Electrostatic Radiation Shielding
Published on Oct 23, 2009 at 7:23 pm.
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Filed under space exploration, space radiation.
Clearly carrying heavy shielding to protect astronauts from radiation in space is expensive. If you havent’ been reading my space radiation series as I’ve been posting, then you can look over the last several of my postings about space radiation to see some of the problems. So, some other strategy may be the way to go to shield against radiation. One idea that I ran across a while ago was electrostatic shielding. Space.com did a good article on that a few years ago.
The basic idea is that like electric charges repel. So, an object with a very large positive charge would repel incoming positively charged particles, like protons. The idea sounds really good at first glance. However, as with many ideas, further study shows that there are some considerable difficulties that need to be worked out before it is truly a viable shield. Working out those difficulties, though, means a lot of time, effort, and hours from scientists and engineers. They need to be paid a salary. It also means experimentation, building simulations, computing, etc. All that requires money, too. Unfortunately, I think that far too little money is being spent at this time on working out all of the problems any time soon. Of course, so little money is being spent on extending space exploration in the US that it will be a very long time before we even need an effective radiation shield.
So, what are the problems with an electrostatic shield idea? Well, the first problem is that it would do nothing to shield against X-rays or gamma rays. Also, if positive charged bodies are used, they would only repel positive particles, like protons. They would even pull in negative particles, such as electrons! So, one strategy would be to have a multiple layer shield. One layer of the shield would deflect protons, and the other would deal with the electrons. Depending upon who you ask, the outer layer might be negative to repel electrons (they are easier to deal with than the heavier protons), and the inner layer would repel the protons. Conversely, the outer layer could be a strong positive charge to repel the protons, and the inner layer negative to handle the electrons that were accelerated by the outer shield. There are advantages and disadvantages to each approach, of course, and I don’t want to go into all of the technical details of each.
But, there are some more serious matters to contend with. First, what sort of charge and voltage would be needed to repel the charged particles? Well, that depends upon the energy of the incoming particles. Solar particles are much slower than galactic cosmic rays, and are thus easier to deal with. Also, solar radiation is almost entirely protons and electrons, while galactic cosmic rays have a higher percentage of heavier nuclei. The voltage required to shield against the particle increases roughly as the weight of the particle. But, the really big problem is that galactic cosmic rays move much faster than solar radiation particles. Galactic cosmic ray particles often move at very near the speed of light. The voltage needed increases roughly as the square of the velocity, so it would take a very high voltage to block the galactic cosmic rays. The charge on the deflectors is related to the voltage, so that means a very large charge would be needed, and that requires a very big power source. The power requirements of the shield are another issue that has to be dealt with. It may be that electrostatic shielding simply won’t be effective with galactic cosmic rays. But, I think that it could certainly be made to work for solar radiation. It may even work for the most common forms of galactic cosmic rays, but if so, then that’s OK. The heavier particles are far less common, and radiation exposure due to them alone during a mission might be within acceptable limits. Further work is needed.
-Astroprof
Image Credit: NASA/ASRD Aerospace Corp./ Charles Buhler
Space Radiation and Humans
Published on Oct 21, 2009 at 2:25 pm.
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Filed under space exploration, space radiation.
As I continue my series on space radiation, the next topic that I want to address is how radiation harms space travelers. I’ve already alluded to this in earlier posts in the series, but I wanted to mention it again.
In an earlier installment, I said that radiation is a process where energy travels from one body to another in essentially straight lines. Since particles (and even light) can be deflected, that is not entirely correct, but it is close enough for our purposes. The radiation when absorbed can heat things, cause electrical currents, or if enough energy is absorbed by an individual atom, ionize that atom (remove an electron). Ions (charged atoms) behave different chemically than neutral atoms. So, ionizing radiation has potential to do more harm than non-ionizing radiation. In the human body, there are vast numbers of atoms and molecules. Most of the atoms are, in fact, part of molecules. It is the chemistry of how these molecules interact with one another, forming new molecules, releasing molecular energy, absorbing energy, etc, that is the basis of life. Modern biology is not just memorization of species classifications or anatomical terms. Rather, modern biologists are deep into organic chemistry and biochemistry. That is sometimes a shock for students in first year biology at my college. Students are surprised to find that their biology professor is talking about the laws of thermodynamics, electrical transport, chemical potentials, and a host of topics that used to be the domain of physicists and chemists. But, it is here, in the realm of molecular biology, that radiation does its damage: by upsetting the chemistry of life. Ionizing radiation can keep atoms and molecules from behaving as they should in a cell, or they can break molecular bonds, altering a molecule.
Most of the time, the damage done by radiation is simply altering a rather minor molecule. There are plenty of other molecules in the cell to take the place of the disrupted one. In rare cases, the altered molecule bonds to some other molecule(s) that it wasn’t supposed to, taking two or more molecules out of commission. But, there are so many molecules in the cell that this is normally of little importance. However, once in a while, the radiation damages a somewhat rarer and more important molecule. Sometimes an amino acid, a protein, or some other important molecule that has an important function in the cell is damaged. It doesn’t work right, and so the cell doesn’t behave properly. New proteins are being made though, so unless the cell is very unlucky, the damaged protein is eventually replaced by a correct one. But, on very rare occasions, the most important of all molecules in a cell, one of the cell’s strands of DNA (deoxyribonucleic acid) is damaged. DNA controls almost everything about how the cell works. When the DNA is damaged, then the cell no longer works properly. In most cases, that means that the cell dies. In some cases, the cell lives but no longer performs the function in the body that it is meant to perform. If such an altered cell reproduces, and its daughters reproduce, and so on, then a tumor can be formed. If the altered cells are particularly invasive and the body’s immune system doesn’t recognize them as a danger, then the altered cells develop into a cancer.
