Хирошимагийн атомын бөмбөгдөлтийн үлдэгдэл шилэн дотроос шинэ төрлийн металл илрүүлэв

Mixed in with the ordinary sand on the beaches of Hiroshima Bay are tiny glass beads, easy to overlook unless you know what to look for. Some are round. Others have fused together or carry thin tails, as though the molten material was pulled into shape before it had time to harden. Their sizes vary enormously, from a few micrometers to several millimeters across.

They fell from the sky on Aug. 6, 1945.

That morning, the atomic bomb dropped on Hiroshima detonated roughly 580 meters (1,900 feet) above the city. Temperatures inside the fireball exceeded 7,000 °C (12,600 °F), turning buildings, metal, glass, soil and water into a swirling mass of vaporized material. As the cloud cooled, that vapor condensed into droplets. Eventually, they rained back to the ground.

Scientists call these droplets hiroshimaites. And inside one of them, researchers have found something that had never appeared in the scientific record: a microscopic grain of metal containing seven elements arranged in an unfamiliar crystal structure.

The team, led by Luca Bindi, a crystallographer at the University of Florence, described the discovery in a study published in Science Advances. The material is a previously unknown, silicon-rich alloy, with an atomic arrangement not previously documented in that family of metals.

Seven Elements in One Cubic Lattice

Iron accounts for the largest share of the newly identified grain, followed by substantial amounts of chromium, silicon and nickel. Manganese, molybdenum and aluminum are present as well.

The researchers classify it as a multicomponent alloy, a category of metals containing five or more principal elements in significant proportions rather than a single dominant metal with small additions of other ingredients. Much of that material probably came from Hiroshima itself, from the buildings and other urban structures consumed by the explosion.

The chemistry was unusual, but the real surprise lay deeper. Using single-crystal X-ray diffraction, a technique that reveals how atoms are positioned in three dimensions, the team identified a cubic structure belonging to the AlAu4 type.

Ordinarily, metals with this combination of ingredients would be expected to form more familiar arrangements, such as those found in stainless steel.

Bindi told Physics World that the extreme conditions of the explosion, followed by extraordinarily rapid cooling, had effectively trapped the alloy in a metastable state before its atoms could settle into a more conventional structure. The unusual lattice survived.

“These particles are not simply melted debris,” Bindi told Scientific American. “They are physical archives of the explosion.”

One Grain Out of 34

Finding the alloy required a close examination of 34 hiroshimaites using electron microscopy, chemical analysis and X-ray diffraction. In one glassy sphere, the researchers identified four separate metallic grains, even though metal accounted for only about 1 to 3 percent of the entire particle.

Three of those grains were ordinary iron-chromium alloys. The fourth was different.

“So, it appears to be rare,” Bindi told C&EN.

The grain measures only a few micrometers across, far thinner than a human hair. Yet its crystal structure preserved evidence of conditions that are exceptionally difficult to reproduce.

Bindi described the Hiroshima fireball as a “giant accidental material-science laboratory,” and the study draws comparisons between the environment inside the explosion and the extreme conditions produced by hypervelocity planetary collisions, meteorite impacts and lightning strikes.

Michael Widom, a physicist at Carnegie Mellon University who has collaborated with Bindi in the past but was not involved in this research, said Bindi “rightly recognizes that if you want to find new materials, they’re going to be in unusual places.”

A Quasicrystal in the New Mexico Desert

For Bindi, searching for unfamiliar materials in the aftermath of violent events is not new. On July 16, 1945, three weeks before Hiroshima, the first nuclear test, code-named Trinity, took place in the New Mexico desert. The blast melted surrounding desert sand into a greenish glass known as trinitite.

In 2021, Bindi and his colleagues reported finding a quasicrystal inside a fragment of that glass, a solid with an ordered atomic structure that never repeats in a regular pattern.

Trinitite also contains a clathrate, a crystal with a distinctive cage-like structure.

The newly discovered Hiroshima alloy is not itself a quasicrystal, although parts of its atomic arrangement resemble structures seen in quasiperiodic materials. The circumstances of the two nuclear explosions were different, too. Hiroshima’s bomb detonated high above a densely built city, whereas the Trinity device was mounted on a tower above open desert sand.

Those differences are reflected in the glass left behind. A 2019 study of the beach sand, gathered between 4 and 7 miles (6 to 11 kilometers) from Hiroshima, found particles with chemical signatures matching concrete, marble, stainless steel and rubber, materials that had been exposed to temperatures above 1,800 °C (3,300 °F). In some samples, these glassy fragments accounted for as much as 2.5 percent of the sand grains.

“Hiroshimaite particles are much more complex and diverse than trinitites,” said Nobumichi Tamura, a co-author of the 2019 study at Lawrence Berkeley National Laboratory.

The first particles were identified in 2015 by Mario Wannier, a retired geologist examining beach sand collected by his colleague Marc de Urreiztieta on Japan’s Motoujina Peninsula. Wannier noticed the unusual glassy fragments while sorting through the samples and subsequently began separating them from the surrounding sand.

In all, he picked out and sorted approximately 10,000 particles.

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