The release of dangerous radioactive substances on civilian populations by a terrorist as in a dirty bomb is one of the worst-case situations short of a nuclear blast itself.   AristaTek has written an earlier article on “dirty bombs” and the havoc that they may create  (see  March 2003 Newsletter, Technically Speaking).  This newsletter article looks at the problem of locating illicit radioactive materials, of the type that might be used to make a dirty bomb.   Let us first review some radiation basics. 

Radiation Basics

Of the approximately 200 different radioactive isotopes listed in the PEAC tool, only a few stand out as being highly suitable for radiological terror.  These are cesium 137, cobalt 60, californium 252, strontium 90 (at its short-lived daughter product yttrium 90), iridium 192, radium 226, plutonium 238, and americium 241.

Example: Cesium 137:

Let us look at cesium 137 as an example.   We will set the PEAC tool to read metric units, go to “radioactive isotopes”, select “cesium 137” or “Cs 137” and look at the PEAC tool display.   Sometimes cesium 137 is written as ¹³⁷Cs.   The number “137” is the sum of the number of protons (55) plus neutrons (82) in the nucleus of the cesium atom.   The atomic number (55) is the number of protons.   An atom must have the right number of neutrons and protons to be stable.   If there are too few or too many neutrons for the number of protons, the atom will shed some of its excess mass (in the form of alpha or beta particles or even neutrons depending upon the radioactive isotope).  In the process of shedding mass, a minute portion of the atom mass will be converted to energy.   In the case of Cesium 137, much of this energy will be converted to gamma radiation with an energy level of  0.6616 MeV  (million electron volts).   A beta particle will be ejected from the nucleus, which will have a kinetic energy of up to 1.176 MeV.   The beta particle has a mass (same mass and charge as an electron) and will slow down and eventually stop as it interacts with the surrounding air.   The maximum travel distance of the beta particle from the Cesium 137 source can be calculated (e.g. 419 cm in air at sea level).   On the other hand, gamma radiation has no mass and can travel much further.  Theoretically, gamma radiation can travel an infinite distance in the vacuum of outer space.   If there is some material in the way, the gamma radiation will eventually be absorbed as it penetrates the material.

Cesium 137 has a half life of 30.2 years.  This means that if there is a canister containing some cesium 137, half of the cesium 137 will be gone after 30.2 years.   The cesium will be converted to barium 137, which is stable, meaning that there is no further radioactive decay.  A barium 137 atom contains 56 protons and 81 neutrons.   One of the neutrons of the cesium 137 atom has been converted to a proton and a beta particle plus energy (kinetic energy of the expelled beta particle plus the gamma radiation) to give barium 137.   When barium 137 is initially formed (designated barium 137m, with a half life of 2.55 minutes), some x-rays are given off before forming the stable isotope barium 137.  We can’t predict when an individual atom of cesium 137 will undergo radioactive decay, but a few milligrams of cesium 137 will contain many billions of atoms and half of these will be converted to the stable (non-radioactive) barium 137 after 30.2 years. 

The PEAC tool display shows cesium 137 has a radiation activity of 86.7 curies per gram.   Radiation activity can be calculated directly from the half-life, e.g.

Radiation activity = 1.31(10)⁸ /[ (half-life in days) ( # of protons + neutrons)]

For Cesium 137,

            Radiation activity = 1.31(10)⁸ /[(30.2)(365)(137)] = 86.6912 Ci/g

Sometimes radiation activity is expressed in Becquerels (Bq).  To convert Curies to Becquerels, multiply by 3.7(10)¹⁰ .  

People have asked, what does cersium 137 look like?   Is it a powder and what color is it?  How can I recognize it?.    Cesium 137 looks like any other chemical.  It may a powder, an aqueous solution, a solid, or mixed with other chemicals.  It may be in the form of cesium chloride (CsCl), cesium fluoride (CsF), cesium sulfate (Cs₂SO₄), cesium hydroxide (CsOH) or some other chemical.   The only way of detecting it is by the radiation given off, in particular, gamma radiation with an energy level approximately equal to 0.66 MeV.   A first responder investigating a mysterious package or canister might be tipped off if the package is unusually heavy for its size because of lead shielding, but there is no guarantee that a terrorist will adequately shield the package.  The radiation given off would cause the canister to feel warm to the touch, but by that time the responder would already have been exposed to dangerous radiation.

Example Cobalt 60:

Let us look at Cobalt 60  (or Co 60) in the PEAC tool.  A Cobalt 60 atom has a nucleus with 27 protons and 33 neutrons.   The half life of cobalt 60 is 5.271 years.  When a cobalt 60 atom undergoes radioactive decay, a neutron is converted to a proton and a beta particle and a very small amount of mass is converted to energy.   The energy shows up in part as kinetic energy of the beta particle, which is ejected from the atom, and partly as gamma radiation.  Each atom disintegration produces gamma radiation at two energy levels, 1.1732 MeV and 1.3325 MeV.    If a responder had a gamma radiation detector capable of measuring gamma radiation at different energy levels and measured radiation at 1.1732 MeV and 1.3325 MeV, some cobalt 60 would be expected to be nearby.  The cobalt 60 atom undergoing radioactive decay is converted to nickel 60, which is a stable atom and does not undergo further radioactive decay.   Nickel 60 has 28 protons and 32 neutrons.  