As I said in an earlier post of this series, some forms of radiation are more damaging than others. Some, such as X-rays, may damage one atom, and thus one tiny bit of one base nucleotide of the DNA. Other, more energetic radiation may damage a cluster of atoms, damaging more thane one base pair. The more damage is done, the more likely it will be that the cell will not function properly. In addition to direct damage to DNA, the radiation sometimes can alter some other molecule that can then damage the DNA.
But, biology can be amazing. DNA is very important to cell operation. As a consequence, it is not really surprising that there are biological mechanisms that are at work to limit damage to the DNA. The first of these is simply the structure of the DNA itself. DNA is described in textbooks as a “double helix.” What that means is that it is composed of two matched strands arranged in a helical pattern. By matched strands, each nucleotide on one strand is matched with one on the other strand. There are only four nucleotides: adenine, thymine, cytosine, and guanine. Adenine matches with thymine, and cytosine matches with guanine. So, if one of these molecules is damaged, then it is no longer the correct molecule. It doesn’t fit in the DNA. The cell has enzymes that will detect the miss-matched base pair and substitute the damaged nucleotide with the correct match (adenine, thymine, cytosine, or guanine) that corresponds with the other base nucleotide on the undamaged helical strand. This works wonders, and it happens all of the time in our bodies. The problem comes, though, if both nucleotides in a pair are damaged. In that case, the cellular repair mechanism has problems. It can either fill in the gap with random nucleotides, it can splice a fragment of DNA into the chromosome (viruses can do this), or it can simply spice the undamaged ends of the DNA back together, effectively eliminating whatever the damaged portion of the DNA was. In all cases, the cell will no longer work the way that it did before. That is why the heavier and higher energy particles in galactic cosmic rays are so dangerous.
But, during part of a cell’s cycle, particularly during mitosis when it is replicating, the DNA is more vulnerable to damage. At that time, as the DNA is replicating, damage is difficult to repair. Some of the DNA base pairs are involved in separating and forming new base pairs, and so the enzymes that repair damage are unable to find the correct match for base pairs. The damage thus often is not repaired properly. Cells that are in mitosis a lot are thus more easily damaged by radiation. In a normal human body, these include the hair follicles, the linings of the gut, and bone marrow cells. Thus, radiation exposure can cause someone to lose their hair, get nauseated, and become anemic. The anemia is often the first clinical symptom of extreme radiation exposure. If the exposure is at a low enough level, the nausea may not present, but the anemia will often still occur. Low level chronic exposure often causes the hair to gray rather than completely fall out. Incidentally, cancer cells are often multiplying out of control (which is why they are invasive and eventually get in the way of the body functioning, thus killing the patient), so they are also susceptible to radiation damage. That is how radiation therapy works with cancer patients.
So, the effects of radiation occur at the molecular level. Any sort of realistic strategy for protecting astronauts on long duration space missions requires an understanding of radiation at this level. Unfortunately for astronauts, cosmic rays, particularly the galactic cosmic rays, contain some of the most damaging forms of radiation possible. Thus, very low doses of cosmic rays can act like much larger doses of other forms of radiation. It is quite fortunate that such radiation occurs in such low levels in space that astronauts can be exposed to it for many months before the risk of cancer due to the radiation is significant. But, as I said before, long duration missions, such as an extended stay on the Moon or a mission to Mars, could easily reach or exceed NASA imposed limits on cancer risk. But, even an extended mission to Mars would not expose astronauts to a clinically lethal dosage of radiation. Rather, it may create some health issues and would most certainly create a risk for development of cancer that exceeds the level to which NASA is willing to subject astronauts. Thus, some strategy must be developed to protect astronauts from the danger of space radiation.
Researchers at NASA, in industry, in the military, and in universities are working on the problem of dealing with radiation. The simplest solution is to shield against the radiation. Most of the time, such shielding is accomplished by putting enough absorbing material between the source of the radiation and the person being shielded from the radiation. Sometimes, though, that is not practical. After all, the shielding can be expensive, and it is often very heavy. For aircraft or spacecraft, heavy shielding is not realistic. That means that some other strategy is needed. One possibility is to simply move some of the spacecraft systems so that they shield the occupants. Water turns out to be a good radiation shield. So, putting the water storage and waste water handing systems surrounding the crew compartment provides some shielding. But, there is only so much water carried on board a spacecraft, so water shielding can not be the sole solution. Another idea is to perhaps use some other form of shielding than just a physical shield that absorbs the radiation. One strategy being tossed around is an electromagnetic shield that would deflect the radiation. At present, though, electromagnetic shields exist primarily just on paper. Another strategy, though, is biological. The body does have repair mechanisms for DNA. So, perhaps there is some sort of biological approach that may protect astronauts. This could be in the form of drugs that limit the damage from radiation or that boost the cellular repair mechanisms. Some progress has been made on this front. Perhaps the best solution for an extended space mission would be a combination of different types of shielding and drugs. While current technology and spacecraft construction do not provide much radiation shielding for astronauts, it appears that real progress is being made to come up with strategies for protecting astronauts from radiation exposure.
-Astroprof
Image Credit: NASA
Cosmic Rays
Published on Oct 18, 2009 at 4:01 pm.
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Filed under astronomy, space radiation.
Part 3 of my series on space radiation is about the radiation itself. The term cosmic rays is generally used to describe this radiation. Unfortunately, as often happens, the term evolved before the nature of the phenomenon being studied was known, so it may not be the best term to use. Nonetheless, we are stuck with it. In fact, the term has historically been used to describe more than one phenomenon.
