What Is Positron Emission?

A positron, also called a positron, an antielectron, or a positron, is a kind of elementary particle that has a positive charge and has the same mass as an electron. It is an antiparticle of an electron. It was first predicted theoretically by Dirac. On August 2, 1932, Anderson and others at the California Institute of Technology solemnly announced to the world that they had discovered positrons. The positron discovery was observed using a cloud-fog chamber. Fill the cloud and fog room with supersaturated ether gas. When the material emits positrons, the positrons pass through the cloud and fog room, and a droplet line appears in the positron running orbit. You can know its charge symbol and charge-mass ratio (ratio of charge to mass) to determine the properties of the positron. The discovery of positrons has opened up research in the field of antimatter.

Chinese name: Positron
Foreign name: positive electron

nickname: Yang electronics
Subject: physical

Introduction to positron

In fact, before Anderson, a couple of scientists-Jorio Curie (son and daughter of Pierre Curie) first observed the existence of positrons, but this did not attract their attention, and Missed this great discovery. The Curie couple also made outstanding contributions to mankind. In addition to missing the discovery of positrons, they also missed the discovery of neutrons and the discovery of nuclear fission, so that they went to the threshold of the Nobel Prize in Physics three times. And failed to break through. But for their outstanding contributions to radioactivity, they still won the Nobel Prize in Chemistry in 1935.

The positron has a mass of m = 9.1 × 10 ^-31 kg, and an electric quantity of g = + 1.6 × 10 ^-19 Coulomb. The spin is the same as that of the electron. How is positron detected? This requires the help of a cloud chamber in the electromagnetic field.

Positron application

We know that there is a saturated vapor pressure for each substance. When the external pressure is greater than the saturated vapor pressure of the substance, the vapor of this substance starts to condense into droplets. But if the steam is very pure, then even if the external pressure exceeds its saturated steam pressure, the steam will not condense automatically, which becomes a supersaturated gas. If a small disturbance is added to the supersaturated gas at this time, such as the presence of charged particles or other impurities, the gas will quickly condense into small droplets with this impurity as the core. Therefore, when charged particles fly in supersaturated steam, the steam will condense along the track of particle flight, so we can know the movement of particles by observing the trajectory of these droplets. This is the cloud chamber, which is famous by Physicist Wilson invented it, hence the name Wilson Cloud Room.

Positron definition

Positron (e +), also known as positron

A positron is an antiparticle of an electron and has the same properties as an electron except that it has a positive charge. A positron is an unstable particle. When it encounters an electron, it will annihilate with it. It emits two gamma ray photons , each with an energy of 0.511 MeV. When a positron comes into contact with an atomic nucleus, it will annihilate with an electron outside the nuclear. This is the principle of a positron cannon. Positrons are not the basic constituents of matter on Earth. Although positrons are relatively stable, they will soon be annihilated and become photons when they encounter them, so they are not easy to observe.

Positron annihilation (6 photos)

The discovery of positrons is reminiscent of the existence of antiprotons, antineutrons ..., and it has been confirmed that each particle has an antiparticle corresponding to it.

Some people imagine that the use of anti-particles to make anti-matter (such as anti-hydrogen atoms), the acquisition of the above particles is a big step towards the production of anti-hydrogen atoms. The combination of matter and anti-matter (in annihilation) can release a large amount of energy ( A few orders of magnitude higher than nuclear energy), future spacecraft may carry certain substances and anti-matter of such substances as energy.

To realize the great dream of human manned Mars exploration, we need several tons of chemical fuel. On the contrary, if antimatter is used, only tens of milligrams are required, and the theoretical speed limit can reach one tenth of the speed of light. However, in fact, the birth of this motivation comes with a price. Some antimatter reactions generate large amounts of high-energy gamma rays. Gamma rays break down molecules inside cells, so they are harmful to the human body. In addition, high-energy gamma rays can cause the engine itself to be radioactive because the atoms that make the engine are broken.

Positron discovery process

Positron theory discovery

Although positrons have theoretical predictions, they have not been found experimentally. The scientific community in the 1830s did not readily acknowledge the existence of new particles. It is believed that the positively charged particles are only protons, so some people think that the positively charged particles appearing in Dirac's equation are likely to be protons, otherwise why not found in the experiments? This idea includes Dirac himself.

Positron experiment found

Soon after, Dirac's prediction of 1932 was quickly confirmed by experiments, which was discovered by the American physicist Anderson (1905-1991) while studying the deflection of cosmic rays in a magnetic field. At the time, he was working with Millikan (the person who measured the basic charge) on whether cosmic rays were electromagnetic radiation or particles. At that time, most people agreed with Compton's argument that cosmic rays were charged particles, and Millikan was dissatisfied with it. Anderson wondered if the cosmic rays entering the cloud chamber would turn under the influence of a strong magnetic field. He took a picture in the cloud room, which made him close his eyes overnight. He found that after the cosmic rays entered the cloud chamber and passed through the lead plate, the trajectory did bend, and when the high-energy cosmic rays passed through the lead plate, a particle had the same trajectory as the electron, but the direction of the curve was wrong. That is to say, this previously unknown particle has the same mass as the electron but the opposite charge, which is exactly the positron predicted by Dirac. At the time, Anderson did not know Dirac's predictions, and he called the particles he discovered "positrons." The next year, Anderson produced positrons by gamma-ray bombardment, which confirmed the existence of positrons experimentally. Since then, positrons have been officially included in the ranks of elementary particles.

