Cosmic Rays: Concerns on Exposures

#Here's a quick (and also the final segment) continuation from yesterday's post.

Although effects of ground level radiation exposure to cosmic rays are minimal to health, on high altitudes however, the radiation level is becoming a concern. (refer to the first graph on the last post)

Studies in the past have shown that commercial flight crews will receive more than 1 milli-sievert (mSv) which is the general public’s limit of radiation exposure limit. A round trip from Thailand to New York will result 0.28 mSv, which is about 10 times more than an exposure of a lung x-ray machine.Some transatlantic flight crews are even required to wear a radiation dosage badge to indicate the amount of exposures they receive on the trip.

Excessive exposures to radiation will result in the alteration of genetics and hence lead to cancer. This topic will be discussed next time.

Cosmic Rays: Interactions of Particles with Earth's Atmosphere

The count rate of particles vs. the altitude of earth’s atmosphere (hyperphysics.phy-astr.gsu.edu, 2010)

When a primary cosmic ray particle enters earth atmosphere, it will encounter many atoms and collisions will occur. This collisions will send a shower of “secondary” particles which are also subatomic particles such as pions which will quickly decay into relatively stable muons, neutrinos and gamma rays. Muons will soon too decay into electron and positrons which they both will annihilate and the gamma rays will interact with atmospheric atoms.

The numbers of particle that finally reached the earth’s surface depends on the energy content of the "primary" cosmic ray particle. Most secondary cosmic rays that reached the surface of earth are muons with an average intensity of 100 counts per metre square per second.

Although thousands of cosmic ray particles pass through our body every minute, the resulting exposure to that amount of radiation is minimal, safe, and is considered as background. We can say that we are irradiated with fast particles every single moment in our lifetime with negligible health effects.

Cosmic Rays: High Energy Particles

A collision between a high-energy cosmic ray particle and an atom in a photographic emulsion as viewed through the microscope. (Dr. David P. Stern, NASA Goddard Space Flight Center, January 2005)

The energy of cosmic ray particles are measured in electron volts and the first measurement of a cosmic ray particle that has energy exceeding 1e8 TeV is observed at the Volcano Ranch experiment in New Mexico, 1962.

Since then cosmic rays of even higher energy has been observed and the highest so far observed is on the evening of 15th October 1991 at Dugway Proving Grounds using the Fly’s Eye Cosmic Ray Detector which the particle has an energy of 3e8 TeV (50 joules) which is equivalent to the kinetic energy of a 142 g baseball travelling at 96 km/h.

The energy of this particle is 50 million times more than any particle accelerators on earth can produce and the effective energy of collision of this particle would be 750 TeV which is about 50 times the collision energy of the Large Hadron Collider operated by CERN in Geneva.

The sources of such highly energetic particles were a mystery and the discovery was a shock to astrophysicist. There were theories suggesting these particles could have originated from the active galactic core where they come from super-massive black holes.

Cosmic Rays: Origins

Principally, there are two types of cosmic rays – primary and secondary. For the primary cosmic rays, these cosmic particles come from extrasolar astrophysical sources such as supernova explosions and black holes. The primary cosmic ray particles can then travel through space, colliding with the interstellar matter to produce the secondary cosmic rays. The sun also produces relatively low energetic particles from the stellar nucleosynthesis that is related to the so-called “solar wind”.

Because cosmic ray particles are charged, as they pass through any object with magnetic field including earth’s magnetic field, their trajectory is bent and hence randomizing their path, making it impossible to determine the particle’s origin.

Studies of high-energy gamma rays (between 10MeV to 1000MeV) emitted when a cosmic ray collide with interstellar gas atoms indicated that most of the cosmic ray particles are confined at the rim of the galaxy, probably due to the galactic magnetic field. The collision between cosmic rays with light elements also produces nuclear fragments of radioactive isotopes such as Beryllium-10 which possess a half-life of 1.6 million years. By measuring the composition percentage of Beryllium-10 in cosmic rays shows that on average, cosmic ray particles spends about 10 million years in the outer ring of the galaxy before going into intergalactic space.