About a century ago, when physicists were first seriously studying radiation and seeking to know the nature of this phenomenon, these researchers noticed that even when all known sources of radiation were removed from the vicinity of a radiation detection device that device still recorded a low level of radioactivity. Eventually, researchers began to realize that radioactive elements and isotopes were all around them. Even the materials used to shield against radiation, such as concrete blocks or lead plates, contained some radioactive isotopes. Some researches then began to wonder if air itself might contain a very small level of radioactivity. To isolate the radiation of just the air and not terrestrial surroundings, researchers put radiation detectors in balloons. As expected, the background radiation level dropped in the balloon flights at low altitudes. However, as the balloons went higher, the radiation level began to increase with altitude. Victor Hess won the 1936 Nobel Prize in physics for his work in radiation studies, including his explanation for this mysterious increase in radiation with altitude: cosmic radiation. In short, Hess realized that the source of some of the background radiation was from above, having its origins in space. At first, the nature of this cosmic radiation was unknown, only that it originated from beyond the Earth. Today, we know a lot more about cosmic radiation.
As it turns out, much of what Hess and others in his day were measuring was not the radiation that astronauts would experience. In fact, they did not even measure what we now think of as cosmic rays. What they measured were secondary cosmic radiation or particle showers created by the space radiation’s impact with the upper atmosphere. This is the radiation that airline passengers and crew are exposed to. I’ve written about that before. What I want to concentrate on this time is the radiation in space itself.
Remember, from my previous posting, the term radiation actually applies to a wide range of phenomena. All of these different uses of the term are at work in space, so there are multiple sources of radiation. Not only are there multiple sources of radiation, but there are also multiple kinds of radiation and multiple different ranges of energy for that radiation. This makes any kind of effective shield against the radiation for astronauts problematic. However, being difficult to shield against does not mean being impossible to shield against. It is an engineering problem, albeit a rather serious one. Given sufficient time and effort, a solution is likely possible. That will be the topic in a subsequent posting. Now, I wish to talk about the different types of radiation that are in space.
First, I’d like to mention that space radiation can fall into two major categories: electromagnetic radiation and particle radiation. The electromagnetic radiation consists of the whole range of the electromagnetic spectrum, from gamma rays to radio waves. The shorter wavelength (and higher frequency) radiation such as gamma rays and X-rays are considered ionizing radiation, and that is what is a worry to astronauts. There are many different sources of this radiation, just as there are on Earth. However, there are some processes that create ultra-high energy gamma rays. Some of these gamma rays are produced in astrophysical processes far from the Solar System. Gamma ray bursts have been recorded by satellites since the early days of spaceflight. Other ultra-high energy gamma rays can result from interactions of some of the ultra-high energy particles with the Earth’s upper atmosphere. These gamma rays are one type of secondary cosmic rays. As a rule, gamma rays of that energy level are seldom (if ever) produced on Earth in any natural terrestrial process. So, when I was growing up, charts of the electromagnetic spectrum often labeled this ultra-high energy electromagnetic radiation “cosmic rays.” That is how I learned it. Years later, when I was in college, electromagnetic spectrum charts no longer had that label. That ultra-high energy radiation is now included in the overall category of “gamma rays,” unless you look at a old chart or one prepared by someone not familiar with the current usage of the terminology.
Most of what we consider “cosmic rays,” however, rather than being electromagnetic radiation, are high energy particles. Many of these particles are charged particles. There are a few neutral particles in the mix, but they are a very tiny percentage of the cosmic rays. There are three basic sources of cosmic rays. Some are particles originating from the Sun. Others originate from beyond the solar system. And some are believed to originate at the boundary between the solar system and the interstellar medium.
The first that I’ll talk about are the solar cosmic rays. There are always particles streaming away from the Sun. This is the solar wind. Most of these particles are protons (hydrogen nuclei) and electrons. These particles are moving at several hundred kilometers per second. The energy is low enough that ordinary materials of the spacecraft would be effective shielding from the particles themselves. Some of these particles, though, would be problematic for astronauts in spacesuits. However, when charged particles rapidly decelerate, as they would do when they hit the spacecraft, they emit radiation called bremsstrahlung radiation (braking radiation). For most of the solar wind particles, this bremsstrahlung radiation would be soft X-rays — fairly easy to shield against. However, the real danger lies in solar storms. During these storms, generated by a massive explosive release of magnetic energy in the Sun due to a solar flare event, particles can be blasted from the Sun at far higher energies. The radiation storms generated by solar flares can be far more dangerous to astronauts outside of Earth’s magnetosphere. In fact, astronauts exposed outside of a spacecraft would not be sufficient protected by any reasonable spacesuit, and they may experience a lethal dose of radiation during a severe solar radiation storm. Even astronauts inside a spacecraft would experience dangerously high levels of radiation. These solar storms are difficult to predict far in advance. Solar scientists are getting better, but we still don’t do much better than a few days of warning that conditions are right for such storms. That is not sufficient to protect astronauts on an extended mission.
The good news, though, for astronauts in low Earth orbit, though, is that the Earth’s magnetosphere deflects the brunt of these solar radiation storms. However, the blast of particles from the Sun does shift the magnetosphere quite a bit. This has the effect of accelerating charged particles trapped in the magnetosphere (such as in the van Allen radiation belts). These now fast moving particles then cause auroral displays when they crash into Earth’s atmosphere and can pose some radiation problems for astronauts in Earth orbit. For a spacecraft that could be a problem. However, long duration missions, such as on a space station, astronauts will have a lot of water on board. This water makes a decent shield against the trapped particles, thus providing some shielding for astronauts on board the space station during a geomagnetic radiation storm. Still, astronauts will unavoidably be exposed to excess levels of radiation during such an event. Nonetheless, inside of Earth’s magnetosphere, astronauts inside a spacecraft should be sufficiently shielded so that they will not suffer any immediate health problems.
