My understanding of radiation is that is travels in a straight line. So a basement buried on three walls with no windows is a great shelter because you have thick dirt on three sides. but the radio active dust landing on the roof is still radiating right through the wood of the house (1 story) down to the basement.

So would a rooftop sprinkler system wash away the radioactive dust so that it is not accumulating on the roof radiating down into the basement.

Does this make sense or is there a gap in my understanding of how fallout works?

  • 3
    $\begingroup$ Might make things worse. Where is the contaminated water is going to flow? Down? Into the basement where you are? $\endgroup$
    – DKNguyen
    Sep 25 at 20:15
  • $\begingroup$ If you want to use it in a story, you have to have some pool where the water is going , so kind of a drainage $\endgroup$
    – trula
    Sep 25 at 20:58
  • $\begingroup$ @DKNguyen it should flow down hill to the creek. That's were all the rain water goes. is radioactive dust leaching into the ground a concern? $\endgroup$ Sep 25 at 23:43
  • $\begingroup$ Suggest you look up the penetration depth of the radiation you are concerned about (through materials such as roof tiles, wood, air). It is not as penetrating as you seem to think. $\endgroup$ Sep 26 at 8:40

3 Answers 3


So charged particle radiation doesn't travel in straight lines. Obviously, it is an energetic particle so it's moving in some direction, but as it does, it interacts with atomic electrons and forward scatters adding some randomness to the path. It is these atomic electron interactions that are the "ionizing" part of "ionizing radiation".

The interaction is described well by the Bethe-Bloch equation.

$\alpha$ and $\beta$ radiation are the primary charged particle forms of radiation. The alpha is a helium nucleus and it is stopped by a piece of paper. (It's slow moving and has a charge of +2). The biggest danger for alphas is ingesting a source...then it can sit in your body and depositing damaging radiation in vivo for a long time.

Beta rays are beta-decay electrons (or positrons), and they are bit more penetrating...but they should not make it through the roof and the floor, and the air in-between. Electrons are light, so beta rays are generally relativistic, hence they deposit energy at slower rate, and travel farther.

$\alpha$ and $\beta$ radiation are also the most significant for fall-out, as neutron rich fission products work their way back to stability.

Sometimes the decay will leave the daughter nucleus in an isomer, which is just an excited state of the final state nucleus. The isomer will then make an electromagnetic transition towards the ground state, emitting a $\gamma$ ray, which does indeed travel in a straight line. (Cobalt-60 is a classic example).

$\gamma$ rays are neutral, so they penetrate. They do not leave an ionization track (a characteristic of charged particles). When they do interact, via the photoelectric effect, Compton scattering, or pair-production, they produce other charged particles that do damage.

So the best thing to do is: leave them on the roof.

You can look up the "radiation length" of various materials here:


and see what the fall-out emits here:


and get more details about the passage through matter here:


  • $\begingroup$ Thank you! since the gamma rays do penetrate wouldn't it be better to have the fallout on the ground were the gamma radiation has to penetrate all that soil before making it into the basement? $\endgroup$ Sep 25 at 23:51
  • $\begingroup$ @DanAnderson the ground is connected to the water table which connected to your garden which is connected to you, so I’ll go with no. $\endgroup$
    – JEB
    Sep 27 at 15:53
  • $\begingroup$ sure but wouldn't cleaning the soil for food growth be something to worry about after surviving the initial fallout? $\endgroup$ Nov 3 at 20:18

Yes for alpha and beta radiation. Not for gamma or neutron radiation. The US Navy has a system of washdown in case of radioactive contamination.

You are going to have 2 problems.

  1. When you wash the radioactive particles off your roof, they are going to collect around your house, unless you can wash them into a container.
  2. The wind is going to blow more onto your roof and you are going to have to keep doing this until the contamination is low enough to simply allow it to remain.
  • 1
    $\begingroup$ Plus you probably would rather have a bunch of clean water in a closed tank, rather than a bunch of contaminated water on the ground. The US Navy used ocean water for the spray down, so had an unlimited supply. $\endgroup$
    – Jon Custer
    Sep 25 at 22:18
  • $\begingroup$ the 7/10 rule means that after 7 hours a 1000Rad/hour of radiation should only be 100 rad/hour, right? so I just need to do it for the first 7 hours to keep my family safe. or does the gamma radiation last longer than the alpha and beta radiation? Also wouldn't it be better to have the radioactive particles on the ground so that the ground can buffer me in my basement from the radiation? $\endgroup$ Sep 25 at 23:46
  • $\begingroup$ I cannot answer the 7/10 rule, but I would not worry too much about the alpha and beta radiation. Clothing and a surgical mask would suffice to protect you from it. Neutron radiation would be gone within an hour. Gamma radiation is instantaneous... You may die from it in a week, but it is gone literally in a flash. $\endgroup$
    – Rick
    Sep 26 at 12:24
  • $\begingroup$ @Rick So you are saying that the fallout is all gamma and betta radiation and there should be negligible amounts of Gamma radiation in the actual fallout? $\endgroup$ Sep 26 at 21:20
  • $\begingroup$ Dan - No I am saying the most dangerous radiation has the shortest lifespan. $\endgroup$
    – Rick
    Sep 28 at 1:30

Yes, you can. Note that when inhaled or ingested, the alpha- and beta-emitters do not have to pass through your skin in order to poison you from inside. This is why the KGB uses tea spiked with polonium-210 to kill its enemies.

That said, washing down your roof puts the fallout dust in your shrubbery which means it is still at least theoretically possible to inhale or eat it.

  • $\begingroup$ It should go into the downspouts which mostly drain to the creek as we are on a hill. the radio activeness should have mostly died down by the end of the week right? $\endgroup$ Sep 25 at 23:54
  • $\begingroup$ @DanAnderson "by the end of the week..." Um, the half life for Uranium is between two hundred thousand and five billions years... $\endgroup$
    – hft
    Sep 26 at 0:09
  • $\begingroup$ I guess you didn't specify the type of source, but just for comparison a billion years rather longer than a week $\endgroup$
    – hft
    Sep 26 at 0:11
  • $\begingroup$ @hft the sources I've read say that there would be very little uranium in the fall out. the radiation levels from the fallout decrease by 50%. in the first hour and by like 90% in 7 hours, is my understanding wrong? $\endgroup$ Sep 26 at 21:18
  • $\begingroup$ It depends on the type of bomb. For a fusion bomb (a hydrogen bomb) as opposed to a fission bomb there is probably relatively little uranium or other fissionable material (but probably still some that maybe is used as a detonator or some other reason) $\endgroup$
    – hft
    Sep 26 at 21:24

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge that you have read and understand our privacy policy and code of conduct.

Not the answer you're looking for? Browse other questions tagged or ask your own question.