Hi scientists of lemmy, I’m a computer scientist with basic college level physics and an interest in physics.
I was reading Cosmos by Carl Sagan yesterday and he mentions that if you use a Geiger counter next to an uranium ingot you will detect the uranium’s spontaneous decay as a stream of helium nucleei.
Does helium nucleei mean 2 protons and some number of neutrons? What happened to the respective electrons? Does this mean that each uranium atom, with 92 protons, entirely splits into 46 helium nucleei or does it release some number of helium nucleei leaving another element behind? How does the concept of half life play into this? Does it mean that in a uranium half life, half of my ingot would’ve become helium? Finally, how is this stream of helium nucleei so dangerous to living beings?
Thanks for your attention
You must log in or register to comment.
I’m not a scientist, but do teach science.
Uranium decay is a fascinating process with a lot going on. When Sagan talks about a stream of helium nuclei, he is referring to alpha particles. These are helium four nuclei, which means they contain two protons and two neutrons. They are emitted from the nucleus of a uranium atom during radioactive decay.
The missing electrons are an interesting part of this. When an atom of uranium ejects an alpha particle, it loses two protons, but its electrons remain for a moment, making the new atom highly ionized. Over time, the uranium ion will stabilize as it interacts with other atoms. The ejected alpha particle, which is just a bare helium nucleus, will also eventually capture two free electrons from the environment and become a neutral helium atom.
Uranium does not completely break apart into helium nuclei. Instead, it undergoes a slow process where each atom decays step by step. When uranium 238 undergoes alpha decay, it does not vanish but transforms into thorium 234. Thorium is also radioactive and will continue decaying through a long chain of transformations, eventually becoming lead, which is stable. So uranium is not simply breaking into helium, it is gradually changing into different elements.
Half-life plays an important role in this process. The half-life of uranium 238 is about 4.5 billion years, meaning that if you started with a pure chunk of uranium, after that time only half of it would still be uranium. The other half would have decayed into different elements. But it is not just turning into helium. While each decay event emits a helium nucleus, the uranium itself is transforming into new elements along the way. Over long periods, the uranium will be replaced with a mixture of lead and other decay products, while the helium gas may escape into the environment.
The danger of alpha radiation comes from how it interacts with living tissue. Alpha particles are highly ionizing, which means they can cause severe damage at a cellular level. However, they do not penetrate very far. A sheet of paper or even human skin can block them. The real risk comes from inhaling or ingesting radioactive materials. If uranium dust or other alpha-emitting particles enter the body, they can directly damage DNA, increasing the risk of cancer and other health problems. That is why uranium and similar materials are dangerous if they are inhaled or ingested but not as much of a threat when they are simply sitting in solid form.
Uranium decay is a gradual process where atoms lose helium nuclei one by one, transforming into different elements over time. Half-life describes how long this process takes, with only a fraction of the uranium decaying in a given time period. Alpha radiation is dangerous inside the body but does not travel far outside of it.
-
Uranium-238 - Wikipedia:https://en.wikipedia.org/wiki/Uranium-238
-
Uranium Decay Series - ScienceDirect Topics:https://www.sciencedirect.com/topics/chemistry/uranium-decay-series
-
The Timeline of Radioactive Decay for Uranium-238 - IET:https://www.theiet.org/media/8813/radioactive-decay-rate-of-uranium-238.pdf
-
Decay Chain - Wikipedia:https://en.wikipedia.org/wiki/Decay_chain
This is a great response! Now do the ‘why’ it decays, that will surely be the follow up question!
I only didn’t ask this because Sagan actually explains that part in the book.
If I understood correctly, protons repel each other, and the reason atoms don’t usually tear apart is because neutrons exert a force over protons that can counter that repelling and keep the atom’s nucleous together. If an isotope does not have enough neutrons to fully counter the proton’s repelling forces, it is deemed “unstable” and eventually loses nuclear particles. This is the case of Uranium-238.
Why uranium-238 loses specifically 2 protons and 2 neutrons was not explained, but I assume it’s a result of the specific difference between the forces of neutrons holding it together and the forces of protons repelling each other.
Feel free to correct me if I got anything wrong.
Sorry for the late reply, I love geeking out on this. Please note that some of this is off the top of my head coupled with online research and online lectures.
You have a solid understanding of why atoms decay, and you’ve got the big picture right. The nucleus is held together by the strong nuclear force which binds protons and neutrons together. But protons are all positively charged, so they are constantly repelling each other due to the electromagnetic force. Neutrons help by adding extra strong nuclear force without adding more repelling charges, but if an atom has too many or too few neutrons, the balance starts to break down. That’s when an atom becomes unstable and eventually undergoes radioactive decay.
Uranium-238 specifically loses 2 protons and 2 neutrons as an alpha particle instead of, say, just a single proton or neutron due to energy and stability. Large nuclei like uranium are constantly vibrating due to the competing forces inside. Over time, quantum mechanics allows certain particles to “tunnel” out of the nucleus, meaning they escape even though they don’t have enough classical energy to do so. Alpha decay happens because helium-4 is an extremely stable nucleus, so when the uranium nucleus reaches a state where part of it can separate, it naturally forms this tiny, tightly-bound chunk rather than spitting out individual protons or neutrons.
Another factor is that heavier elements tend to decay in ways that release as much energy as possible while still leaving a stable nucleus behind. If uranium-238 lost just one proton or neutron at a time, it would still be unstable and just keep decaying step by step. But losing an alpha particle is a more efficient way for it to drop to a lower-energy, more stable state quickly. Thorium-234, the element left behind after uranium-238 decays, is still radioactive, but it is slightly more stable than uranium, so this decay step makes sense in the long run.
So uranium-238 decays via alpha emission because it is the most energetically favorable way for it to become slightly more stable. The strong nuclear force holds nuclei together, but once an isotope gets too large and the balance between attraction and repulsion is off, the nucleus finds a way to shed some mass to stabilize itself. The reason why it does so by specifically ejecting a helium nucleus rather than some other particle comes down to quantum mechanics and the fact that helium-4 is an incredibly stable arrangement of protons and neutrons.
Also, thank you for all of the wonderful comments!
Great response! Thank you
I’m not the OP, but this is a great answer, I bet your students are grateful for you.
As the OP, I definitely agree
holy cow great explanation, thanks
Someone asks a question and an expert in the field appears to give a detailed answer. Total reddit moment ducks
-