Trinity revisited

In the midst of a global pandemic, how do you commemorate the 75th anniversary of one of the most impactful scientific achievements of all time? In the spring of 2020, Mark Chadwick, chief operating officer and chief scientist for the Weapons Physics directorate at Los Alamos National Laboratory, asked himself that question.

The scientific achievement under consideration was the Trinity test—the detonation of the world’s first atomic device in New Mexico’s Jornada del Muerto (“Journey of the Dead Man”) desert on July 16, 1945. Manhattan Project scientists based at a then-secret laboratory in Los Alamos, New Mexico, had designed, assembled, and detonated the device. The resulting explosion changed the course of history in numerous ways. The scientists developed a weaponized version of the device that was detonated above Nagasaki, Japan, on August 9, 1945. Several days later, an armistice was declared, and on September 2, World War II officially ended. The then-secret laboratory eventually morphed into Los Alamos Scientific Laboratory and then Los Alamos National Laboratory. Designing and maintaining nuclear weapons has remained its primary mission.

With this legacy in mind, Chadwick organized a virtual lecture series to explore the science behind the Trinity test. To orient listeners, one lecture focused on history, but most dug into deeply technical aspects of the test. Lecturers explained the science behind Trinity— techniques and experiments that had never been done before because atomic weaponry had never been pursued previously. They also explained the various ways that applying modern-day data analysis and computer simulation techniques to 75-year-old data had helped them to understand the test better.

As each lecture unfolded, Chadwick was inspired to capture and share the information. At the conclusion of the series, Chadwick challenged the speakers to write up their presentations as technical papers. He also invited colleagues at other nuclear science institutions (including Lawrence Livermore National Laboratory, Sandia National Laboratories, and the United Kingdom’s Atomic Weapons Establishment [AWE]) to contribute additional, related papers.

In a few short weeks, 120 people had agreed to write or collaborate on what would become 46 individual papers. “The project became contagious,” Chadwick recalls. “As people saw their colleagues signing up to write on different topics, they were inspired to join in the project with their own papers.”

“This was our chance to go into more detail than what the existing scholarship addressed,” he continues. “The goal of these papers was to clarify the nature of the breakthroughs made, correct previous misunderstandings in the open literature, illuminate fascinating aspects of the underlying research, and illustrate how science from 75 years ago has proven foundational for the peaceful use of nuclear energy and today’s nuclear technology.”

The end result is twofold. In May 2021, all 46 papers (including 27 classified papers) were published in a 550-page, Trinity-focused edition of Weapons Review Letters (WRL)—a monthly electronic journal internal to Weapons Physics. Later this year, 23 unclassified papers will be published in a special issue of the American Nuclear Society’s Nuclear Technology journal.

Exploring the archives

For the majority of the coronavirus pandemic, the Laboratory conducted “normal operations with maximized telework,” which meant that many researchers who had previously worked in classified environments were working at home several days a week. Researching and writing their papers for Chadwick was a good way to fill that time. (Classified papers, of course, had to be written in a secure area at the Laboratory.) If and when researchers did go on site, some visited the National Security Research Center (NSRC), the Lab’s classified library. The NSRC contains approximately 20,000 documents relating to Project Y—the Los Alamos branch of the Manhattan Project. With assistance from people including Senior Historian Alan Carr—who authored a paper on the history of Trinity and its impacts—and Senior Archivist Danny Alcazar, researchers pored over pages of hand-typed records, manuscripts, photographs, and more. Often, they found more than they bargained for.

“Once you look and dig, you find things that allow you to point to other things that haven’t been appreciated,” Chadwick says. “There’s so much information; you often don’t quite know what you’re going to stumble across.”

Even Carr, who has worked for the Lab for almost 20 years, was surprised by some of the finds. “I was able to use records I had never seen before,” Carr says. “I imagine some of them had not seen the light of day in decades.”

To facilitate reviews of all the information collected, Chadwick dispatched each paper to experts at Los Alamos, Livermore, and beyond. Their feedback identified gaps or weaknesses in the writing that could then be rectified through additional investigation at the NSRC.

“The project introduced a new way to do peer review—a crowd sourcing approach in which all draft papers were sent to all coauthors participating in the project, inviting their feedback,” Chadwick says. “This meant that we were able to take advantage of a broad range of expertise as the papers were being developed.”

Many of the unclassified papers are highlighted by subject area below (click here for the complete list of paper titles and authors).

Nuclear science and technology

Chadwick, of course, contributed to the project. He authored a paper that documents the neutron cross-sections (used to express the likelihood of interaction between a neutron and a nucleus) measured with increasing accuracy during the Manhattan Project.

“Project Y scientists did some very clever measurements to infer what the key quantities might be,” Chadwick explains. “It was fascinating to figure this out. I said, ‘I need to turn this into a paper—it’s important and needs to be documented.’”