The most important thing about solar radiation storms is that they don’t last long. It may be too expensive to shield a spacecraft outside of Earth’s magnetosphere completely from solar radiation, but it may be possible to make small shielded areas where astronauts can retreat to in the event of a radiation storm. They’d just have to wait out the storm in what may be an uncomfortable setting. But, the storm will subside. A greater threat, though, are the higher energy cosmic rays that originate outside of the solar system. These are much tougher to shield against, and they provide a steady background level of radiation.
The cosmic rays that Hess discovered were largely the byproducts of what we now call galactic cosmic rays. These are extremely high energy particles, most moving at near the speed of light. At such high velocities, when they slam into atoms near the top of Earth’s atmosphere, they create a cascade of subatomic particles that rain down towards Earth. Some of these particles make it to the ground, and others slam into other things creating yet more particles to rain downward. A single high energy cosmic ray can thus create an incredible number of secondary particles that create a shower of particles across hundreds or thousands of square meters on the surface of the Earth. Not all of the particles make it all the way to the ground, so the higher one goes into the atmosphere, the more of these secondary particles that you encounter. That is what airline passengers and crew experience: secondary cosmic rays. Cosmic rays of this energy striking the wall of a spacecraft would likewise create a shower of particles that would hit astronauts inside the spacecraft. Even some of the primary cosmic rays would pass through the wall of the spacecraft to hit the astronauts directly, creating a shower of particles inside the astronaut. As you can see, these primary galactic cosmic rays can be incredibly dangerous. About 89 to 90% of the galactic cosmic rays are high energy protons. About 9 to 10% are helium nuclei. About 1% of the particles are heavier nuclei. High energy electrons are also likely generated in the mechanism that produces galactic cosmic rays, but electrons are much lighter than atomic nuclei and are thus easier to deflect and slow down. The nucleus of practically any fairly stable known element in the periodic table has been found in cosmic rays, though nuclei of elements heavier than iron are rare. The heavier the cosmic ray particle, the more damage it typically does when it hits something.
Even very low levels of galactic cosmic radiation can act like much higher levels of lower energy radiation in terms of biological damage. Worse, how do you shield against these things? A fairly dense and heavy (and thus expensive to launch) shield on a spacecraft would slow the particles, but create bremsstralung radiation of very high energy: hard X-rays and gamma rays. Any effective physical shield would be too heavy to launch. To date, the most effect way of dealing with such high energy radiation is to simply limit exposure. If you are exposed to such radiation for short periods of time, then it would be manageable. NASA regulations limit astronaut radiation exposure to a level that is estimated to subject the astronauts to no more than about a 3% risk of cancer induced by the radiation exposure. Since we don’t really know the exact relative biological effect for this radiation, then we don’t really know how long of an exposure that might be. Estimates range from as short as 100 days exposure to as long at 300 days, but most falling in the 200 day exposure range. That is a little over six months exposure to galactic cosmic rays. That is far longer than the Apollo missions to the Moon, so they were not at great risk due to cosmic rays. However, NASA is talking about long duration missions to the Moon to man a permanent or semi-permanent base there. Those missions would last 4 to 6 months. That puts the length of the proposed missions right in the middle of the exposure limit range. A single mission would thus expose astronauts to their entire career maximum radiation limit, excluding any radiation from solar storms. Worse, a mission to Mars would take 7 to 8 months just to get to Mars, and nearly 3 years round trip (Astronauts cannot simply come back from Mars whenever they want. They have to wait until the planets are aligned in such a way that the return trip is possible. That forces them to wail over a year at Mars until Earth and Mars are back in the proper configuration, making any possible round trip almost 3 years in length). So, just the trip to Mars would exceed the 3% cancer risk that is the NASA imposed limit to astronaut radiation exposure. Then, you have the trip back. And, the thin atmosphere of Mars would make the surface of Mars experience a radiation exposure from secondary cosmic rays similar to what high altitude aircraft experience on Earth, which is near the peak level of secondary radiation. Thus, astronauts on a mission to Mars would experience a significant health risk, perhaps in excess of a 10% risk of cancer, far in excess to NASA’s radiation limits. But, this is also a limit for astronauts working anywhere in space, even on the Moon or in Earth orbit.
Incidentally, you might wonder why Earth’s magnetosphere protects us from galactic cosmic rays. Well, it does, a little. When charged particles move in a magnetic field, they are deflected. This tends to make them move in circular or helical paths. The radius of the circular motion is dependent upon the velocity of the particles and their mass. So the high energy (speed) and mass of the galactic cosmic ray particles makes their circular orbits so large that they still run into Earth. Ironically, until sufficient studies had been done by unmanned spacecraft above Earth’s atmosphere, some scientists had speculated that the trend of increase in radiation levels with altitude observed in balloon and sounding rocket experiments might continue. Had this trend in radiation levels indeed continued, then the environment of outer space would have been far too high in radiation levels for manned spaceflight to even be possible. Fortunately, that trend does not hold. Once you get above the level of peak secondary radiation, then the radiation levels begin to drop to background level that is manageable (though still much higher than experienced on the surface of Earth).
