nominal particle. Meaning of the word b-particle in medical terms A b particle
They have been trying to find the Higgs boson for decades, but so far without success. Meanwhile, without it, the key provisions modern theory microcosms hang in the air.
The study of particles began not so long ago. In 1897, Joseph John Thomson discovered the electron, and 20 years later Ernest Rutherford proved that hydrogen nuclei are part of the nuclei of other elements, and later called them protons. In the 1930s, the neutron, muon and positron were discovered and the existence of the neutrino was predicted. At the same time, Hideki Yukawa built a theory of nuclear forces carried by hypothetical particles hundreds of times heavier than an electron, but much lighter than a proton (mesons). In 1947 traces of pi meson (pion) decays were found on photographic plates exposed to cosmic rays. Later, other mesons were discovered, and some of them are heavier than not only the proton, but also the helium nucleus. Physicists have also discovered many baryons, heavy and therefore unstable relatives of the proton and neutron. Once upon a time, all these particles were called elementary, but such terminology has long been outdated. Now only non-composite particles are considered elementary - fermions (with half spin - leptons and quarks) and bosons (with integer spin - carriers of fundamental interactions).
Elementary particles of the Standard Model
The fermion group (with half-integer spin) consists of leptons and quarks of the so-called three generations. Charged leptons are the electron and its massive counterparts the muon and the tau particle (and their antiparticles). Each lepton has a neutral partner in the person of one of the three varieties of neutrino (also with antiparticles). The family of bosons, whose spin is 1, are particles that carry interactions between quarks and leptons. Some of them do not have mass and electric charge - these are gluons, which provide interquark bonds in mesons and baryons, and photons, quanta of the electromagnetic field. Weak interactions, manifested in the processes of beta decay, are provided by a trio of massive particles - two charged and one neutral.
The individual names of elementary and compound particles are usually not associated with the names of specific scientists. However, almost 40 years ago, another elementary particle was predicted, which was named after a living person, the Scottish physicist Peter Higgs. Like carriers of fundamental interactions, it has an integer spin and belongs to the class of bosons. However, its spin is not 1, but 0, and in this respect it has no analogues. For decades, they have been looking for it at the largest accelerators - the American Tevatron, which was closed last year, and the Large Hadron Collider, which is now functioning, under the scrutiny of the world media. After all, the Higgs boson is very necessary for the modern theory of the microworld - the Standard Model of elementary particles. If it cannot be found, the key provisions of this theory will hang in the air.
Gauge symmetries
The beginning of the path to the Higgs boson can be counted from a short article published in 1954 by the Chinese physicist Yang Zhenning, who moved to the United States, and his colleague at the Brookhaven National Laboratory, Robert Mills. In those years, experimenters discovered more and more new particles, the abundance of which could not be explained in any way. In search of promising ideas, Yang and Mills decided to test the possibilities of a very interesting symmetry, which is subject to quantum electrodynamics. By that time, this theory had proved its ability to give excellent results in agreement with experience. True, in the course of some calculations, infinities appear there, but you can get rid of them using a mathematical procedure called renormalization.
Symmetry, which interested Yang and Mills, was introduced into physics in 1918 by the German mathematician Hermann Weyl. He called it a gauge, and this name has survived to this day. In quantum electrodynamics, gauge symmetry manifests itself in the fact that the wave function of a free electron, which is a vector with real and imaginary parts, can be continuously rotated at each point in space-time (which is why the symmetry is called local). This operation (in the formal language - changing the phase of the wave function) leads to the fact that additives appear in the equation of motion of the electron, which must be compensated in order for it to remain valid. To do this, an additional term is introduced there, which describes the electromagnetic field interacting with the electron. The quantum of this field is a photon, a massless particle with a unit spin. Thus, the existence of photons (as well as the constancy of the electron charge) follows from the local gauge symmetry of the free electron equation. We can say that this symmetry dictates that the electron interact with the electromagnetic field. Any phase shift becomes an act of such an interaction - for example, the emission or absorption of a photon.
The relationship between gauge symmetry and electromagnetism was discovered as early as the 1920s, but did not arouse much interest. Yang and Mills were the first to use this symmetry to construct equations describing particles of a different nature than the electron. They took up the two "oldest" baryons - the proton and the neutron. Although these particles are not identical, in relation to nuclear forces they behave almost identically and have almost the same mass. In 1932, Werner Heisenberg showed that the proton and neutron can formally be considered different states of the same particle. To describe them, he introduced a new quantum number - the isotopic spin. Because the strong force does not distinguish between protons and neutrons, it conserves total isotopic spin, just as the electromagnetic force conserves electric charge.
