An introduction to cryogenics

These notes aim to give a somewhat basic introduction to the hows (and occasionally whys) of cryogenics. Cryogenics is not a very well defined term; here I have taken it to mean systems that operate at temperatures of 77 K and below, with the emphasis on systems operating below 4.2 K and above 1 K. The first two temperatures are, of course, the boiling points of nitrogen and helium respectively. A cryogenic experiment or system is normally dominated by the need to get something cold and keep it cold, with other elements of the design subservient to that. Generally, the need to operate at cryogenic temperatures makes even an otherwise simple experiment complicated, and the colder you need to go, the harder life generally gets. Since it is so time consuming to get things cold, the ability to experiment with new techniques is somewhat limited, and most low temperature labs cling to a set of tried-and-trusted methods. Often these differ from lab to lab, and in the most interesting cases are contradictory. First, a few basic definitions. In these notes, I shall use the following words in the way defined here (these are not the only definitions) – cryostat: the system that a low temperature experiment goes in – dewar: something for storing and (possibly) transporting liquid cryogens Helium comes in two isotopes; the common helium-4 (He), which is obtained from oil and gas wells (it starts life as alpha particles) and the much more exotic helium-3 (He) which does not occur in nature, but is produced by the decay of tritium. There are several books to which you can turn to expand on these notes, and after some years of neglect, old books are being reprinted in new editions, and new books appearing. The following lists the ones that I refer to the most; there are other books which are also well worth reading. – Ekin (Ref. [1]); this is a new book, and gives lots of useful, detailed, down-to-earth advice on many aspects of cryogenics, though it is generally limited to temperatures above 1 K, and has an emphasis on making measurements on superconductors (you may or may not think this is a good thing). – White and Meeson (Ref. [2]) is also a very worthwhile read, and is up to date. – Pobell (Ref. [3]) is another standard textbook, and a new edition has recently appeared. – Richardson and Smith (Ref. [4]) is a collection of seminars from the Low Temperature Group at Cornell University, and although somewhat out of date in places gives a lot of hands-on advice. – Wigley (Ref. [5]) gives useful information on mechanical properties of materials.