Accurate neutron cross-sections were needed to determine critical masses—the minimum amounts of fissile material needed to maintain nuclear chain reactions—of plutonium and highly enriched uranium. At Project Y, this work was necessary for the development of a plutonium implosion bomb—a device that would use high explosives to rapidly compress and increase the pressure and density of a spherical plutonium core, pushing the core to critical mass. The resulting nuclear chain reaction produced a powerful explosion.

To measure neutron cross-sections, 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 plutonium and enriched uranium-235.

In just two years, advances in experimental methods led to measured nuclear data that are surprisingly close to today’s best values in the Evaluated Nuclear Data Files—America’s nuclear reaction database that is developed by national laboratories and universities in collaboration with the International Atomic Energy Agency (IAEA). Many of the key original papers and numerical values have now been archived through a collaboration with the IAEA and Brookhaven National Laboratory in the internationally available Experimental Nuclear Reaction Data database.

A paper on the first fast critical assemblies (metal assemblies—reactor cores—that do not contain materials that can moderate and slow down the neutrons) by Jesson Hutchinson (of the Los Alamos Advanced Nuclear Technology group) and colleagues and another on pulsed and solution assembly experiments (which involve neutrons moderated by hydrogen atoms in the assembly) by Robert Kimpland (also of the Advanced Nuclear Technology group) and others provide details about how critical masses were determined, and how they influenced subsequent research across the world on nuclear criticality and criticality safety. The Los Alamos “water boiler” assembly was the world’s third reactor to become operational (in 1944, after Chicago’s CP-1 and Oak Ridge’s X-10 piles), the first to use a solution, and the first to use enriched uranium fuel. It assembled a critical mass of enriched uranium in a solution, with a chain reaction of neutrons slowed down to thermal energies.

During the Manhattan Project, bare critical masses (experiments that made metal nuclear material critical without using a reflector around it) were not measured directly because of a lack of time and material; instead, the bare critical masses were estimated using subcritical measurements and reflected assemblies— spherical critical masses surrounded by neutron reflectors. Using neutron reflectors, scientists could make critical masses that used smaller amounts of fissile material, providing a snapshot of what a final bare critical mass would look like.

Avneet Sood, of the Lab’s Radiation Transport Applications group, describes the evolution of neutronics calculational capabilities—computer simulations that describe the motion and chain reactions of the neutrons—from early neutron diffusion work to subsequent refinements by physicists Robert Serber and Alan Wilson and the postwar innovations of so-called “Sn deterministic’’ and Monte Carlo neutron-transport simulations.

Stephen Andrews (of the Lab’s Verification and Analysis group), Madison Andrews (of the Lab’s Radiation Transport Applications group), and Laboratory Director Thom Mason (who was born in Canada) describe the Canadian work at the Montreal Laboratory and Chalk River and the essential role Canada played in supplying nuclear materials for the Manhattan Project. The authors also tell of the contributions of the Canadians who came to work on the Manhattan Project in the United States. The Montreal Laboratory’s work was focused on neutronic criticality theory and heavy-water-moderated reactor experimentation—research that proved to be important for postwar CANDU reactor development.

Without British (see below) and Canadian expertise and resources, Mason argues, Trinity might not have happened until much later, or it might not have worked. “An unsuccessful test might have had far-reaching implications,” he says. “It would have taken time to understand the origins of failure, any changes needed to mitigate, and the conduct of a second test to confirm this analysis and design or fabrication changes. Furthermore, the materials ultimately used in World War II would, instead, have to be used for a second test.”

Of course, Trinity was a success, and the design used to pull it off helped bring a quick end to World War II in the form of Fat Man. Little Boy, a gun-type nuclear weapon that scientists were confident would work, was detonated above Hiroshima, on August 6, 1945. “It has been said that the most significant nuclear weapons secret was—for the time period between Trinity and Hiroshima—that they work,” Mason says. “Trinity revealed that a nation with sufficient resources and persistence could develop a weapon.”

Hydrodynamics

Nathaniel Morgan (of the Lab’s Applied Mathematics and Plasma Physics group) and Bill Archer (of the Lab’s Weapons Physics directorate) describe Los Alamos’ Theoretical Division’s Lagrangian hydrodynamic shock calculations. Performed on IBM punched-card machines, the calculations helped scientists model the physical motion of metals that flow like liquid when heated under pressure; this helped them understand what happens inside a nuclear device when it’s detonated.

Their paper presents the algorithmic advances made during the Manhattan Project by mathematician and physicist John von Neumann who led to the late-1940s formulation of computational fluid dynamics. Today, the algorithms developed by von Neumann and physicist Robert Richtmyer are the basis of simulations for everything from climate change to nuclear reactor design.