Yang and Mills wondered which local gauge transformations preserve isospin symmetry. It was clear that they could not coincide with the gauge transformations of quantum electrodynamics, if only because we were already talking about two particles. Young and Mills analyzed the totality of such transformations and found that they generate fields whose quanta supposedly carry the interactions between protons and neutrons. There were three quanta in this case: two charged (positively and negatively) and one neutral. They had zero mass and unit spin (that is, they were vector bosons) and traveled at the speed of light.
The theory of B-fields, as the co-authors dubbed them, was very beautiful, but did not stand the test of experience. The neutral B-boson could be identified with the photon, but its charged counterparts were left out. According to quantum mechanics, only sufficiently massive virtual particles can be mediators in the transfer of short-range forces. The radius of nuclear forces does not exceed 10–13 cm, and the massless Yang and Mills bosons clearly could not claim to be their carriers. In addition, experimenters have never detected such particles, although in principle charged massless bosons are easy to detect. Yang and Mills proved that local gauge symmetries "on paper" could generate force fields of a non-electromagnetic nature, but the physical reality of these fields was pure conjecture.
Electroweak duality
The next step towards the Higgs boson was taken in 1957. By that time, theorists (the same Yang and Li Zundao) assumed, and the experimenters proved, that parity is not conserved in beta decays (in other words, mirror symmetry is violated). This unexpected result interested many physicists, among whom was Julian Schwinger, one of the founders of quantum electrodynamics. He hypothesized that weak interactions between leptons (science had not yet reached quarks!) are carried by three vector bosons - a photon and a pair of charged particles similar to B-bosons. It followed that these interactions are in partnership with electromagnetic forces. Schwinger did not deal with this problem anymore, but suggested it to his graduate student Sheldon Glashow.
The work spanned four years. After a row failed attempts Glashow built a model of the weak and electromagnetic interactions based on the unification of the gauge symmetries of the electromagnetic field and the Yang and Mills fields. In addition to the photon, it featured three more vector bosons - two charged and one neutral. However, these particles again had zero mass, which created a problem. The radius of a weak interaction is two orders of magnitude smaller than that of a strong one, and it all the more requires very massive mediators. In addition, the presence of a neutral carrier required the possibility of beta transitions that do not change the electric charge, and at that time such transitions were not known. Because of this, after publishing his model in late 1961, Glashow lost interest in unifying the weak and electromagnetic forces and switched to other topics.
Schwinger's hypothesis also interested the Pakistani theorist Abdus Salam, who, together with John Ward, built a model similar to Glashow's. He also encountered the masslessness of gauge bosons and even came up with a way to eliminate it. Salam knew that their masses could not be entered "by hand" as the theory became non-normable, but he hoped to get around this difficulty by spontaneous symmetry breaking, so that the solutions to the equations of motion of bosons did not have the gauge symmetry inherent in the equations themselves. With this task, he interested the American Steven Weinberg.
But in 1961, the English physicist Geoffrey Goldstone showed that in relativistic quantum theories field spontaneous symmetry breaking seems to inevitably generate massless particles. Salam and Weinberg tried to disprove Goldstone's theorem, but only strengthened it in their own work. The riddle looked unsolvable, and they turned to other areas of physics.
Higgs and others
Help came from specialists in condensed matter physics. In 1961, Yoichiro Nambu noted that when a normal metal goes into a superconducting state, the former symmetry is spontaneously broken, but no massless particles appear. Two years later, Philip Anderson, using the same example, noted that if the electromagnetic field does not obey the Goldstone theorem, then the same can be expected from other gauge fields with local symmetry. He even predicted that the Goldstone bosons and the Yang and Mills field bosons could somehow cancel each other out, leaving behind massive particles.
This prediction turned out to be prophetic. In 1964, it was acquitted by François Englert and Roger Broat, physicists at the Free University of Brussels, Peter Higgs, and Jerry Guralnik, Robert Hagen, and Thomas Kibble at Imperial College London. Not only did they show that the conditions for the applicability of the Goldstone theorem are not met in Yang–Mills fields, but they also found a way to provide excitations of these fields with a nonzero mass, which is now called the Higgs mechanism.
