The Ultimate Pursuit of Alchemy - Turning Stone into Gold

Produced by: Science Popularization China

Author: Wu Yue (Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences)

Producer: China Science Expo

Gold (Au), as a well-known member of the heavy metal elements, is a common and popular precious metal in life. It has the advantages of strong stability, good ductility, and high catalytic performance. It is widely used in jewelry, currency, medical treatment, catalysis and many industrial fields.

In China, there is an idiom "turning stone into gold" long ago. In the West, when we mention turning stone into gold, we can easily think of the alchemists in the Middle Ages: they were committed to transforming ordinary metals into precious metals, such as gold, to satisfy the desire to create wealth. Not only Western alchemists pursued artificial gold, but people in Chinese history also made related attempts, but all failed.

From the perspective of modern chemistry, we know that the path taken by ancient alchemists is not feasible. But if we look at the life of "gold" from the perspective of modern science, can we realize the dream of turning stone into gold?

The growth story from hydrogen to iron

In the early universe, the density of matter was very high everywhere. It was like a thick soup of various particles, and there were no atomic nuclei. After the Big Bang, some particles stuck together and captured electrons to create atomic nuclei, but at first there were almost only very light atomic nuclei such as hydrogen and helium. Under the influence of gravity, these light nuclei gathered into clusters, and their core temperature and density continued to rise, eventually reaching the high temperature and pressure required for nuclear fusion, and hydrogen and helium atoms began to undergo nuclear fusion - stars were born.

The evolution of the universe

(Photo source: veer photo gallery)

For stars like the sun, hydrogen nuclei first undergo fusion reactions to generate helium nuclei, and the energy released by them prompts the helium nuclei to continue to undergo nuclear fusion, generating carbon nuclei and oxygen nuclei. The released energy continues to promote the fusion of carbon and oxygen nuclei to form heavier elements... Therefore, many people call stars like the sun "alchemy furnaces of elements." In this way, if nuclear fusion continues inside stars, the elements will become heavier and heavier, and eventually, won't rich gold mines be formed?

Actually, this is not the case. When the mass of a star is large enough, the nuclear reaction inside it can continue until an iron nucleus is generated. However, iron has the highest specific binding energy, so it becomes very difficult to "squeeze" nucleons into the iron nucleus, which requires a lot of energy. When iron fusion occurs inside a star, a lot of energy is consumed, which is equivalent to adding a "fire extinguisher" inside the star, causing the star to collapse. So how are elements heavier than iron (super-iron elements) formed?

The specific binding energies of various elements, among which iron has the highest

(Image source: Wikipedia)

The Midas Touch

Previous nuclear fusion was to fuse two lighter nuclei together to form a heavier nucleus. Based on the observations of the abundance of nuclear nuclei in the solar system and the nuclear shell model, scientists have deduced that the formation of super-iron elements is due to the "neutron capture" process - after the elements are fused into iron, the heavier elements are formed by directly inserting neutrons into the iron nucleus.

If only the number of neutrons increases, the type of atom does not change, but only heavier isotopes are produced. However, some isotopes are not stable and usually undergo β decay, that is, the neutrons in them have a certain probability of decaying into protons by releasing an electron. In this way, the nucleus will have one more proton and become a heavier element.

The process of converting iron into gold in stars through slow neutron capture

(Image source: Institute of Modern Physics, Chinese Academy of Sciences)

Neutron capture inside stars is very weak and inefficient, also known as "slow neutron capture" (Slow-process). Is there a way to accelerate it to achieve large-scale production of heavier elements?

In order to achieve "fast neutron capture" (Rapid-process), the density of the neutron flux must be high enough, there must be enough neutrons and enough energy to prompt the neutron capture process to occur quickly. What kind of environment can meet these conditions?

Scientists have discovered that supernova explosions can cause neutron capture to occur quickly. When iron fusion occurs inside a star, it absorbs energy and triggers a nuclear explosion, which is a supernova explosion. Supernova explosions can illuminate a galaxy and also produce elements with atomic numbers larger than iron atoms. In this "fireworks show" that can illuminate the galaxy, a large number of neutrons are released in an extremely high-energy form. Under the impact of these high-energy neutrons and the effect of beta decay, the previous atomic nuclei are constantly "upgraded", thus forming a large number of heavy metal elements. However, most of these heavy metal elements are concentrated in the core of the star, and not many of them are used by us.

