by 1945. We end by describing the post-war measurements in the 50s, including the PFNS data used by Ford and Wheeler in their simulations in 1951, the Bonner 1952 data, the seminal 1952 Watt paper with a new empirical parametrization of the PFNS, and the accurate PFNS measurement undertaken at Los Alamos by Cranberg et al. in 1956. We compare the measurements with our best understanding today as embodied in the Evaluated Nuclear Data File ENDF/B-VIII.0. Some images from historical documents in our Los Alamos National Security Research Center (NSRC) archives are shown.

In the last decade, there has been a renaissance of fission research resulting in new high-precision experiments and advanced fission modeling. For instance, the Chi-Nu and CEA teams supplied, for the first time, the 239 Pu prompt fission neutron spectrum (PFNS) for broad ranges of incident and outgoing neutron energies. The CEA team also measured 239 Pu average prompt neutron multiplicities, ν̄p , with lower statistical uncertainties and a technique significantly different than the one used in the past. The NIFFTE collaboration provided 239 Pu( n ,f)/ 235 U( n ,f) cross section shape ratios with uncertainties below 1% utilizing a novel detector type. Advanced fission event generators were developed, among them CGMF, FIFRELIN, FREYA, and GEF, which calculate post-scission fission observables in a correlated manner. These new experimental data and more consistent fission models change the evaluated PFNS, ν̄p , and ( n ,f) cross sections, some only modestly, compared to ENDF/B-VIII.0. In turn, the individual new nuclear data distinctly change simulated effective neutron multiplication factors of fast critical assemblies, but their combined impact is small, while affecting the prediction of LLNL pulsed sphere neutron leakage spectra and reaction rates only within experimental uncertainties. Also, the parameters obtained from fitting to ν̄p reproduce various post-scission fission observables within the uncertainties of experimental data. This indicates that new differential experiments and consistent fission modeling reduce compensating errors present in ENDF/B-VIII.0.

In big bang nucleosynthesis (BBN), the deuterium-tritium (DT) fusion reaction, D(T,n)$\alpha$, enhanced by the 3/2$^+$ resonance, is responsible for 99% of primordial $^4$He. This has been known for decades and has been well documented in the scientific literature. However, following the tradition adopted by authors of learned articles, it was stated in a matter-of-fact manner and not emphasized; for most people, it has remained unknown. This helium became a source for the subsequent creation of $\geq$25% of the carbon and other heavier elements and, thus, a substantial fraction of our human bodies. (To be more precise than $\geq$25% will require future simulation studies on stellar nucleosynthesis.) Also, without this resonance, controlled fusion energy would be beyond reach. For example, for inertial confinement fusion (ICF), laser energy delivery for the National Ignition Facility (NIF) would have to be approximately 70 times larger for ignition. Because the resonance enhances the DT fusion cross section a hundredfold, we propose that the 3/2$^+$ $^5$He excited state be referred to as the "Bretscher state" in honor of the Manhattan Project scientist who discovered it, in analogy with the well-known 7.6 MeV "Hoyle state" in $^{12}$C that allows for the resonant 3$\alpha$ formation.

and discuss complementary and collaborative international efforts. Detailed descriptions of nuclear data needs for basic science, nonproliferation, national security, nuclear energy together with medical and space applications are also presented. Lastly, a set of specific cross-cutting nuclear data needs with relevance for multiple applications areas are also identified for further discussion in a follow-on report planned for release at the end of January 2023.

action items from this and past CSEWG meetings. Please take a moment and note any actions for which you or others at your institution may be responsible.

The new paradigm of nuclear reaction evaluation postulates that differential and integral experiments along with the reaction models should be used in concert to produce the evaluated data files. The results of a previous assimilation project are summarized as a proof of principle that adjusting reaction model parameters to the results of integral experiments is feasible. The new paradigm requires vast moderniza-tion of the nuclear data infrastructure. Such modernization is actually carried out by several national and international efforts. We describe those dedicated to: (i) handling of differential and integral data, (ii) providing and operating the sensitivity profiles, (iii) ensuring reproducibility of the evaluations, and (iv) enabling automated verification of the entire library. We also point to the importance of the nuclear reaction modeling and discuss advantages and disadvantages of the new paradigm. (c) 2021 Elsevier Ltd. All rights reserved.