These wonderful works were noticed and appreciated by no means immediately. It was only in 1967 that Weinberg built a unified model of the electroweak interaction, in which the trio of vector bosons gain mass based on the Higgs mechanism, and Salam did the same a year later. In 1971, the Dutch Martinus Veltman and Gerard "t Hooft proved that this theory lends itself to renormalization and, therefore, has a clear physical meaning. She firmly stood on her feet after 1973, when in a bubble chamber Gargamelle(CERN, Switzerland) experimenters registered the so-called weak neutral currents, indicating the existence of an uncharged intermediate boson (direct registration of all three vector bosons was carried out at CERN only in 1982–1983). Glashow, Weinberg and Salam got it for her Nobel Prizes in 1979, Veltman and "t Hooft - in 1999. This theory (and with it the Higgs boson) has long been an integral part of the Standard Model of elementary particles.
Higgs mechanism
The Higgs mechanism is based on scalar fields with spinless quanta - Higgs bosons. It is believed that they arose moments after the Big Bang and now fill the entire Universe. Such fields have the lowest energy at a non-zero value - this is their stable state.
It is often written that elementary particles acquire mass as a result of braking by the Higgs field, but this is an overly mechanistic analogy. The electroweak theory involves four Higgs fields (each with its own quanta) and four vector bosons - two neutral and two charged, which themselves have no mass. Three bosons, both charged and one neutral, each absorb one Higgs and as a result acquire mass and the ability to carry short-range forces (they are denoted by the symbols W + , W - and Z 0). The last boson does not absorb anything and remains massless - it is a photon. "Eaten" Higgs are unobservable (physicists call them "spirits"), while their fourth cousin should be observed at energies sufficient for its birth. In general, these are exactly the processes that Anderson managed to predict.
elusive particle
The first serious attempts to catch the Higgs boson were made at the turn of the 20th and 21st centuries at the Large Electron-Positron Collider ( Large Electron-Positron Collider, LEP) at CERN. These experiments were truly the swan song of a remarkable facility, on which the masses and lifetimes of heavy vector bosons were determined with unprecedented accuracy.
The Standard Model makes it possible to predict the channels of creation and decay of the Higgs boson, but it does not make it possible to calculate its mass (which, by the way, arises from its ability to self-force). According to the most general estimates, it should not be less than 8–10 GeV and more than 1000 GeV. By the beginning of the sessions at LEP, most physicists believed that the most likely range was 100–250 GeV. The LEP experiments raised the lower threshold to 114.4 GeV. Many experts believed and believe that if this accelerator had worked longer and increased the energy of colliding beams by ten percent (which was technically possible), the Higgs boson could have been registered. However, the CERN leadership did not want to delay the launch of the Large Hadron Collider, which was to be built in the same tunnel, and at the end of 2000 LEP was closed.
Boson pen
Numerous experiments, one after the other, ruled out the possible mass ranges of the Higgs boson. The lower threshold was set at the LEP accelerator - 114.4 GeV. At the Tevatron, masses exceeding 150 GeV were ruled out. Later, the mass ranges were refined to 115–135 GeV, and the upper limit was shifted to 130 GeV at CERN at the Large Hadron Collider. So the Higgs boson of the Standard Model, if it exists, is locked into fairly narrow mass bounds.
The next search cycles were carried out at the Tevatron (on the CDF and DZero detectors) and at the LHC. As Dmitry Denisov, one of the leaders of the DZero collaboration, told PM, Tevatron began collecting statistics on Higgs in 2007: “Although there was enough energy, there were many difficulties. The collision of electrons and positrons is the "cleanest" way to catch the Higgs, because these particles do not have an internal structure. For example, during the annihilation of a high-energy electron-positron pair, a Z 0 -boson is born, which emits the Higgs without any background (however, in this case, even dirtier reactions are possible). We, on the other hand, collided protons and antiprotons, loose particles consisting of quarks and gluons. So the main task- highlight the birth of the Higgs against the background of many similar reactions. A similar problem exists for the LHC teams.”
Traces of unseen beasts
There are four main ways (as physicists say, channels) for the birth of the Higgs boson.
The main channel is the fusion of gluons (gg) in the collision of protons and antiprotons, which interact through loops of heavy top quarks.
The second channel is the fusion of virtual vector bosons WW or ZZ(WZ) emitted and absorbed by quarks.
The third channel for the production of the Higgs boson is the so-called associative production (together with the W or Z boson). This process is sometimes called Higgsstrahlung(similar to the German term bremsstrahlung- bremsstrahlung).
And finally, the fourth one is the fusion of a top quark and an antiquark (associative production together with top quarks, tt) from two top quark-antiquark pairs generated by gluons.
“In December 2011, new messages came from the LHC,” Dmitry Denisov continues. - They were looking for Higgs decays either on top-quark and its antiquark, which annihilate and turn into a pair of gamma quanta, or into two Z 0 -bosons, each of which decays into an electron and a positron or a muon and an antimuon. The data obtained suggest that the Higgs boson pulls about 124–126 GeV, but this is not enough for final conclusions. Now both our collaborations and physicists at CERN continue to analyze the results of experiments. It is possible that we and they will soon come to new conclusions, which will be presented on March 4 at an international conference in the Italian Alps, and I have a presentiment that you will not be bored there.”
