Positron, also known as positronAntielectron, positron,Elementary particleIt is the antiparticle of the electron. It is positively charged and has the same mass as the electron.It was first predicted theoretically by Dirac.In 1930, Zhao Zhongyao was the first to discover positrons when he studied antimatter phenomena in an American university.[2]On August 2, 1932,California Institute of TechnologyAnderson and others announced to the world that they had discovered the positron.The discovery of positrons was observed in a cloud chamber.The cloud chamber is filled with supersaturated ether gas. When the matter emits positrons, the positrons pass through the cloud chamber and drop lines appear in the positrons' running orbit. The charged sign of the positrons can be known by measuring the deflection direction and radius of the positrons through the external magnetic field, andCharge mass ratio(ratio of charge to mass) to determine the properties of positrons.The discovery of positrons opened up the field of antimatter research.
In fact, before Anderson, there was a couplescientist——Jolio·Curie couple(Pierre·Curie couple) first observed that ZhengElectronicsHowever, they did not pay attention to it and missed this great discovery.The Curie couple also made outstanding contributions to mankind. They missed not only the discovery of positrons, but also the discovery of positronsneutronDiscovery andNuclear fissionThe discovery ofThe nobel prize in physicsBut he failed to break the door.However, because of their outstanding contributions to radioactivity, they still obtainedNobel Prize in Chemistry。
We know that every substance existsSaturated vapor pressureWhen the external pressure is greater than the saturated vapor pressure of the substance, the vapor of the substance startscoagulationDroplet formation.But if the steam is very pure, even if the external pressure exceeds its saturated steam pressure, the steam will not automatically condense, which becomes an oversaturated gas.If a small disturbance is added to the supersaturated gas at this time, such as the existence of charged particles or otherimpurityThe existence of,GasThis impurity will quickly condense into small droplets.Therefore, when charged particles fly in supersaturated steam,steamIt will condense along the tracks of particles, so we can know the movement of particles by observing the tracks of these droplets. This isCloud chamber, a famous physicistWilsonIt is also called Wilson Cloud Chamber.
The positron is electronicAntiparticle, stripingpositive chargeOther properties are the same as those of electrons.The positron is unstableparticle, will annihilate and release two electronsGamma photon(gamma ray photo), eachenergyIs 0.511MeV.When positrons andNucleusWhen contacted, it will annihilate with extranuclear electrons, which is called positiveElectronic gunPrinciple of.Positrons are not the basic component of matter on earth.Although the positron is relatively stable, it will annihilate and change intophoton, so it is not easy to observe.The discovery of positrons makes people think about whether they existAntiproton,AntineutronIt has been confirmed that each particle has an antiparticle corresponding to it.
Some people imagine that it is made of antiparticlesAntimatter(e.gAntihydrogen atom）, the above particles are obtained by manufacturingAntihydrogen atomA big step forward. The combination of matter and antimatter (annihilation) can release a lot of energy (compared withnuclear energySeveral orders of magnitude higher), the futurespacecraftIt is possible to carry some substance and its antimatter as energy.
If we want to realize human carryingMarsThe great dream of exploration, we need several tons of chemical fuel, on the contrary, if antimatter is used, it only needs tens of milligrams, and the theoretical speed limit can reachlight speedOne tenth of.However, in fact, the birth of this power is accompanied by costs.Some antimatter reactions generate a lot of high-energyGamma ray。Gamma rayDecomposableCellswithinmoleculeTherefore, they are harmful to human body.In addition, high-energy gamma rays will makeengineMaterialatomCracking will make the engine itself haveradioactivity。
Discovery process
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Theoretical discovery
Although positrons have theoretical predictionsexperimentNot found on.In the 1830s, the scientific community did not readily admit the existence of new particles.It is believed that only positively charged particlesproton, so some people think thatDirac equationThe positively charged particles that appear in are probably protons, otherwise why are they not found in the experiment?This idea included Dirac himself.
Experimental findings
Not long after that, Dirac's prediction in 1932 was quickly confirmed by experiments. It was an American physicistAnderson(1905-1991) was discovered when studying the deflection of cosmic rays in a magnetic field.At that time, he was working with Millikan（elementary charge) togethercosmic rayyeselectromagnetic radiationIt's still a matter of particles.At that time, most people agreedComptonAnd believes that cosmic rays are charged particles,MillikanVery dissatisfied with this.Anderson wanted to find outCloud chamberOfcosmic raysWill it turn under the action of strong magnetic field.He isCloud chamberHe took a picture in, which kept him awake all night.He discovered that cosmic rays enterCloud chamberAfter passing through the lead plate, the track did bend. Moreover, when high-energy cosmic rays passed through the lead plate, the track of a particle was exactly the same as that of an electron, but the direction of the bend was "wrong".This means that this previously unknown particle has the same mass as the electron, butchargeOn the contrary, this is exactly the positron predicted by Dirac.At that time, Anderson did not know Dirac's prediction. He called the discovered particle "positron".The next year, Anderson usedGamma rayThe bombardment method produced positrons, which fully confirmed the existence of positrons experimentally.Since then, positrons have been officially listedElementary particleThe ranks of.