The third type of cosmic rays are what we call anomalous cosmic rays. These anomalous cosmic rays are of mid-energy range between the solar particles and the galactic cosmic rays. The origin of the anomalous cosmic rays are a bit of a mystery, but they are believed to come from the boundary region between the solar system and the interstellar medium, a region known as the heliomagnetosheath, of the heliosphere’s magnetosheath. This is where the solar wind particles are slowing down and bunching up and where the interstellar medium particles are being deflected as they pass by the solar system. It is the edge of the Sun’s magnetic influence. It has been suggested that perhaps particles trapped in the magnetosheath get buffeted and accelerated to high enough energies to escape this region and become the anomalous cosmic rays, but we don’t know that for sure. They are mostly protons.
So, that is a brief primer on cosmic rays. Next, I’d like to talk about how radiation does damage to biological systems, but that will be the topic of my next post.
-Astroprof
What is Radiation?
Published on Oct 10, 2009 at 10:15 pm.
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Filed under physics, space radiation.
As the second installment in my space radiation series, I thought that I’d take a moment to ask (and answer!) the question, “What is radiation?” After all, at heart, I am a physics and astronomy professor, so it makes little sense to me to talk about something without first introducing the topic defining the terms.
At first thought, it would seem that everyone should have at least some idea what radiation is. After all, radiation has been something that is listed in science curricula for most of the Twentieth Century. It is a topic that is in everyday life. People eat irradiate food. They go to the dentist and the X-ray machine has a radiation warning trefoil on it. If you open a smoke detector to change the batteries, you’d often see a small metal container with a radiation trefoil warning sticker. Most people cook food using microwave radiation. And, of course there are occasional news reports that involve nuclear radiation topics. For that matter, since the 1940s, radiation has been in the news. So, with all of this, people should know what radiation is, right? I’ve been a college professor for far too long to believe that! I know that a great many of my students (in fact most of my non-science majors) don’t know what radiation is, or they have serious misconceptions about radiation. The demographics of my regular readers are such that they probably have some knowledge about radiation. However, a lot of people are finding their way to Astroprof’s Page using search engines, and they may be here simply because they typed in the question “What is radiation?” just to find the answer to the question. So, it is useful to do my professorial thing and teach about radiation.
If you try to read about radiation on the internet, you find a bewildering assortment of things. One of the first difficulties is that the term appears to be used in many different ways to refer to different physical phenomena. Secondly, and perhaps even more difficult for the layman trying to understand radiation is that there is a bewildering assortment of ways of measuring and reporting radiation. You wind up with units such as Grays, Sieverts, Rads, Roentgens, REMs, Curies, Becquerels, Kermas, Counts, Watts, Electron Volts, and a host of others. I’ve got some friends who are pilots and flight attendants. They are often exposed to quite a bit of radiation on the job, but the few who have tried to figure out how much exposure they get quickly become confused trying to sort through all of the literature. The problem is that there are many different ways of measuring radiation, and each of the different ways of measurement measures a different thing. There are often ways to go from one measure to the other, but sometimes it is not easy to do that. So, it is no wonder that there is so much confusion about radiation.
First of all, we need to cover just what we mean by the term radiation. Whenever radiation is used in the media, it normally comes with a negative connotation. But, in physics the term is completely neutral. The term radiation ultimately comes from the term ray. Remember, physics is a very mathematical science. 
In mathematics, a ray can be thought of as half of a line. While a line extents infinitely far in both directions, a ray starts at one point and goes off in an infinite direction in the other direction. Light appears to move in rays. Light travels away from a light emitting body in straight lines. (Note: technically, this is not true. The path of light can be altered by passing from one medium to another via a process known as refraction, and general relativity shows that the path of light can be bent by gravitational effects. However, for most purposes, light appears to travel in straight lines.) When something moves away from a source in rays, it is said to be radiating. The following image (courtesy Grant Goodge and Wikimedia Commons) shows crepuscular rays radiating from the Sun.
To a physicist, this process of radiating is known as radiation. Unfortunately, not only is the act of radiation called radiation, but so is the thing being radiation. So, light, in this instance, is radiation. In fact, visible light is only one instance of radiation. Ultraviolet light, infrared light, and even radio waves are all different forms of electromagnetic radiation — waves of electric and magnetic fields. Microwaves are also examples of electromagnetic radiation.
In the Nineteenth Century, another form of radiation was discovered. Certain vacuum tubes were found to be emitting radiation of some sort from the cathode, or negative electrode, of the tube. Eventually, it was shown that these cathode rays were negatively charged particles streaming away from the cathode in straight lines. In essence, they were radiating from the cathode, and thus were a form of radiation. Physicist began to think of radiation as both being from electromagnetic waves and from particles streaming away from a source. In 1895, Wilhelm Roentgen was experimenting with Crooke’s tubes, the devices in which cathode rays had been discovered, and found that some mysterious radiation was being emitted by the anode (the positive electrode) of the tube. This mysterious radiation was designated X-radiation (or X-rays, for short). This X-radiation was far more penetrating than the other forms of radiation known. But, it was absorbed by some materials. So, it would pass through flesh easier than through bone. This led to the possibility of using X-rays as non-invasive diagnostic techniques for medicine and industry.
