Gamma ray

Electromagnetic wave with frequency higher than X-ray

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Gamma ray, also called gamma ray Particle Flow , Yes Nucleus Energy level transition The radiation emitted during de excitation is shorter than 0.1 wavelength Angstrom Of electromagnetic wave （1 Angstrom =10 -10 m）， Energy above 124 keV , the frequency exceeds 30EHz (3 × 10 nineteen Hz). Gamma ray has strong penetration and can be used in industry flaw detection Or automatic control of pipeline. Gamma ray is harmful to cells and is used for medical treatment tumour 。

Gamma ray is a kind of electromagnetic wave with frequency ratio X-ray Higher. Gamma ray is first generated by France Scientist P 5. Willard's discovery is the third one after alpha and beta rays Nucleus Ray.

Chinese name: Gamma ray
Foreign name: Gamma ray
Alias: γ Particle Flow

Discoverer: P. V. Willard
Wavelength: Less than 0.1 angstrom
Energy: Above 124keV
Frequency: Over 30EHz (3 × 10 nineteen Hz）

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Research history

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First observation

Gamma ray

It was first observed by human beings in the 1970s. The US military launched Vela satellite to detect "nukeflash" (evidence of unauthorized atomic bomb explosion), but Vela did not recognize the nuclear flash, but found a strong burst of radiation from space. This discovery initially caused a panic at the Pentagon: was it the Soviet Union that tested a new nuclear weapon in space? Later, it was determined that the radiation came from all directions of the sky evenly, meaning that it actually came from outside the Milky Way. But if they come from outside the Milky Way, they must release a real amount of astronomical energy, enough to light up the entire visible universe.^[1]

Origin theory

There is a theory about the origin of gamma ray bursts - they have infinite energy“ Polar supernova ”(hypernova), leaving a huge black hole 。 It seems that gamma ray bursts are giant black holes lined up.

Space production

The gamma rays produced in space are fixed star Core nuclear fusion Because it cannot penetrate the earth atmosphere Therefore, it is impossible to reach the lower atmosphere of the earth and can only be detected in space. The gamma ray in space was first observed by a man-made satellite named "Villas" in 1967. Gamma ray images detected by different satellites in the early 1970s provide information about hundreds of previously undiscovered stars and possible black hole 。 Launched in the 1990s Artificial satellite (including Compton Gamma Ray Observatory), providing information about supernovae cluster 、 a quasar And so on Astronomy Information.

Manual manufacturing

Professor Tino Janosensky of Strathclyde University

September 2011, UK university of strathclyde A scientific research team led by him recently produced a beam of the brightest gamma ray on the earth - 1 trillion times brighter than the sun. This will usher in a new era of medical research.^[2]

Physicists found that Ultrashort laser pulse Can and Ionized gas It reacts and produces an extremely powerful laser beam, which can even penetrate the lead plate with a thickness of 20 cm. It can be completely shielded by a 1.5 meter thick concrete wall.

This ultra intense laser beam has many uses, including medical imaging, radiotherapy, and positron emission tomography（ PET ）Scan. At the same time, this radiation source can also be used to monitor the safety of the sealed nuclear waste. In addition, because this laser pulse is extremely short, lasting only one billionth of a second, fast enough to capture the reaction excited by the atomic nucleus, it is very suitable for nuclear research in the laboratory.

The emission source used in this study is smaller and cheaper than the common gamma ray emission equipment. The experiment was conducted in Rutherford, affiliated to the British Science and Technology Facilities Association Appleton In addition to scientists from Strathclyde University University of Glasgow And scientists from the Institute of Advanced Technology in Lisbon, Portugal participated in the experiment.

This research has obtained British Engineering and Physical Science Research Association ， British Association of Science and Technology Facilities , Laser Lab - European Union and Extreme Optical Facilities Project Team.

Production principle

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radioactivity Nucleus Occurs on Alpha decay 、 Beta decay The new nucleus generated after the accident is often high energy Level, to transition to low energy level and emit gamma photons. atom Nuclear decay and nuclear reaction Can produce gamma rays. It is wavelength Less than 0.2 angstrom electromagnetic wave 。 The wavelength of gamma ray is shorter than that of X-ray, so gamma ray has X-ray More powerful pierce through Ability.

