Sunday, April 17, 2022

Tin Can Shielding

This indicates that 10 inches of steel and 4 inches of lead are effectively identical in gamma ray reduction.  2 inches is the usual standard for fallout shelters.  How many inches of steel are there in a case of food cans?  I cannot seem to find the thickness of cans and I am in bed so going to the garage to get a micrometer waking up the dogs to go measure cans in the kitchen is silly.  I doubt they more than  .050" thick.  A gamma ray going straight through six cans will therefore cross at least .6" of steel plus the food contents which can ignore for this exercise.  That would be a 1.5% reduction in radiation.  So 26 cases stacked high enough to shield you from the outside ground would likely be sufficient protection from the external radiation and on the tables above you 13 cases single stacked of canned food. 

Just make sure it is something you can eat over the next five years once we can stop worrying about thus.  While most canned foods have "best by" dates, I understand this is primarily texture, color, and vitamin degradation, not spoilage.

Someone asked if irradiated canned food is safe.  Civil Defense says it is.  An exploration of why follows.  Most of this I remember from junior high, but I looked it up to be sure.

Alpha particles will not penetrate the steel (or your skin, for that matter).

Beta particle capture causes a a proton to transform to a neutron: atomic mass is unchanged; atomic number number drops by one.  C12 turns into B12, which decays by beta particle emission to stable C13 or C12.  C13 goes to B13 again decaying to either C12 or C13 (both stable).

N14 turns to C14, which is slightly radioactive (half life long enough that it is not going to do anything but make your bones carbon datable).

O16 turns to N16 which decays rapidly to C12 or O16.

H1 turns into a neutron which decays to H1 (short half-life).

The rest of the elements common to food are in small quantities and need not concern us here.

Neutrons are also low risk for food.  H1 + neutron gives H2 stable.  

Neutron + H2 (quite rare) gives tritium which is radioactive.

Neutron + C12 gives C13, stable.

Neutron + c13 gives C14; relatively long half-life.  Our distant descendants can use it it to radiocarbon date your bones.

Similarly, the common isotopes of nitrogen and oxygen do neutron capture to stable isotopes.  

Neutron capture is also dependent on nuclear cross section, which is measured in barns (10^24 sq. cm.).  I am not worried about my food capturing neutrons.

Gamma rays excite electrons to higher energy levels ("You come here often?") which decay back to a lower energy level producing X-rays.  As I recall this is a pretty fast process so your beef stew should be safe.

Of course, you do not get full absorption of beta particles or gamma rays by any shielding, nor full absorption by your body.  In civil defense matters you are only trying to get the amount getting through to you down to tolerable levels.  There is enough background radiation that your freeze-dried banana chips (K40) might be your biggest hazard in a proper fallout shelter.

UPDATE: One of you asked about aluminum (for ammo boxes, I think).  This Radiation Halving Chart (what thickness of material will halve your dose) matches what I said in the comments, aluminum is about 1/3 of steel: 2.7 inches of aluminum and 1" of steel both halve your radiation exposure by half.  Ideally, you want 14.3" of aluminum to get a PF40 (1/40th of the outside exposure).


  1. THe contents of the can, being largely water and protein or cellulose are actually pretty good shielding as well.

  2. Certainly Santa Monica is in the middle of things, so Shelter-in-Place seems the only reasonable response. How thick are Mil surplus ammo cans? How much Gamma radiation does brass stop?
    So, 24 cans should do the ten inches? I suppose we just need to make sure it isn't aluminum, or does aluminum work something like steel?

    1. Aluminum is less effective than steel. Shielding is mostly mass dependent. Aluminum is roughly 1/3 the density of steel. Lead is good because it is cheap and dense. (Cheap because lead is the stable decay product of everything from uranium 238 down.) Gold would be almost twice as effective as lead (19.3 g/cc vs. ~11 g/cc). Just stack a few inches of gold bullion on the outside.

  3. According to this, water has roughly 1/16th to 1/7th the gamma shielding properties of iron (depending on gamma energy). That's enough that I don't think you can ignore the can content when estimating the shielding ability of a case of food cans.

    1. Indeed. But compared to the can, pretty minor.

    2. And it argues for lots of water jugs in your shielding; you are going to need it.

    3. Um, my BOTE calculation says that the shielding from the food contents would likely be somewhat more than the shielding from the can steel. Six cans at 3" per can would be 18" of food content that a gamma ray would have to pass through, along with the 0.6" of steel for the cans themselves. Divide the inches of food by 16 to get an "equivalent inches of steel" measure, and that's 1.25" - twice that of the 0.6" of actual can-steel.

    4. That's interesting. The next case lot sale of chili...

  4. I would have to check, but I'm not sure you would want to eat food that you used as radiation shielding.

    Also, some gamma radiation will go through the entire planet.

    The "Expedient Fallout Shelter" (Oak Ridge National Lab - 1976) indicates the 12 inches of dirt would be effective for an expedient shelter.

    For a more permanent shelter more is better. If you have a basement, well that solves some of the problem.

    Sheltering for 4 days is good. 21 days should be the max - unless you are hit with extreme fallout. You may need to bug-out after 4 days

    1. Gama rays are not stopped; you just block enough that it does not matter. The ones coming up from the Antipodes are the problem.