Protection from Radioactive Fallout by Tennessean

Part One

About me: I’m a retired Ph.D. biochemist who has worked with beta emitters in the laboratory.  I’ve no financial interest in any of the firms or products mentioned below.

This post is dedicated to the late Cresson Kearny and his wonderful hard work in preparing the most essential book Nuclear War Survival Skills available at and other sources.  

Dr. Stephen Hawking was once told by his publisher that for every equation he placed in his book the readership would drop by half.  Dear reader the author will harass you with only three equations.  The late great thermodynamicist  Dr. Josiah Willard Gibbs would often observe…”Words are no substitute for equations.”  Dr. Gibbs came up with the “Gibbs free energy change” which describes in which direction any chemical reaction will go at constant temperature and press.

In this essay we do not concern ourselves with the “prompt” radiation from the nuclear fireball itself, nor from the thermal pulse which may set fires, nor the possible EMP effects, nor the blast wave which will damage or destroy structures.  This piece is limited to the dangers of local fallout and the reduction of those dangers.

Perhaps the best guide we have to what we might expect is from reference.  In 1953 there was a series of above ground nuclear tests at Yucca Flat in Nevada.  This series was given the name of Upshot-Knothole.  A total of twelve detonations were made over a ninety-day period.  The nuclear bombs were placed atop a 300 foot tall steel tower.  This short tower allowed the resulting fireball to suck up a lot of soil and rock, adding to the intensity of the fallout.  Each detonation was given a name:  “Nancy” yielded an explosion equivalent to 21 kilotons of TNT.   “Dirty Harry” was 32 kilotons;  “Simon” was 43 kilotons.  The sum of all the yields of the twelve tests was 253 kilotons of TNT equivalent.   On some of these detonations the wind unexpectedly shifted, giving St. George Utah moderate fallout.  This test series caused the deaths of about 4,000 sheep that had grazed in areas west of St. George and ingested lethal doses of fallout.  The main forage of the sheep was sagebrush.  It was later found that the sagebrush leaves retained more fallout than the leaves of any other plant.  The descriptions of the effects on the sheep are quite dramatic and grisly.   Cows and horses suffered beta burns but few died.   Several sheepherders became quite ill.  Many residents in the area lost all their hair.  The subsequent incidence of childhood leukemia almost tripled, as did the deaths of adults from cancer.  Dr. Knapp estimated that from the “Dirty Harry” shot alone that infants in the St. George area received as much as 440 rads to their thyroid glands from the accumulation of radioactive iodine, first by cows from forage into milk, then from milk into the thyroid glands of the babies.   A later analysis estimated that from the entire series of the Upshot-Knothole tests that children up to five years old received a total dose of about 1,200 rads to their thyroid glands

The Atomic Energy Commission(AEC) repeatedly assured the inhabitants that the amount of radioactivity in the fallout was harmless and did everything possible to hide the fact that the sheep died from radiation poisoning.   In 1954 the movie “The Conqueror” was filmed about 15 miles west of St. George.  The Atomic Energy Commission assured the director and actors that there was absolutely no danger from the residual fallout.   In subsequent years many of the film stars and staff died of cancer.  Some years ago there was a “Mythbusters” episode on Discovery TV on this.  The producers went to the places in Hollywood where many of the costumes used in the movie were kept and found no significant radioactivity and concluded that fallout from the 1953 series of tests played no part in the cancer deaths.  Well, this was some 55 years later.  Any fallout not removed when the clothes were washed would have nearly totally decayed.  Note that huge fans were used in the battle scenes to stir up dust storms and all the costumes would have been extremely dusty by the end of the film.  The costumes shown on the “Mythbusters” show had all been obviously laundered.                                                                                        

The “Simon” detonation in April of 1953 caused a “hot spot” of fallout in Troy in New York State.  As the radioactive cloud from “Simon” passed over Pennsylvania, the Hudson Valley, Vermont, and Massachusetts’s heavy rainstorms caused much of the radioactivity in smaller particles to reach the ground.  It so happened that Renssellaer Polytechnic Institute in Troy, NY, measured the amount of fallout hitting the ground.   No doubt there were other “hot spots” along the way, but no data are available as few monitoring devices were in use at the time.  Dr. Harold Knapp testified that infants in Troy could have received a total dose of 30 rads due to drinking milk containing radioiodine.  An updated version of a report from “Project Gabriel” estimated that if the radioactive cloud from a 13 kiloton blast ran into an intense snowstorm or rainstorm enough radioactivity would be dumped to kill most of the inhabitants in an area as large as 100 square miles. 

The idealized, simplified, fallout patterns discussed below and shown in many books are about nearly worthless.  For one thing, winds at different altitudes blow at different directions most of the time.  This fact tends to spread out the fallout deposition.  The central problem is that local, severe, hot spots can occur literally thousands of miles from the detonation event.    One cannot assume safety if the nuke event is far away.  This is why instruments to measure radioactivity are absolutely necessary.  In the former Soviet Union when the Chernobyl accident occurred it was illegal for an ordinary citizen to possess any type of radiation meter.  Many reports have come from Japan after the Fukushima disaster that police confiscated the radiation meters from ordinary citizens who were doing their own measurements.   I, for one, do not trust governments to tell us the truth about dangers from a terror nuke event.  I cannot count the times I’ve seen folks on television say it would be easy and simple to clean up radioactivity after a dirty bomb attack.  A “dirty bomb” is the descriptive term for a conventional high explosive mixed with radioactivity.  Any chemist who has actually spilled a bit of radioisotope on a laboratory bench top will tell you that removing the radioisotope is far from an easy, trivial, task.  Been there, done that.

If you believe that a terror nuke explosion of a fission weapon with a yield equivalent to more than a few kilotons of TNT equivalent is probable, then the book “The Day We Bombed Utah” is surely the best guide out there to risks from radioactive fallout.  I cannot recommend this book too highly.

