Emissivity of Materials


[/caption]In the last few centuries, in which time we have had several scientific revolutions, our understanding of heat, energy and the exchange thereof has grown exponentially. In particular has been the increasing ability to gauge the amounts of energy involved in particular processes and in turn create theoretical frameworks, units, and even tools with which to measure them. One such concept is the measurement known as Emissivity. Essentially, this is the relative ability of a material’s surface (usually written ? or e) to emit energy as radiation. It is expressed as the ratio of the emissivity of the material in question to the radiation emitted by a blackbody (an idealized physical body that absorbs all incident electromagnetic radiation) at the same temperature. This means that while a true black body would have an emissivity value of 1 (? = 1), any other object, known as a “grey body”, would have an emissivity value of less than 1 (? < 1). In general, the duller and blacker a material is, the closer its emissivity is to 1. The more reflective a material is, the lower its emissivity. Emissivity also depends on such factors as temperature, emission angle, and wavelength of the radiation. At the opposite end of the spectrum is the material’s absorptivity (or absorptance), which is the measure of radiation absorbed by a material at a particular wavelength. When dealing with non-black surfaces, the relative emissivity follows Kirchhoff's law of thermal radiation which states that emissivity is equal to absorptivity. Essentially an object that does not absorb all incident light will also emit less radiation than an ideal black body. An important function for emissivity has to do with the Earth’s atmosphere. Like all other “grey bodies”, the Earth’s atmosphere is able to absorb and emit radiation. The overall emissivity of Earth's atmosphere varies according to cloud cover and the concentration of gases that absorb and emit energy in the thermal infrared (i.e. heat energy). In this way, and by using the same criteria by which they are able to calculate the emissivity of “grey bodies”, scientists are able to calculate the amount of thermal radiation emitted by the atmosphere, thereby gaining a better understanding of the Greenhouse Effect. Every known material has an emissivity coefficient. Those that have a higher coefficient tend to be polished metals, such as aluminum and anodized metals. However, certain materials that are not metals and are non-reflective, such as red bricks, asbestos, concrete and pressed carbon, have equally high coefficients. In addition, naturally occurring materials such as ice, marble, and lime also have high emissivity coefficients. We have written many articles about emissivity of materials for Universe Today. Here's an article about heat rejection systems, and here's an article about absorptivity. If you'd like more info on emissivity, check out these articles from Engineering Toolbox and Science World.

We’ve also recorded an entire episode of Astronomy Cast all about Electromagnetism. Listen here, Episode 103: Electromagnetism.


What Are Gamma Rays

Fermi mapped GeV-gamma-ray emission regions (magenta) in the W44 supernova remnant. The features clearly align with filaments detectable in other wavelengths. This composite merges X-ray data (blue) from the Germany/U.S./UK ROSAT mission, infrared (red) from NASA’s Spitzer Space Telescope, and radio (orange) from the Very Large Array near Socorro, N.M. Credit: NASA/DOE/Fermi LAT Collaboration, NASA/ROSAT, NASA/JPL-Caltech, and NRAO/AUI


In the universe there are kinds of energy and different ways it manifests itself. One common form is radiation. Radiation is the wave energy produced by electromagnetic forces. There are different kinds and their strength can be divided into three categories. There are alpha rays, beta rays, and finally gamma rays. Essentially each example is high energy particles traveling in a straight line. However, there are limits for level. Alpha rays are the weakest and can be blocked by human skin and gamma rays are the strongest and only dense elements like lead can block them.

So what are gamma rays? Gamma rays are the strongest from of radiation. This is what makes nuclear radiation so dangerous. This high energy form of radiation can damage human tissue and cause mutations. In circumstances where gamma radiation is plentiful most life forms would be killed within a short amount of time.

Gamma rays differ from alpha and beta waves in their composition. Alpha and beta rays are composed of discrete subatomic particles. This is part of the reason why these rays are more easily deflected by less dense matter. Gamma rays are on a whole different level. They are pure energy and radiation so only the most dense kind of matter can deflect it.

Gamma rays can be found practically anywhere in the universe. The best example is celestial bodies like the sun, pulsars, and white dwarfs. Each of these are massive sources energy burning off hydrogen in massive nuclear reactions. This produces massive amounts of radiation in the form of rays. Outside of the Earth’s protective atmosphere the radiation manifests itself in cosmic rays. Cosmic rays carry tremendous amounts of energy but what makes them pack such a punch are the gamma rays that they are made up of.