Gamma rays are electromagnetic photons with frequencies higher than 1.5 trillion hertz. Gamma ray No charge And quiet quality Therefore, it has weaker ionization ability than alpha particles and beta particles. Gamma ray has extremely strong penetrability and high energy. Gamma ray can be high Atomic number Nuclear stoppage, for example lead or Depleted uranium 。

measuring method

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Gamma photons are not charged, so their energy cannot be measured by magnetic deflection method. Generally, the above secondary effects caused by gamma photons are used to calculate indirectly, for example, by measure The energy of photoelectron or positron negative pair is calculated. In addition, the energy of gamma photons can also be measured directly with a gamma spectrometer (using the interaction between gamma rays and matter).

from fluorescence Crystal Photomultiplier tube And electronic instruments Scintillation counter Is to detect γ Ray intensity Common instruments of.

Main hazards

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Gamma ray has very strong penetration ability. When the human body is exposed to gamma rays, gamma rays can enter the interior of the human body and occur with cells in the body Ionization , ionized Ions Able to erode complex Organic molecule , such as protein 、 nucleic acid and enzyme , they all constitute living Cellular tissue Once they are destroyed, the normal chemical processes in the human body will be disturbed, and cells will die in serious cases.

application

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nuclear explosion

Penetrating radiation

In general, nuclear explosion (e.g Atomic bomb 、 Hydrogen bomb The killing power of) is composed of four factors: shock wave 、 ray radiation 、 radioactivity Contamination and penetrating radiation. The penetrating radiation is mainly composed of strong gamma rays and neutron streams. It can be seen that nuclear explosion itself is a gamma ray source. Through the ingenious design of the structure, other hard killing factors of nuclear explosion can be reduced, so that the energy of explosion is mainly released in the form of gamma rays, and the action time of gamma rays can be prolonged as far as possible (which can be three times as that of ordinary nuclear explosions). This kind of nuclear bomb is Gamma ray bomb 。

And others nuclear weapon In contrast, the power of gamma rays is mainly shown in the following two aspects: first, the energy of gamma rays is large. Because the wavelength of gamma ray is very short and the frequency is high, it has very large energy. high energy The γ - ray of radiation dosage When it reaches 200-600 rem, human hematopoietic organs such as bone marrow will be damaged, white corpuscle Serious reduction, internal bleeding, hair loss, and the probability of death within two months is 0-80%; When the radiation dose is 600-1000 rem, the probability of death within two months is 80-100%; When the radiation dose is 1000-1500 rem, the human gastrointestinal system will be damaged, and diarrhea, fever Endocrine disorder The probability of death within two weeks is almost 100%; When the radiation dose is more than 5000 rem central nervous system Damaged spasm The probability of death in two days is 100%. The second is that gamma ray has a strong penetration ability. Gamma ray is a killing weapon Bullet Is much more powerful. neutron The bomb uses neutron flow as the means of attack, but the yield of neutron is small, accounting for only a small part of the energy released by nuclear explosion, so the killing range is only 500-700 meters, and it is generally used as a tactical weapon. The killing range of gamma ray is said to be 1 million square kilometers, which is equivalent to The Alps As the center of the whole southern Europe. Therefore, it is a very deterrent Strategic weapons 。

Silent weapon

In addition to its high lethality, the gamma ray bomb has two outstanding characteristics: first, it does not need explosives to detonate. General nuclear bombs are equipped with high explosives and detonators, so they are prone to accidents during storage. The gamma ray bomb did not detonate explosives, so it is much safer to store it at ordinary times. Second, the gamma ray bomb has no explosion effect. Do this nuclear test It is not easy to be measured, even if it explodes over the enemy. So gamma ray bombs are very difficult to defend, as U.S.A Defense Minister Cohen is accepting Germany《 Le Monde 》"This kind of weapon is silent and has instantaneous effect". It can be seen that once this "silent" killer enters the battlefield, it will become an important factor affecting the battlefield pattern.

Encyclopedia x confusion: illustration of nuclear radiation

Basic effects

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When gamma rays pass through material And atom When interacting photoelectric effect 、 Compton effect And positive and negative Electronics For three effects.