The Higgs boson and the end of the world
So, this year we can expect either the discovery of the Higgs boson of the Standard Model, or its cancellation, so to speak. Of course, the second option will create a need for new physical models, but the same can happen in the first case! In any case, one of the most authoritative experts in this field, John Ellis, professor at King's College London, thinks so. In his opinion, the discovery of a "light" (not more massive than 130 GeV) Higgs boson will create an unpleasant problem for cosmology. It will mean that our Universe is unstable and someday (perhaps even at any moment) will move into a new state with less energy. Then the end of the world will happen - in the very full meaning this word. It remains to be hoped that either the Higgs boson will not be found, or Ellis is mistaken, or the Universe will delay the suicide a little.
Baryons (from the Greek "baris" - heavy) are heavy elementary particles, strongly interacting fermions, consisting of three quarks. The most stable baryons are the proton and the neutron. The main baryons are: proton (uud), antiproton, neutron (ddu), antineutron, lambda hyperion, sigma hyperion, xi hyperion, omega hyperion.
Employees of the DZero international collaboration at the Fermi National Accelerator Laboratory, which is part of the system of US research centers, have discovered a new elementary particle, the baryon. The particle, dubbed "xi-bi-minus baryon" (Ξ-b), is unique in its own way. This is not just another baryon containing a b-quark, but the first particle containing three quarks of three different families - a d-quark, an s-quark and a b-quark.
She also has another name - "cascade-bi". A baryon carries a negative charge and is about six times as massive as a proton (particle mass 5.774±0.019 GeV).
To register a new particle, scientists had to analyze tracks over five years of accelerator operation. As a result, 19 events were found that indicated the formation of a new baryon.
Previously, scientists have already obtained a baryon consisting of three different quarks - a lambda-bi baryon, consisting of a u-, d- and b- quark, but it contains only two generations of quarks (see inset).
Thus, for the first time in the history of high-energy physics, a baryon consisting of quarks of three generations or families has been discovered. The bi-cascade consists of one d-quark (the "down" quark belonging to the first family), one s-quark (the "strange" quark, the second family) and one b-quark (the "charm" quark, the third family). That is why the new Ξ-b particle is truly unique.
Interestingly, although the collaboration is based at Fermilab, which has a powerful Tevatron accelerator, the current discovery was made in Europe - at the Large Electron-Positron Collider at CERN (LEP)
Thus, scientists continue their search on the “second floor” of the baryon pyramid, discovering baryons containing one “beautiful” or “bottom” quark (b).
For the first time such particles received also a team from Fermilab. Last year, the CDF International Collaboration, conducting experiments at the Department of Energy's Fermi National Accelerator Laboratory, announced the discovery of two new elementary particles belonging to the baryon class. The particles were called Σ + b and Σ-b.
In experiments, physicists collided protons with antiprotons, accelerating them at the Tevatron, the most powerful accelerator at the moment.
Experiments are carried out at this accelerator when a beam of protons with an energy of 1 TeV collides with a colliding beam of antiprotons of the same energy. In a collision with such an energy, a b-quark appeared, which then, interacting with quarks of protons and antiprotons, formed two new particles.
The experiment registered 103 events associated with the birth of positively charged u-u-b particles(Σ+b) and 134 births of negatively charged d-d-b particles(Σ-b). To detect so many events, scientists had to analyze tracks from 100 trillion collisions over the five years of the Tevatron's operation.
From approximately 1000 seconds (for a free neutron) to a negligible fraction of a second (from 10 −24 to 10 −22 s for resonances).
The structure and behavior of elementary particles is studied by elementary particle physics.
All elementary particles obey the principle of identity (all elementary particles of the same type in the Universe are completely identical in all their properties) and the principle of corpuscular-wave dualism (each elementary particle corresponds to a de Broglie wave).
All elementary particles have the property of interconvertibility, which is a consequence of their interactions: strong, electromagnetic, weak, gravitational. Particle interactions cause the transformation of particles and their aggregates into other particles and their aggregates, if such transformations are not prohibited by the laws of conservation of energy, momentum, angular momentum, electric charge, baryon charge, etc.
Main characteristics of elementary particles: lifetime , mass , spin , electric charge , magnetic moment , baryon charge , lepton charge , strangeness , isotopic spin , parity , charge parity , G-parity , CP-parity .
