A blast from the past

A 1940s-era explosives site is now a protected archaeological site.

By Whitney Spivey | December 1, 2020

Rudolf Peierls X Ray Opt
German-born British physicist Rudolf Peierls (far left) observes the installation of the x-ray detector post at P-Site, part of the wartime Laboratory (Project Y), in the early 1940s. Los Alamos National Laboratory

In October 2016, engineers John Benner and Jonathan Morgan were driving around a remote part of Los Alamos National Laboratory. A new road had recently been cut along the edge of a canyon to provide access to groundwater-monitoring wells, and they wanted to see if it encroached on the areas where high-explosives experiments take place. “We wanted to verify that all road access to these areas had appropriate warning signs and locked entry gates,” Morgan remembers.

“My hope is that we can make progress on the archaeology of P-Site so that it’s more formally recognized as contributing to the Manhattan Project.” “My hope nhattan Project.”
- Jeremy Brunette

Looking out the passenger window, Benner saw a large, rusted steel post lying on the ground. “I said ‘Stop!’” Benner remembers. “‘That looks really old!’”

The two got out to inspect the post, the top of which was in the shape of a cross. “We could tell it was designed to withstand blasts and that it had experienced blasts,” Benner says. Beyond that, he wasn’t quite sure what the post was … until a Laboratory historian recalled a Manhattan Project photo of the very same post being suspended by a crane.

During the Manhattan Project, scientists wanted to study implosion—compressing a plutonium core by detonating high explosives around it, which results in a nuclear explosion. Small Geiger counters (instruments used to detect radiation) were used to detect x-rays passing through the high explosive. Several Geiger counters, each about an inch in diameter, were placed inside the post (a hollow construction) and arranged in a grid on the inner-back surface of the heavily reinforced cross-shaped section. The post was strong enough to survive nearby explosions while allowing x-ray penetration to the detectors inside it. The resulting data was used to study components of the implosion device, although this particular method did not work as well as scientists had hoped and was abandoned in March 1945.

The x-ray detector post, and all of P-Site, where this and other explosives work had taken place, was also quite literally abandoned. The remote 10-acre area was seemingly forgotten until 2005, when the  Laboratory decided to demolish the remaining buildings, which were old and unsafe.

Bob Webster John Benner P Site
Deputy Laboratory Director for Weapons Bob Webster (left) and Weapons Executive Officer John Benner inspect the 1940s-era x-ray detector post at P-Site in July 2020.

Even then, it was another decade before Benner and Morgan stumbled upon the post. The men went back to P-Site with some colleagues on October 31, a few days after their initial discovery. “By sticking a phone camera inside the structure, we were able to get a picture of all the electrical cabling that tied the detectors together into an array,” Morgan says. “As we left the site on Halloween, the team had a pretty good idea that this was a setup to radiograph detonating high-explosives objects and measure the wave shape in the cruciform x-y directions.”

Laboratory archaeologist Jeremy Brunette explains how fortuitous it was that Benner and Morgan were the ones to find the post. “Due to their extensive experience in the Weapons programs, they knew what they were looking at, and could convey the significance of what they found,” Brunette says. “The detector post sits right next to a seldom-used road, but to most people—myself included— it doesn’t look like anything special. It really took people who understand the science and engineering components of the Manhattan Project to understand what it was.”

Shortly thereafter, the Laboratory’s cultural resources staff surveyed P-Site and submitted documentation to the New Mexico State Historic Preservation Office, which then declared the area an archaeological site. “The fact that it is now an archaeological site is helping to protect the site and is a critical part of the preservation of the site,” Brunette explains. “My hope is that we can make progress on the archaeology of P-Site so that it’s more formally recognized as contributing to the Manhattan Project. Many areas of the Laboratory are already part of the Manhattan Project National Historical Park, but P-Site is not one of them—yet.”

In the meantime, Brunette and his fellow archaeologists are trying to decide what to do with the x-ray detector post. “I would like to see it displayed at P-Site, at the Laboratory’s Bradbury Science Museum, or near the Lab’s Dual-Axis Radiographic Hydrodynamic Test (DARHT) facility, which is where the Lab does implosion experiments today,” he says, noting that any option will require quite a bit of effort: “At probably more than 3,000 pounds, the post is very heavy!”

Darht Facility
Today, the Laboratory’s DARHT facility is a much improved descendant of the 1940s-era x-ray detector post. DARHT consists of two large x-ray machines that produce freeze-frame radiographs (high-powered x-ray images) of mock-nuclear weapons that implode at speeds greater than 10,000 miles an hour. Completed in April 2020, a new weather enclosure (pictured) around the site’s firing point (where implosions take place) keeps experiments on schedule and improves worker safety.