h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

There's another thread on nuclear power and the environment in the Forum at the moment. One thing that comes up in those discussions is the relative danger or not of radiation.

Would it be possible to do a kind of tutorial here that starts with the basics of what radiation is and what different forms of radiation are so that non-technical people have some knowledge to understand the nuclear debate? We could then move onto a discussion of what the relative dangers are.

thanks,
kea.

Coo, big subject, but I'll give it a go.

At its most basic level, radiation is anything that's radiated. Could be light, sound, heat. But in this context is usually taken to mean one of two things, which are often confused:

1. Electromagnetic radiation - coupled alternating electric and magnetic fields travelling at the speed of light. The rate they alternate at is called the frequency, and the sum of all frequencies is called the electromagnetic spectrum. We give different parts of the spectrum different names arbitralily, according to how we use them, for example (from low to high frequency): radio waves, microwaves, terahertz waves, infrared, visible light, ultra-violet, X-rays, gamma rays.

A single packet of EM energy is called a photon. The higher the frequency, the more energy carried by the photon. Very high energy photons can knock electrons out of the atoms in your body, ionising them. This is then called ionising radiation and can cause DNA damage leading to cell death or cancer. Sunburn is a result of ionising radiation. The cut off point is somewhere in the UV range or above, depending on the atom.

EM radiation below this range is generally pretty harmless (this'll be controversial , providing it is at low intensities. Crank up the power high enough and it'll burn you though (microwave ovens, lasers).

2. Radioactivity (see A661727) - the stuff emitted by a large, unstable atom such as uranium is also called radiation. This consists of alpha (a helium nucleus) and beta (an electron) particles, and just to confuse matters, usually some EM radiaton too such as gamma rays. This is also ionising.

The harm it causes depends on levels again. We have evolved to repair a certain amount of DNA damage (e.g. from sunlight, cosmic rays and radioactive materials in the ground) and there is evidence to show that low levels are beneficial - a bit like priming the immune system.

What often causes confusion is the fact that radiation is transient, whereas radioactivity can persist as long as unstable atoms exist. If you ingest radioactive dust, it'll sit inside your body radiating away for ages, making it particuarly dangerous. At Chernobyl for example, a lot of the workers died quickly from direct exposure (radiation sickness is a simple overload of toxins as a result of may cells dying at one), whereas most of the civilian casualties were victims of radioactive iodine, ingested in milk (via grass, cows etc) lodging in their thyroids and causing cancer.

Hope that'll do to kick off the debate, there is loads that can be added....

Thanks Dave, nice start.

Can you give a brief definition of an electric field and a magnetic field?

Is Uranium inherently unstable i.e. does it emit radioactivity wherever it is before humans get anywhere near it?

How can a nucleus be emitted? Doesn't that destroy the atom?

In general, is ionisation when an electron is removed or added to an atom so that it's no longer balanced?

>>Can you give a brief definition of an electric field and a magnetic field?<<

Uff, that's a really hard one, I'll leave that to someone else I think mostly. Simply between two separated charges is an electric field. One end is positive and the other end is negative. Positive things are attracted towards the negative source and vice versa. The more charges at either source point (electrode for example) the more powerful the field. Magnetic fields are much the same only magnetic north and south are the 'electrode' equivalents and only other magnetic things will be attracted to either pole.

>Is Uranium inherently unstable i.e. does it emit radioactivity wherever it is before humans get anywhere near it?<

Yes It decays at an exponential rate with a half life of 3.5 billion years (ish) so it's a very long lived and fairly inactive radiation source. However it's an alpha emitter and so is fantastically dangerous if ingested (although mostly it's poisonous I think). Alpha particles don't penetrate far in air (less than 1cm generally) but are very destructive to tissue if in direct contact.

>How can a nucleus be emitted? Doesn't that destroy the atom?<

Yes. You get two new atoms - the process is generally called fission if two relatively big nuclei are formed (e.g. in a nuclear reactor this occurs). But all ionising radioactive decay of unstable nuclei produces new atoms (that are more stable than the original) - even if the decay is gamma decay. Gamma emission is often associated with a neutron becoming a proton (or vice versa) and so changing the element up or down one atom number (e.g Carbon has six protons -change it to seven and it is now a nitrogen atom).

>In general, is ionisation when an electron is removed or added to an atom so that it's no longer balanced?<

Yes, it forms ions which are charged atoms or molecules. A positive ion is called a cation (attracted to the cathode) and a negatively charged one is called an anion (attracted to the anode in an electric field).

>> Can you give a brief definition of an electric field and a magnetic field? <<

>Uff, that's a really hard one, I'll leave that to someone else I think mostly. <

I was just preparing my reply, which was basically "I'll leave that one to a physicist". An engineer just needs to know that if you vary them both at the same frequency they kind of join up at a distance of about a wavelength from the emitter and shoot off as two perpendicular wavy lines. As to what they actually *are*...