In 1994, scientists photographed a supernova explosion (SN 1994D, the bright white spot in the lower left corner) (Image source: Wikipedia)

In addition, there is another way to produce elements with atomic numbers larger than iron, and that is the merger of neutron stars. Massive stars that undergo supernova explosions have two fates depending on the mass of their cores. One is to form a black hole, and the other is to become a neutron star with extremely high density. If two neutron stars in the universe meet, they will approach each other under the influence of gravity and eventually merge together. In the process of merging, strong gravitational waves will be generated, and elements with larger atomic numbers such as gold and silver will also be produced. However, this nuclear reaction is complex and diverse, so finding a celestial body of pure gold in the universe can only be an unattainable dream.

Isotope abundance distribution and corresponding nucleosynthesis processes in the solar system

(Image source: Reference 2)

A gift from the stars

The matter produced during supernova explosions and neutron star mergers will be thrown into the universe, mixed with existing interstellar gas and dust clouds, and form new stars under the action of gravity. In other words, the next generation of stars, including stars and planets, are born in the "graveyard" of the previous generation of stars. These stars, planets and other celestial bodies will continue to grow and tend to balance under the action of gravity, forming new galaxies, which include our solar system.

Therefore, part of the gold that exists on the earth now is the composition of the early miniature earth: the early earth material was in a molten state, and the density of gold was relatively large, so it would continue to sink into the core, and geological activities would transport part of the gold from the core to the surface in the form of volcanic eruptions; the other part is a gift from outer space: after the earth solidified and stabilized, the earth's gravity would "capture" passing meteorites, which are rich in heavy metals such as gold, tungsten, and lead. After coming to the earth, they will sink and accumulate under the influence of geological activities, waiting for the day when they will be discovered by people.

Artificial gold, the road is difficult and arduous

After understanding the "past and present" of gold, the theoretical basis of "turning stone into gold" already exists. The difference between various elements lies in the number of protons, neutrons and electrons in their atoms, especially the number of protons. If we can artificially change the number of protons in the nucleus, we can transform one element into another and use "cheap" elements to make "precious" elements. Nature provides us with two methods, representing the addition of nuclear fusion and the subtraction of nuclear fission.

There are 118 elements in the periodic table, 92 of which are found in nature, and 26 are artificial elements obtained by nuclear fusion from lighter elements. The production of artificial elements is to do addition, but to achieve this change requires extremely high temperature and high pressure, generally requiring a temperature of hundreds of millions of degrees or a pressure of hundreds of billions of atmospheres. To do subtraction, you need to find a way to remove the protons in the nuclei heavier than the gold nucleus, such as mercury.

The human dream of "turning stone into gold" finally came true in 1941. Dr. Bainbridge of Harvard University in the United States bombarded mercury atoms (mercury) commonly used by ancient alchemists with neutrons, knocking out a proton in its nucleus, and at the same time some neutrons were captured by the new nucleus, successfully turning mercury, which has an atomic number 1 greater than gold, into gold. In 1980, the Lawrence Berkeley Institute in the United States used a high-energy accelerator to bombard bismuth nuclei (atomic coefficient 83) with particles close to the speed of light, causing 4 protons to break out of the nucleus, leaving 79 protons, turning it into a gold atom. However, the gold element created in this process is extremely rare, and the energy consumption is extremely huge. It is estimated that the cost of gold produced in this way exceeds one trillion US dollars per ounce, but at that time, one ounce of gold mined only cost 560 US dollars.

Bismuth metal, which forms beautiful crystals naturally and is available in many science kits

(Image source: Wikipedia)

The human, material and financial resources consumed in the process of artificial gold far exceeded the value of the gold itself, which was undoubtedly a failure from an economic perspective. However, the purpose of these experiments was not to obtain monetary gold, but to enhance human cognition of super-iron elements in the process of creating gold, which is of great significance.

Although the experiment of turning stone into gold has been successful, there is still a long way to go in the research on the origin of super-iron elements. These studies will also greatly promote the development of disciplines such as nuclear physics, astrophysics and celestial evolution, and add bricks to the edifice of modern science.

References:

[1] Li Chao, Xie Zhongfei, Yang Shubin. Prosperity starting from “0” - Gold Element[J]. Guangdong Chemical Industry, 2020, 47(09): 93-94.

[2] Tang Xiaodong, Li Kuoang. The origin of elements in the universe[J]. Physics, 2019, 48(10): 633-639.