Classification
By life time
- Stable elementary particles - particles that have an infinitely long lifetime in a free state (proton, electron, neutrino, photon and their antiparticles).
- Unstable elementary particles - particles decaying into other particles in a free state in a finite time (all other particles).
By weight
All elementary particles are divided into two classes:
- Massless particles - particles with zero mass (photon, gluon).
- Particles with non-zero mass (all other particles).
The size of the back
All elementary particles are divided into two classes:
By type of interaction
Elementary particles are divided into the following groups:
Composite particles
- Hadrons are particles involved in all kinds of fundamental interactions. They consist of quarks and are subdivided, in turn, into:
- mesons - hadrons with integer spin, that is, being bosons;
- baryons are hadrons with half-integer spin, i.e. fermions. These include, in particular, the particles that make up the nucleus of the atom - proton and neutron.
Fundamental (structureless) particles
- Leptons are fermions that look like point particles (that is, they do not consist of anything) up to scales of the order of 10 −18 m. They do not participate in strong interactions. Participation in electromagnetic interactions has been experimentally observed only for charged leptons (electrons, muons, tau leptons) and has not been observed for neutrinos. There are 6 types of leptons known.
- Quarks are fractionally charged particles that make up hadrons. They were not observed in the free state (the confinement mechanism was proposed to explain the absence of such observations). Like leptons, they are divided into 6 types and are considered structureless, however, unlike leptons, they participate in strong interaction.
- Gauge bosons - particles through the exchange of which interactions are carried out:
- photon - a particle that carries electromagnetic interaction;
- eight gluons, particles that carry the strong force;
- three intermediate vector bosons W + , W− and Z 0 , carrying weak interaction ;
- graviton is a hypothetical particle that carries the gravitational interaction. The existence of gravitons, although not yet experimentally proven due to the weakness of the gravitational interaction, is considered quite probable; however, the graviton is not included in the Standard Model of elementary particles.
Sizes of elementary particles
Despite the great variety of elementary particles, their sizes fit into two groups. The dimensions of hadrons (both baryons and mesons) are about 10 −15 m, which is close to the average distance between their quarks. The sizes of fundamental, structureless particles - gauge bosons, quarks and leptons - within the limits of the experimental error are consistent with their point character (the upper limit of the diameter is about 10 −18 m) ( see explanation). If the final sizes of these particles are not found in further experiments, then this may indicate that the sizes of gauge bosons, quarks and leptons are close to the fundamental length (which may very likely turn out to be the Planck length equal to 1.6 10 −35 m).
It should be noted, however, that the size of an elementary particle is a rather complex concept, not always consistent with classical concepts. First, the uncertainty principle does not allow strictly localizing a physical particle. A wave packet, representing a particle as a superposition of precisely localized quantum states, always has finite dimensions and a certain spatial structure, and the packet sizes can be quite macroscopic - for example, an electron in an experiment with interference on two slits “feels” both interferometer slits separated by a macroscopic distance. Secondly, a physical particle changes the structure of the vacuum around itself, creating a "fur coat" of short-term virtual particles - fermion-antifermion pairs (see Vacuum Polarization) and bosons-carriers of interactions. The spatial dimensions of this region depend on the gauge charges that the particle possesses and on the masses of the intermediate bosons (the radius of the shell of massive virtual bosons is close to their Compton wavelength, which, in turn, is inversely proportional to their mass). So, the radius of an electron from the point of view of neutrinos (only weak interaction between them is possible) is approximately equal to the Compton wavelength of W-bosons, ~3 × 10 −18 m, and the dimensions of the region of strong interaction of a hadron are determined by the Compton wavelength of the lightest of hadrons, the pi-meson (~10 −15 m), which acts here as a carrier of interaction.
Story
Initially, the term "elementary particle" meant something absolutely elementary, the first brick of matter. However, when hundreds of hadrons with similar properties were discovered in the 1950s and 1960s, it became clear that at least hadrons have internal degrees of freedom, that is, they are not, in the strict sense of the word, elementary. This suspicion was later confirmed when it turned out that hadrons were made up of quarks.
Thus, physicists have moved a little deeper into the structure of matter: the most elementary, point parts of matter are now considered leptons and quarks. For them (together with gauge bosons) the term " fundamental particles".