"How can a nucleus be emitted? Doesn't that destroy the atom?"

I think this is in reference to an alpha particle being a helium nucleus. It was meant in the sense that the two are made up the same, ie two neutrons and two protons; it didn't mean that an alpha particle is actually part of a helium atom.

Well to be fair, an alpha particle *is* part of a helium atom.

It's a helium atom that is bereft of its electrons.

But it never was and never will be in the middle of a helium atom.

One thing that hasn't been mentioned so far is isotopes. Whilst all the atoms of an element have the same number of protons they may differ in neutrons. Thus carbon-12 has six of each, but carbon-13 has seven neutrons and carbon-14 eight. This can affect the stability of an atom - C-12 and C-13 are stable, but C-14 experiences beta decay to become nitrogen (N-14 to be precise, which is the most common form and stable). On the other hand uranium has no stable isotopes.

Perhaps to save any confusion maybe its best to examine a little bit how atoms are made up.

They consist of a nucleus which is positively charged surrounded by electrons which are negatively charged.

The positive part of the nucleus is made from particles called protons. The number of protons must equal the number of electrons to ensure neutrality although chemically most elements gain or lose electrons when bonding with other atoms - but that is another story.

Now as we should all know I hope, positively charged things repel one another very strongly so any atom with more than one proton (i.e. anything heavier than hydrogen) has a problem.
The positively charged protons are glued together in the nucleus in a complex way involving weird stuff called the nuclear strong and weak forces (?) and this involves the creation of uncharged particles called neutrons.

The balance of neutrons and protons is important for stability, a stable nucleus will have approximately equal numbers of neutrons and protons. To horribly simplify the mutual repulsion of the electrons is balanced best in these cases by the nuclear binding forces.

It should be noted here that most elements can have what are called different isotopes. An element (atom) is defined by the number of protons it has. This defines the number of electrons in orbit and hence the chemistry it does since chemistry is all to do with the behaviour of the outermost electrons.
Isotopes are atoms of the same sort that contain different numbers of neutrons and so are of different masses. Carbon 12 is the most stable isotope of carbon for example and contains six protons and six neutrons in its nucleus. Carbon 14 contains 8 neutrons and in this case the atom is not stable as the nuclear binding forces are outweighed by the repulsion of the protons and so the nucleus decays after a while.

For carbon 14 we get beta emission from the nucleus which therefore emits an electron. This creates a new proton in the nucleus which is now stable and now has 7 protons (and 7 neutrons) making a Nitrogen 14 atom.

Most of the common elements you or I are familiar with possess an abundant stable isotope but as the elements get bigger and heavier in their nuclei their stability gradually reduces (well past the iron nucleus anyhow) and past Bismuth (which has atomic number [=number of protons] 83) they are *all* unstable fundamentally, albeit often with a very long half life.

So the three ways they decay as Daveblackeye explained earlier are:

Alpha decay = loss of two protons and two neutrons (a helium nucleus) to give a new atom that has - you guessed it - two less protons and two less neutrons and thus greater stability. This doesn't necessarily lead directly to a stable nucleus, there may be several steps of decay until a stable nucleus is made.

Beta decay - loss of an electron (or anti-electron [positron]) leading to a nucleus with one greater proton and one fewer neutron.

Electromagnetic emission - this is usually accompanied by one or other of the above. The new nucleus formed from other types of decay is not necessarily in its lowest energy state so the nuclear binding forces readjust themselves to a lower energy state and a high energy photon is released - usually a gamma or X-ray.

Four: Yes there's others... - neutron emission....


Hope that largely helps on radiactive decay of atoms. I think it is important that to understand radioactive decay one understands the make up of the atom as one cannot understand one without the other.


Incidentally the fission of Uranium nuclei in a nuclear reactor is enhanced by firing neutrons at them. U-235 nuclei absorb neutrons very well but then fall into two heavy bits - Xenon atoms for example (roughly half the mass of Uranium) and release even more neutrons.

This process happens normally with uranium but the concentration of U-235 is so small that it fizzles out. If you purify U-235 enough and shape it into spheres so that neutron emission and reabsorption is maximised then it becomes a self-sustaining chain reaction and is accompanied by massive heat output. This can be used to heat water, turn a turbine...

Or.... purify U-235 even more and shape it even better then the chain reaction becomes explosive and things can get toasty as they did over Christmas island, Nagasaki, Hiroshima etc.

>But it never was and never will be in the middle of a helium atom.<


Incorrect sadly. _All_ the helium you find on earth was formed by alpha decay.

Helium is too light a gas to remain trapped in the atmosphere - earth has insufficient gravity. The natural helium of the solar system is found in the Sun (and possibly Jupiter, not sure).

The helium you buy by the cylinder to blow up party balloons and make your voice squeaky is only around because heavy elements in the crust continually emit it and it sometimes get trapped in deep mines and in places where oil and natural gas deposits too.

Christmas island may have been subject to H-bombs of course which are another story.