Types of radiation from nuclear fallout.

First it may be helpful to note two integers associated with the isotope of any element.  The first is the “atomic number” that gives the number of protons in the nucleus.  The second is the “mass number” that gives the numbers of both protons and neutrons in the isotope.  The atomic number is unique to each element.  However any element can have a number of isotopes with differing mass numbers.  Some of these isotopes will be stable.  The radioisotopes have unstable nuclei and will decay (perhaps in a series of steps) to an element with a stable nucleus.  Hydrogen has an atomic number of one.  This means there is one proton in the nucleus.  The most abundant isotope of hydrogen has a mass number of 1, leading to the conclusion that for this isotope has no neutrons in its nucleus.  This isotope has the symbol H1.    A somewhat rare isotope has one neutron in the nucleus and is stable.  Its common name is deuterium.  Often the symbol D is used to represent deuterium.  Heavy water is D2O.  The radioisotope of hydrogen with two neutrons in the nucleus is often called tritium, and symbolized by the letter T.  This is an unstable nucleus.  It is a weak beta emitter with half-life of 12.1 years.  More on decay modes and half lives below

The U235 (the isotope of the element uranium with a mass number of 235) nucleus when hit by a neutron will split into two atoms and several neutrons.  There are at least forty different ways in which this fission occurs, leading to more than eighty possible daughter products (Gl).  The majority of the fission events give rise to two new elements of similar mass.  Generally these new elements are unstable, with a neutron to proton ratio that is too large for the mass of the nucleus.  These will decay over time to a stable nucleus that poses no radiation danger.  It is sometimes the case that several separate decay events must occur before stability is reached.

  • Alpha particles.  These are essentially helium nuclei, two protons and two neutrons.  These have little penetrating power.  For example an alpha particle from plutonium has a fairly high energy and an average penetration in air of about 1.5 inches (1).   Once the energy has dissipated through collisions this particle will capture two electrons and become a harmless helium atom.  The penetration into skin is about 1000 times less than for air.   The energy possessed by alpha particles will cause the formation of ionized molecules and atoms, splitting chemical bonds.  If ingested or inhaled alpha emitters pose a very real health risk.
  • Beta particles.  These are just very fast moving electrons ejected from the nucleus.  These result from the conversion of a neutron into a proton in the nucleus of the radioisotope.  These fast electrons will be absorbed in about ten feet of air (1).  Direct contact of beta particles with unprotected skin causes burns that may be quite severe.  Reasonably heavy clothing will absorb most beta particles before they reach skin (1).  Beta emitters that are ingested or inhaled also pose a very real health risk. Beta burns on the skin could pose a very real problem if the person’s immune system is damaged due to gamma radiation.
  • Positron emission.  A positron has the same properties as an electron, but carries a positive (not negative) charge.   This particle is emitted when a proton in a nucleus becomes a neutron.   Otherwise quite similar to a beta particle.
  • Gamma rays.  A photon with very short wavelength, shorter than X-ray photons.  This can be the result of electron capture, wherein a proton captures an electron to become a neutron.  Note that the energy content of a photon is inversely proportional to its wavelength (Planck’s Law). This is why gamma photons break chemical bonds and infrared photons only change the vibrational states of molecules. Calculation of the attenuation of gamma photons is much more complex.   One reason is the wide variation in energy of gamma photons.   The high penetrating ability of gamma photons means that a substantial mass must be interposed between the source(s) and humans.  More on this later.  And any gamma emitter ingested or inhaled also poses a serious health risk.

I must note here that none of the aforementioned particles can cause the production of a radioisotope in any material impacted by them.   Neutrons absorbed by nuclei often result in an unstable isotope.  This is why ground bursts produce much hotter fallout than air bursts.  In a ground burst tons of surface material are sucked up into the fireball and bombarded with neutrons from the fission processes in the fireball, causing the production of radioisotopes which are rarely produced in fission events alone.  Which radioisotopes, you may ask.  Well, that totally depends on the elements that are most abundant in the surface material and is quite complex and will not be further explored here.  Na24 (the isotope of sodium with a mass number of 24) is the greatest problem as a rule.

Note that only an extremely tiny fraction of the fallout will emit neutrons, a fraction so small as to be insignificant.  That said, if fallout comes into direct contact with material the material may well absorb some fraction of the radioactivity.  To put this in perspective, imagine that fallout has somehow landed on canned food in a grocery store.  The outside of the can may be radioactive, but the contents inside will not be (unless the metal in the can has a hole in it).

Estimating the size and distance of the event.

If you can keep your wits about you, begin counting off the seconds between the start and end of the bright flash (this accompanies the thermal pulse) and you will have an approximate yield.  These data come from the circular slide rule in some early editions of (?).  This will be easier done with a digital watch with a stopwatch function.   As a general rule, the greater the distance from the event, the less fallout.  Please note the modifier “general.”    Some idea of both yield and distance will help you decide whether or not to shelter in place or evacuate.

Estimated yield from                Estimated range from
Illumination time:                     flash to bang time
                                                                       miles to
Seconds:    Yield:                  min: seconds          ground 0:
 < 1              1-2 kt                       :05                1.1
    1              2.5 kt                       :10                 2.2
    2              10 kt                        :15                 3.5
    3              22 kt                        :20                 4.5
    4              40 kt                        :25                 5.5
    5              60 kt                        :30                 6.7
    6              90 kt                        :45                 9.9
    7            125 kt                       1:00                13.7
    8            160 kt                       1:15                16.8
    9            200 kt                       1:30                19.9
  10            250 kt                       1:45                23.0
  12            325 kt                       2:00                26.7
  14            475kt                        2:15                29.8 
  16            700kt                        2:30                32.9
  20            1 mt                         3:00                39.8
  24          1.5 mt                         4:00                52.8
  27            2 mt                         5:00                65.9
  40           5 mt                        10:00                130
  50          10 mt                       15:00                200
  70          20 mt                       30:00                400


Biological effects of gamma photon exposure.

