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Fully Understanding Half-Life in Radiation

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  1. my first question would be, how often does U-235 as an example, shoot out a ray of alpha radiation. Alpha radiation is a helium atom, but how often does that happen? because the half-life of U-235 is 700 million years, it’d take 100 g that many years to become 50 g. But throughout those 700 million years, is the alpha decay a constant drip?
  2. If I only have 1 atom of U-235, does that mean its just neutral for 700 million years, until it eventually shoots out 1 helium atom and decays?

Comments

  1. PickingEnthusiast Avatar
    PickingEnthusiast

    I don’t know for sure but I would assume the rate of decay is constant and the half deteriorates over the time span – I can’t do the maths based on those numbers but if we round up to 100m I’d assume a constant rate of decay of 0.142 (rounded up) over each of the 700m years.

    I’m not sure using the grams is the right way to measure as the actual material doesn’t disintegrate but the isotope decays into one which is stable?

    I have made a number of assumptions that may be wrong and I am happy to be corrected.

    Edit: 1.42 to 0.142

  2. EventHorizonbyGA Avatar
    EventHorizonbyGA
    1. my first question would be, how often does U-235 as an example, shoot out a ray of alpha radiation. Alpha radiation is a helium atom, but how often does that happen? because the half-life of U-235 is 700 million years, it’d take 100 g that many years to become 50 g. But throughout those 700 million years, is the alpha decay a constant drip?

    This is not correct. You will have 50 g of U-235 and ~50 g of other elements.

    1. If I only have 1 atom of U-235, does that mean its just neutral for 700 million years, until it eventually shoots out 1 helium atom and decays?

    How often does a single U-235 atom spontaneously decay via alpha emissions? You have about a one in 3.12×10−17 chance of that happening per second. What that means is every second from now there 1.4B years from now you have the exact same chance of it happening.

  3. BananaResearcher Avatar
    BananaResearcher
    1. it’s a random process, described statistically such that we can say that any given mass of uranium-235 will be half that mass in a certain amount of time (half-life). This obviously then means that how many alpha particles are being released per unit time depends on how much u-235 you have. The math would be (mass of uranium -> convert to atoms of uranium -> take half and divide by half-life = number of alpha particles per unit time).

    I have nothing better to do so 100g of uranium is .42mol Uranium. A mole is 6.02×10^23 atoms.

    So: 100g U-235 = 0.42mol U-235 atoms. .42*6.02e23 = 2.53e23 atoms. Half will decay in 700mil years, each releasing an alpha particle (we’re ignoring everything else). So 1.265e23 alpha particles. 1.265e23/7e8 = 1.807e14 alpha particle per year = 5.73e6 alpha particles per second, i.e., 5.73 million helium atoms per second. Or something close enough.

    1. It’s a random process. The single u-235 atom could decay in a second, it could decay in a billion years. It’s random. Half-life is usually understood as a bulk property of materials but it’s a reflection of the statistical rate of decay, which is random.
  4. Quantumtroll Avatar
    Quantumtroll

    An intuitive way to think about radioactive decay is that each individual atom of U-235 has an equal chance of decaying in a given time-span. That chance is equal to 50% in 700 million years. So in 700 million years, you’d expect about half of the U-235 you have today to have decayed.

    Now to your questions:

    1. The total flux of alpha particles will decrease over time, because the amount of U-235 will decrease over time. After 700 million years, because half of the uranium has decayed, you’d measure half as much alpha radiation coming off your hunk of metal.

    2. Yes. That single atom will just sit there until it randomly falls apart without warning.