Zhao ZhongyaoHe was the first person to observe the annihilation of matter and antimatter, and the first physicist to discover antimatter in the history of physics.This discovery was enough to win Zhao Zhongyao the Nobel PrizeSwedenThe Royal Society also seriously considered awarding him a Nobel Prize.Unfortunately, there was aGermanyThe working physicists raised questions about Zhao Zhongyao's achievements. Although the facts later proved that Zhao Zhongyao's results were completely accurate and the scientists who raised questions were wrong, this affected the further confirmation of Zhao Zhongyao's achievements.In 1936, in recognition of the important achievement of positron discovery,Royal Swedish Academy of SciencesThe Nobel Prize in Physics was awarded to Anderson, who observed positron tracks in the cloud chamber in 1932, rather than Zhao Zhongyao, who first discovered positron annihilation in 1930.(Anderson also admitted that when his classmate Zhao Zhongyao's experimental results came out, he was in the office next to Zhao Zhongyao. At that time, he realized that Zhao Zhongyao's experimental results had shown that there was a new substance that people did not know about. His research was inspired by Zhao Zhongyao.) Of course, later,As the discoverer of positrons, it was proved (one year before the death of Zhao Laoying, it could finally comfort the world and Zhao Laoying's soul).
The generation and annihilation of electron pairs have greatly changed people's understanding of elementary particles, and people have to reconsider what is the problem of elementary particles.Originally, elementary particles meant that these particles were the most basic and indivisible unit of matter, and elementary particles such as electrons could neither be produced nor destroyed.However, it is found that under appropriate conditions, positive and negativeElectron pairThey can be produced or annihilated in pairs, that is, they can transform 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 found, so more facts have repeatedly confirmed this rule.
At the temperature of T=5 * 10 ^ 9K, photons can react to generate positron negative pairs to a higher degreeheat balanceThe number of positrons is roughly equal to the number of photons.
nuclear fusion
The main nuclear reaction of stars will release positrons, such as our sun, in which the following reaction occurs every moment: four protons converge into one helium nucleus, and two electron neutrinos and two positrons are released at the same time.
decay
For example, the radioactive isotope phosphorus 30 will release positrons through beta decay.
Other methods
It is possible to observe the appearance of positrons by using gamma rays with energy higher than 1 MEV to radiate lead plates, thin metal foils, gaseous media, etc.
Positron emission tomography
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Positron Emission Tomography (PET) system uses the phenomenon of annihilation effect between positrons produced by the decay of positron isotopes and negative electrons in the human body, injects compounds with positron isotope markers into the human body, and uses coincidence detection methods to detect gamma photons generated by the annihilation effect, so as to obtain the distribution information of isotopes in the human body,The computer carries out reconstruction and combination operations to obtain three-dimensional sectional images of the distribution of marker compounds in the human body[1]。
PET is a technique for direct imaging of brain function. Its basic principle is: inject the subject with a tracer containing radioisotopes, and the positive electrons emitted by the isotope will annihilate with the negative electrons in the brain to release gamma rays.The changes of regional cerebral metabolic rate (rCMR) and regional cerebral blood flow (rCBF) can be measured by recording the location distribution of gamma rays in the brain, so as to reflect the changes of brain functional activities[1]。
PET can be used for the differential diagnosis of schizophrenia, depression, drug addiction, etc., and to understand the brain metabolism and functional status of patients, such as the functional abnormalities of frontal lobe, temporal lobe, and hippocampal basal ganglia in schizophrenic patients.PET imaging can detect dopamine and other receptors in the brain, revealing the essence of disease at the molecular level.This is incomparable with other methods[1]。
Limitations of PET: the imaging time is long (at least tens of seconds), and the experimental mode of block design can only be used;The imaging is limited by radioisotopes, which is not suitable for repeated research of a single subject.The same subject should not participate in the experiment frequently, which is not conducive to the research that requires the subject to participate in the experiment many times;The system cost is very high. In addition to PET scanner, an accelerator is generally required to prepare 15O isotopes with a half-life of only 123s[1]。
Relevant contributions
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In 1928, Dirac introduced relativity into quantum mechanics and established the Schrodinger equation in the form of relativity, that is, the famous Dirac equation.This equation has two characteristics: one is that it meets all the requirements of relativity and is applicable to electrons no matter how fast they move;Second, it can automatically derive the electronspinConclusion.The solution of this equation is very special, including both positive and negative energy states.Dirac thus predicted the existence of positrons, believing that positrons are a mirror image of electrons, and they have exactly the same mass, but the charge sign is opposite.According to this picture, Dirac also predicted that an electron and a positron would annihilate each other and emit photons;On the contrary, the reverse process of this process, that is, the process of two photons annihilating to produce an electron and a positron, may also exist.In 1932, when American physicist Anderson (1923 -) studied the track of high-energy electrons in the cosmic ray shower, he strangely found that half of the electrons in the strong magnetic field deflected in one direction and the other half in the opposite direction. After careful identification, this was the positron predicted by Dirac.It was soon discovered that gamma rays produced electron pairs, positive and negative electron collisions“Annihilation”ChengphotonSuch phenomena fully confirmed the correctness of Dirac's prediction.Dirac's work createdAntiparticleandAntimatterTheoretical and experimental research.The positron is thus deduced theoretically.