The measurement of radiation can be very confusing, with a multitude of units used.  For biological purposes the unit of interest is the RAD (radiation absorbed dose) which is a measure of the degree of damage to the body through processes that split chemical bonds.  Since water is the major compound in our bodies, much of the damage from gamma photons comes from the splitting of water into two “free radicals.”   A “free radical” is an atom or molecule with an unpaired electron.  When water is split generally an OH free radical and an H free radical are produced.  Free radicals are extremely reactive, and if they are in the neighborhood of one’s genes, breaks in the DNA will occur.  We have enzymes that can repair certain radiation damage to our DNA, like excision and repair of thymine dimers.  Thymine is on of the four “bases” in our DNA’s genetic code.  If there are two thymines adjacent to each other in the same strand these can become chemically crosslinked by a passing gamma photon.  A gene with such a crosslink cannot be transcribed into messenger RNA (with subsequent synthesis of the protein coded for by that gene) nor duplicated (leading to cell division) until the defect is repaired.  The same is true for a single strand break.  If the DNA in the cell does not have double strand breaks it is likely to be repaired and the cell can then function and divide.  If double strand breaks are in the cell the cell cannot divide.  Ever. 

Now we connect failure of cells to be able to divide with the overt symptoms of radiation exposure.  It is critically important that stem cells in bone marrow and the circulating B cells that make antibodies to be able to divide so the immune system can mount an effective response to an infection.  The other issue is that if the bone marrow cells are heavily damaged the production of platelets will be reduced.  If this reduction is too great, one’s blood will fail to clot properly and spontaneous bleeding will become a serious issue.

Nausea is produced by the failure of cell division of the epithelial cells in our alimentary canal.  Severe diarrhea and vomiting can rapidly cause fatal dehydration.   Doses above 300 rems will cause hair to fall out due to (again) failure of hair follicle cells to properly divide.

Now we consider the effects of 200 rem (or rad) total dose. The lymphocyte (immune cells) count is reduced to about 25% of normal with the minimum count at about 30 days (EWS).  By 50 days the bone marrow has recovered enough so that the count is near 90% of normal.  Platelet counts are minimal at about 30 days also, dropping to about 50% of normal and recovering to about 85% of normal after 50 days (EWS).  Red blood cell counts drop off more slowly and recover more slowly and will likely reach a minimum of about 30% of normal at 50 days (EWS).  At this dose fatality is not likely.  At least half of those receiving this dose will feel quite sick with vomiting and feeling very lethargic (4). For this dose level symptoms will become apparent within 2 to 3 hours (Gl).

As the total absorbed dose becomes higher, the incidence of death from bleeding and infection goes up so that at 450 rem  mortality is about 50% in 2 to 12 weeks. (4).  The symptoms will appear within about an hour.  Incident of vomiting is 100%.  Clearly having a supply of antibiotics is likely to be life saving for doses higher than about 300 rem.  

A few relevant comments here…  very few novels or movies about fallout deal with actually measuring the dose.  Two exceptions are the book “Pulling Through” by Dean Ing (later included in The Rackham Files) in which a Kearny Fallout Meter (KFM) is used to great advantage and the movie “The Day After” which shows putting a remote Geiger tube on top of a building. Alas, Babylon by Pat Frank skirts the importance of knowing the dose rate and has the mistake of saying that gold jewelry will become radioactive on exposure to fallout.  In the PBS movie “Testament” no one in the affected town has any means to measure radiation doses and no idea as to how to minimize exposure.  In the Ray Milland movie “Panic in the Year Zero” the father is smart enough to get his family into a cave.  However, they have no way of knowing when it might be safe to exit the cave and to go outside.  And no way of knowing how long they can stay outside without serious radiation exposure.

As the Dean Ing book clearly shows being able to monitor the dose rate is likely to make the difference between life and death.   Simply being able to ascertain the part of your basement getting the least gamma photons could well be a lifesaver.  Note that if one is able to shelter one’s leg bones so that the radiation dose they receive is minimal one will have protected a sizable fraction of one’s stem cells from damage and preserve to some extent a competent and working immune system

So here is the problem… how to reduce one’s total rem dose to less than 100 and avoid significant damage to our bone marrow and gut epithelial cells.


Part Two

Units of radiation amounts and rates.

For most folks the prefixes to scientific units present a confounding challenge.  It is hoped this table will help in that regard.  We use as an example the unit of mass.

1 megagram (Mgm)                  = 1,000,000 grams
1 kilogram (kgm)                      = 1000 grams
1000 milligrams (mgm)              = 1 gram
1,000,000 micrograms (mgm)     = 1 gram
1,000,000,000 picograms (pgm) = 1 gram

There are, of course, other prefixes, but these are the ones used below.

First we need to distinguish between the amount of a radioisotope and the rate at which it produces radiation.  One measure of the amount of a radioisotope is the Curie.   One Curie has a decomposition rate of 3.7×1010 disintegrations per second.  This unit arose because it is the rate at which one gram of radium atoms will disintegrate.  One Bequerel (Bq) is one disintegration per second.  Contamination of liquids is often given in Bequerels or in picoCuries per liter of liquid.   One Bequerel is equal to 37 picoCuries.   Note that a picoCurie is 37 disintegrations per second.  A small amount for sure.

For civil defense purposes one Roentgen is equal to one rem and equal to one rad.  Since it is gamma photon damage that is the main problem this makes the math a bit simpler.  Rem stands for “Roentgen Equivalent in Man” and rad stands for “Radiation Absorbed Dose.”

Yet another unit has become commonplace, the Sievert  (Sv), another measure of dose equivalence and for gamma photons 1 Sievert = 100 Rems.

