Cosmic radiation

Simulation of the impact of a 1 TeV particle (10)¹² eV) coming from outer space, and from consequent cosmic radiation on Chicago.

Representation of the different cosmic rays detectors.

Cosmic rays, also called cosmic radiation, are subatomic particles from outer space whose energy is very high due to their high speed. They were discovered when it was found that the electrical conductivity of the Earth's atmosphere is due to ionization caused by high-energy radiation.

In 1911, Victor Franz Hess, an Austrian physicist, showed that atmospheric ionization increases proportionally with altitude. He concluded that the radiation must come from outer space.

The discovery that radiation intensity depends on altitude indicates that the radiation particles are electrically charged and deflected by Earth's magnetic field.

Ernest Rutherford and his collaborators, contrary to and prior to Hess's experiences, assumed that the ionization observed by the spectroscope was due to terrestrial radioactivity, since measurements made in 1910 at the base and top of the Eiffel Tower That's how they detected it.

Robert Andrews Millikan coined the expression cosmic rays after his own measurements that concluded that, indeed, they were of very distant origin, even outside the solar system.

History

After the discovery of radioactivity by Henri Becquerel in 1896, it was accepted that atmospheric electricity —air ionization— was caused exclusively by radiation generated in turn by radioactive elements in the soil and by radioactive gases or isotopes of radon that they produce. Subsequent measurement, during the decade from 1900 to 1910, of the ionization rate (rhythm of air ionization) with respect to altitude showed a decrease that could be explained by the absorption of ionizing radiation by the intervening air.

Discovery

In 1909 Theodor Wulf developed the first electrometer. This was an instrument designed to measure the rate of ion production within a hermetically sealed container. Wulf used this instrument to show that the levels of ionizing radiation at the top of the Eiffel Tower were higher than at its base. However, his article, published in Physikalische Zeitschrift, did not find wide acceptance. In 1911, Domenico Pacini observed simultaneous variations in the ionization rate over a lake, over the sea, and at a depth of 3 meters below the surface. From the observed drop underwater, Pacini concluded that a portion of the ionization is due to sources other than terrestrial radioactivity.

Pacini performing a measurement in 1910.

Later, in 1912, Victor Hess flew three Wulf-enhanced precision electrometers to an altitude of 5,300 meters using a hot air balloon and found that the ionization rate was approximately four-fold compared to that which could be measured at ground level. from the ground. Hess also ruled out the Sun as the responsible radiation source by a new balloon ascent during a near-total solar eclipse. When the Moon was blocking most of the visible solar radiation, Hess was still able to measure an increasing ionization rate with height, and concluded: "The best explanation for the result of my observations is given by the assumption of that radiation of enormous penetrating power enters our atmosphere from above". In 1913-1914, Werner Kolhörster confirmed Hess's first observations by measuring the increase in the ionization rate at 9 km altitude.

Increased ionization rate with the measured altitude by Victor Hess in 1912 (left) and Kolhörster (right).

Hess received the Nobel Prize in Physics in 1936 for his discovery.

The flight of the Hess balloon took place on August 7, 1912. Exactly 100 years later, on August 7, 2012, the Mars Science Laboratory rover measured ionizing radiation levels for the first time on another planet using your RAD (Radiation Assessment Detector).

Hess lands after his 1912 balloon flight.

Origin

The origin of cosmic rays is still unclear. It is known that, in periods when large solar flares are emitted, the Sun emits low-energy cosmic rays, but these stellar phenomena are not frequent. Therefore, they are not a reason for explaining the origin of this radiation. Neither are the eruptions of other Sun-like stars. Large supernovae explosions are, at least, responsible for the initial acceleration of much of the cosmic rays, since the remnants of such explosions are powerful radio sources, involving presence of high energy electrons.

In 2007, a group of Argentine scientists from the Pierre Auger Observatory made a spectacular discovery that inaugurated a new branch of astronomy. This group found evidence that most of the cosmic ray particles come from a nearby constellation: Centaurus. This constellation contains a galaxy with an active nucleus, whose nucleus is due to the existence of a black hole (probably supermassive), falling matter to the ergosphere of the black hole and rotate rapidly.