String theory, which has been actively developed since the mid-1980s, assumes that elementary particles and their interactions are consequences of various kinds vibrations of especially small "strings".
standard model
The Standard Model of elementary particles includes 12 flavors of fermions, their corresponding antiparticles, as well as gauge bosons (photon, gluons, W- And Z-bosons), which carry interactions between particles, and the Higgs boson discovered in 2012, which is responsible for the presence of inertial mass in particles. However, the Standard Model is largely regarded as a temporary theory rather than a truly fundamental one, since it does not include gravity and contains several dozen free parameters (particle masses, etc.) whose values do not follow directly from the theory. Perhaps there are elementary particles that are not described by the Standard Model - for example, such as the graviton (a particle that hypothetically carries gravitational forces) or supersymmetric partners of ordinary particles. In total, the model describes 61 particles.
Fermions
The 12 flavors of fermions are divided into 3 families (generations) of 4 particles each. Six of them are quarks. The other six are leptons, three of which are neutrinos, and the remaining three carry a unit negative charge: the electron, the muon, and the tau lepton.
| First generation | Second generation | third generation |
|---|---|---|
| Electron: e- | Muon: μ − | Tau lepton: τ − |
| Electron neutrino: v e | Muon neutrino: ν μ | Tau neutrino: ν τ (\displaystyle \nu _(\tau )) |
| u-quark ("top"): u | c-quark ("enchanted"): c | t-quark ("true"): t |
| d-quark ("bottom"): d | s-quark ("strange"): s | b-quark ("charming"): b |
antiparticles
There are also 12 fermionic antiparticles corresponding to the above twelve particles.
| First generation | Second generation | third generation |
|---|---|---|
| positron: e + | Positive muon: μ + | Positive tau lepton: τ + |
| Electronic antineutrino: ν ¯ e (\displaystyle (\bar (\nu ))_(e)) | Muon antineutrino: ν ¯ μ (\displaystyle (\bar (\nu ))_(\mu )) | Tau antineutrino: ν ¯ τ (\displaystyle (\bar (\nu ))_(\tau )) |
| u-antiquark: u ¯ (\displaystyle (\bar(u))) | c-antiquark: c ¯ (\displaystyle (\bar (c))) | t-antiquark: t ¯ (\displaystyle (\bar(t))) |
| d-antiquark: d ¯ (\displaystyle (\bar (d))) | s-antiquark: s ¯ (\displaystyle (\bar (s))) | b-antiquark: b ¯ (\displaystyle (\bar (b))) |
Quarks
Quarks and antiquarks have never been found in a free state - this is explained by the phenomenon
Alpha(a) rays- positively charged helium ions (He ++), flying out of atomic nuclei at a speed of 14,000-20,000 km / h. The particle energy is 4-9 MeV. a-radiation is observed, as a rule, in heavy and predominantly natural radioactive elements (radium, thorium, etc.). The range of an a-particle in air increases with an increase in the energy of the a-radiation.
For example, a-particles of thorium(Th232), having an energy of 3.9 V MeV, run 2.6 cm in air, and a-particles of radium C with an energy of 7.68 MeV have a run of 6.97 cm. The minimum thickness of the absorber required for complete absorption of particles is called the run of these particles in a given substance. The ranges of a-particles in water and tissue are 0.02-0.06 mm.
a-particles absorbed completely by a piece of tissue paper or a thin layer of aluminum. One of the most important properties a-radiation is a strong ionizing effect. On the way of motion, an a-particle in gases forms a huge number of ions. For example, in air at 15° and 750 mm of pressure, one a-particle produces 150,000-250,000 pairs of ions, depending on its energy.
For example, specific ionization in air a-particles from radon, having an energy of 5.49 MeV, is 2500 pairs of ions per 1 mm path. The ionization density at the end of the α-particle run increases, so the damage to cells at the end of the run is approximately 2 times greater than at the beginning of the run.
Physical Properties a-particles determine the features of their biological effect on the body and methods of protection against this type of radiation. External irradiation with a-rays is not dangerous, since it is enough to move away from the source by a few (10-20) centimeters or install a simple screen made of paper, fabric, aluminum and other common materials so that the radiation is completely absorbed.
the greatest danger a-rays represent when hit and deposited inside radioactive a-emitting elements. In these cases, the cells and tissues of the body are directly irradiated with a-rays.
Beta(b)-rays- a stream of electrons ejected from atomic nuclei at a speed of approximately 100,000-300,000 km / s. The maximum energy of p-particles is in the range from 0.01 to 10 MeV. The charge of the b-particle is equal in sign and magnitude to the charge of the electron. Radioactive transformations of the b-decay type are widespread among natural and artificial radioactive elements.
b-rays have a much greater penetrating power than a-rays. Depending on the energy of b-rays, their range in air ranges from fractions of a millimeter to several meters. Thus, the range of b-particles with an energy of 2-3 MeV in air is 10-15 m, and in water and tissue it is measured in millimeters. For example, the range of b-particles emitted by radioactive phosphorus (P32) with a maximum energy of 1.7 MeV in tissue is 8 mm.
b-particle with energy, equal to 1 MeV, can form about 30,000 pairs of ions on its way in the air. The ionizing ability of b-particles is several times less than that of a-particles of the same energy.