Radioisotopes decay according to the first order rate law.  The consequence of this is that for any given radioisotope the half-life is constant.  The first order rate law may be expressed as:

Ln(X) = ln(X0) – k*t.

Where ln is the natural logarithm function, X0 is the amount of radioisotope at time zero, X is the amount remaining after time t, and k is the first order rate constant.  Now the half-life is the time at which X =X0/2.   From this we can solve that t1/2 = ln(2)/k or = 0.693/k. 

A one megaton fission air burst will produce about  0.1 MegaCurie of Sr90 (the isotope of the element strontium with a mass number of 90) (Glasstone).  Thus Sr90 is one of the major radioactive fission products.   It has a half-life of 28.1 years and decays by beta emission.   At the end of each half-life the amount of the radioisotope (and the rate at which it produces radiation) drops by half.  If one began with 1 Curie of Sr90 after 28.1 years 0.5 Curies would remain.  After 56.2 years 0.25 Curies would remain.  After 84.3 years 0.25 Curies would remain.  After 112.4 years 0.125 Curies would remain.  It is the radioisotopes with the longest half-life that pose the greatest long-term threat.

In some cases it is the product of the first decay that is the problem.  For example Cs137 (the isotope of the element Cesium with a mass number of 137) is another major fission product.  It undergoes beta decay with a half-life of 30.2 years.  The decay product is Ba137 (the isotope of the element barium with a mass number of 137).   This nucleus then undergoes gamma decay with a half-life of 2.55 minutes with the gamma photon having an energy of 0.6616 MeV (mega electron volts).  While the initial beta decay particle cannot travel far in one’s body, the subsequent gamma decay produces a very energetic photon.   After this gamma photon emission a stable nucleus results that does not undergo further decay. 


Local and delayed fallout… the importance of the size of the fallout particles and the type of weapon.

Ground bursts would be used against missile silos and certain hardened targets.  Air bursts would be used against cities and airfields.  The nature of the attack would have a substantial effect on the amounts of deadly fallout.  A terror nuke attack might take either form.

“Local” fallout comes from the larger particles that settle to the ground in the first few days.   As fallout particles get smaller their rate of descent to the surface decreases. If much of the fallout is injected into the stratosphere these small particles will partially decay well before reaching the ground.  This is termed delayed fallout and will be spread widely over a range of latitudes.  Of course the radioisotopes with long half-lives will eventually reach the ground.  The distribution of this will be over a very wide area.  Very small particles that are injected high into the stratosphere will remain there for a quite long time as the air is far less dense and Brownian motion will greatly slow the rate of descent.

 However, local weather conditions can produce very hot spots in certain areas.  If one is evacuating an area after a nuke event being able to monitor the radiation rate could be life saving in avoiding “hot spots.”

The overall decay rate of fallout over time.

The mix of radioisotopes produced in a fission event and their differing half-life results in a rate of decay that follows the 7-10 rule (Glasstone).  This rule states that for every seven fold increase in time the radiation rate will drop by ten fold.  Suppose that one hour after the fission event the radiation rate from fallout on your roof is 100 rem per hour.  After seven hours the rate would be 10 rem per hour.  After 49 hours the rate would be 1 rem per hour.  After 343 hours (2 weeks) the rate would be 0.1 rem per hour.  After 2401 hours (100 days) the rate would be 0.01 rem per hour.  This rule holds for about the first six months of decay.  After this time (due to the mix of radioisotopes and daughter products that are also radioisotopes with widely varying half-lives) the radiation rate decreases much faster.  After one year the actual amount of remaining radioactivity is only 40% of that predicted by the 7-10 rule (Glasstone).  After five years the remaining is only about 7% of that predicted by the 7-10 rule.  After 25 years the remaining radioactivity is only 0.02% of that predicted by the 7-10 rule.

Now this rule is true for the fallout from only one fission event.  Suppose there are two such events in your vicinity separated by, e.g., three days.  Then the 7-10 rule must be applied to both events.

It would be very useful to have a supply of 4 cycle log log graph paper. If one is handy with Excel one may generate this paper.  For a single fission event the plot of ln(radiation rate) vs. ln(time) will be about linear for the first six months.  If your plot shows an upward break in the linearity then you are receiving fallout from at least two separate events.

The bottom line is that it will be critical to reduce your exposure to radiation from fallout very especially for the first several days.  Cresson Kearny’s book is extremely valuable for several reasons.  First is the complete set of directions and templates for a home made dosimeter that is really quite accurate (the Kearny Fallout Meter or KFM).  This book has instructions for pre event preparations (the most important of which is to make several KFMs). 

Reducing the exposure to gamma photons from fallout.    We cannot control time, but it is our ally in reducing radiation exposure.  Distance from the radiation source is also important.  Consider a spot source of gamma photon production.  If the dose rate 1 foot from the source is 100 rem per hour at two feet the rate would be ¼ of that, or 25 rem per hour.  At 3 feet the dose would be 1/9 of that, or 11 rem per hour.  This is the radial square law.  However. the most important factor is shielding.  Gamma photons are absorbed by electrons.  The greater the density of the shielding the more dense the electrons.  This means it is the mass between you and the fallout that matters.  The relevant equation is (and is very similar to the first order rate law for radioactive decay):

Ln(I) = ln(I0)  –  m.x 

where I is the radiation rate experienced at thickness x of shielding, I0 is the rate outside the shielding and m is termed the ‘linear absorption coefficient.” (Glasstone).  If the thickness is measured in units of feet the value of m would have units of feet-1.  For example, suppose we are behind 3 feet of shielding with a m = 1 foot-1.  Suppose the radiation rate outside the shielding is 100 rem/hr, then the rate behind the shielding would be 4.98 rem/hr.  Now suppose we are behind 6 feet of shielding.   The rate behind the shielding turns out to be 0.2 rem/hr.

The “protection factor” of shielding represents the reduction in radiation.  For example the 3 feet of shielding would have a protection value of 20 and 6 feet of shielding a protection factor of 500.

One consequence of the above equation is that one may define a quantity not unlike the half-life of a radioisotope: the amount of radiation is reduced by one half for every additional “halving thickness.”  From Glasstone we can compute the halving thickness for gamma photons of differing energies:


MeV energy           air     concrete       iron            lead
   1.0                  285’      0.10 feet     0.049 feet   0.029 feet
   5.0                  660’      0.32 feet     0.092 feet   0.046 feet
 10.0                  880’      0.38 feet     0.10 feet     0.038 feet.

So if all our gamma photons had 5.0 MeV energy and the rate outside the shielding was 100 rem/hour the rate behind 0.049 feet of iron would be 50 rem/hour, the rate behind 0.098 feet of iron would be 25 rem/hr, the rate behind 1.47 feet of iron would be 12.5 rem/hr, the rate behind 0.196 feet of iron would be 6.25 rem/hr, and so on.

These data are for ordinary concrete with a density of 2.3 g/cm.  Iron has a density (at 20C) of 7.784 (CRC) and lead a density of 11.35 g/cm.  If one wanted to construct a fallout shelter the overhead concrete should contain not pea gravel, but pieces of iron or lead.

For the mix of gamma photons produced from a fission event the halving thickness of packed earth is 0.30 feet (CK).   If one had, e.g., 0.6 foot of packed earth between you and the fallout the protection you would have two halving thicknesses, and the radiation rate would be reduced by1/4 (protection factor of 4), for 0.9 feet the rate would be 1/8 that of outside (protection factor of 8), for 1.2 feet the rate would be 1/16 of outside (protection factor of 16) and so on.  A thickness of 1.5 feet of packed dirt would provide a protection factor of 32.   It is not impossible that a shielding thickness providing a protection factor of a 1000 would be needed to survive. 

Glasstone provides idealized, simplified, fallout patterns.  Suppose the event is a surface burst of a 1 Megaton fission weapon and that at all altitudes the wind is directly toward you from ground zero at 15 mph. 

                                   Radiation         Total dose
Miles downwind       rate at               in the first
From ground zero    18 hours           18 hours:

  •                         70 R/hr              3,000 R
  •                        20 R/hr              1,000 R

65                          9 R/hr                300 R
120                        3 R/hr                  50 R

If you are unlucky enough to be closer than 50 miles from ground zero for this event without an excellent shelter you would soon be dead (if winds at all altitudes are from the direction of the event).  For reasons discussed above, do not rely on these estimates, they can easily be much higher or lower.

Houses themselves provide a minimum protection factor (Gl).  Consider a house with a first floor area of 2,000 square feet.  A one story brick veneer house would provide a protection factor of 3.0 for the center of the first floor, and 14 in the center of the basement.  It is assumed that the floor of the first floor is level with the ground all around the house and the basement has no windows nor doors to the outside.   A two-story brick veneer with a first floor area of 2000 square feet would provide a protection factor of 4.1 in the center of the first floor and 34 at the center of the basement.  The protection factors for a two-story 2000 square feet first floor house without a brick veneer would be less, 2.4 in the center of the first floor and 29 in the center of the basement.

Part Three

So… let us suppose that a fission event has taken place and you live in a two story house with a basement that has some small windows on one side, and is only half covered with dirt on that side, being all the way covered up to the first floor on the other three sides.  Exactly where in this basement has the highest protection factor?  The only way to discover the best spot to sit in would be if one had a device to measure the radiation rate.

There are many other reasons why a radiation measuring device is necessary.  In references (RDH) and (BC) are nomograms showing  how to estimate the radiation rate at future times from current rate data.  And a nomogram for giving the maximum time one could be outside at future times only if one knows the radiation rate outside.  As Cresson Kearny points out a device to measure radiation rate is absolutely essential.

Now we move on to a comparative analysis of instruments to measure the radiation rate.

Again we must distinguish between an instrument that measures the rate at which radiation is being experienced (a rate meter) and a dosimeter, which measures the total amount of radiation that the dosimeter has received since the dosimeter was reset or zeroed.  The Kearny Fallout Meter is a dosimeter that may be constructed at home with ordinary materials in a few hours.  If kept dry it is a remarkably accurate dosimeter with a range of zero to at least 43 Roentgens/hr.   By timing the separation between the two leaves of ordinary aluminum foil with a watch one can compute the Roentgens per hour.

For example, if the separation between the bottom of the two leaves of aluminum foil decreases by 14 mm over a time period of 15 seconds the radiation rate is about 43 Roentgens per hour.  If the separation decreases by 2 mm over the time span of 16 minutes the rate is only 0.1 Roentgens per hour.

A table of times, separation distance decreases, and the R/hr is cut out from the instructions and pasted onto the KFM.  This instrument requires no calibration of any kind.  After making this instrument charge up the aluminum leaves as directed and make sure the leaves stay separated over time when only background radiation exists.    The usual background radiation rate is about 20 counts per minute at sea level (unless you have radon gas in your home).

A kit for making the KFM is available from some suppliers listed at  A comment I have about this kit is that the metal can is far too flimsy.  A can from a small can of beans is much more suitable. 

Other choices for dosimeters include:

1. The RADSticker: a chemical dosimeter whose change in color reflects the absorbed dose. These are available from suppliers listed at  At the time of this writing (May 2013) the cost was $25 for five of these. These suffer from the fact that they are single use dosimeters and cannot be reset (rezerod). In addition their shelf life at room temperature in the dark is only about two years.  If kept in a freezer the service life is about ten years.  There are color patches to indicate a total exposure to gamma photons and beta particles with energies above 1 MeV.  The patches indicate if the sticker has received 25, 50,100, 200,400, and 1000 rad exposure.

