The Field Guide to Particle Physics https://pasayten.org/the-field-guide-to-particle-physics©2021 The Pasayten Institute cc by-sa-4.0The definitive resource for all data in particle physics is the Particle Data Group: https://pdg.lbl.gov.The Pasayten Institute is on a mission to build and share physics knowledge, without barriers! Get in touch.A few References and Resources for you.Isotopes of Helium:https://en.wikipedia.org/wiki/Isotopes_of_heliumHelium Fact Sheet from NIST:https://webbook.nist.gov/cgi/inchi/InChI%3D1S/HeCDC Fact sheet on Uranium-238:https://www.cdc.gov/nceh/radiation/emergencies/isotopes/uranium.htmUS Information Agency Facts on Energy Consumption:US : https://www.eia.gov/energyexplained/us-energy-facts/World : https://www.eia.gov/todayinenergy/detail.php?id=49876Berkeley National Lab Essay on Earth's Heathttps://newscenter.lbl.gov/2011/07/17/kamland-geoneutrinos/The Alpha ParticlePart 4 : Inside the EarthDensityHelium, Neon and Argon are all noble gases. None of them react to form chemicals. As the chemists are fond of saying, their electron valence shells are filled.So why is Helium such a scare resource, while Argon in particular makes up a sizable fraction of our atmosphere?In a word, Density.Helium is much less dense a gas than Neon or Argon. With only two protons and two neutrons, helium is about ten times less dense then the air we typically breathe on Earth.Just like air bubbles under water, balloons full of helium rise in the atmosphere because they’re less dense than the surrounding media.Something similar can be said about heavy things. Rain falls from the sky when water vapor condenses to form droplets of liquid water. That liquid is much more dense than the air around it, so it falls. All the way down. To Earth.A similar thing happens INSIDE the Earth. It’s a little less intuitive because rocks and dirt are typically solid. But denser material tends to sink towards the center of the earth. Of course, that sinking takes place on geological time, not human time.Really heavy elements, like Uranium and Thorium aren’t terribly common up near the surface of the Earth. But inside. There’s quite a bit of both. And we’re very lucky that is so.Helium CreationDeep in the Earth, Uranium and Thorium decay. They’re radioactive, and as we discussed last time, they occasionally decay by spitting out an alpha particle. An alpha particle, you might recall, is nothing but a Helium nucleus.That alpha particle eventually soaks up a couple of electrons from the surrounding environment, turns to helium and begins to rise, literally, through the cracks. It eventually pools in underground wells, as discussed in part two of this series.This is a long process, and not just because it takes a while for the Helium atoms to migrate up to those subsurface wells. The real bottleneck is alpha decay. In some sense, its a pretty uncommon event. A typical Uranium-238 nucleus lives for about 4.5 billion years. And each event only produces ONE helium atom.Helium Production RateTrying to build an intuition for atoms and molecules is difficult because they’re so small. They are so small AND there are so many of them.  What does it mean to say that a typical MRI machine might go through a million billion billion helium atoms in a given year?Frankly, I’m not sure, but it’s a lot. Especially considering how long it takes for a Uranium nucleus to decay. The reason we have any helium around at ALL is because we have SO MUCH URANIUM in the earth.Rather than attempt to wildly estimate the rate at which helium is produced deep underground, it’s better to explain that uranium lifetime - that 4.5 billion years -  a bit more precisely.In part three of this series, we explained that alpha decay was a random sort of thing. It was a “quantum tunneling event” - as physicists are fond of saying. It’s not guaranteed to happen at any given point in time. But it happens at a typical rate. For uranium 238, that’s about once every 4.5 billion years.But th