Exposure to b-rays on the body can manifest itself both with external and internal irradiation, in case of ingestion of active substances emitting b-particles into the body. To protect against b-rays during external irradiation, it is necessary to use screens made of materials (glass, aluminum, lead, etc.). The radiation intensity can be reduced by increasing the distance from the source.
What are nuclei made of? How are the parts of the nucleus held together? It was found that there are forces of enormous magnitude, which hold the constituent parts of the nucleus. When these forces are released, the energy released is huge compared to chemical energy, it's like comparing the explosion of an atomic bomb with the explosion of TNT. This is explained by the fact that an atomic explosion is caused by changes inside the nucleus, while during the explosion of TNT, only the electrons on the outer shell of the atom are rearranged.So what are the forces that hold neutrons and protons together in the nucleus?
Electrical interaction is associated with a particle - a photon. Similarly, Yukawa suggested that the attractive forces between a proton and a neutron have a special kind of field, and that the oscillations of this field behave like particles. This means that it is possible that, in addition to neutrons and protons, there are some other particles in the world. Yukawa was able to deduce the properties of these particles from the already known characteristics of nuclear forces. For example, he predicted that they should have a mass 200-300 times greater than an electron. And, oh, a miracle! - a particle with such a mass was just discovered in cosmic rays! However, a little later it turned out that this was not the same particle at all. They called it the muon, or muon.
And yet, a little later, in 1947 or 1948, a particle, the π-meson, or pion, was discovered that met Yukawa's requirements. It turns out that in order to obtain nuclear forces, a pion must be added to the proton and neutron. "Wonderful! - you will exclaim. - With the help of this theory, we will now build quantum nuclear dynamics, and pions will serve the purposes for which Yukawa introduced them; Let's see if this theory works, and if so, we'll explain everything." Vain hopes! It turned out that the calculations in this theory are so complicated that no one has yet managed to do them and extract any consequences from the theory, no one has had the luck to compare it with experiment. And it's been going on for almost 20 years!
Something does not stick with the theory; we do not know whether it is true or not; however, we already know that something is lacking in it, that some irregularities lurk in it. While we were trampling around the theory, trying to calculate the consequences, the experimenters discovered something during this time. Well, the same μ-meson, or muon. And we still don't know what it's good for. Again, many “extra” particles were found in cosmic rays. To date, there are already over 30 of them, and the connection between them is still difficult to grasp, and it is not clear what nature wants from them and which of them depends on whom. Before us, all these particles do not yet appear as different manifestations of the same essence, and the fact that there is a bunch of disparate particles is only a reflection of the presence of incoherent information without a tolerable theory. After the undeniable successes of quantum electrodynamics - some set of information from nuclear physics, scraps of knowledge, semi-experienced, semi-theoretical. They are asked, say, by the nature of the interaction of a proton with a neutron and look at what will come of it, without really understanding where these forces come from. Beyond what has been described, there has been no significant progress.
But chemical elements after all, there were also many, and suddenly between them it was possible to see the connection expressed by the periodic table of Mendeleev. Let's say that potassium and sodium - substances that are similar in chemical properties - in the table fell into one column. So, we tried to build a table like the periodic table for new particles. One such table has been proposed independently by Gell-Mann in the US and Nishijima in Japan. The basis of their classification is a new number, like an electric charge. It is assigned to each particle and is called its "strangeness" S. This number does not change (just like the electric charge) in the reactions produced by nuclear forces.
In table. 2.2 shows new particles. We won't talk about them in detail for now. But the table at least shows how little we still know. Under the symbol of each particle is its mass, expressed in certain units called megaelectronvolts, or MeV (1 MeV is 1.782 * 10 -27 G). We will not enter into the historical reasons that forced the introduction of this unit. Particles are more massive in the table above. In one column are particles of the same electric charge, neutral - in the middle, positive - to the right, negative - to the left.
Particles are underlined with a solid line, "resonances" - with strokes. There are no particles in the table at all: there is no photon and graviton, very important particles with zero mass and charge (they do not fall into the baryon-meson-lepton classification scheme), and there are no some new resonances (φ, f, Y *, etc.). The antiparticles of mesons are given in the table, and for the antiparticles of leptons and baryons it would be necessary to compile a new table similar to this one, but only mirrored with respect to the zero column. Although all particles, except for the electron, neutrino, photon, graviton and proton, are unstable, their decay products are written only for resonances. The strangeness of leptons is also not written, since this concept is not applicable to them - they do not interact strongly with nuclei.