At enormous speeds, centrifugally, part of that matter escapes, made up of protons and neutrons. Upon reaching the Earth (or other planets with sufficiently dense atmospheres) only protons arrive, which, after colliding with the upper atmospheric layers, fall in cascades of cosmic rays. The discovery observed in Centaurus seems to be extrapolated to all galaxies with nuclei activated by black holes.

It is also believed that additional acceleration is generated as a result of shock waves from supernovae propagating into interstellar space. There is no direct evidence that supernovae contribute significantly to cosmic rays. However, it is suggested that X-ray binary stars may be sources of cosmic rays. In these systems, a normal star loses mass to its complement, to a neutron star or to a black hole.

Radio astronomical studies of other galaxies show that they also contain high-energy electrons. The centers of some galaxies emit radio waves of much greater intensity than the Milky Way. This indicates that they contain high energy particle sources.

Components at sea level

Cosmic rays that reach the atmosphere in its upper layer are mainly (98%) protons and high-energy alpha particles. The rest is made up of electrons and ionized heavy particles. These are called primary particles.

These charged particles interact with the atmosphere and the Earth's magnetic field, become secondary particles (they are the product of the interaction of the primary particles with the atmosphere) and are distributed in such a way that, Due to the magnetic field, the greatest intensity of the particles that reach the ground occurs at the poles.

Therefore, the component of particles that reach the ground varies by altitude (the higher the altitude, the less atmosphere with which to interact) and by latitude (the higher the latitude, the greater the number of particles deflected by the magnetic field), and they favor certain variation with the solar cycle (of 11 years).

At sea level and at a latitude of about 45º N, the important components of these particles are:

• muons: 72%
• photons: 15%
• neutrons: 9%

Dose received from cosmic rays varies between 300 μSv (microsieverts) and 2,000 μSv per year. Averaged by population, occupancy data, and other factors, an average value of 380 μSv/year is found.

Typical Doses

• The normal dose due to ambient radiation on Earth is average 2.4 mSv per year, with significant differences between countries. At sea level the contribution of cosmic rays is approximately 0.3 mSv.
• The radiation dose received during a medical X-ray ranges from 0.1 to several dozens of mSv, depending on the type of X-ray. They are high levels, so protection is used.
• The typical dose received during a transatlantic flight (Europe – North America) due to galactic cosmic rays is 0.05 mSv. It can be significantly increased in the case of energy particle events (up to 10 factor increases have been reported in the case of very strong solar events, but these events are very rare and have a very short duration to influence the annual dose). Over the years, frequent travelers or flight cabin crews can accumulate doses of a few mSv. Airline personnel (pilots and stewardesses) in recent years have taken the request to perform routine radiation controls during flights. Obviously the cosmic rays are losing intensity as they approach the earth's surface (they are disintegrating into weaker particles), but at high altitudes, they are dangerous.

Cosmic Ray Cascades

The showers or cascades of subatomic particles originate from the action of primary cosmic rays, whose energy can be greater than 10²⁰ eV (electronvolts): one hundred million times higher than what can be imparted to a subatomic particle in the most powerful particle accelerators.

When a high-energy cosmic ray reaches Earth's atmosphere, it interacts with atoms in the Earth's atmosphere, collides with gases, and releases electrons. This process excites the atoms and generates new particles. These, in turn, collide with others and cause a series of nuclear reactions, which give rise to new particles that repeat the cascade process. Thus, a cascade of more than 10¹¹ new particles can be formed. The corpuscles that make up the cascades can be measured with different types of particle detectors, generally based on the ionization of matter or the Cherenkov effect.

Detection methods

The Cherenkov Air Telescope Set VERITAS

There are two main classes of detection methods. First, direct detection of primary cosmic rays in space or at high altitudes by balloon-borne instruments. Second, the indirect detection of secondary particles, that is, the extensive air showers at higher energies. Although there have been proposals and prototypes for the detection of air showers in space and aboard balloons, currently running experiments for high-energy cosmic rays are ground-based. In general, direct detection is more accurate than indirect. However, the cosmic ray flux decreases with energy, making direct detection difficult for the energy range above 1 PeV. Both direct and indirect detection are performed using various techniques.

Direct detection

Direct detection is possible by all kinds of particle detectors on the ISS, on satellites, or in high-altitude balloons. However, there are weight and size restrictions that limit detector options.