The Christy Gadget is the informal name for the plutonium device detonated in the Trinity test on July 16, 1945. In September 1944, Robert Christy, working in the theoretical implosion group, proposed a novel concept that altered the design of the nuclear core in Fat Man. While scientists originally intended to use a hollow sphere of plutonium, this design entailed substantial risk, due to the likelihood of asymmetries resulting from implosion. Christy proposed changing the design to a solid sphere of plutonium with a modulated neutron source, and the design was eventually adopted, tested at Trinity, and used in the attack on Nagasaki. While there is no question regarding the important role that Christy played in demonstrating its feasibility as a reliable design, there is a debate as to who initially proposed the idea; though most sources have attributed this invention to Christy, some historical sources have attributed credit to Christy's group leader, Rudolf Peierls, or indeed other scientists. This paper seeks to outline and resolve this dispute. We present new unclassified evidence extracted from previously unavailable sources (to unclassified audiences) from the National Security Research Center archives at Los Alamos National Laboratory. This evidence consists of 1945-1946 patent documentation, oral history interview tapes of Christy and Peierls, and monthly 1944 progress reports from the Theoretical Division. Though Christy and Peierls share joint credit on the patent, both Christy's and Peierls' words and writings, together with sources from Hans Bethe and Edward Teller, support the traditional view that Christy was indeed the originator of the idea. While Christy does deserve the majority of the credit for the invention and design, we acknowledge the important role Peierls and von Neumann played in its development.

The National Nuclear Security Administration (NNSA)/DP French Alternative Energies and Atomic Energy Commission (CEA)/DAM agreement on cooperation on fundamental science is a U.S.-French collaborative effort to combine intellectual and experimental resources and further the relevant nuclear science. Recently, both the NNSA and CEA experimental teams performed high-statistics measurements of the 239Pu(n, f) prompt fission neutron spectrum (PFNS) at the Los Alamos Neutron Science Center, both of which were recently published in the journal Physical Review C. These separate measurements used the same experimental area and a common neutron detector array, but differ in many aspects, including background assessments, data acquisition systems and philosophies, fission detectors, and PFNS extraction techniques. Hence, some aspects of the experimental methods and associated uncertainties are highly correlated while others are independent. The results from both measurements broke new ground for PFNS measurements given their higher accuracy and more detailed study of corrections necessary for the measured quantity compared to existing literature measurements, and both will significantly impact PFNS nuclear data evaluations for the foreseeable future. The focus of this work is to document a comparison of the results from these distinct measurements in terms of the acquired data, the PFNS results, and the measured average PFNS energies. While systematic differences between the PFNS results are present on the 1–3% level, the acquired data relative to each respective measurement at low incident neutron energies are in remarkable agreement, as are the conclusions regarding the magnitude and position of features in the PFNS relating to second-chance fission, third-chance fission, and pre-equilibrium neutron emission.

Nuclear physics advances in the US and Britain, from 1939-1945, are described. The Manhattan Project's work led to an explosion in our knowledge of nuclear science. A conference in April 1943 at Los Alamos provided a simple formula used to compute critical masses, and laid out the research program needed to determine the key nuclear constants. In short order, four university accelerators were disassembled and reassembled at Los Alamos, and methods were established to make measurements on extremely small samples owing to the initial lack of availability of enriched 235 U and plutonium. I trace the program that measured fission cross sections, fission emitted neutron multiplicities and their energy spectra, and transport cross sections, comparing the measurements with our best understanding today as embodied in the Evaluated Nuclear Data File ENDF/B-VIII.0. The large nuclear data uncertainties at the beginning of the project, which often exceeded 25--50%, were reduced by 1945, often to less than 5-10%. 235 U and 239 Pu fission cross section assessments in the fast MeV range were reduced with more accurate measurements, and the neutron multiplicity increased. By a lucky coincidence of cancelling errors, the initial critical mass estimates were close to the final estimated masses. Some images from historical documents from our Los Alamos archives are shown. Many of the original measurements from these early years have not previously been widely available. Through this work, these data have now been archived in the international experimental nuclear reaction data library (EXFOR) in a collaboration with the IAEA and Brookhaven National Laboratory.

A collection of articles reviewing Trinity on its 75th anniversary.

action items from this and past CSEWG meetings. Please take a moment and note any actions for which you or others at your institution may be responsible.