The most interesting characteristic of gamma rays is that they don’t have a uniform energy level. In some cases the energy levels vary so much you can have gamma rays that meet every criterion for the term but in the end have less energy than an x ray from a X ray machine at the hospital. The energy of the gamma ray largely depends on the source and production of the radiation.

In the end Gamma rays are one the many interesting energy phenomena in our universe and scientist are constantly looking to learn more about them and gain a better understanding of their properties.

We have written many articles about Gamma Ray for Universe Today. Here’s an article about Gamma Rays, and here are the Top Ten Gamma Ray Sources from the Fermi Telescope.

If you’d like more info on Gamma Rays, check out the NASA Official Fermi Website. And here’s a link to NASA’s Article on Gamma Rays.

We’ve also recorded an episode of Astronomy Cast all about planet Earth. Listen here, Episode 136: Gamma Ray Astronomy.

What is Beta Radiation?

Beta radiation is radiation due to beta particles, which are electrons (or, sometimes, positrons); mostly, when you come across the words ‘beta radiation’, what is meant is what is produced by beta decay (radioactive decay which produces beta particles … either electrons or positrons).

Within a few years of Becquerel’s discovery of radioactivity (in 1896), its heterogeneous nature was discovered … and the three (then) known components given the memorable names alpha radiation, beta radiation, and gamma radiation. And, in 1900, Becquerel showed that beta radiation was composed of particles which have the same charge-to-mass ratio as electrons (which had been discovered only a few years’ earlier). The realization – by Irène and Frédéric Joliot-Curie, in 1934 – that some beta radiation is composed of positrons, rather than electrons, had to wait until positrons themselves were discovered (in 1932).

Some fun facts about beta radiation:

* beta radiation is in between alpha and gamma in terms of its penetrating power; typically it goes a meter or so in air

* like all kinds of radioactive decay, beta decay occurs because the final state of the nucleus (the one decaying) has a lower energy than the initial one (the difference is the energy of the emitted beta particle and neutrino)

* beta decay involves only the weak interaction (or force), unlike alpha and gamma decay

* the key to the specifics of beta decay is the emission of a neutrino (or antineutrino), postulated by Pauli (in 1931) and combined into a model by Fermi, in 1934 (though it wasn’t until 1956 that the neutrino was detected, and the 1960s for the existence of carriers of the weak force – the three bosons W, W+, and Z0 – to be hypothesized).

* beta radiation has the characteristics we observe it to have because key constants in the weak interaction have the values they have (no theory in physics predicts what those values are … yet); had those values been just a teensy bit different in the early universe, we would not be here today (this is part of an idea called the anthropic principle).

Here are some of the Universe Today stories that are related to beta radiation New Insights on Magnetars, Superstrings Could Be Detectable As They Decay, and Don’t ‘Supermassive’ Me: Black Holes Regulate Their Own Mass.

Two Astronomy Cast episodes are well worth a listen, as they provide further insights into beta radiation The Strong and Weak Nuclear Forces, and Nucleosynthesis: Elements from Stars.

Sources: EPA, Wikipedia

Beta Particles

Beta particles are electrons (symbol β), or positrons (symbol β+), emitted in beta decay (a kind of radioactivity); beta radiation in other words. Sometimes ‘beta particles’ refers to high energy electrons, irrespective of their source (e.g. the beta particles in the Van Allen radiation belts around the Earth; very few are produced by beta decay).

Of the three kinds of radioactivity (alpha, beta – both of which are particles – and gamma (which is electromagnetic radiation)), beta particles have intermediate penetrating power.

Beta particles have an important role in medicine … as diagnostic tools, to treat some diseases (notably various cancers, particularly via radionuclide therapy), in biochemical analysis, etc. For example, 18F (the fluorine-18 isotope) is used as a positron (β+) emitter in positron emission tomography (PET).

Beta particles – or rather the weak interaction which is the cause of their emission – were crucial in Big Bang Nucleosynthesis … as the early universe cooled, reactions between the protons, neutrons, electrons, and photons produced many light nuclides, but the balance between many reactions left only hydrogen, deuterium, helium-3, helium-4, and lithium-7 when the universe became too cool for any nuclear reactions to continue (of course, isolated neutrons and unstable nuclides – such as tritium – were also left, but they decayed well before the time of cosmic microwave background).

