Gamma rays

Gamma radiation.

gamma radiation or gamma rays is a type of electromagnetic radiation, and therefore made up of photons, generally produced by radioactive elements or by subatomic processes such as the annihilation of a positron-electron pair. It is also generated in astrophysical phenomena of great violence.

Due to their high energies, gamma rays are a type of ionizing radiation capable of penetrating matter more deeply than alpha and beta radiation. They can cause serious damage to the nucleus of cells, which is why they are used to sterilize medical equipment and food.

Energy of this nature is measured in megaelectronvolts (MeV). One MeV corresponds to gamma photons with wavelengths less than 10^-11 m or frequencies greater than 10¹⁹ Hz.

Gamma rays are produced by de-excitation of a nucleon from one level or excited state to another of lower energy and by the decay of radioactive isotopes. They differ from X-rays in their origin. These are generated at the extranuclear level, by electronic braking phenomena. Radioactivity is generally linked to nuclear energy and nuclear reactors, although it exists in the natural environment: to cosmic rays, expelled from the sun and from outside our solar system: from galaxies; radioactive isotopes in rocks and minerals.

In general, gamma rays produced in space do not reach the Earth's surface, as they are absorbed by the upper atmosphere. To observe the universe at these frequencies it is necessary to use high-altitude balloons or exospace observatories. To detect them, in both cases the Compton effect is used. These gamma rays originate from high-energy astrophysical phenomena, such as supernova explosions or active galaxy nuclei.

In astrophysics, GRBs (abbreviation for "gamma ray bursts") are called sources of gamma rays that last a few seconds or a few hours, followed by a decreasing brightness in the source by X-rays for a few days. They occur at random positions in the sky. Its origin remains still under scientific discussion. In any case, they seem to constitute the most energetic phenomena in the universe.

Exceptional are the gamma rays of energy greater than a few gigaelectronvolts (GeV, thousands of MeV) which, when they hit the atmosphere, produce thousands of particles (extensive atmospheric cascade), which, since they travel at speeds close to light in the air, they generate Cherenkov radiation. This radiation is detected at the Earth's surface using a Cherenkov telescope.

Story of discovery

The first source of gamma rays discovered historically was the radioactive decay process called gamma decay. In this type of decay, an excited nucleus emits a gamma ray almost immediately after its formation (this is now understood as a nuclear isomeric transition, although inhibited gamma decay with a measurably much longer half-life can also occur). Paul Villard, a French chemist and physicist, discovered gamma radiation in 1900 while studying the radiation emitted by radium. Villard knew that the radiation from him was more powerful than the previously described types of radiation from radio rays, such as beta rays, first observed as "radioactivity"; by Henri Becquerel in 1896, and alpha rays, discovered as a less penetrating form of radiation by Rutherford in 1899. However, Villard did not consider them to be a different fundamental type in naming them. Villard radiation was recognized in 1903 by Ernest Rutherford as a fundamentally different type of rays, being also the one who named them as "gamma rays", by analogy with alpha and beta rays that he himself had differentiated in 1899. The rays emitted for radioactive elements were named based on their power to penetrate various materials, using the first three letters of the Greek alphabet: alpha rays, the least penetrating, followed by beta rays and gamma rays, the most penetrating. Rutherford also realized that gamma rays were not deflected (or at least not easily deflected) by a magnetic field, another property that differentiated them from alpha and beta rays.

Gamma rays were originally thought to be particles with mass, like alpha and beta rays. Rutherford believed that they might be extremely fast beta particles, but the inability to deflect them by a magnetic field indicated that they had no charge. In 1914, gamma rays were observed to reflect off glass surfaces, showing that they were electromagnetic radiation. Rutherford and his partner Edward Andrade measured the wavelengths of radium gamma rays, and found that they were similar to those of X-rays, but with a shorter wavelength and (thus) a higher frequency. This was eventually recognized by also being given more energy per photon, as soon as the latter term became generally accepted. Gamma decay was then understood as the emission of a single gamma photon.

Protection

To shield from gamma rays, a large amount of mass is required, depending on the energy, since distance is also a barrier, as well as exposure time, it is not always a "great barrier" In medical use, 99mTc does not require a high barrier and has not been shown to induce cancer. Materials with high atomic number and density protect better; and the higher the energy of the rays, the thickness of the protection must be greater. Such materials are classified according to the thickness necessary to reduce the intensity of gamma rays by half, known as the HVL (half-value layer). For example, gamma rays that require 1 cm of lead to attenuate their intensity by 50% also decrease it in the same proportion when passing through 6 cm of concrete or 9 cm of compacted earth.

Interaction with matter

Total absorption coefficient of gamma rays of aluminum (atomic number 13) according to different gamma rays energies, and contributions of the three effects. In most of the region of energy shown dominates the Compton effect.

Total absorption coefficient of lead gamma rays (atomic number 82) according to different gamma rays energies, and contributions of the three effects. Here the photoelectric effect dominates in low energies. From 5 MeV it begins to dominate the creation of pairs.

When a gamma ray passes through matter, the probability of absorption in a thin layer is proportional to the thickness of the layer. This implies exponential decrease in intensity.

I(d)=I0⋅ ⋅ e− − μ μ d{displaystyle I(d)=I_{0}cdot e^{-mu d}

being:

• μ = n × σ, the absorption coefficient, measured in cm^-1,
• n the number of atoms per cm3 of the material,
• σ the absorption spectrum in cm2, and
• d the thickness of the material in cm.

Passing through matter, gamma radiation ionizes mainly in three ways: photoelectric effect, Compton effect, and pair creation.

