Development of an “ultra-low-radiation flexible printed cable” expected to be used in dark matter research, etc. | Sorae Space portal website

「Dark matter(Dark Matter) Collision/Collapse” and “Neutrino-free double beta decay“(described below) and other physical phenomena are believed to be closely related to the mysteries of the universe. It is said that these phenomena can only be detected by signals that occur extremely rarely, but in order to do so, it is necessary to remove signals that have nothing to do with the phenomenon as much as possible .

A research team led by Isaac J. Arnquist of PNNL (Pacific Northwest National Laboratory) has developed a “new”Ultra low radiation flexible printed cable(Flexible printed cables with very low radioactivity). This cable could be used not only for physics research but also for quantum computers in the future.

■Radioactive materials are a problem when capturing rare physical phenomena

“Radiation” emitted by the decay of unstable atomic nuclei and high-energy environments is all around us. Cosmic rays falling from outside the Earth, uranium and thorium in rocks, radon in the air, tritium in water, potassium-40 and carbon-14 in living organisms, etc., are examples that are familiar to us all around us. – It is a source of radiation. The level of environmental radiation found in nature is low in most places and does not pose a problem to the human body.

However, such radiation can pose a major problem in physics research. In particular, research aimed at exploring the true nature of “dark matter”, which is said to make up the majority of matter in the universe, and the mysterious elementary particle “neutrino”. (※1) This is a major problem in research observing “neutrino-free double beta decay,” which is said to lead to clarification of the properties of neutrinos.

Dark matter is an unknown substance whose existence can only be known through gravity. Unlike ordinary matter, such as stars and planets, which can be observed at electromagnetic waves (including visible light), dark matter cannot be observed at electromagnetic waves. Since electromagnetic interactions operate in matter that can be observed with electromagnetic waves, electromagnetic interactions do not operate in dark matter, so it is expected that it will not interact with ordinary matter except through gravity. However, depending on its identity, dark matter may collide with ordinary matter with very low probability. It is also possible that the energy released when the particles that make up dark matter decay can be observed in the form of electromagnetic waves.

Another phenomenon, neutrino-free double beta decay, is important in exploring the properties of neutrinos. Some atomic nuclei undergo “double beta decay.” (※2) It decays through rare radioactive decay, typically emitting two neutrinos. However, in extremely rare cases, neutrinos have certain properties. (※3) It is thought that double beta decay, which does not produce neutrinos, may occur if the above conditions are met. This rare beta decay is a phenomenon called neutrino-free double beta decay. Whether or not double beta decay of a neutrino is observed, it imposes major constraints on the properties of neutrinos, and his research will lead to a greater understanding of the mysterious nature of neutrinos.

*1…A neutrino is a group of elementary particles that includes three types of elementary particles. Researchers have nicknamed them “ghost particles” because they rarely interact with other matter and can pass through giant objects like the Earth or the Sun. Due to the difficulty of observing neutrinos, many mysteries remain regarding the properties of neutrinos, and elucidating them is one of the major challenges of modern physics.

*2…The phenomenon in which one of the neutrons in the nucleus of an atom decays into one proton, an electron, and an anti-electron neutrino is called beta decay. Some nuclei, such as calcium 48 and germanium 76, are prevented from undergoing single beta decay, but they may not be prevented from undergoing double beta decay at the same time, which is called double beta decay. In double beta decay, beta decay occurs, and thus two neutrinos are generated.

*3…In double beta decay, two neutrinos are always generated, but if the neutrinos meet and annihilate each other immediately after the decay, it looks like a decay in which no neutrinos are generated, so this double beta is called non-neutrinos. It is called collapse. For neutrino-free double beta decay to occur, neutrinos must not be separated into particles and antiparticles (matter and antimatter), but must be “Majorana particles” in which particles and antiparticles are indistinguishable. Whether neutrinos are Majorana particles or not has a major impact on theories describing elementary particles as a whole.

Research monitoring dark matter and neutrino-free double beta decay includes observing the energy released by the decay directly, or monitoring the energy released when particles from the decay collide with other atoms. but,Environmental radiation that occurs in nature produces noise that produces signals very similar to that produced by this energy.Moreover, since the probability of observing the physical phenomenon is very low, there is a risk of missing the true signal hidden in the noise.