Soon, it was found that certain elements tended to emit forms of radiation. This radiation comes from the nuclei of the atoms, so it is called nuclear radiation. The nuclear radiation was found to be either positive, negative, or neutral in electrical charge. The positive charged radiation was called alpha radiation. The negative charged radiation was labeled beta radiation. And, the neutral was called gamma radiation. Alpha, beta, and gamma are the first three letters in the Greek alphabet. Later studies showed that alpha radiation was the result of the emission of what we now regard to be essentially helium nuclei moving at very high speeds. Beta radiation is the emission of very high energy electrons. As it turns out, cathode rays are also high energy electrons. The biggest difference between beta rays and cathode rays is simply the source. The neutral gamma rays turn out to be more electromagnetic radiation. X-rays, too, turn out to be electromagnetic radiation. But, so is visual light. So, what’s the difference? The only difference between the different forms of electromagnetic radiation turns out to be the frequency at which they oscillate (and hence the energy that they carry). All forms of electromagnetic radiation are oscillating electric and magnetic fields, and all travel at the speed of light.
So, as you can see, there is more than one type of radiation. Over time, even more forms of radiation have been found. Some of that radiation is in the form of oscillating fields (electromagnetic radiation, gravitational radiation, etc), and some of the radiation is in the form of highly energetic particles (alpha radiation, beta radiation, neutron radiation, proton radiation, etc). There are quite different physical properties involved in the generation of all of the different forms of radiation, so why do we use the term radiation to describe all of it? Well, in physics, we use the term radiation to describe any process in which energy radiates away from a source in rays. For electromagnetic radiation, the waves travel in straight lines. For extremely energetic particles, the particles travel in straight lines. Naturally, the paths of the particles can be altered by fields (electric or magnetic fields for particles with charge, gravitational fields for particles with mass, etc), but they move in straight lines away from their source. Their deflection occurs later. The particles are moving, so they have kinetic energy. The electromagnetic fields can affect charged bodies so they, too, carry energy. Conservation of energy, a basic premise in physics, tells us that if energy is being carried by the radiation, then it must have come from the source, which thus must lose energy in this process. Electromagnetic radiation can be absorbed when it runs into something. The particles can collide with something and slow down. Energy is thus transferred. In a macroscopic view, all of this looks superficially similar, so it is useful to call it all the same thing: radiation. It doesn’t matter exactly what the source of the radiation is, nor the physical process in which the energy is transferred (particles moving carrying kinetic energy or electromagnetic waves carrying energy). What matters is that energy is transferred in rays, straight lines (excluding external influences), as in this diagram.
When the rays of the radiation stop, that is when the energy carried by the radiation is absorbed by something, that energy can change the object being absorbed. The degree of change depends upon how much energy is absorbed. Visual light only carries enough energy to adjust energy levels of electrons in atoms. That can trigger chemical reactions in some cases. Of course, that is ultimately how you see! Radio waves can cause electrons to run up and down electrical conductors, and if you have the appropriate electronics connected, you can detect them. X-rays, gamma rays, and the high energy particles can carry plenty of energy that can knock electrons out of atoms. This leaves the atom ionized. The ionized atoms then behave differently than they did before being ionized. Thus, ionization can cause damage to complex molecules (such as DNA) or to physical structures (such as crystal structures). Certain gemstones can be colored by exposure to radiation of sufficient energy, and some integrated circuit chips can be damaged. But, from a human point of view, the damage to molecules such as DNA is important. When DNA is damaged, then cellular damage can occur. Such damage can cause either cell death or mutation. That can lead to radiation sickness or death. Mutations can also lead to cancer at a later time. I’ll talk more about the radiobiological issues later in this series on space radiation.
Radiation can be in the form of electromagnetic waves, particles, or any other means that carries energy away from a source in an essentially straight line. It can be subatomic particles such as electrons, protons, or neutrons. Radiation can also be in the form of fast moving atomic nuclei, such as alpha particles. Many of the forms of cosmic rays are high energy atomic nuclei. Many of these nuclei are quite heavy nucleons, such as iron nuclei. Because these nuclei are so heavy, they carry much more energy than lighter particles moving the same speed. However, cosmic rays move very fast indeed, many of them moving at very close to the speed of light. This is much faster than many more familiar forms of radiation, such as alpha or beta radiation. The energy is very sensitive to the speed of the particles. The kinetic energy goes as the square of the speed. So, going twice as fast has four times the energy. Going 10 times faster gives 100 times the energy. And, going 1000 times faster gives 1,000,000 times the energy. So, you can see how the cosmic rays, moving near the speed of light, can be very energetic indeed!
Now, I want to finish up this particular posting on the measure of radiation. How would you go about measuring radiation? Since radiation carries energy, one unit of measure would be the amount of energy carried. The SI unit for energy is the Joule. Often the rate of energy transmittal is more important than just the amount of energy. In that case, the appropriate unit is the Watt. One watt equals one joule per second. Typically, the energy is distributed over an area, so a better measure might be the intensity, which is the energy per second per area. The units of intensity would be watts per square meter. These units are fine, and they are typically quite sufficient for most forms of radiation, such as visible light or microwaves. However, they fail to convey the damage potential of ionizing radiation, so there are different units typically used to measure ionizing radiation.
But, what counts as ionizing radiation? Clearly, from what I’ve said earlier, alpha rays, beta rays, gamma rays, and X-rays can be considered ionizing radiation. But, ultraviolet light can also ionize some atoms. Since UV is not as strongly ionizing as X-rays, UV is not typically considered ionizing radiation by most people (though there is little difference between high energy UV and low energy X-rays). But, you may wonder if intensity makes any difference. After all, if low energy UV does not ionize well, could you just crank up the intensity to ionize something? The short answer is “No!” As it turns out, light also behaves like it travels as particles, called photons. The shorter the wavelength of light, the higher energy each light photon has. So, gamma rays have inherently more energy than X-rays, and X-rays are inherently more energetic than ultraviolet light. In fact, some gamma rays are so energetic that they can cause electrons to be ejected from atoms with enough energy to ionize other atoms. The energy of individual photons can be measured in terms of a tiny bit of energy called an electron volt (eV). An electron volt is 1.602×10-19 Joules. X-rays are often in the range of thousands of electron volts (keV) and gamma rays are often in the range of millions of electron volts (MeV). This tells how ionizing the radiation is, but not how much ionization the radiation produces.