Fast forward to today … beta (β+) particles from the decay of potassium-40 is one source of internal heat for the Earth – giving us plate tectonics, its magnetic field, etc – … the decay of carbon-14 and beryllium-10 (both of which produce beta (β) particles) provide us with tools to do radioactive (or radiometric) dating (these are two of the nuclides produced by cosmic ray spallation).

Beta Particle Radiation is a good, introductory webpage (from the University of California, Davis), and Weak Interactions explains how beta particles and the weak (nuclear) force are related (from SLAC’s Virtual Visitor Center)

Universe Today has several stories which cover the role of beta particles in astronomy; for example A Prototype Detector for Dark Matter in the Milky Way, and Fermilab Putting the Squeeze on Higgs Boson.

The Strong and Weak Nuclear Forces, and Nucleosynthesis: Elements from Stars are two Astronomy Cast episodes which will help you understand beta particles better.

What is Cherenkov Radiation?

Cherenkov radiation is named after the Russian physicist who first worked it out in detail, in 1934, Pavel Alekseyevich Cherenkov (he got a Nobel for his work, in 1958; because he’s Russian, it’s also sometimes called Cerenkov radiation).

Nothing’s faster than c, the speed of light … in a vacuum. In the air or water (or glass), the speed of light is slower than c. So what happens when something like a cosmic ray proton – which is moving way faster than the speed of light in air or water – hits the Earth’s atmosphere? It emits a cone of light, like the sonic boom of a supersonic plane; that light is Cherenkov radiation.

The Cherenkov radiation spectrum is continuous, and its intensity increases with frequency (up to a cutoff); that’s what gives it the eerie blue color you see in pictures of ‘swimming pool’ reactors.

Perhaps the best known astronomical use of Cherenkov radiation is in ICATs such CANGAROO (you guessed it, it’s in Australia!), H.E.S.S. (astronomers love this sort of thing, that’s a ‘tribute’ to Victor Hess, pioneer of cosmic rays studies), and VERITAS (see if you can explain the pun in that!). As a high energy gamma ray, above a few GeV, enters the atmosphere, it creates electron-positron pairs, which initiate an air shower. The shower creates a burst of Cherenkov radiation lasting a few nanoseconds, which the ICAT detects. Because Cherenkov radiation is well-understood, the bursts caused by gamma rays can be distinguished from those caused by protons; and by using several telescopes, the source ‘on the sky’ can be pinned down much better (that’s what one of the Ss in H.E.S.S. stands for, stereoscopic).

The more energetic a cosmic ray particle, the bigger the air shower it creates … so to study really energetic cosmic rays – those with energies above 10^18 ev (which is 100 million times as energetic as what the LHC will produce), which are called UHECRs (see if you can guess) – you need cosmic ray detectors spread over a huge area. That’s just what the Pierre Auger Cosmic Ray Observatory is; and its workhorse detectors are tanks of water with photomultiplier tubes in the dark (to detect the Cherenkov radiation of air shower particles).

However I think the coolest use of Cherenkov radiation in astronomy is IceCube, which detects the Cherenkov radiation produced by muons in Antarctic ice … traveling upward. These muons are produced by rare interactions of muon neutrinos with hydrogen or oxygen nuclei (in the ice), after they have traveled through the whole Earth, from the Artic (and before that perhaps a few hundred megaparsecs from some distant blazer).

ICAT: imaging Cherenkov Air Telescope
CANGAROO: Collaboration of Australia and Nippon (Japan) for a Gamma Ray Observatory in the Outback
H.E.S.S.: High Energy Stereoscopic System
VERITAS: Very Energetic Imaging Telescope Array System
UHECR: ultra-high-energy cosmic ray

This NASA webpage gives more details of how ICATs work.

Quite a few Universe Today stories are about Cherenkov radiation; for example Astronomers Observe Bizarre Blazar with Battery of Telescopes, and High Energy Gamma Rays Go Slower Than the Speed of Light?.

Examples of Astronomy Casts which include this topic: Cosmic Rays, and Gamma Ray Astronomy.