2.  Quartz fiber dosimeters.  There are several sources.  New or used civil defense dosimeters and chargers are often available from military surplus outfits.  Typically these dosimeters have a 0 to 200 Roentgen scale. The surplus CD dosimeter chargers require one D cell battery.  I do not know if the lower voltage (1.2V) rechargeable NiCad or NiMH will work in these chargers.   The American Civil Defense Association ( has rugged 0 to 200 milli Roentgen (mR) dosimeters.  These are likely to be more useful than the CD surplus 0 to 200 R dosimeters. sells a piezo electric dosimeter charger that needs no batteries.  Check with the vendor for current pricing. It is critical to ascertain that one’s dosimeters are not electrically “leaky” by charging them to read zero and going back after a week to see if the dosimeter still reads zero.  If the dosimeter fails this test I discard it.   One reads a quartz fiber dosimeter by holding one end toward a light, and looking through the other end at a scale on which a vertical line is seen.  This is the quartz fiber.  The accumulated dose is given by the position of this fiber on the scale. also sells quartz fiber dosimeters with a total range of accumulated dose of 200 mR, 500mR, 2R, 5R, 20R, 2 mSv, 5 mSv ranges.  These are alleged to have an accuracy of better than 10% of the dose from either Cs137 or Co60 (the isotope of the element cobalt with a mass number of 60).   The temperature range is –20 C to 50 C.  An option for these is a protective sapphire lens, much more scratch resistant than glass lenses.   Over time the lens tends to get scratched up with use.  If one is using a dosimeter to record total received dose when one is outside it is important to keep the dosimeter in a plastic bag and clip it onto your belt to record the average dose received by your body.  Since most of the radiation will arise from fallout on the ground, your feet will encounter a higher dose of gamma photos than your head.  The plastic bag keeps fallout from contaminating the dosimeter.  This same idea is useful for any radiation measuring device taken outside after an event.   Note that conventional “electroscope” dosimeters do not suffer from the “saturation effect” that conventional Geiger-Muller tubes do as explained below.


Rate Meters

A very important problem with Geiger Muller tubes is that at higher radiation rates they become saturated so that the meter reading can be seriously in error on the low side.  Cresson Kearny cites the example of a dose rate meter sold in 1982 that when exposed to a dose of 150 R/hr it gave a reading of 13.9 R/hr.  Dr. Bruce Clayton tested another of these same meters at a dose of 400R/hr and the reading was only 16 R/hr.

Some beta emitters, e.g. tritium, produce such an extremely weak beta particle and are not generally detectable by any rate meter.  Even with a liquid scintillation counter the efficiency is only about 30%.

One may often see for sale the yellow surplus CD rate meters from military surplus sources.  These in general have NOT been calibrated and are likely to have very serious error even if the “check” function reads ok. The surplus CD meters were specifically designed to work properly up to 500 R/hr and in general only measure gamma photons.  The vendors listed at sell CD meters that have been properly calibrated by the ki4u lab.  I’ve not been able to discover the frequency with which these CD meters need to be recalibrated. 

The online store sells several types of rate meters.  Their M4 radiation rate meter is said to be accurate up to 50 mR/hr with an accuracy of +-15% for gamma photons from Cs137.  It detects gamma photos down to 10 keV through the window and down to 40 keV through the case (plastic) of the meter.   Detection efficiency for 50 keV beta radiation is about 35%.  Detection efficiency for 150 keV beta radiation is about 75%.   Detects gamma photons down to 10 keV through the mica window of the small Geiger tube. The analog meter has a useful logarithmic scale that provides easier reading at the low end of the ranges of 0 to 0.5 mR/hr, 0-5 mR/hr, and 0-50 mR/hr.  A 9 V battery is used and there is a battery check function.  With continuous use an alkaline battery will last about 2000 hours and there is a battery check function.  There is an audible click for each particle detected.  This may be turned off for silent use.  There is a headphone jack for earphones for the audible clicks.  The manual for this meter is quite comprehensive and useful.  This rate meter is available as a kit.  I’ve assembled two of these kits and both worked the first time.  These are NOT kits for beginners.  The circuit board traces are small and the soldering tricky.  It is easy to have a cold solder joint or a solder bridge between traces.

In the March, 2013, issue of “Nuts and Volts” magazine is an article describing a homemade radiation meter.   As of this writing a complete kit is available to construct this device from  This device features a voltage output that can be continuously logged onto a PC with software which uses a $29 Dataq voltage data logger that connects to the PCs USB port.   The device contains a built in timer capable of counting seconds, minutes, hours, or days and a LCD display.  One may select the time interval for recording radiation by means of a momentary switch.  An audible alarm is given if the radiation rate exceeds 30 uR/hr.  An alkaline 9 V battery will last about two days with continuous use.  The unit is mounted in a small plastic Serpac case, which has a built in holder for the 9 V battery.  This is not a kit for the beginner.  I had trouble identifying which transistor was which.  The circuit board has tiny traces.  Two capacitors were missing. 

A very interesting and useful rate meter is the RADAlert, small enough to fit on one’s key ring.  Available from some vendors listed at the site.  We keep one of these in the glove compartments of our vehicles.   This detector does not use a Geiger-Muller tube.  Rather there is a compound inside that produces a small flash of light when struck by a gamma photon.  This flash of light strikes a cadmium sulfide photocell, which produces a small voltage spike that is processed in a way that the number of chirps emitted by the RADAlert reflects the amount of radiation.  This detector arrangement is not easily saturated.  At 50 (and above) R/hr 10 chirps will be heard every thirty seconds.  The number of chirps decreases with decreasing radiation rate measured so that if the radiation is 0.1 R/hr only one chirp is heard every thirty seconds.  The internal battery is said to be good for about 10 years.  One may test the device by chilling and then returning to room temperature.  This process produces chirping until the innards of the device all reach ambient temperature.  Note that the upper reading this device will properly measure is 1/10 that of the CD meters and 1000 times that of the M4 meter discussed above.  The RADAlert does not respond to alpha or beta particles, but I do not consider this limitation of any real importance.  The great thing is that this device is constantly on.