Cosmic Rays: Composition

Solar System and Galactic Cosmic Ray (GCR) composition (NASA Goddard Space Flight Centre, Feb 2010)

By definition from California Institute of Technology (Caltech), cosmic rays are highly energetic charged particles that are travelling near to the speed of light that is coming to earth from space in all directions. The general composition of cosmic ray is mostly hydrogen nuclei (the lightest and most abundant element in the universe) which are essentially protons, but cosmic rays can also include electrons, positrons and other subatomic particles, in addition, nuclei of other heavier elements in the periodic table are also detected. The proportions are 89% of the particles being the hydrogen nucleus (proton), 10% helium and 1% other elements.

The common heavier elements present in the cosmic rays such as (silicon, carbon, oxygen, iron and magnesium) are in a proportion to the abundance of such elements in the solar system, but there are important differences in the proportion of element and its isotopes that that gives information of the origin and history of cosmic rays of galactic origin.

Electrons constitute about 1% of the galactic cosmic rays and the reason why accelerating electrons are less efficient than light nuclei is still an unsolved mystery.

In the graph above, silicon is taken as the standard (reference point) and comparing to the relative abundances of other elements in the solar system and in galactic cosmic rays. Silicon is used as a reference because of its common intermediate-weight that is relatively easy to measure.

Cosmic Rays: A Little Energetic History

Today, most people who know about the existence of cosmic rays take it for granted that the earth is constantly being bombarded by particles coming from outer space since all of existence and the time to come. This is a story of the discovery of cosmic rays and human’s knowledge about it.

The journey starts from the “father” of cosmic rays studies, Victor Hess. He was a graduate in physics and during his work as an assistant in the institute of Radium research at the Austrian Academy of Sciences in the early 1900’s, he was astounded by the fact that there are residue charges trapped inside a sealed electroscopes no matter how good is the quality of the instrument. Scientist of his day proposed that it was caused by terrestrial ionising radiation – radioactivity from rock minerals. Hence the ionisation measured by the electroscopes should reduce if brought up higher into the atmosphere.

Oddly, previous experiments seems to indicate that the ionisation level is actually increasing with altitude, for example in 1910, Theodore Wulf measured the ionisation levels at the bottom and on top of Eiffel tower in Paris and it is found that the ionising radiation levels detected at higher level is much higher compared to the ground which is contradicting to the proposed terrestrial ionising radiation theory. Many other scientists at that time tried other methods to send instruments to record the ionisation levels in higher altitude using balloons but the data retrieved are inconclusive due to instrument defects under the conditions of high altitude.

Now speculating that the radiation is actually coming from the sky instead of the ground, Hess improvised the experiment by designing instruments that can withstand the pressure and temperature conditions of high altitude. He also determined that terrestrial radiation will no longer produce ionisation effect in altitudes higher than 500 meters.

Hess then mounts his instrument on balloons and sent to the skies ten times in the course of three years during (1911-1913). He found that at the height of several kilometres into the sky, the ionisation level is a few times higher than the surface of earth. Hence he concluded that “a radiation of very high penetrating power enters our atmosphere from above.”

Another conclusive data comes from one of Hess’s experiment on 12 April 1912, during a near-total eclipse of the sun. The ionising level did not reduce under the eclipse means that the radiation could not be from the sun itself; it has to come from further out in space. Hess’s experiments are later confirmed by Robert Milikan in 1925 who coined the term “cosmic rays”. And for this discovery, Hess shared the Nobel Prize in physics with the discoverer of the Positron, Carl D. Anderson in 1936.

For a period of time, cosmic rays are referred to as rays because it was believed to be a part of the electromagnetic spectrum, but during the 1930’s it was discovered that the cosmic rays must be composed of charged particles because they are affected by earth’s magnetic field.