• Photoelectric effect. When a gamma photon interacts with an atomic electron it transfers its energy and expels it from the atom. The resulting kinetic energy, of the photoelectron, is equal to the energy of the gamma photon incident less the link energy of the electron.
  The photoelectric effect is the process of transferring dominant X-ray energy and gamma-ray photons of energies below 0.5 MeV (millions of electronvoltios). Higher energies are less important.

• Compton effect. Interaction where an incident gamma photon increases the energy of an atomic electron enough to provoke its expulsion. The remaining energy of the original photon emits a new low-energy gamma photon with emission direction different from that of the gamma photon incident. The probability of the Compton effect decreases as the power of the photon increases.
  The Compton effect is considered to be the main gamma-ray absorption procedure in the intermediate energy range between 100 kiloelectronvoltios or electronic kilovoltios keV to 10 MeV (Megaelectronvoltio), energy range that includes most of the gamma radiation present in nuclear explosions. The Compton effect is relatively independent of the atomic number of the absorbing material.

• Creation of pairs. Due to the interaction of the strength of Coulomb, in the vicinity of the nucleus the energy of the incident photon becomes spontaneously the mass of an electron-positron pair. A positron is the antiparticle equivalent to an electron. Its mass is of equal magnitude. The electric charge is thus of equal magnitude, but of opposite sign than that of an electron.
  The surplus energy (1.02 MeV) of the equivalent of the resting mass of the two particles appears as kinetic energy of the pair and the kernel. The “life” of the positron is very short: of the order of 10^-8 seconds. At the end of his period he combines with a free electron. The entire mass of these two particles then becomes two gamma photons of 0.51 MeV of energy, each.

Frequently the energy of the secondary electrons (or positrons) produced in any of these three processes is enough to generate many ionizations until their conclusion (of the processes).

Strictly speaking, the exponential absorption described above holds only for a narrow range of gamma rays. If a wider ray passes through a thin concrete block, the scattering on the sides reduces the absorption.

Gamma rays often occur between other categories of radiation, such as alpha and beta. When a nucleus emits an α or β particle, sometimes the decay product becomes excited and can jump to a lower energy level and emit a gamma ray. Similarly, an atomic electron can jump to a lower energy level and emit visible light or ultraviolet radiation.

Decomposition scheme ⁶⁰Co.

The possible types of electromagnetic radiation are: gamma rays, X-rays, visible light and ultraviolet rays: UV (UVA and UVB). UVB are more energetic. There is also visible light, microwave waves, and radio waves. The only difference between them is in the frequency, and therefore in the energy of the photons, from which it follows that gamma rays are the most energetic. An example of gamma ray production is shown below.

First ⁶⁰Co breaks down to ⁶⁰Ni excited:

60Co→ → 60Ni*+e− − +.. ! ! e.{displaystyle}{hbox{Co}}{;to ;^{60}{hbox{Ni*}}};+;e^{-};+;{overline {nu }}}{e}. !

Then ⁶⁰Ni enters its ground state and emits two consecutive gamma rays.

60Ni*→ → 60Ni+γ γ .{displaystyle}{hbox{ ni*}};to ;^{60}{hbox{Ni}}};+gamma. !

These gamma rays are 1.17 MeV and 1.33 MeV, respectively.

Another example is the alpha decomposition of ²⁴¹Am, to produce ²³⁷Np. This decay generates gamma emission. In some cases, this cast is quite simple, eg ⁶⁰Co/⁶⁰Ni. In cases like ²⁴¹Am/²³⁷Np and ¹⁹²Ir/¹⁹²Pt the gamma emission is complex. It reveals that a number of different levels of nuclear energy can exist. The fact that in an alpha spectrum there can be a diversity of peaks, of different energies, reinforces the idea of the possibility of many levels of nuclear energy.

Because a beta decay emits a neutrino, which in turn subtracts energy, there are no sharp lines in the beta spectrum, but rather a broad peak. Therefore, from a single beta decay it is not possible to determine the different energy levels of the nucleus.

In spectropic optics it is well known that an entity that emits light can also absorb light of the same wavelength (photon energy). For example a sodium flame can emit yellow light. It can also absorb yellow light from a sodium vapor lamp. In the case of gamma rays, it can be observed in Mössbauer spectroscopy, where a correction can be obtained for the energy lost due to the recoil of the nucleus and, through resonance, the exact conditions of gamma ray absorption.

Usage

The power of gamma rays makes them useful for sterilizing medical equipment. They are often used to kill bacteria and insects in food products such as meat, mushrooms, eggs and vegetables, in order to maintain their freshness.

Due to the ability to penetrate tissues, gamma rays or X-rays have a wide spectrum of medical uses, such as tomography and Nuclear Medicine studies. However, due to their condition as ionizing radiation, if DNA is affected, they carry the ability to cause molecular changes that can have repercussions in carcinogenic effects.

Despite its cancer-causing properties, gamma rays are also used to treat certain types of cancer. In a procedure called gamma-knife surgery, multiple concentrated beams of gamma rays are directed at cancer cells. The beams are emitted from different angles to focus the radiation on the tumor, while minimizing damage to surrounding tissues.

Gamma rays are also used in nuclear medicine to make diagnoses. Many gamma-emitting radioisotopes are used. One of them is technetium 99m: ^99mTc. When administered to a patient, a gamma camera can use the emitted radiation to obtain an image of the distribution of the radioisotope. This technique is used in the diagnosis of a wide spectrum of diseases, for example in the detection of bone cancer.