For this reason, efforts are being made to completely eliminate environmental radiation in this type of research. For example, research facilities and experimental equipment are installed deep underground to take advantage of rocks that block cosmic rays. Representative examples include the “Xenon” direct dark matter detector installed underground at Gran Sasso, Italy, and the neutrino-free double beta decay detector “Camelland Zen” installed at the former Kamioka mine in Gifu Prefecture.

■ Difficulty in preventing radiation from the experimental equipment itself

but,The experimental equipment itself contains a radiation sourceThis has been a difficult problem to solve so far. Equipment that captures rare physical phenomena consists of electronic equipment that requires cables to transmit signals. These cables use the same “copper polyimide” standard used in regular electronic equipment, which is a combination of highly conductive copper foil and insulating polyimide.

However, commercially available copper polyimide contains uranium and thorium to several parts per billion of its total weight. It is at a level that does not pose any problem for normal use, howeverFor equipment that captures extremely rare physical phenomena, even this small source of radiation poses a problem.

In order to eliminate radiation sources as much as possible, it was necessary to use other cables that did not use copper polyimide, or to reduce the amount of copper polyimide used. However, manufacturing cables made of materials other than copper polyimide is expensive because they must be newly developed, and the reliability and cleanliness that are advantages of copper polyimide may not be guaranteed. Another problem is that these cables can only transmit a very small amount of signal.

In addition, if we wanted to reduce the use of copper polyimide, we would have to design a circuit that would shorten the overall length of the cable, so we would have to unlearn the knowledge we had accumulated and redesign it from scratch. The problem too.

■ A copper polyimide cable containing very low radioactive material has been developed

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PNNL’s Arnquist and his team collaborated with Q-Flex, a US company specializing in custom flexible cables, to develop copper-polyimide cables with ultra-low radiation content. By carefully evaluating the degree of contamination with radioactive materials at each stage, we have finally succeeded in developing a cable that contains extremely low levels of radioactive materials.

In research class 10000 (※4) A prototype copper polyimide printed circuit board was placed in a clean room, and the amount of radioactive material mixed at each stage of manufacturing was measured for each product, material, and solution. The copper polyimide substrate was fabricated into detachable strips to facilitate measurements at each step.

*4…Clean room cleanliness value according to US Federal Standard Fed.Std.209. It is equivalent to Class 7 in ISO 14644-1, a common hygiene class in electronic equipment and food plants.

As a result of the prototype research, it was discovered that there are some manufacturing steps where radioactive materials tend to accumulate or not be removed. For example, the adhesive used in the thin layer covering copper to prevent oxidation was found to contain significant amounts of radioactivity. It has also been found that ultrasonic cleaning of copper surfaces is unable to remove much radioactive material. Conversely, we have found that carbonless electroplating is less susceptible to radioactive material buildup than regular electroplating, leading to process improvements.

Based on the results of this experiment, Mr. Arnquist and his colleagues created a new way to manufacture “ultra-low radiation flexible printed cables” by adding or changing some of the manufacturing processes. This cable is compared to commercially available copper polyimide cables.The thorium content is reduced to about 1/10, and the uranium content is reduced to about 1/100.In addition to being able to keep production costs relatively low, it is also possible to take advantage of the advantages of copper polyimide, which is advantageous in terms of the amount of transmission signals.

■Could it be used not only for physics experiments but also for future quantum computers?

The newly developed cable could be used in the next generation of experimental equipment aimed at observing dark matter and neutrino-free double beta decay. Not only that, it could also be used in high-performance quantum computers that will emerge in the future.

Quantum computers make use of an unstable state called “quantum entanglement” that can be easily broken by external stimuli. Although there are no reports that current quantum computers are affected by small amounts of radioactive material found in cables, it could become an issue when developing high-performance quantum computers. Ultra-low radiation flexible printed cables can be used in practical environments outside of physical experiments.

source

• Isaac J. Arnquist, et al. “Flexible printed cables with extremely low radioactivity.” (EPJ techniques and devices)
• Karen Headey. “Shh! Quiet cable array to help detect rare physical events. (Pacific Northwest National Laboratory)

Written by Riri Aya