User:Armondikov/sandbox2

The following is a simple bluffers guide to NMR hyperpolarisation. It follows on from Trend enquiring about it a few weeks ago as well as several others wanting to know so I thought I'd write it down somewhere. It assumes no prior knowledge of chemistry, quantum mechanics, spectroscopy or any other technique.

Introduction
NMR, nuclear magnetic resonance, is a powerful analytical technique used in chemistry and medicine. It works by placing samples in a magnetic field and seeing how they absorb and re-emit radio waves. The medical application is known as MRI, or magnetic resonance imagining; so called partially because 3-letter acronyms are catchier than 4-letter ones, and because people tend to freak out at the word "nuclear". "Nuclear" in this sense refers to the atomic nucleus, the central core of an atom, which is what is interacting with the radio waves. NMR was developed through the 1930s and 1940s, partially as an offshoot of radar research, and its imaging applications have been available since the early 1970s.

Magnetism
First imagine a compass needle; on Earth, a compass points north and has been of great use to sailors and explorers for centuries. In a more strict sense, the compass needle doesn't really point north, it just aligns with a magnetic field. The Earth happens to have a fairly strong and consistent magnetic field generated by its rotation and its iron core. This alignment can be seen with gimbal mounted compasses that can point slightly up or down in certain parts of the world, rather than just north or south, reflecting the overall shape of the field, which comes up out of the magnetic poles and wraps around the Earth to form a loop.

The reason that compass needles "point" north is because someone who knew in advance what direction the needle would align with and had the decency to mark one end with an "N" so you can see easily, just by looking, which way is north. The needle also has a magnetic field and this aligns with the invisible field of the Earth, its "North" end going with the Earth's North and its "South" end going towards the Earth's south. But what if someone didn't actually paint the "N" on the right end, and there was actually a mixture of compasses that "pointed" in opposite directions? Obviously, this wouldn't be a very good compass manufacturer and ramblers the world over might be having words with them later. What if there was a statistical tendency for them to be right? In this case you'd just have to look at many compass needles to see where the majority point; the weaker that statistical tendency, the more needles you have to look at, but you could still determine north. You would take one needle that is pointing one way, pair it up with one pointing the other direction, take them both away and continue until you only have needles pointing in one direction. This seems like a strange situation, and this probabilistic approach to finding North leads to what one could call a "quantum compass", as quantum mechanics is often based on similar statistical and probabilistic approaches. It is a highly relevant analogy, however, because the atomic nucleus behaves in just this sort of manner, as a "quantum compass".

The atomic nucleus is the core of an atom, where the positive charge and most of its mass is stored, around which the electrons orbit. The nucleus is tiny, the equivalent of a grain of sand in a stadium but contains 99.99% of the mass. These cores often have magnetic fields, and can act like compass needles, aligning with a magnetic field just like the "quantum compass" described above, with some pointing North and others pointing South – NMR theory usually refers to these as "up" and "down", or sometimes alpha and beta, but the terms don’t matter, what is important is that the point in opposite directions. In the case of an atomic nucleus, however, it isn’t just careless labelling that causes these atomic quantum compasses to point in the "wrong" direction; the magnetic fields really do point "up" or "down". A real compass can be flipped away from magnetic North with a kick of energy; they can be easily deflected by a stronger magnetic field placed nearby, for instance. But with atoms, with their weaker field and small size, it doesn’t take much energy at all for them to flip, and they can do it spontaneously just with the inherent energy they have stored as heat. The difference in energy between a nucleus pointing "up" and "down" is about 0.1 Joules; about half a million times smaller than the energy it takes to break a chemical bond. In perspective, if breaking a chemical bond was equivalent to Superman leaping over the Empire State Building in a single bound, flipping a nucleus to point "down" instead of "up" would be like an insect nonchalantly stepping over a grain of salt. So it is unsurprising that almost just as many atomic nuclei point "down" as they do "up" and they almost cancel each other out.

Weak signals
The energy gap between "up" and "down" atoms is small, but it can be increased by exposing the atoms to a much more powerful magnetic field; it takes more energy for the nuclei to flip against a stronger field. Hence NMR and MRI instruments use magnetic fields many times stronger than the Earth's magnetic field. However, there is a limit to the size that a suitable magnetic field can be and one that would be strong enough to prevent any atoms pointing "down" would produce a field so strong that it would have to be built in deep space. Despite the strong fields used in NMR and MRI, there is still only a small proportion of excess atoms with their magnetic fields pointing "up" - about 1 in 15,000 "up" pointing atoms aren’t cancelled out by a corresponding "down" pointing atom. Because the "up" pointing magnetic field isn’t entirely cancelled out by the "down" field, it can be detectedin an NMR experiment. This is often interpreted in the sense that only 1 out of every 30,000 atoms is detected in an NMR experiment, which is incredibly small and means that any NMR sample has to either be A) concentrated or B) scanned repeatedly. In the world of MRI in medicine, doctors can only look at fat and water, as these are the most concentrated collections of NMR-active atoms in the body, and many procedures can take up to an hour as they consist of dozens of repeated scans in order to detect the very weak effect. How this up and down relates to MRI and gathering actual NMR data is another story, but what should be remembered is that there are "up" pointing atoms and "down" pointing atoms and these almost cancel, leaving a very small residual magnetic field that NMR instruments can detect.

If there was a way of changing this "quantum compass" so that all atomic nuclei pointed the same direction, aligning only with or only against a magnetic field, an NMR experiment would be able to detect not just a few, but every atom simultaneously. This is termed "hyperpolarisation" and how this is achieved is discussed in the next section.