This is by no means an exhaustive list of available rate meters. For example, has a number of rate meters for sale.

 Blocking uptake of radioactive elements:

The concept here is competition.  The body cannot distinguish between a stable isotope of, e.g., iodine and a radioisotope of iodine as they are chemically identical.  If we ingest 1 picogram of radioactive iodine along with 1 microgram of stable iodine we would expect that 1,000 times less radioiodine would be taken up by the thyroid gland than stable iodine, and the subsequent radiation damage would be reduced by a factor of a thousand as opposed to the situation in which only 1 picogram of radioactive iodine were ingested.   The blocking competition below is for ingested radioactivity with the exception of iodine, which exists as a gas, and if inhaled will make its way to the thyroid gland.   The other radioisotopes listed will be inhaled as small particles and for the most part will remain stuck in the lungs.

  • Radioiodine is perhaps one of the greatest threats.  The principal  radioisotope produced is I131 (the radioisotope of iodine with mass number 131) whose half-life is 8.07 days (6).  This is a beta and gamma photon emitter with 90% of the beta particles emitted having an energy of 0.606 MeV.  Iodine is rather rare and is actively concentrated by the body into the thyroid gland, as it is essential for the synthesis of thyroxin.  The other problem with iodine is that it exists as a gas under ordinary conditions. Please note that this radioisotope becomes concentrated in cow’s milk when the cows’ pasture has been contaminated with this radioisotope.  So there are two subsequent concentrating processes that make milk especially hazardous for children after a nuke event.  I’ve read that 40% of children in the Fukushima area have thyroid abnormalities.  Two bad outcomes are possible.  The thyroid gland may literally die or it may develop a malignancy.  The Chernobyl disaster also resulted in a huge number of children with damaged thyroid glands.  For this reason one essential prep to have is a large supply of nitrogen packed nonfat dry milk.  One may protect one’s thyroid gland by taking potassium iodide or potassium periodate pills.  Experiments in the UK have shown that the bioavailability of the iodine is essentially identical for the two compounds.  It is recommended that folks buy and store sufficient tablets for a dosing lasting several months.  For an adult about 100 milligrams of potassium iodide per day is about correct.  Some people develop allergic reactions to iodine and this must be considered in the dosing regimen.
  • Bone seeking elements.  Strontium, barium, and radium are chemically similar to calcium and become concentrated in bone tissue.  And where are stem cells located that turn into red blood cells, immune system cells, and platelets are located?  In long bones.  A serious problem here.  A supply of calcium citrate tablets would be an excellent idea. (4)
  • Plutonium.  This is an extremely dangerous radioisotope if ingested or inhaled.  Uptake from ingested material can be blocked to a limited extent by iron supplements, as these two elements have similar chemical behavior.
  • Cesium.  Cs137 is  chemically resembles potassium and foods rich in potassium will help block absorption.  The good news is that Cs will be slowly excreted and is not concentrated in any one tissue. (4).
  • Zinc.  Zn65 (the radioisotope of the element zinc with a mass number of 65) is a gamma photon emitter with a half-life of 243 days.  Seaweed and certain nuts are good sources of Zn and will help block uptake.  Zn is to some degree more long lived in the body than cesium because it is tightly bound to many enzymes.
  • Cobalt.  Co60 is a very minor product of fission reactions.  If, however, an enemy wanted to produce heavy fallout damage having the bomb casing made of cobalt would produce enormous amounts of Co60  which is a gamma photon emitter with a half-life of 5.26 years.   As far as I am aware cobalt is only needed as the central atom in vitamin B12.  It may be that increasing the amount of B12 in one’s supplements might reduce the uptake of Co60.


When it becomes safe to take short trips to the outside.

The nomograms referenced above will, along with the known radiation rate outside, allow you to decide how long you can be outside your shelter each day.  A dosimeter in a plastic bag worn on one’s belt when outside will serve the same purpose but it would be great to know in advance how long you could stay outside.  The critical thing here is NOT to bring fallout back into your shelter with you.  A “duster” type coat with a hood should be kept outside the shelter, but protected from falling fallout, rain, and snow, along with boots.   A covered porch would meet these requirements. On leaving the shelter put on the duster and boots.   One may buy tyvek thin hazmat suits.  If you wear one of these over the duster you will have even less contamination available to enter the shelter with you.

How not to contaminate the shelter when you return to your shelter.

Before returning to the shelter at the very least remove the duster and boots.   I further suggest keeping an entire set of clothes out on your covered porch and leaving the shelter buck-naked and return the same way.   Obtain a solar heated shower device and keep it handy on your covered porch and shower before entering the shelter naked.  And getting dressed in the shelter with clothes that have not been out of the shelter.  Immediately check your dosimeter and write down the accumulated dose you have received on this outing.  A radiation rate meter should be used once you are inside to check your naked body for residual contamination.  Safety before modesty.  Important places to check are the ends of fingernails, hair, and nasal entrances.  I suggest men stay clean shaven with hair cut as short as possible.   An N-95 facemask should protect you from inhaling fallout.  Remember that the extremely fine particles are (in the absence of a “hot spot” produced by local rain or snowfall) are going to be a very minor issue.

Frequently survey all around the inside of your shelter to insure you’ve not contaminated anything inside.  Pay particular attention to the floor.   Yet another very important reason to have a low-level rate meter in your shelter.

My Suggestions:

Visit and download their piece on what to do if you believe that nuke event(s) may soon happen.  Then read it.  Many most excellent suggestions.