Particles that stand together with a neutron and a proton are called baryons. This is a “lambda” with a mass of 1115.4 MeV and three other “sigmas”, called sigma-minus, sigma-zero, sigma-plus, with almost the same masses. Groups of particles of almost the same mass (difference by 1-2%) are called multiplets. All particles in a multiplet have the same strangeness. The first multiplet is a pair (doublet) proton - neutron, then comes the singlet (single) lambda, then the triplet (triple) sigma, doublet xi and singlet omega-minus. Beginning in 1961, new heavy particles began to be discovered. But are they particles? They have such a short life span (they decay as soon as they are formed) that it is not known whether to call them new particles or to consider them a "resonant" interaction between their decay products, say, Λ and π at some fixed energy.
For nuclear interactions, in addition to baryons, other particles are needed - mesons. These are, firstly, three varieties of pions (plus, zero and minus), forming a new triplet. New particles were also found - K-mesons (this is a doublet K+ and K 0 ). Every particle has an antiparticle, unless the particle happens to be its own antiparticle, say π+ and π- are each other's antiparticles, a π 0 is its own antiparticle. Antiparticles and K- with K + , and K 0 with K 0 `. In addition, after 1961 we began to discover new mesons, or sort of mesons, which decay almost instantly. One such curiosity is called omega, ω, its mass is 783, it turns into three pions; there is another formation from which a pair of pions is obtained.
Just as some rare earths have fallen out of the very successful periodic table, so some particles fall out of our table. These are particles that do not strongly interact with nuclei, have nothing to do with nuclear interaction, and also do not interact strongly with each other (strong is understood as a powerful type of interaction that gives atomic energy). These particles are called leptons; these include the electron (a very light particle with a mass of 0.51 MeV) and the muon (with a mass 206 times that of an electron). As far as we can judge from all experiments, the electron and muon differ only in mass. All the properties of the muon, all its interactions are no different from the properties of the electron - only one is heavier than the other. Why it is heavier, what good it does, we do not know. In addition to them, there is also a neutral mite - a neutrino, with a mass of zero. Moreover, it is now known that there are two kinds of neutrinos: one associated with electrons and the other with muons.
Finally, there are two more particles that also do not interact with nuclei. One we already know is a photon; and if the gravitational field also has quantum mechanical properties (although the quantum theory of gravitation has not yet been developed), then, perhaps, there is also a graviton particle with zero mass.
What is "mass zero"? The masses that we have given are the masses of particles at rest. If a particle has a mass of zero, then it means that it does not dare to rest. A photon never stands still, its speed is always 300,000 km/sec. We will still understand the theory of relativity and try to delve deeper into the meaning of the concept of mass.
So we've come across a whole array of particles that together seem to be a very fundamental part of matter. Fortunately, these particles do not all differ in their interaction from each other. Apparently, there are only four types of interactions between them. We list them in order of decreasing strength: nuclear forces, electrical interactions, (β-decay interaction and gravitation. A photon interacts with all charged particles with a force characterized by some constant number 1/137. The detailed law of this connection is known - this is quantum electrodynamics. Gravity interacts with any energy, but extremely weakly, much weaker than electricity. And this law is known. Then come the so-called weak decays: β-decay , due to which the neutron decays rather slowly into a proton, an electron and a neutrino. Here the law is only partially clarified. And the so-called strong interaction (the bond of a meson with a baryon) has a force equal to unity on this scale, and its law is completely obscure, although some rules are known, such as the fact that the number of baryons does not change in any reaction.
The situation in which modern physics finds itself must be considered terrible. I would sum it up in these words: outside the core, we seem to know everything; inside it, quantum mechanics is valid, violations of its principles were not found there.
The stage on which all our knowledge operates is relativistic space-time; it is possible that gravity is also associated with it. We do not know how the Universe began, and we have never set up experiments to accurately test our ideas about space-time at small distances, we only know that outside these distances our views are infallible. One could also add that the rules of the game are the principles quantum mechanics; and, as far as we know, they apply to new particles just as well as to old ones. The search for the origin of nuclear forces leads us to new particles; but all these discoveries only cause confusion. We do not have a complete understanding of their mutual relations, although we have already seen some striking connections between them. We, apparently, are gradually approaching the understanding of the world of beyond-atomic particles, but it is not known how far we have gone along this path.