Compton effect

1923 American physicist Compton (A.H. Compton) discovered X-ray and electron scattering The wavelength will shift when Compton effect 。

Gamma photons and atoms' outer electrons (can be regarded as free electron ）Occurs Elastic collision Gamma photons only transfer part of the energy to the outer electrons in the atom, making the electrons break away from the binding of the nucleus and shoot out from the atom. Photon itself changes Direction of movement 。 The emitted electrons, called Compton electrons, can continue to interact with the medium. The angle between the directions of scattering photons and incident photons is called scattering angle, which is generally recorded as θ. The angle between the recoil direction of recoil electrons and the direction of incident photons is called the recoil angle, which is generally recorded as φ. When the scattering angle θ=0 °, the energy of the scattered photon is the maximum, then the energy of the recoil electron is 0, and the photon energy is not lost; When the scattering angle θ=180 °, the incident photon collides with the electron head-on and scatters back in the opposite direction, while the recoil electron flies out in the direction of the incident photon. This situation is called backscattering. At this time, the energy of the scattered photon is the smallest.

Like ordinary electrons, photoelectrons can continue to generate excitation, ionization and other effects with the medium. Due to the vacancy in the electron shell, the outer electrons fill the vacancy and emit characteristic X-ray. But the light is invisible to human eyes, with the highest frequency and the shortest wavelength (the wave in vacuum v=c light speed, c=λ f, λ wavelength, f frequency).

Electron pair effect

When the γ photon with energy greater than 1.02MeV passes by the atomic nucleus, under the action of the coulomb field of the atomic nucleus, the γ photon transforms into an electron and a positron. Part of the energy of photons is converted into the static energy (1.02MeV) of positive and negative electrons, and the rest is taken as their kinetic energy 。 The emitted electrons can continue to excite and ionize with the medium; After losing energy, positrons will combine with the negative electrons in the material to become gamma rays, that is, annihilation, to detect this Annihilation radiation It is a reliable experimental basis for judging positron production.

Coherent scattering

For low energy photons (energy is much less than Electronic static energy ）For example, the inner electrons are tightly bound by the atomic nucleus and cannot be regarded as free electrons. If a photon collides with this bound electron, it is equivalent to colliding with the whole atom. In the collision, the energy transmitted by the photon to the atom is very small, almost keeping its own energy unchanged. such Scattered light The original wavelength is retained. be called Thomson scattering (Thomson scattering) or rayleigh Rayleigh scattering or coherent scattering. Since the number of inner electrons increases with the increase of the atomic number of the scatterer, and the proportion of outer electrons decreases, the intensity of scattering photons with constant wavelength increases, while the intensity of Compton scattering photons with longer wavelength decreases.

rayleigh Coherent scattering The scattering photons caused are limited in a small angle range. That is, the angular distribution of photons has sharp peaks in the direction of the photons, and the energy loss of deflecting photons can be ignored. As the scattering angle φ of the scattered photons increases, the wavelength of Rayleigh scattering The relative intensity of photons decreases gradually, while the relative intensity of Compton scattering photons with longer wavelength increases gradually, and the change of wavelength also increases gradually.

Photonuclear reaction

It is also known as photonuclear absorption. Gamma photons larger than a certain energy interact with the atomic nucleus of the material atom and can emit particles, such as (γ, n) reaction. However, the magnitude of this interaction is small compared with other effects, so it can be ignored. Photonuclear absorptive Threshold energy At 5MeV or higher, this process is similar to the atomic photoelectric effect, but in this process, photons are absorbed by the atomic nucleus rather than surrounded by the nucleus turn Photonuclear absorption of shell electrons generally causes neutron emission. The most remarkable feature of optical nucleus absorption is "giant resonance ”（giant resonance）。 Photonuclear reaction In Giant resonance It is a kind of dipole resonance, which comes from the electric dipole excitation of the nucleus caused by gamma photons, called Giant Dipole Resonance (GDR). For light nuclei, Absorption cross section The center of is about 24MeV. along with Target nucleus Mass number Increase, central energy decreases, huge Formant Of position It also decreases. The heaviest stability is 12 MeV, and the width of the giant resonance (corresponding to the energy difference of the half maximum height section) varies with the target core, about 3-9 MeV. Even for the resonance peak, the cross section of the optical nucleus is smaller than that of the previously mentioned photoelectric cross section contribution Less than 10%, however, when Radiation shielding Photonuclear absorption is important in design because the emitted neutrons have greater penetrability in heavy nuclei than the incident photons. stay Irradiation technology The radioactivity caused by is more important.