Zhao Zhongyao was the first person to observe the extinction of positive and antimatter, and the first physicist to discover antimatter in the history of physics. This discovery was enough for Zhao Zhongyao to win the Nobel Prize. At that time, the Royal Society of Sweden also seriously considered granting him the Nobel Prize. Unfortunately, a physicist working in Germany questioned Zhao Zhongyao's results. Although it turned out that Zhao Zhongyao's results were completely accurate and the questioning scientist was wrong, this affected Zhao Zhongyao's results. further confirm. In 1936, in recognition of the important achievement of the discovery of positrons, the Royal Swedish Academy of Sciences awarded the Nobel Prize in Physics to Anderson, who observed positron trails in a cloud chamber in 1932, instead of discovering positrons first in 1930 Zhao Zhongyao with negative electron annihilation. (Anderson also acknowledged that when his classmate Zhao Zhongyao's experimental results came out, he was in the office next door to Zhao Zhongyao. At that time, he realized that Zhao Zhongyao's experimental results had shown that there was a new substance that people had not yet known. His The research was inspired by Zhao Zhongyao.) Of course, later, as the discoverer of positrons, was proved (one year before Zhao Lao's death, he could finally comfort the world and Zhao Laoying soul).

The generation and annihilation of electron pairs has caused a great change in people's understanding of elementary particles, and people have to reconsider what the elementary particles are. Elementary particles originally meant that these particles were the most basic and indivisible unit of matter. Elementary particles such as electrons could neither be generated nor destroyed. However, it was found that under appropriate conditions, positive and negative electron pairs can be generated or annihilated in pairs, that is, they can be transformed into each other. The various forms of matter can transform each other, which is undoubtedly a huge leap in understanding. Since then, more antiparticles have been discovered, so more facts have repeatedly confirmed this law.

In 1936, Anderson was awarded the Nobel Prize in Physics for the year for discovering positrons, when he was only 31 years old.

Positron generation

Positron photon transition

At a temperature of T = 5 * 10 ^ 9K, photons can react to a higher degree to generate positron-negative electron pairs. The number of positrons and photons are approximately equal when the system is in thermal equilibrium.

Positron fusion

The main nuclear reactions of stars will release positrons, such as our sun. The following reactions occur at every moment: four protons are aggregated into a helium nucleus, and two electron neutrinos and two are released at the same time. Positron.

Positron decay

For example, the radioactive isotope phosphorus 30 will release positrons through positive beta decay.

Other methods of positron

It is possible to observe the emergence of positrons by using gamma rays radiating energy above 1 megaelectron volts to radiate lead plates, thin metal foils, and gaseous media.

Positron Positron Emission Tomography

Positron emission tomography (PositronEmissionTomography) system uses the phenomenon of annihilation of positrons generated by positron isotope decay with negative electrons in the human body. By injecting a compound labeled with a positron isotope into the human body, Method: Detect the gamma photons produced by the annihilation effect, obtain the distribution information of isotope in the human body, and perform reconstruction and combination operation by computer to obtain a three-dimensional tomographic image of the distribution of labeled compounds in the human body ^[1] .

PET is a technology that directly contrasts brain function. Its basic principle is: to inject a subject with a tracer containing radioactive isotopes, the positrons emitted by the isotopes are annihilated with the negative electrons in the brain, and -rays are released. By recording the location distribution of -rays in the brain, changes in regional cerebral metabolic rate (rCMR) and regional cerebral blood flow (rCBF) can be measured to reflect changes in functional activity of the brain ^[1] .

PET can be used for the differential diagnosis of schizophrenia, depression, drug addiction, and to understand the patient's brain metabolism and functional status, such as dysfunction of the frontal lobe, temporal lobe, and hippocampal basal ganglia in patients with schizophrenia. Using PET imaging, multiple receptors such as dopamine in the brain can be measured, and the nature of the disease is revealed at the molecular level. This is unmatched by other methods ^[1] .

Limitations of PET: The imaging time is long (at least several tens of seconds), and only the block design experimental mode can be used; imaging is limited by radioisotopes, and it is not suitable for repeated research by a single subject. It is not advisable for the same subject to participate in the experiment frequently, which is not conducive to the research that requires the subject to participate in the experiment multiple times. The system is very expensive. In addition to the PET scanner, an accelerator is generally required to prepare 15O equivalent isotopes with a half-life of only 123s. ^[1] .

Positron related contributions

In 1928 Dirac introduced relativity into quantum mechanics and established the Schrödinger equation in the form of relativity, also known as the Dirac equation. This equation has two characteristics: one is to meet all the requirements of the theory of relativity, which is suitable for electrons no matter how fast they move; the other is that it can automatically derive the conclusion that electrons have spins. The solution to this equation is special and includes both positive and negative energy states. Dirac therefore made a prediction of the existence of a positron, thinking that the positron is a mirror image of the electrons, they have exactly the same mass, but the opposite sign of the charge. According to this image, Dirac also expected that there is a process in which an electron and a positron annihilate each other and emit a photon; on the contrary, the reverse process of this process, that is, the process of annihilation of two photons to produce an electron and a positron may also exist of. In 1932, when the American physicist Anderson (1923-) studied the high-energy electron trails in a cosmic ray burst, he strangely found that half of the electrons in a strong magnetic field were deflected in one direction and the other half were deflected in the opposite direction. After careful identification This is the positron predicted by Dirac. Later, the phenomenon of electron pairs generated by -rays and the annihilation of positrons and negative electrons into photons was discovered soon, which fully confirmed the correctness of Dirac's prediction. Dirac's work pioneered theoretical and experimental research on antiparticles and antimatter. The positron is thus theoretically deduced.