An example of the direct detection technique is a nuclear track-based method developed by Robert Fleischer, P. Buford Price, and Robert M. Walker for use in high-altitude balloons. In this method, the sheets of Clear plastic, such as 0.25mm Lexan polycarbonate, are stacked and directly exposed to cosmic rays in space or at high altitudes. The nuclear charge causes the breaking of chemical bonds or ionization in the plastic. At the top of the plastic pile, ionization is less, due to the high speed of cosmic rays. As the speed of the cosmic rays decreases due to the slowdown in the stack, the ionization increases along the way. The resulting plastic sheets are "etched" or they are slowly dissolved in a hot caustic solution of sodium hydroxide, which removes material from the surface at a known slow rate. Caustic sodium hydroxide dissolves plastic at a faster rate along the path of ionized plastic. The net result is a conical pitting in the plastic. Pitting is measured with a high-powered microscope (typically 1600× oil immersion), and the rate of etching is plotted as a function of depth into the stacked plastic.

This technique produces a unique curve for each atomic nucleus from 1 to 92, making it possible to identify both the charge and the energy of the cosmic ray passing through the plastic stack. The more extensive the ionization along the path, the greater the charge. In addition to its uses for cosmic ray detection, the technique is also used to detect nuclei created as nuclear fission products.

Indirect detection

There are several terrestrial methods of cosmic ray detection currently in use, which can be divided into two main categories: the detection of secondary particles that form extensive air showers (EAS) by various types of particle detectors, and the detection of the electromagnetic radiation emitted by EAS in the atmosphere.

Extensive air shower arrays formed by particle detectors measure the charged particles that pass through them. EAS arrays can observe a wide area of the sky and can be active more than 90% of the time. However, they are less capable of separating the background effects of cosmic rays than air Cherenkov telescopes. Most of the latest generation EAS sets use plastic scintillators. Water (liquid or frozen) is also used as a detection medium through which the particles pass and produce Cherenkov radiation to make them detectable. As a result, several matrices use water/ice-Cherenkov detectors as an alternative or in addition to scintillators. By combining several detectors, some EAS arrays have the ability to distinguish muons from lighter secondary particles (photons, electrons, positrons). The fraction of muons among secondary particles is a traditional way of estimating the mass composition of primary cosmic rays.

A historical secondary particle detection method that is still used for demonstration purposes involves the use of the cloud chamber to detect the secondary muons created when a pion decays. Cloud chambers, in particular, can be built from widely available materials and can even be built in a high school lab. A fifth method, involving a bubble chamber, can be used to detect cosmic ray particles.

More recently, CMOS devices in the ubiquitous cameras of smartphones have been proposed as a practical distributed network for detecting air showers from ultra-high-energy cosmic rays. The first app to exploit this proposal was the CRAYFIS experiment (Cosmic RAYs Found in Smartphones).edu/paper.pdf CRAYFIS detector array paper. In 2017, the CREDO (Cosmic Ray Extremely Distributed Observatory) collaboration released the first version of its fully open source app for Android devices. Since then, the collaboration has attracted the interest and support of many scientific institutions, educational centers, and members of the public around the world. Future research has to demonstrate in which respects this new technique can compete with dedicated EAS arrays.

The first detection method of the second category is called an airborne Cherenkov telescope, designed to detect low-energy (<200 GeV) cosmic rays by analyzing their Cherenkov radiation, which for cosmic rays are emitted gamma rays by traveling faster than the speed of light in their medium, the atmosphere. Although these telescopes are extremely good at distinguishing between background radiation and cosmic ray source radiation, they can only work well on clear nights without bright light. Luna, have very small fields of view and are only active for a small percentage of the time.

A second method detects light from nitrogen fluorescence caused by excitation of nitrogen in the atmosphere by particles moving through it. This method is the most accurate for cosmic rays at the highest energies, particularly when combined with EAS particle detector arrays. Like Cherenkov light detection, this method is limited to clear nights.

Another method detects radio waves emitted by air showers. This technique has a high duty cycle similar to that of particle detectors. The precision of this technique has been improved in recent years, as demonstrated by several experimental prototypes, and it may become an alternative to the detection of atmospheric Cherenkov light and fluorescence light, at least at high energies.