Recently some people have disputed the existence of the NEUTRON
, which if this had any sound basis would cast doubt on some of my recent activities. Maybe this is because the neutron has not had much of an impact in popular culture. The only item that I remember is a song Yes to the Neutron Bomb
(1981) by the Liverpool Group “Moderates”.
I first went a-neutron scattering
about three years ago. I arrived at the Rutherford Appleton Laboratory
near Oxford, and went to the ISIS
facility. After getting my badge and doing the safety test, I walked through the experimental hall (picture
) and found myself entering a room entitled “LOQ CABIN
”, although it did not look at all as if Abraham Lincoln had been born there. But what is the point of scattering neutrons? A common natural scattering experiment that is easily visible is the Lunar Corona (picture
), where droplets of water in a thin cloud are scattering light. One can also scatter X-rays
, in order to look at objects and structures almost as small as atoms. But in regard to extracting information from those structures, neutrons can go places that electromagnetic radiation can’t.
**Hail to thee, blithe neutron!**!
Indeed “you can do such a lot with a neutron”. (Aficionados will know that I am claiming the neutron to be almost as versatile as the legendary WOMPOM
of Flanders and Swann.) It was discovered in 1932 by James Chadwick
, but 12 years earlier Ernest Rutherford
discussed the possibility of an atom with zero charge. Said he
Such an atom would have very novel properties. Its external field would be practically zero, except very close to the nucleus, and in consequence it would be able to move freely through matter. Its presence would probably be difficult to detect with the spectroscope, and it may be impossible to contain it in a sealed vessel. On the other hand, it should enter readily into the structure of atoms.
In 1930 the first hint of their existence came when their atom-smashing effects were observed, but it was two years before they themselves were actually detected. Rutherford’s predictions proved accurate, but even more was to come. A neutron is itself a tiny magnet, its field being produced, we now understand, by those three tiny quarks inside it going round and round. One of my colleagues has taken advantage of this to study speromagnetism
(what’s that?) in glasses at ISIS. Over two decades other colleagues were studying oxygen in semiconductor-grade silicon, which has to be got really just right – so move over Goldilocks! But the speciality of neutrons is that through the residual strong force
they interact directly with nuclei. The biggest bonus here is perhaps the very different interaction of the neutron with different isotopes of the same element, especially on the one hand the hydrogen 1
H and on the other deuterium 2
H or D nuclei. This enables one to look even at one of the most “difficult” molecules, that of water, by mixing D2
O or “heavy water” with ordinary water. As an example of a recent study, a group from Rome have been to ISIS and observed how water interacts with trehalose
, the “sugar” that enables many organisms to survive extreme desiccation.
** Einstein woz right! **
On July 1st 1933, Science News Letter
announced that Weighing New-Found atoms Proves Einstein was Right
. It concluded “… it is just slightly lighter than the light hydrogen atom. Its weight is 1.0065 while hydrogen’s weight is 1.00778”. This was attributed to the mass loss when the hypothetical proton and electron were squashed together, releasing binding energy
of course) which was lost to the system. Since it carries no charge, one could not “weigh” a neutron directly in a mass spectrometer, but had to estimate its mass from the difference between deuterium and hydrogen. However in 1935, more accurate measurements allowed Chadwick to derive a neutron mass of between 1.0084 and 1.0090 units; the modern estimate lies almost exactly in the middle of this range, and so the neutron is appreciably heavier than the proton. Was Uncle Albert wrong? Chadwick immediately suggested (following Einstein again) that neutrons should have an excess of energy and be beta radioactive in common with other nuclei under like circumstances. It was not so easy, however, to verify this experimentally. But, partly owing to World War II, it was not until 1948 that neutron decay was verified – indeed, 12 minutes after they’ve been kicked out of the nucleus half of them will have split apart as Chadwick had suggested. So if you want to use neutrons, you can’t keep ’em in a bucket. Fortunately, the times-of-flight as ISIS are, even for the longest run, less than one-tenth of a second …… but if you want them to stay stable long-term, you need either the nuclear force or the intense pressure inside a neutron star to stop the protons and electrons from flying apart. (Incidentally, I don’t think Einstein will ever be proved wrong. Any variation on the theme will be analogous to knowing that the Earth goes round the Sun, then later finding that the Sun is orbiting the centre of the Milky Way.)
Will it be war?
Last week U. S. chemists and physicists girded up their loins for war on their British colleagues. A U. S. discoverer's right to name his own discovery had been challenged from abroad. Scientific relations between the two countries were described as "very tense."
This is the opening of a news item by Time Magazine
in 1934. Harold Clayton Urey, the American discoverer of “heavy hydrogen”, had named it “deuterium”, but Lord Rutherford wished to call it “diplogen”, for fear that the name of its nucleus, called a “deuteron”, might be confused with “neutron”. It may be that the development of today’s indistinct syllable-swallowing English English was already under way!
Why ISIS, TS2, and LOQ?
Why is Ye Olde Neutron Lab called ISIS? It was named after the Ancient Egyptian goddess because much of the equipment in it was recycled from earlier projects. In AE mythology, her husband-and-brother Osiris had been murdered by the bad guy Seth, who cut him into pieces which he then scattered all over the Nile. Isis gathered up the pieces, re-assembled them, and brought them back to life again. So it is not an acronym for “Intense Safety Instruction Seminar”! TS2
stands for the Second Target Station
, which will be a second neutron source. The first protons were delivered to the target on December 14, 2007. (Wikipedia says neutrons but I’m pretty sure they’re wrong. You can’t accelerate neutrons with electric fields; it’s protons that are speeded up before smashing into the target to knock neutrons out of the appropriate nuclei.) The first neutron beam came down the line a couple of weeks ago on August 3, 2008 at 13:15.
I could tell you that q
is the symbol for the scattering vector, but what does that mean? Following the work of Sir William Lawrence Bragg
and his law of diffraction
, X-ray crystallographers have referred to the crystalline lattice spacing as d
and the reciprocal of this (spatial frequency) as k
. But ‘proper’ physicists (don’t worry, I’m a very improper physicist!) with their quantum mechanics don’t like that, and prefer to talk in terms of q
, or momentum transfer. In the case of neutrons (as opposed to X-rays) that does make sense, because neutrons interact with matter in all sorts of ways that X-rays don’t. But when you’re simply bouncing neutrons off your specimen the way we do, you can simply regard it as a measure of the recoil of the neutron against the specimen.
Why low q
, then? If you look at a street light through a net curtain, you will see a pattern something like this one
from a laser shining through a wire mesh, though not quite so fancy. The coarser the net curtain, the finer the pattern will be. If it were a really coarse net, the pattern would be too fine to see. The polymers that we study have much bigger molecules than most compounds, and so act as a very coarse net. However, with neutrons there are ways round that which allow one to observe low q
To round off, let me inform y’all that they also produce muons
at ISIS. Muons are a kind of transient heavy lepton (lifetime 2.2 microseconds, though particle physicists would regard that as being very long-lived). The physics of muon interactions is, however, much too hairy for me.