After Loss of Lunar Orbiter, India Looks to Mars Mission

India Moon Mission

After giving up on re-establishing contact with the Chandrayaan-1 lunar orbiter, Indian Space Research Organization (ISRO) Chairman G. Madhavan Nair announced the space agency hopes to launch its first mission to Mars sometime between 2013 and 2015. Nair said the termination of Chandrayaan-1, although sad, is not a setback and India will move ahead with its plans for the Chandrayaan-2 mission to land an unmanned rover on the moon’s surface to prospect for chemicals, and in four to six years launch a robotic mission to Mars.

“We have given a call for proposal to different scientific communities,” Nair told reporters. “Depending on the type of experiments they propose, we will be able to plan the mission. The mission is at conceptual stage and will be taken up after Chandrayaan-2.”

On the decision to quickly pull the plug on Chandrayaan-1, Nair said, “There was no possibility of retrieving it. (But) it was a great success. We could collect a large volume of data, including more than 70,000 images of the moon. In that sense, 95 percent of the objective was completed.”

Contact with Chandrayaan-1 may have been lost because its antenna rotated out of direct contact with Earth, ISRO officials said. Earlier this year, the spacecraft lost both its primary and back-up star sensors, which use the positions of stars to orient the spacecraft.

The loss of Chandrayaan-1 comes less than a week after the spacecraft’s orbit was adjusted to team up with NASA’s Lunar Reconnaissance Orbiter for a Bi-static radar experiment. During the maneuver, Chandrayaan-1 fired its radar beam into Erlanger Crater on the moon’s north pole. Both spacecraft listened for echoes that might indicate the presence of water ice – a precious resource for future lunar explorers. The results of that experiment have not yet been released.

Chandrayaan-1 craft was designed to orbit the moon for two years, but lasted 315 days. It will take about 1,000 days until it crashes to the lunar surface and is being tracked by the U.S. and Russia, ISRO said.

The Chandrayaan I had 11 payloads, including a terrain-mapping camera designed to create a three-dimensional atlas of the moon. It is also carrying mapping instruments for the European Space Agency, radiation-measuring equipment for the Bulgarian Academy of Sciences and two devices for NASA, including the radar instrument to assess mineral composition and look for ice deposits. India launched its first rocket in 1963 and first satellite in 1975. The country’s satellite program is one of the largest communication systems in the world.

Sources: New Scientist, Xinhuanet

Alpha Particle

An alpha particle is a particle made up of two protons and two neutrons. Since this configuration is similar to that of a helium nucleus, it’s often referred to as a helium nucleus. The term is commonly used in nuclear physics, and is one of the three particles commonly emitted during a radioactive decay, i.e., alpha, beta, and gamma particles.

Alpha particles gained prominence during the early days of particle physics when scientists used them as projectiles to bombard certain targets. One of the most widely celebrated experiments that made use of alpha particles was that of Ernest Rutherford’s that led to the discovery of the atom’s structure.

Using alpha particles as projectiles and gold foils as targets, Rutherford was able to come to the conclusion that atoms were made up of very dense positively charged cores with the much lighter negatively-charged electrons orbiting around it. His conclusion was based on the observation that the trajectories of the alpha particles were slightly deviated (as expected) at most times but in rare instances bounced off like ping-pong balls thrown against a wall.

The alpha particles went through the gold foils unhindered when they passed through the large but sparsely filled region around the nucleus. However, when, during much rarer instances, they happened to collide head on or even came close to the very dense and positively charged nucleus, they were deflected at very wide angles.

Through this information, there was no other option but for Rutherford to conclude that the atom must have a very dense nucleus which is very much smaller compared to the entire atom.

In terms of atomic proportions, alpha particles are considered very massive because of the existence of the two protons and two neutrons. Furthermore, they are also positively charged due to the protons. As such, they can easily wreak havoc to most targets. That is, they have high ionization properties.

Alpha particles are released during alpha decay processes which can happen most especially to ultra-heavy nuclei like uranium, thorium, actinium, and radium. Since they’re not as fast (due mainly to their masses) as betas and gammas, they can’t travel across large distances and can be easily stopped by a piece of paper or human skin.

However, again because of their huge masses, alpha particles can be very dangerous whenever they can somehow enter the body through inhalation or ingestion. Minus that possibility, you don’t have to worry much about this heavyweight of a particle.

Universe Today has some interesting related content that you might want to read. Want to know about how the Opportunity rover got sidelined by a charged particle hit? And here’s an article about alpha radiation.

There’s more about it at NASA. Here are a couple of sources there:

Here are two episodes at Astronomy Cast that you might want to check out as well: