Tikalon Blog by Dev Gualtieri

High Energy Cosmic Rays

November 13, 2017

While the night sky may look peaceful, we live in a very energetic universe. The most energetic bits of matter in the universe are the invisible cosmic rays that travel at nearly the speed of light (300,000 km/sec). I've written about cosmic rays in several articles (High Altitude Radiation, March 2, 2017, Cosmic Ray Source, February 20, 2013, and Local Dark Matter/A Hundred Years of Cosmic Rays, August 17, 2012.

Cosmic rays are nuclei of the elements, usually just hydrogen nuclei (protons). These apparently travel for millions of years at near-light speed from their distant sources before reaching the Earth. Supernovas have been the on again, off again, leading contenders as cosmic ray emitters.[1-2] Observations by the orbiting Fermi Gamma-ray Space Telescope have shown that supernovas are the principal source of cosmic rays, but there are other sources.

The first evidence for cosmic radiation was found in the late 18th century when French physicist Charles-Augustin de Coulomb, found that an isolated charged sphere in supposedly insulating air would slowly lose its charge.[2] Wilhelm Röntgen discovered X-rays in 1895, and it was found that X-rays would induce an electrostatic charge. In my days of chemical photography, I had a film negative dusting brush that incorporated a mild radiation source as a means of detaching dust from the acetate film by the same principle. Experiments were performed to examine the charge state of air in a lead vessel that would shield X-radiation, the air still became ionized, so another form of radiation was suspected.

This radiation was discovered by the Austrian scientist, Victor Hess, who is credited with the discovery of cosmic rays, a discovery for which he shared the 1936 Nobel Prize in Physics. Hess decided to test whether this ionizing radiation was generated by a substance in the Earth, or elsewhere. On August 7, 1912, Hess ascended in a balloon to an altitude of five kilometers, where he found that the ionization happened at three times the rate at ground level.[2]

Victor Hess, returning from his balloon flight on August 7, 1912.

(American Physical Society photograph, via Wikimedia Commons)

While supernovas produce high energy cosmic rays, there are some very high energy cosmic rays thought to emanate from jets associated with black holes.[3] Some ultra-high energy cosmic rays are thought to have an extragalactic origin.[4]. One example of an extremely energetic cosmic ray is the Oh-My-God particle, detected to have an energy of 3 x 10²⁰ eV, which is 40 million times that of a proton in the Large Hadron Collider.

Are cosmic rays just a scientific oddity; or, is their presence important to Earth? While space suits and spacecraft provide a measure of shielding against high intensity radiation, these don't shield against all cosmic radiation. Astronauts first noticed cosmic rays as random flashes of light. On lunar missions, the Apollo astronauts reported these flashes about once every three minutes, other astronauts in Earth orbit have seen these about half as often, and a 2001 study showed a higher incidence of cataracts in former astronauts who have flown in space, some cataracts having appeared as soon as 4-5 years after the missions.[5]

Incidence of cataracts for NASA astronauts as a function of age.

The radiation dose effect is clearly evident, and getting cataracts in your thirties is unusual for ordinary people.

(Created with Inkscape from data in ref. 5.[5]

Cataracts are a minor risk compared with cancer, as shown in a recent article that assesses the cancer risk from cosmic rays for travel to Mars.[6] The health hazard of radiation exposure is typically estimated using a linear dose model, a model that's been controversial for at least the last half century. It appears that the radiation exposure estimate for travel to Mars based on conventional models, while still quite high, might be a two-fold underestimate of the actual hazard.[6]

As shown in the figure, cosmic rays at Earth's surface are a considerable portion of radiation from all sources. As I wrote in an earlier article (Semiconductor Radiation Damage, August 3, 2012), there are about 10,000 of the lowest energy particles (1 GeV = 10⁹ eV) impinging on a square meter's area every second, dropping to one per square meter at a TeV (10¹² eV). Such a high flux has an affect on electronic materials as well as biological materials.

Environmental radiation sources.

Surprisingly, the human body, itself, emits radiation.

From NCRP Report #93, "Ionizing Radiation Exposure of the Population of the United States" (1987), via Lawrence Berkeley National Laboratory.