For visual light, the power of light can be determined by multiplying the intensity by the area. However, ionizing radiation tends to be somewhat penetrating (particularly X-rays, gamma rays, and several other forms). Alpha radiation is one exception. It has very little penetrating capability (which makes it easy to shield against). Low energy beta radiation is not very penetrating, but high energy beta radiation is penetrating. Sometimes penetrating radiation goes completely through a target without interacting with it. Other forms of radiation of similar power and energy may be less penetrating, and thus completely absorbed. Likewise, some radiation may cause more ionization than the same intensity of other radiation. So, the best measure may be some other measure than intensity.
One unit of measure used is Radiation Exposure. The basic unit of radiation exposure is the Roentgen. One Roentgen (R) is the amount of radiation that produces 0.000258 Coulombs of ionization in one kilogram of dry air. Another, smaller exposure measure is the Air Kerma, which is equal to 0.01 R. But, you can sometimes get the same ionization in air from different energies of radiation absorbed. So, it is often better to measure the amount of energy absorbed. That is called the Absorbed Dose. The unit of absorbed dose that I learned and used years ago when I did work in the area was the Rad (standing for radiation absorbed dose). However, the rad is now a deprecated unit. The preferred SI unit for absorbed dose is the Gray (Gy). One Gray is one Joule of energy absorbed per kilogram of material. A Gray is 100 rad.
There are many forms of ionizing radiation. In fact, radiation is one area of physics where the terminology seems a bit sloppy to me. We use the same term for electromagnetic radiation such as gamma rays and for particulate radiation like beta radiation. The thing is that some radiation can do more damage than an equivalent amount of radiation of some other form. For example, alpha particles (the particles of alpha radiation) are very heavy compared with things like electrons. As alpha particles pass through a material, they knock electrons off of atoms ionizing them. But, the alpha particles are so heavy that they only somewhat slow down and they keep moving, ionizing other atoms. X-rays, however, behave differently. A single X-ray or gamma ray photon is absorbed to ionize an atom. It is completely absorbed, so it can’t keep going to ionize other atoms. Thus, a certain amount of energy absorbed of alpha radiation might do more damage than the same amount of energy of X-rays. So, an another measure of radiation is the Effective Dose or sometimes called the Dose Equivalent. To find the effective dose, you multiply the absorbed dose by a term that is related to how many atoms a single particle of radiation can affect. This is sometimes called the quality factor of the radiation. The quality factor is also sometimes known as the Relative Biological Effect (RBE). Numerically, the RBE is the ratio of the dose of 200 keV X-rays needed to produce a certain biological effect to the absorbed dose of some other radiation to produce the same effect. So, alpha radiation has a far bigger RBE than beta radiation. The effect of radiation on biological systems also depends upon the particular body part exposed. Some parts of the body are more susceptible than others to radiation damage. A weighting factor taking into account the body part irradiated also needs to be considered when determining the effect of radiation. By multiplying the RBE and the weighting factor, the actual effect of the radiation can be determined. Then, multiplying the absorbed dose by the RBE yields the effective dose. The preferred unit for effective dose is the Sievert. One Sievert is one Gray times the RBE for the radiation. An older (now deprecated) unit is the REM (standing for Roentgen Equivalent Man). There are 100 rem in one Sievert. Most permitted radiation dose limits are given in Sieverts. The confusion, though, is that the Sievert is a measure of effective radiation absorbed dose, not actual radiation absorbed dose, which is what most detectors measure. And, the radiation that you absorb is not equal to the radiation to which you are exposed. So, it is no wonder that many people in occupations where they are exposed to radiation really don’t even understand the units or how much radiation they receive!
This has been a bit more technical post than I sometimes make. However, I feel that it is relevant to the discussion of space radiation to actually define the terms of radiation. And, after all, I am a college professor, and many of my regular readers expect a certain level at Astroprof’s Page. That is part of what makes this site a bit different from many others on the internet.
-Astroprof
Image Credit: Wikimedia Commons
Space Radiation (Part I)
Published on Oct 6, 2009 at 10:36 am.
1 Comment.
Filed under cosmic rays, space exploration, space radiation.
This past weekend, I attended a symposium on space radiation. This is an interesting topic, and I’ve written about it before. But, space radiation is not confined to space. Cosmic rays raining down on Earth also create secondary radiation that impacts air travelers. I have received a number of comments on several of my posts over time regarding radiation in space, so I thought that it was time to revisit the topic. Further, I thought that it might be interesting to stretch this into a series of several posts. So, this is the first of that series.
First of all, there are many different types of radiation in space. A description of that will be in my next posting, along with a description of radiation in general. This radiation can cause physical and biological damage. Engineers can design systems resilient to physical damage from radiation in space. But, the problem is in the biological component of manned spaceflight: humans. The radiation induces damage at the sub-cellular level, in the very chromosomes of the astronauts. The cells then seek to repair the damage. The design of the cells, though is amazing. In many cases, if the damage is not too severe, the damage can, indeed, be repaired. In that case, life goes on, unaffected by the radiation. However, sometimes the damage is too severe, or it comes at an inopportune time in the life cycle of the cell, and it can not be repaired. If the cells are unable to repair the damage, the cell normally dies. Sometimes a partial, or a mistaken repair happens. Usually that, too, is fatal for the cell. Once in a while, though, the cell continues to live, but in a mutant form that no longer behaves like it is supposed to behave in the body. If the cell continues to grow and divide, then a tumor develops. This can lead to the disease that we know as cancer. The human body, though, is amazing in how its own immune system often recognizes cancers and attacks them. In the vast majority of cases, this system works. Once in a while, though, something gets past the immune system, and the patient develops cancer. That cancer often progresses until the patient dies. This is the danger and the worry of space radiation for both astronauts and passengers and crew of high flying aircraft.
