New study helps uncover the nature of cosmic rays
September 9, 2013
Using data from the IceCube Neutrino Observatory at the South Pole, scientists have reported progress in understanding the longstanding mystery of how and where cosmic rays originate, in a development that might help us find ways to shield astronauts and electronics from cosmic radiation.
What is a cosmic ray?
The term cosmic ray is a misnomer. In truth, these are not rays but rather charged particles made up primarily of high-energy protons and atomic nuclei that travel throughout interstellar space at extremely high speeds.
We still don't know how cosmic rays accelerate as they travel through space, and we also know very little about where they originate. Since they are charged particles, their trajectory is bent multiple times by the interstellar magnetic fields they find along their way, which makes it much harder to deduce where they originally came from.
Cosmic rays are thought to be the highest-energy particles in the universe. Some can reach energies of 300 EeV, which is forty million times the energy at which particles collide at the Large Hadron Collider and approximately the kinetic energy of a tennis ball being served at 115 km/h (70 mph).
Why do we care about cosmic rays?
You may argue that the money for building powerful new telescopes is ill-spent because the knowledge they produce is essentially useless to us, but in the case of cosmic rays, you'd be very wrong. Cosmic rays are already a concern for the space and the electronics industry today, and their impact is destined to become even more serious over the next few decades – which is why any development in this area is generally welcomed with open arms.
The Earth's magnetic field shields us from the vast majority of cosmic particles, but outside the comfort of our little bubble the threat of cosmic radiation can be very real. These particles can be dangerous because their incredibly high energy is enough to knock off DNA molecules and disrupt electronics.
Thanks to recent experiments, we can now estimate that unshielded humans in space should expect a dose of 400 to 900 milliSieverts a year (compared to 2.4 on Earth). Radiation doses over 4 Sieverts are considered extremely dangerous and potentially lethal, meaning that while short trips in space don't pose a real radiation threat, a long-term mission to Mars would necessitate effective ways to shield the crew from radiation.
As for electronics, high-energy cosmic rays have enough force to alter the bits inside integrated circuits and cause transient errors to occur. Back in the 1990s, an IBM study estimated that cosmic rays induce one error per 256 megabytes of RAM per month, and the problem will only get tougher as electronics become smaller and smaller. (In 2008, Intel went as far as patenting a cosmic ray detector to integrate inside their next-gen CPUs.)
What did we find out?
The flux of cosmic rays that hit the surface of the Earth can be analyzed and classified both by energy and chemical composition in order to learn more about their origin, the way in which they are accelerated, their energy spectrum and their composition: information that might help us build an effective shielding mechanism.
When ultra high energy cosmic rays are produced, they are accompanied by ultra high energy neutrinos. Neutrinos are chargeless and almost massless particles that rarely interact with other matter. As a result, unlike cosmic rays, they travel in a straight line and can be traced back to their source.
"On rare occasions when neutrinos interact with matter, they produce charged particles," University of Delaware physicist Bakhtiyar Ruzybayev, corresponding author of a paper on this study, tells Gizmag. "And when this charged particles go through transparent medium at speeds faster than light (in that medium), they emit light. Most neutrino detectors are built to detect that light."
Using neutrino detectors such as the one at IceTop, scientists have observed a relationship between the energy of cosmic rays and their flux (that is, how often they hit a given area). Particles thought to originate from the Milky Way generally have lower energy but are much more frequent, while particles from much further away are harder to come by and are also higher-energy because they had been accelerating for a much longer time before they finally hit Earth.
A constant acceleration would dictate a simple power law between flux and the energy of the particle (which would look like a single straight line in the logarithmic graph above); however, things aren't that simple, and scientists have detected a steepening of the curve, which they call the "knee." To the left of the knee are lower-energy particles that are mostly coming from our own galaxy, while to its right are mostly extra-galactic cosmic rays. A second feature, the "ankle," describes the graph where higher-energy particles are involved.
The recent findings by the University of Delaware scientists are that things are even more complicated than they look. The cosmic-ray energy spectrum does not follow a simple power law between the knee around 4 PeV (peta-electron volts) and the ankle around 4 EeV (exa-electron volts), as previously thought, but exhibits features like hardening around 20 PeV and steepening around 130 PeV.
The graph above illustrates the findings, with the data zooming in around the knee of the previous graph. Their significance is that the acceleration and propagation of cosmic rays adheres to laws that are less predictable than previously thought. "These measurements provide new constraints that must be satisfied by any models that try to explain the acceleration and propagation of cosmic rays," Ruzybayev explained.
The findings appear in the journal Physical Review D.
Source: University of Delaware
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