(Rendered using Gnumeric)

Electronic devices are susceptible to "soft errors" that occur when a cosmic ray strikes an integrated circuit. This generate millions of electron-hole pairs, and these stray charges can change the state of transistor logic gates and memory circuits. Error-correcting software can compensate to some extent, but our shrinking transistors make such corrections more of a problem, since more than a single bit might be affected.

It's been more than a hundred years since Victor Hess discovered cosmic rays, but scientists are still trying to determine the source of the most energetic rays. I mentioned earlier that the detected energy of one cosmic ray, the appropriately named, "Oh-My-God particle," had an energy of 3 x 10²⁰ eV. Now, the Pierre Auger Collaboration, an international team of more than 400 scientists from a hundred institutions in eighteen countries responsible for the Pierre Auger Observatory has determined to a high certainty that the highest-energy cosmic rays have an extragalactic origin.[7-11]

The principal detectors of the Pierre Auger Observatory are 1600 Cherenkov detectors distributed over 1,200 ssquare miles (3,000 square kilometers) in Argentina (see figure). These detectors utilize an effect noted by Nobel Physics Laureate, Pavel Cherenkov, in 1934. In this effect, a charged particle will emit light when moving in a dielectric medium (such as water) at a speed greater than the phase velocity of light in that medium.

Layout of the Pierre Auger Observatory. The dots are the 1,600 Cherenkov detectors, which are huge tanks of water monitored for flashes of light. Image by Darko.veberic, via Wikimedia Commons

Such a massive detector is required for such a study, since few high energy cosmic rays reach the Earth. Cosmic rays with an energy greater than two joules have a flux at the top of Earth's atmosphere of just one per square kilometer per year.[11] These particles are detectable only because their impact with air molecules produces a cascading shower of electrons, photons and muons containing more than 10 billion particles.[11] These air shower particles will cover an area in excess of 40 square kilometers by the time they reach the ground.[9]

The 1,600 Cherenkov detectors of the Pierre Auger Observatory in Argentina, which has been gathering data since 2004, are sited in a 3,000 square kilometer area, equivalent to the size of Rhode Island near the town of Malargüe in western Argentina.[9] GPS receivers detect the arrival times for the air shower particles, and these times determine the direction within about one degree.[9-11]

One of 1,600 Cherenkov detectors of the Pierre Auger Observatory.

Pierre Auger Observatory image.

(Click for larger image.)

The Pierre Auger Collaboration found a slightly dipolar distribution of 30,000 detected cosmic rays with energies greater than 8 x 10¹⁸ electron volts detected at more than a 5.2-sigma level of significance.[7] This anisotropy had an amplitude of about 6.5 percent toward right ascension 100 ±10 degrees and declination -24 (+12, -13) degrees. That direction indicates an extragalactic origin for these ultrahigh-energy particles.[7] The statistical significance is a chance of about two in ten million.[11] This is a region of the sky located 120 degrees from the galactic center, so it cannot be associated with any galactic source. These cosmic rays have an extragalactic origin.[9]

Cosmic ray anisotropy as found by the Pierre Auger Observatory. The galactic center is at the center of the ellipse, and the central region of excess cosmic rays is circled. (Pierre Auger Collaboration image.)

Says Karl-Heinz Kampert, a professor at the University of Wuppertal and spokesperson for the Auger Collaboration,

"We are now considerably closer to solving the mystery of where and how these extraordinary particles are created—a question of great interest to astrophysicists... Our observation provides compelling evidence that the sites of acceleration are outside the Milky Way."[10]

While there is clearly an extragalactic origin for the particles, the observations are not precise enough to pinpoint specific cosmic ray sources.[11] magnetic fields in space will still modify the trajectory of even at these high energy particles, but observations of even higher-energy cosmic rays, planned for an observatory upgrade to be completed in 2018, will help.[10-11]

Artist's impression of an air shower over a Cherenkov detectors at the Pierre Auger Observatory, seen against a starry sky. Cosmic rays create a shower of electrons, photons and muons through successive interactions with the nuclei in the atmosphere. These air showers can contain more than 10 billion particles.[11] (Illustration by A. Chantelauze, S. Staffi, and L. Bret.)

High Energy Cosmic Rays

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