Nuclear War Survival Skills by Cresson H. Kearney is a must have book for a great many reasons.  The first and foremost are the field-tested instructions and templates for a KFM.   Included are field-tested instructions for the construction of expedient fallout shelters.  Note that there will not be enough time after the event to construct a high protection factor shelter.  Kearny points out the extreme importance of adequate ventilation in an enclosed shelter and shows that the CD plans for shelters given out during the Cold War would result in the deaths of the occupants from inadequate ventilation.  Living humans emit an overall energy of about 100 watts and a substantial amount of water.  In an unventilated shelter the temperature and humidity levels will rise and become lethal.  His book provides plans and instructions for making a quite usable air pump for ventilation of a shelter interior. 

Build two KFMs and practice using them.  Obtain several dosimeters of various ranges and a dosimeter charger.   Check the dosimeters for leakage.  Consider getting two rate meters: one a calibrated surplus CD meter and a more sensitive rate meter for less than life threatening radiation levels.  All electronics of any kind need to be placed in an adequate Faraday Cage.  See reference (AB) for complete information on EMP and Faraday Cage protection. 

Get a two-month supply of KIO3 tablets for each of your family members.

[JWR Adds: The now dominant school of thought in medical circles is that Potassium Iodate (KIO3), Potassium Iodide (KI) or other thyroid blockers for radiological events are not recommended for anyone over 40.]

I expect most of the readers of this excellent blog already know the importance of having stored a lot of food and water.

Understand how to make an expedient shelter in your basement.  Reference (BC) shows sketches of how to arrange household articles around an area to provide some protection. 

Consider making a basement shelter with filled concrete blocks as shown in reference (BC).  Note that protection factors are multiplicative.  If a location in the center of an interior basement gives a protection factor of 35 then a stack of filled concrete blocks 8 inches wide would itself provide a protection level itself of four for 5 MeV gamma photons.  Inside the 8 inches of concrete around and above oneself the total protection factor would be 280.   Note that any entryway into the area of stacked concrete blocks must have a 90-degree turn so that gamma photons cannot enter the “safe” area of the shelter.   One would have to do careful engineering to design such a shelter whose 8 inches of concrete on the top would not collapse and crush the inhabitant.

If you are concerned about potential blast wave or fire damage the obvious choice is a dedicated underground shelter.   You might consider something along the lines of our shelter.  We are fortunate to be on a 35-degree slope, which made things easier.  We acquired a piece of a large abandoned diesel fuel tank some 64 inches inside diameter, six feet long, with one end cut off.  These may often found in farming areas for the price of the scrap metal.  First I cleaned off the old diesel from the inside and painted the inside white with an oil alkyd paint.  I painted the outside with a similar paint (which bonded well to a slightly rusty surface) and then coated the outside with roofing tar.  I excavated (with a shovel and pickaxe) a place on the slope to place this piece.  With three come-alongs I positioned it into the excavated site. 

The inside was braced vertically by PT 4×4 posts; cut tapered to just fit inside.  Horizontal bracing was with PT 2x4s.   This bracing was used to build shelves at the back end and sides of the enclosure.  A door opening inside (and the door) was framed with PT 4×4 posts for some blast resistance.  The bottom horizontal braces were used to provide a flat floor of deck boards.  A small shed was built over the entrance and camo tarp used to enclose the entrance to the shed.

The tank was situated so that its three openings were at the top.  Two of these were 2” in diameter and were fitted with PCV pipe for ventilation, ending in a 90-degree angle pointing down with a screen wire block for insects.  A third hole, about 1” diameter, was used to run in 12 VDC from a solar array nearby charging a bank of golf cart batteries.  In addition a short-wave antenna lead was passed though this hole and the hole sealed.  I’ve a Drake SW-2 digital radio in a Faraday Cage inside.  There is a 12 VDC lighting system, using vehicle bulbs, which can be turned on one light at a time if need be.  I keep a mechanical windup alarm clock, lots of 4 cycle log log graph paper, KIO3 tablets, first aid kits, HazMat booties, gas masks, dosimeters and charger, sleeping bag, spare clothes, water, food, and other goodies.  If the balloon goes up the radiation meters go into the cave with us.  A solar shower and a small port a potty reside in the covered entrance.

This doubles as a root cellar.  Our saved seeds are stored there in evacuated glass jars, in the cool and dark.  A 12 VDC fan in the door provides adequate ventilation when needed.  This project is not yet complete, as I need more shielding on the top of the tank.


  • The Effects of Nuclear Weapons, Samuel Glasstone and Philip J. Dolan, 670 pages, November 2006  (in print)  (Note: It may be possible to find one of the used early editions that contains a pocket in the back cover with an incredibly useful circular slide rule.)
  • The Day we Bombed Utah, John G. Fuller, New American Library Books, New York, 1984
  • Nuclear War Survival Skills by, 1987 revised and updated edition
  • Life After Doomsday,  Dr. Bruce Clayton, The Dial Press, New York, 1980
  • Emergency War Surgery (NATO Handbook:- Third United States Revision, 2004) by Dr. Martin Fackler, et al. U.S. Defense Department
  • Radiological Defense: Planning and Operations Guide, Department of Defense, Office of Civil Defense, SM-11.23.2 June 1966
  • CRC Handbook of Chemistry and Physics”, 56th edition, 1975 CRC Press
  • Disaster Preparedness for EMP Attacks and Solar Storms, Dr. Arthur Bradley, 2012 edition


  1. Very good article. A couple of diagrams of fallout plumes might have help folks understand the impacts of yield, height of burst, wind direction and strength on fallout patterns. FEMA produced a US map with “Sample Radioactive Fallout Pattern” that show one possible big picture. The impact of potential surface bursts on the Minuteman silos in Wyoming, N. Dakota and Montana are unmistakable.

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