Physics of Medical Scans
Helium
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Helium was first discovered by J. C. Janssen in 1868, when he was observing the spectral lines from the sun. [1] Lockyer, who also made the same observation, assumed that the element was a metal and thus named it helium: helio means the sun in Greek and the suffix -um was typically used for metals. [2]
Helium have two common isotopes. Helium-4 makes up of more than 99% of natural helium, and Helium-3 makes up the rest. [3]
Why does helium have such a low boiling temperature?
Helium has a low boiling temperature because it is a noble gas, which means it has a full outer shell of electrons and it exists as neutral helium atoms. Interactions between helium atoms are weak, and hence not very much energy is required to overcome them.
How to cool things down to such a low temperature?
Helium was liquefied for the first time by a Dutch physicist, Heike Kamerlingh Onnes, for which he was awarded the Nobel Prize in Physics in 1913.[4]
Helium is cooled by the Joule-Kelvin effect, also known as the Joule-Thomson effect. This is done by forcing gas at high pressure through a plug. The gas expands as it is pushed into a region with a lower pressure.
However, the Joule-Kelvin effect only cools down the gas when it is at a temperature below its inversion temperature. This is because at higher temperatures, atoms may collide so hard that their potential energy increases, and when the atoms move away from each other, potential energy is transferred into kinetic energy. The inversion temperature of helium is 45K.[5]
Therefore, helium must be pre-cooled to below 45K before the Joule-Kelvin process can take place, for example, by using a liquid hydrogen bath.[6]
For a mathematical treatment of the Joule-Kelvin effect, click here:
Are we running out of helium?
Helium is extracted from natural gas, which is a non-renewable resource. Once helium is released into the atmosphere, the earth's gravitational field is not strong enough to trap it, so helium escapes into space. At present, it is very expensive to extract helium from the atmosphere.[7]
Scientists estimate that demand for helium will continue to rise, and by 2030, there will be a deficit in the production of helium.[8]
Did you know...
Helium does not turn into a solid at atmospheric pressure at absolute zero
At atmospheric pressure, helium does not solidify even at absolute zero. It only solidifies at a pressure about 25 times greater than the atmospheric pressure, at [9]
At this pressure, the melting temperature is 1.5K.
References:
[1] Royal Society of Chemistry, "Helium: Element Information", Retrieved from http://www.rsc.org/periodic-table/element/2/helium [Accessed: 24.02.19]
[2] ibid.
[3] Thomas Jefferson National Accelerator Facility - Office of Science Education, "Isotopes of the Element Helium", Retrieved from https://education.jlab.org/itselemental/iso002.html [Accessed: 23.02.19]
[4] Blundell S J and Blundell K M, "Concepts in Thermal Physics: Second Edition", Oxford University Press, Oxford (2010)
[5] Hock K, "Cooling Techniques", University of Liverpool, Liverpool (2012) Retrieved from http://hep.ph.liv.ac.uk/~hock/Teaching/2011-2012/4-cooling-techniques-handout.pdf [Accessed: 23.02.19]
[6] de Waele A, "Basics of Joule-Thomson Liquefaction and JT Cooling", Journal of Low Temperature Physics, vol 186, issue 5-6 (2017)
[7] Grenshko, M, "We Discovered Helium 150 Years Ago. Are We Running Out?", National Geographic, Retrieved from https://www.nationalgeographic.com/science/2018/08/news-helium-mri-superconducting-markets-reserve-technology/ [Accessed: 24.02.19]
[8] Gazprom, "Producing Helium from Natural Gas", Retrieved from http://www.gazprominfo.com/articles/helium/ [Accessed: 24.02.19]
[9] Basu D, "Dictionary of Pure and Applied Physics: A Volume in the Comprehensive Dictionary of Physics", Boca Raton, CRC Press, (2001)