Thanks, Orcus - but you did ask for this: Tell us about the H-bomb? Pretty please?

The H-bomb relies on fusion of hydrogen isotopes; little atoms into bigger ones instead of splitting large atoms into little ones. It's what the sun does, and is much more powerful than what you get from fission reactions

Yes, I mentioned that iron (57) is the most stable isotope of all. Lighter elements are less stable than this and so if you can manage to fuse these nuclei together to form something heavier (but less stable than Fe-57...) then there is a release of energy.

For hydrogen atoms fusing to form helium this energy release is the biggest of all forms of nuclear reaction and so H + H --> He is the most powerful of all.

An H-bomb uses a precisely fired fission bomb to compress Hydrogen isotopes enough to fuse them and --- KABOOM (to use the techinical phrase).

The Sun has enough mass to force this fusion reaction to happen naturally at its core. So although nasty in the H-bomb, it is also what keeps all life going.

Here's a sort of related question: if everything heavier than iron is unstable, why does the periodic table on my daughter's wall (I know, I insisted) not list all of these as radioactive? Is there an accepted limit for something being considered radioactive, like a minimum half-life?

Interesting, interesting...

When Dave's Q has been attended to:

So far as my knowledge goes, we (we? well, some people) are trying for the He>, which "energy release is the biggest of all forms of nuclear reaction".

Biggest by what factor(s)?
Why?

Intuitively, "the biggest of all" sounds as if it *should* also be the most difficult, but - ?

I didn't say everything after iron was unstable, I said all nuclei after Fe-57 were less stable than it. It is some time before they actually become fully unstable and therefore radioactive.

I was taught that everything after Bismuth is radioactive. But someone told me on here once that bismuth is technically only metastable and is in fact unstable. However its half life is approximately 10-20 billion years or so, meaning hardly any of it created will have decayed yet.


The really interesting question is: If Fe-57 is the most stable nucleus , how is anything heavier than it made at all?

Or a side question sort of related (perhaps for another thread), why is Iron (Fe-57) the most stable?

>>The really interesting question is: If Fe-57 is the most stable nucleus , how is anything heavier than it made at all?

They're formed as byproducts of the explosions of supernovas. In the massive blast, atoms get pushed together that should not really be together. Gold and all the heavier stuff are formed in the death-throes of stars.

Ah, had to do a bit of work so your question beats my explanation of that. It also ties in a bit I think with maybe my use of the words stable and unstable may have been a bit confusing judging by the jist of the current questions ()

Although as to exactly *why* it is the most stable is one for the most deeply theoretical I feel.

I mentioned earlier that a nucleus is made up of protons and neutrons and they are bound together by nuclear binding forces.

If one knows the mass of a proton and a neutron (and they have been measured quite precisely) and then one measures the mass of a nucleus one finds that the mass of the nucleus is actually *less* than the sum of its protons and neutrons.

This is called the mass defect of the nucleus and is the sum of its nuclear binding energy. The larger the mass defect the larger the binding energy of the nucleus. The energy of binding can be determined from the old equation

E = mc^2

where E is energy, m is the mass (in this case the mass defect - the 'missing mass' of the nucleus) and c is the speed of light.

Of course the speed of light is a big number, so the speed of light squared is a REALLY BIG NUMBER. So the binding energy, even of a small mass defect is HUGE.

So when you subject a massive nucleus to fission or you fuse two small nuclei to make a big one then the binding energy released is the difference between the total mass defects of the starting nuclei and the final nuclei.

Here's a wee graph of the mass defects of each nucleus as you progress in atomic number (remember, number of protons in the nucleus)...
http://en.wikipedia.org/wiki/Image:Binding_energy_curve_-_common_isotopes.svg

And here's a wee tutorial on the questions being asked here
http://www.eas.asu.edu/~holbert/eee460/massdefect.html


So from that I hope you can see that I was wrong in saying Fe-57 was the most stable nucleus - actually it's Fe-56 (slap my hand ). And when I say most stable, I mean it has the highest binding energy per nucleon of any nucleus.

Fusion of hydrogen nuclei to give helium gives the biggest release of binding energy per nucleon possible that any other nuclear reaction if you examine the left hand side of the linked to graphs and this is why the H-bomb has the highest energy yield of any nuclear bomb.

And to answer my own question. Fusion at the heart of stars creates all the elements from Helium to Iron.
A normal star will spend the vast majority of its time fusing H to He. But as it gets older it starts to run out of Hydrogen and so starts fusing heavier elements together. This yields less energy and so the star's spectrum shifts to the lower energy band of the electormagnetic spectrum - red giant stars are such stars.
But you cannot release energy by fusing anything higher than Fe so this is far as it is possible to get in star-born fusion.

The heavier elements can only be formed by rapid absorption of vast quantities of neutrons at extremely high energy and only one force in the universe has such power. Supernova.

So all elements heavier than Fe were born in Supernovae in the deep past.
I think that's a really thought.