Morgan and Archer also illuminate the less appreciated, but very influential, roles of Manhattan Project physicists Rudolf Peierls and Tony Skyrme. The authors describe that the first usage of “artificial viscosity,” a concept central to computational hydrodynamics. In fact, based on a letter from Peierls to von Neumann found in the NSRC archives, artificial viscosity appears to have originated with Peierls in 1944.

When examining fluid flow to learn how explosions behave, shock propagation processes severely complicate the research because they add discontinuities to mathematical equations for explaining fluid motion. In other words, because the discontinuities are not mathematically continuous, it’s virtually impossible to get a totally accurate value. By introducing viscosity, Peierls demonstrated that thickening and flattening these otherwise unruly shock zones could help resolve them computationally.

As Project Y moved toward the design of an implosion-type weapon, hydrodynamic modeling became crucial to understanding how the Trinity device (and Fat Man) would work. Today, hydrodynamic modeling remains essential for maintaining the U.S. nuclear stockpile.

Skyrme is well known to nuclear and particle physicists, but few know of his research in shock physics (the study of materials in extreme conditions). Indeed, it was Project Y Director J. Robert Oppenheimer seeking expertise in this area that brought two dozen British scientists to New Mexico in 1944.

Other papers describe the history of the Los Alamos computing facility; Nicholas Lewis of the NSRC wrote about the Laboratory’s human computers (many of them women). Archer described the IBM punched-card computations needed for hydrodynamics and neutronics.

High explosives

The implosion design of the Fat Man atomic bomb relied on precision-engineered high explosives (HE) to symmetrically compress a plutonium core. Focusing the effect of an explosive’s energy like this is called shaping a charge. Eric Brown and Dan Borovina describe this work, its subsequent impact on broader shaped-charge technology, and its use in mining, oil recovery, and even SpaceX multistage rocket separation.

AWE’s Richard Moore describes pioneering British work on shaped charges that influenced von Neumann, Seth Neddermeyer, and James Tuck’s HE lens design that controlled the shape and velocity of the HE detonation around the plutonium core of an implosion device. The trio determined that simultaneously exploding faster- and slower-burning explosives in a certain configuration within a weapon would produce a specific compressive wave that would focus enough shock inward on the plutonium core to increase its density several times over. Doing this would reduce the core’s critical mass and make it supercritical at the right time to start a chain reaction.

Jonathan Morgan, of the Laboratory’s Integrated Weapons Experiments group, authored a paper that describes Jumbo, the steel vessel pictured on the cover of this magazine that would contain the Trinity test if the HE detonated but the plutonium core did not implode. Jumbo would allow scientists to recover the precious plutonium. In the end, Jumbo was not used for Trinity, but the vessel was valuable for later containment-vessel work and reactor engineering.

Plutonium materials and metallurgy

Joseph Martz (of the Lab’s Materials Science and Technology group), Franz Freibert, and David Clark (both of the Lab’s National Security Education Center) trace the process through which plutonium was discovered in 1940 at University of California–Berkeley. The first plutonium was characterized there and at Chicago’s Metallurgical Laboratory, before U.S. research efforts were consolidated at Los Alamos in 1943. Particularly interesting is the early confusion caused by the widely varying density measurements and the subsequent discovery of the many complex phases of plutonium. This work collects the historical records and reconstructs the history of the rapidly advancing field of plutonium metallurgy and chemistry. The authors show that the idea of using gallium as an alloying agent to stabilize the malleable delta phase of plutonium was first raised only a few months before the Trinity test, a reflection of the intense pace of the project.

Another paper by Scott Crockett (of the Lab’s Physical Chemistry and Metals group) and Freibert describes the rapid wartime expansion of experimental and theoretical work on the equation of state (a quantity that describes a given set of physical conditions, such as pressure, volume, or temperature) of plutonium, uranium, and other materials. The authors describe the foundational equation of state research needed to understand the hydrodynamic behavior of these materials.

Today, Los Alamos is the nation’s Plutonium Center of Excellence for Research and Development and is the only place in the United States that can use plutonium to make everything from plutonium cores to heat sources that power Mars rovers. These capabilities are a direct result of the scientific achievements of the Manhattan Project—specifically the Trinity test.

Nuclear energy and yield

Susan Hanson and Warren Oldham (both of the Lab’s Nuclear and Radiochemistry group) review the foundational radiochemistry methods developed to measure the yield of Trinity and how the techniques evolved in subsequent years. David Mercer (of the Lab’s Physical Chemistry and Applied Spectroscopy group), Katrina Koehler (of the Safeguards, Science, and Technology group), and others describe recent measurements of radionuclides in trinitite rock from Alamogordo. Their paper describes traditional radiation-detection methods used in the training of IAEA inspectors at Los Alamos as well as the novel decay energy spectroscopy method.