By policy, astronauts are limited to no more radiation exposure than that which would be expected to yield a 3% chance of cancer mortality. Now, a 3% chance of death by cancer might seem unacceptably high for the general population, but there are plenty of people who would risk this for a chance to fly in space. In fact, there are many people who would risk a far higher chance of cancer to be an astronaut. The radiation level in space varies with the type of mission, but it always remains fairly tolerable for short duration missions. The radiation absorbed dose is computed by multiplying the radiation level by the time of exposure. Not all radiation has the same effect, so you next have to multiply by a relative biological effect for the particular type of radiation (which can even be different for different body parts). I’ll discuss this in more detail in a later installment of this series.
There are several sources of radiation in space. These different sources result in different radiation levels, and the depend upon a number of factors. The particular radiation exposure is a combination of the mission type (where a spacecraft is going), the duration of the mission, and external factors such as solar activity. That makes predicting radiation exposure complicated, particularly since we lack some data on radiation from certain sources. Again, I intend to discuss this more in a future installment of this series.
We have a lot of data on radiation exposure on Earth, but not so much on radiation exposure in space. We know that the very high energy galactic cosmic rays do different things than the lower energy radiation typical on Earth. So, estimating the risk to astronauts is difficult. There are several different ways of doing this sort of estimation, and they yield different results. Some estimates are that the 3% mortality tolerance may be reached in as few as 100 days outside of Earth’s protective magnetosphere. Other estimates put the danger level at over 200 days. The range comes from the fact that different factors are taken into account (sex of the subject, type of mission, etc). This is quite a wide range of estimates. To be on the safe side, you’d want to keep to the lower end of the estimate. Unfortunately, that level is reached in as few as three months in some cases. Even for the longer estimates, a six month tour of duty on a lunar outpost would reach or exceed safety limits. Even the Earth’s magnetosphere, while protecting astronauts, does not give complete protection. Three tours of duty on the International Space Station might reach the lifetime maximum radiation exposure for astronauts. All of these estimates are for galactic cosmic rays. The Sun also sometimes spits out particles in radiation storms. That complicates the matter.
So, we’ve got space radiation. What do we do about it? On Earth, there are three ways of dealing with radiation. First, you can leave the area of the radiation. That is actually the best solution. But, sometimes you can’t do that. For example, if you work with radioactive materials (such as with a nuclear reactor), then you can’t really get away from the radiation. Likewise, unless you decide simply not to fly in space or in an airplane, then you can not get away from the space radiation. So, in comes the second strategy to deal with radiation: shield against it. With many stationary radioactive sources, this is possible. For example, workers can operate a reactor from behind the safety of a heavy shield. But sometimes, that strategy fails. Once in a while, you simply have to go into the radioactive area to do something. The shielding is too heavy to wear, so workers will simply get exposed to radiation. But, shielding being heavy is another problem for those in vehicles. Radioactive materials can be transported on the ground inside heavy shielded containers by large trucks, but it is much more difficulty to shield an aircraft or spacecraft. Traditional shielding is simply not possible. The craft would be too heavy to get off of the ground. The final strategy for dealing with radiation is to use biological means of protection. There are some drugs that provide some protection to limited radiation exposure. There are different strategies for how these drugs operate. Unfortunately, most of them are pretty toxic, and the side effects are nearly as unpleasant as the radiation exposure itself. Work is progressing, though, on using drugs to combat radiation exposure. I’ll talk about radiation countermeasures in an upcoming post in this series.
I still have plenty of other duties this semester, and some days are pretty booked. So, I won’t be able to post every day, but I’ll try to get several posts per week to complete this series before the month is out.
-Astroprof
Astroprof’s Page
Published on Oct 5, 2009 at 11:36 pm.
1 Comment.
Filed under blogging.
Hey, I just found out that Astroprof’s Page has been listed again as one of the top science blogs, this time by Online Universities Weblog. I have occasionally made it into a top 100 list before. Lately, I’ve been so busy with other duties that blogging has been put on the back burner. I guess that I need to pay more attention and get back to posting things regularly. Trying to post quality things, though, is tough. It takes time. And, I’ve got other duties, too (going to meetings, publishing, research, filling out paperwork, teaching, etc). I have written most of the labs that we use in our physics and astronomy classes here at the college. I have about 40 other publications. And, I am also planning on writing a book. I wonder if anyone would buy it, though. So, I don’t need to blog as an outlet to writing. Ironically, when I was a student, I thought that my job would be mostly in the lab and classroom. Now, I find that a large part of what I do is writing! Still, the blog is fun. And, I make enough on the advertising to pay the hosting fees for my site (it’s a good thing that I don’t do this for a living, though!). But, with being listed again, I guess that I need to buckle down and finish some posts that I’ve started and never actually finished. I also use the blog as a teaching tool. It gives me a chance to talk about things that are a bit outside of the scope of my classes.
-Astroprof
CoS 120
Published on Sep 13, 2009 at 5:49 pm.
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Filed under blogging.
The 120th edition of the Carnival of Space is now up. This week it is being hosted by Flying Singer at the Music of the Spheres. So, go on over and take a look.
-Astroprof