Nuclear resonance reaction

The incident photon excites the atomic nucleus to the excited state, and then emits gamma photons when it is de excited.

Figure 1 Schematic diagram of three main interactions between gamma rays and matter

The first three interactions have the greatest impact, as shown in Figure 1. For narrow beam gamma rays (that is, the gamma photons passing through the absorption sheet are only composed of photons that are not interacted or collided), μ is recorded as the bus attenuation of gamma rays passing through the absorption medium coefficient , which includes two contributions of γ - photons that are truly absorbed by the medium and scattered away from the collimation. Some studies directly express μ as the total absorption coefficient, which is equivalent to the macro absorption cross section of the medium for gamma rays dimension Is the reciprocal of the length. Obviously, μ value reflects the absorption capacity of the medium for gamma rays.

Fig. 2 Three main interactions between γ - rays and matter

For low-energy gamma rays and Atomic number High absorption material, photoelectric effect is dominant; For medium energy gamma rays and absorption materials with low atomic numbers, Compton effect is dominant; For high-energy gamma rays and absorption materials with high atomic numbers, Electron pair effect dominant. The relative strength of the three can be expressed as Figure 2. The photon energy is in the range of 100keV to 30MeV, and the contribution of the last three secondary interaction modes to the absorption of gamma rays is less than 1%

Gamma ray burst

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phenomenon

Gamma burst

In astronomy, gamma ray bursts are called“ Gamma ray burst ”。 What on earth is a gamma ray burst? Where does it come from? Why does it produce such huge energy?

"Gamma ray bursts are a phenomenon in the universe where gamma rays suddenly increase." National Astronomical Observatory, Chinese Academy of Sciences Zhao Yongheng researcher Said that the gamma ray has a wavelength of less than 0.1 nm electromagnetic wave , is more than X-ray The one with higher energy radiation ， Gamma burst The energy of is very high. But most gamma rays will be blocked by the earth's atmosphere, and observation must be carried out outside the earth.

Cold War During the period, the United States launched a series of Military satellite To monitor global nuclear explosion tests, gamma ray detectors are installed on these satellites to monitor a large number of high-energy rays generated by nuclear explosions. Reconnaissance satellites discovered in 1967 Cosmic space It is called "gamma ray burst" because of the sudden increase of gamma ray in a short time. Due to military secrecy and other factors, this discovery was not published until 1973. This is a phenomenon puzzling astronomers: some gamma ray sources will suddenly appear for a few seconds and then disappear. This burst releases energy power Very high. A gamma ray burst“ brightness ”It is equivalent to the total "brightness" of all gamma ray sources throughout the day. Then, there is constant high energy Astronomical satellite Gamma ray bursts can be observed once or twice every day.

Gamma ray burst The energy released can even match Big Bang put on a par with. The duration of gamma ray bursts is very short, usually tens of seconds long and only a few tenths of a second short. And its brightness changes are complex and irregular. However, the energy emitted by gamma ray bursts is huge. The energy emitted by gamma rays in a few seconds is equivalent to the total energy emitted by hundreds of suns in their lifetime (10 billion years)!

The gamma ray burst occurred on December 14, 1997. It is 12 billion light years away from the Earth, and the energy released is hundreds of times larger than the supernova explosion. The gamma ray energy released in 50 seconds is equivalent to the total radiation energy of the entire Milky Way galaxy for 200 years. In a second or two, this gamma ray burst is as bright as the whole universe except it. Within hundreds of kilometers around it, the high temperature and density at one thousandth of a second after the Big Bang are reproduced.^[3]

However, on January 23, 1999 Gamma ray burst It is even more violent than this one. The energy released by it is ten times of that in 1997, which is also the most powerful gamma ray burst known to mankind so far.

debate

Up to now, there is no conclusive conclusion about the cause of gamma ray bursts in the world. Some people speculate that it is two neutron star Or two black hole Generated when collision occurs; Others speculate that it is the process of massive stars forming black holes when they die, but this process is much more violent than the supernova explosion, so some people also call it "supernova".

In order to explore the cause of gamma ray bursts, two astronomers have initiated a great debate.