Immediately after the Trinity test, the first estimate of the device’s yield was about 18 kilotons (the equivalent of 18 kilotons of trinitrotoluene, or TNT), with an estimated 20 percent uncertainty. In the years following World War II, the yield of the device was recalculated to be 21 kilotons. However, after examining archival calculations and recent measurements of radionuclides in rock samples originally taken from near the city of Alamogordo—located just outside the blast zone—Hugh Selby (of the Lab’s Nuclear and Radiochemistry group) and others share in their paper that Trinity’s yield was higher still, approximately 24.8 kilotons.

“The new value comes from the powerful combination of advanced inorganic separations chemistry with high precision mass spectrometry—an analytical tool useful for measuring the mass-to-charge ratio of molecules,” Selby explains. “The former purifies the element containing fission fragments of interest from the sea of chemical interferences present in debris. The latter quantifies the minute amounts of bomb-produced isotopes of the element present in the purified sample, relative to the natural background. The level of precision necessary to make such measurements and to reanalyze 75-year-old data was made possible by major advances in both chemistry and mass spectrometry.”

Other papers examine early prompt assessments of Trinity’s yield: Jonathan Katz (of the Weapons Physics directorate) sought to understand how physicist Enrico Fermi might have determined the yield when he observed the blast wave’s impact on small pieces of falling paper; Roy Baty and Scott Ramsey (of the Lab’s Theoretical Design division) revisit G. I. Taylor’s 1950 determination of the yield from the growth of the fireball. Using Lie group symmetry techniques, they rederive Taylor’s two-fifths law relating a blast wave’s position, time, and explosive energy.

During the Manhattan Project, physicists Hans Bethe and Richard Feynman developed an analytic formula to predict the yield of a fission explosion from some elegant considerations. This work has had an enduring influence over the past 75 years; six classified papers were written on different aspects of the formula— three from Los Alamos, one from Livermore, and one from AWE. A short “tri-lab” paper by John Lestone (of Los Alamos’ Radiation Transport Application group), Mordy Rosen (of Livermore’s Design Physics division), and Peter Adsley (of AWE) describes, for the first time, the formula and its relationship to earlier wartime British work by physicists Rudolf Peierls, Otto Frisch, Paul Dirac, Maurice Pryce, and Klaus Fuchs.

For Rosen, the research hit close to home. “As someone who loves studying history, it was fun to ‘re-live’ Trinity through modern eyes,” he says. “On a personal note, my late father-in-law served in the U.S. Navy in the Pacific Theater of Operations in World War II and was preparing for the invasion of Japan. Trinity and its aftermath saved millions of U.S. and Japanese lives by bringing the war to a rapid end. Thus, my wife and our 11 grandchildren are likely beneficiaries of this event.”

Technical history

The Trinity papers include several technical history papers, including those mentioned above on the beginnings of computing. Another discusses the origins of the plutonium core design and describes the invention patent located in the NSRC archives, resolving longstanding disputes about who originated the idea. (It was Robert Christy.)

In a second paper, Moore introduces Peierls’ 1945 summary of the British contributions to the “Tube Alloys” project, the codename for the British effort to build an atomic device, before the project transferred to Los Alamos. The author provides Peierls’ summary in full—with Sir James Chadwick’s marginal notes. Moore’s useful introduction and footnotes shine light on the activities of the time and the progress by the British researchers toward establishing the feasibility of an atomic bomb.

Trinity’s legacy

In 1947, physicist Robert Wilson wrote that the Trinity test “was for a specific military purpose. It will be gratifying to all those who participated in the work when it takes its more proper place as a contribution to the general structure of scientific knowledge.”

More than seven decades later, Trinity’s legacy is indeed felt across numerous technical fields, and Chadwick’s Trinity papers project helps solidify its influence. The papers likely represent the most in-depth analysis of the test ever completed; they include never-before-seen information and data that further ratify the event as one of the most important scientific experiments of all time. The papers also solidify Los Alamos’ place in history.

“In the process of researching and writing these papers, we confirmed that Los Alamos largely invented the field of nuclear science,” Chadwick says. “That was somewhat known, but this catalog of research shows the outside world exactly how much was invented here. The basic weapons science, physics, and engineering we use at the Lab today comes from that first breakthrough 75 years ago.”

“I can speak for all the authors when I say that we had fun writing these papers and that we learned many new things in the process,” Chadwick continues. “I trust that this collection is indeed a contribution to both the history of science and to the advancement of science.”

Individuals with permission and security clearances may access the Laboratory's Weapons Review Letters papers by emailing editor Craig Carmer . Many papers will also be published and accessible to all later this year in the American Nuclear Society's Nuclear Technology journal.

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