Example  Californium 252:

The PEAC tool display for Californium 252 (or Cf 252) shows that a lot of things happen with this radioactive isotope as it undergoes radioactive decay.  A californium 252 atom contains 98 protons and 154 neutrons.  The half life of Californium 252 is 2.65 years.   Most (96.9%) of the atoms undergoing radioactive decay shed a mass equal to two protons and two neutrons (called an alpha particle, or alpha radiation) from the atom nucleus forming curium 248.   The curium 248 nucleus has 96 protons and 152 neutrons, and also undergoes further radioactive decay.   The other (3.1%) of the atoms of Californium 248 undergo spontaneous fission, that is, the atom splits apart forming smaller fragments.    There are many different ways that Californium 248 can split into smaller atoms.  These fragments are also radioactive, that is, they may shed alpha and beta particles plus more gammas radiation.   During the fission process itself, neutrons and beta particles will be expelled from the atoms plus accompanying gamma radiation.   Neutrons can travel far from the source and can inflict severe damage to the human body.   Prompt gammas refer to gamma radiation emitted at the time the atom undergoes fission; delayed gammas refer to later gamma radiation.    Almost all (99+%) of the neutrons emitted at the time the atoms undergoes fission are prompt, that is, they are emitted at the time of fission.   Very few (<1%) are delayed.

If cerium 248 is pulled up on the PEAC tool, the display shows a half life of 340000 years.  When cerium 248 undergoes radioactive decay, 92% of the atoms shed an alpha particle producing plutonium 244 and the other 8% undergo spontaneous fission producing various smaller atom fragments, neutrons, gamma radiation, and beta particles.  Plutonium 244 has a very long half life (82 million years), but eventually it too will shed an alpha particle (99.9% of the atoms) producing uranium 240; a small fraction (0.1%) will undergo spontaneous fission.   Uranium 240 has a short half life (14.1 hours) shedding a beta particle producing neptunium 240, and then plutonium 240, and so on, until stable elements are eventually produced many millions of years later.

What Does Radiation Basics Tell Us?

Two things given off by radioactive materials can be measured by detection equipment located at a distance.   One is gamma radiation, the other is neuron particle radiation.   Gamma radiation is given off by radioactive isotopes, which can be used to make a dirty bomb.   Neutron radiation is given off by certain heavy radioactive isotopes (atomic number 92 or greater) undergoing at least some fission, and could mean the presence of  ingredients to make a nuclear bomb.  These heavy radioactive isotopes also give off gamma radiation. 

Alpha and beta particles do not have enough kinetic energy to travel from the source. Alpha particles can travel at best only a few centimeters and beta particles only a few meters in air.   Gamma radiation and neutrons do not have that restriction.   Some of the natural gamma radiation reaching the earth originated from galaxies millions of light years away. 

Gamma and neutron radiation decrease inversely approximately as the square of the distance from the source.   This means for example there is a flux of a million neutrons per square centimeter (or a million gamma photons per square centimeter) at a ten meters distance from the source, there would be approximately 10,000 units per square centimeter 100 meters from the source.   The word “approximately” is used rather than “exactly” because some scattering takes place as the radiation travels through the air.  

Another thing radiation basics tell us that each radioactive isotope is different.   The various energy levels associated with gamma radiation represent a signature for that isotope allowing it to be identified.  Mixtures of several radioactive isotopes will be more difficult to identify, but even these may have recognizable signatures at distinct gamma energy levels.

Gamma and Neutron Signatures

Let us look at some gamma and neutron radiation measurements that might be recorded by a monitoring system.   The detection systems for gamma radiation and neutrons are different so we will look at gamma radiation first.   Gamma radiation detectors are available which simply give a total count rate, but we will look at a system that gives the total gamma-ray spectrum as a function of energy level.   This will help in identifying of the radioactive isotopes and to distinguish between the target illicit material and natural radiation background.

Figure 1 shows a hypothetical tracing of what background radiation might look like for a detector allowed to record counts at different energy channels  [1000 KeV = 1 MeV] for some specified period of time.   The natural radiation comes from many sources, the rocks and soils of the ground, building materials, radiation from outer space, and some minute residuals from nuclear tests conducted decades ago.

Figure 2 shows a plot at the same location and for the same time period but with some cobalt 60 located perhaps 100 meters away.    The increased counts at energy levels near 1.173 and 1.333 MeV represent a signature for cobalt 60.  

Recall that gamma radiation decreases inversely as the square of the distance from the source.  As the detector moves further from the cobalt 60 source, the cobalt 60 counts will blend into background.  But if the detection system does identify peaks, the instrumentation system can be set to measure counts at these energy levels and move into the source.