Astronomical observatory

In the 1970s and 1980s, people generally believed that Gamma ray burst It happened in Galaxy And speculate that it is related to neutron star The physical process of the surface. However, poland American astronomer Patzinski is unique. In the mid-1980s, he proposed that gamma ray bursts were located in cosmology Distance, and a quasar A distant object, in fact, means that gamma ray bursts occur outside the Milky Way. However, at that time, people had been ruled by the theory that "gamma ray bursts occur in the Milky Way" for many years, so they often laughed at Patchinsky's view.

But a few years later, things changed. In 1991, the United States“ Compton The "Gamma Ray Observatory" was launched to conduct a comprehensive and systematic monitoring of gamma ray bursts. After several years of observation, scientists have found that gamma ray bursts appear in all directions of the sky, which is very similar to the distribution of galaxies or quasars, which is completely different from the distribution of stars in the Milky Way. As a result, people began to take seriously the view that Patchinsky's gamma ray bursts might be distant objects outside the Milky Way. This also led to Paczynski and another astronomer who held the opposite view in 1995 Ram The big debate.

However, at that time in 1995, there was no way to measure Gamma ray burst Therefore, the two sides of the debate could not convince each other. The occurrence of gamma ray bursts is spatially random, and the duration is very short, so subsequent observations cannot be arranged. Moreover, there is no corresponding object in other bands except for the transient gamma ray bursts, so it cannot be verified by objects with known distances in other bands. The debate is still up in the air. Fortunately, in 1997 Italy Launched a high-energy astronomical satellite, which can quickly and accurately determine the position of gamma ray bursts Optical telescope and radio telescope Then we can make follow-up observations. Astronomers first successfully discovered the Optical counterpart This optical counterpart is called the "optical afterglow" of gamma ray bursts; Then we saw the corresponding galaxies, which fully proved the phenomenon of gamma ray bursts in the cosmological distance, and thus made a conclusion for the debate between Patchinsky and Lam.

So far, more than 20 Gamma ray burst The "optical afterglow", most of which are located at a certain distance, are all distant objects outside the Milky Way. Researcher Zhao Yongheng said that the discovery of "optical afterglow" has greatly promoted the research work of gamma ray bursts, making the observation band of gamma ray bursts developed from gamma ray to optical and radio bands, and the observation time has been extended from tens of seconds to months or even years.

Supernovae once again caused controversy, one after another. March 24, 2003, in Canada Quebec At the meeting of the High Energy Astrophysics Branch of the American Astronomical Society, some researchers announced that they had found some of the most powerful signs so far, indicating that ordinary supernova explosions may lead to violent gamma ray bursts within weeks or months. Once this statement was put forward, it caused heated controversy at the meeting.

Gamma ray burst instant

In fact, in a 2002 issue of the British journal Nature, a British research team reported that they had Gamma ray burst According to the latest research results of, gamma ray bursts are related to supernovae. The researchers studied the observation data of a gamma ray burst in December 2001 Newton Space telescope The "afterglow" of the 270 second X-ray band of the gamma ray burst was observed. Through X-ray observation, researchers found that magnesium, silicon, sulfur, etc element with Sublight velocity Escape outward, usually caused by supernova explosion.

Most astrophysicists believe that the strong gamma ray emission is caused by the collapse of the star's core Supernova explosion And form a black hole. Massachusetts Institute of Technology Of researchers passed the Chandra X-ray telescope A supernova explosion that lasted less than one day in August 2002 was tracked. In this 21 hour burst, people observed much more X-rays than similar cases. X-rays are widely regarded as the emission of unstable neutron stars initially formed after supernova explosion. A large number of observations have shown that there are always massive materials produced by supernova explosion near the gamma ray emission source.

Those who oppose the above views believe that these statements do not exclude the possibility of abnormal increase or decrease of X-ray. Moreover, there is a relationship between supernova explosion and gamma ray eruption time interval The reason is still unknown.

In any case, human beings pursue the mysterious energy from the vast universe -—— Gamma ray burst The momentum will not be reduced by a series of doubts, on the contrary, scientists will work harder to explore. As the basic research of astronomy, this kind of exploration can help people understand the universe and observe the physical phenomenon It is meaningful to discover new laws.

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