Detection of Nuclear Weapons of Mass Destruction NWMD

The primary objective of any radiation measurement regimen would be to scan the most territory in the shortest period of time for the least cost. Detection at a distance remains the primary problem. How does one detect a radioactive object and distinguish it from the naturally occurring background radiation (NORM)? Sources such as the naturally occurring minerals consisting primarily of uranium and thorium and their decay isotopes in rock and soil along with cosmic rays from outer space contribute approximately 10 to 25 microrad per hour. Any measurements must compete against this "background noise".

A man made, compact "point" source of intense radiation radiates in all directions. The strength of the radiation will decrease with the "inverse square" of the distance from the source. If you double the distance between the detector and the point source, the radiation level drops to one fourth. Triple the distance and the field strength is now one ninth. In addition, radiation is adsorbed to varying degrees by any intervening matter, including air. Alpha particles are stopped by a few centimeters (less than 2 inches) of air or a sheet of paper. The alpha particle could not be detected at any realistic distance. The beta particle would travel a greater distance but the majority of these high-speed electrons are captured in a thin sheet of lead or a thicker sheet of aluminum shielding. This leaves the detection of gamma rays (high energy "x-rays") to be the primary means of detecting radioactive materials. Thermal neutrons emitted by weapons grade material can be detected provided that neutron shielding is absent.  

Three primary portable tools exist to detect gamma rays. The most expensive and sensitive detectors use a sensitive crystalline semiconducting material that will directly adsorb the gamma energy and convert it into free electrons which are captured and measured. These crystals are also very sensitive to background gamma radiation. Of equal or greater sensitivity, the scintillation counter relies upon an optically clear crystal coupled to a photomultiplier tube. The scintillation crystal give a weak burst of light (scintillates) with each gamma detected and the light enters the photomultiplier tube where it is converted to electrons which are amplified billions of times. This forms an electrical pulse which is proportional to the gamma ray energy. Most radioactive elements emit one or more characteristic gamma rays at known energy levels. If enough gamma rays are captured, the radioactive elements can be identified.

The third and least expensive method uses the gas-filled Geiger-Muller tube to primarily detect gamma rays. A few percent of the gamma rays that cross through the volume of gas within this Geiger tube are converted into a pulse of electrons for each detected gamma ray.  A Geiger tube about the size of lipstick container will detect approximately one gamma ray each minute in a radiation field of a microrad per hour. This results in 10 to 25 counts per minute in a 10 to 25 microrad/hour NORM field with the Geiger tube used in the RadScanner Model 500VBR.

No matter which detection method is used, at a certain distance, it becomes unlikely that a "point source" can be distinguished from the background radiation. If directional shielding is used around the detector with an opening pointed toward the suspected source, some relief from detecting gamma radiation of cosmic or terrestrial origin may be had at the expense of needing to always keep moving the detector to scan new territory. Alternatively, at checkpoints, such as entryways to ports, bridges and tunnels, an array of directionally shielded detectors can be pointed at railcars or truck containers and scanned for "hotspots".

The detection of neutrons with specialized equipment would require a better knowledge of the possible sources of neutrons from within a "suitcase bomb". Since neutrons are used to start the chain reaction, the number emitted would have to be under some threshold to prevent detonation. This will not be covered in this review. Since uranium and thorium atoms emit neutrons upon decay there is always some small background neutron flux. Certain shielding materials will absorb neutrons and prevent their detection.

The primary task of detecting a point source of nuclear radiation should be divided into at least three categories.

First, consider the "suitcase bomb" that most probably will be the size of a heavy footlocker or larger and should not emit much radiation with heavy shielding. The thick shielding would make the weapon opaque to x-ray imaging. Suspicion should be aroused upon finding a heavily shielded container the size of a foot locker or 55 gallon drum that cannot be easily moved because of the weight of the shielding and that does not allow an x-ray photograph to be taken.  

The second scenario involve the so-called "dirty bomb", a conventional explosive wrapped with intense radioactive isotopes, like those encapsulated powders used in medical or industrial applications. Strict inventory and security of all known hospital and industrial sources must be maintained to protect against inadvertent or intentional spread of highly radioactive dust. The capsule of highly radioactive material is always stored in thick metallic lead shields to protect hospital and industrial workers. The capsule is easily detected without the shielding.

The third concern would be distinguishing "false positive" responses to medical tests using trace amounts of radioactive compounds routinely used for cardiac stress tests and thyroid scans and treatment.

Hand held, pocket sized, Geiger counters like the Model 500VBR RadScanner from PROGRESSIVE SYSTEMS COMPANY of Houston, Texas ( are capable of detecting those persons undergoing medical tests with short-lived, radioactive trace elements. The detection range is 2 to 3 meters (6 to 12 feet) dependant upon the radioactive isotope used and the elapsed time since administration. Most of these medical tracers have half-lives of hours or days, but emit intense fields 1000 to 2000 times the normal background radiation levels directly after administration.

As an example: "In one case last spring, a man being treated for an overactive thyroid gland was stopped at Pennsylvania Station. In another case, a woman who had undergone a diagnostic heart study was stopped while trying to drive out of Manhattan through a tunnel. In both cases, the people involved had been treated with radioactive materials. And in both cases, doctors said, they were stopped by law enforcement officers armed with radiation detectors used to track possible terrorists."-- "High Security Trips Up Some Irradiated Patients, Doctors Say" By AL BAKER, NYTimes December 4, 2002

In those war zones where depleted uranium (DU) shells have been deployed, large areas have higher than normal background radiation. Shell fragments have a count rate 1000 to 2000 times the background rate. DU is approximately 75% as radioactive as naturally occurring uranium metal. These tank-busting weapons have been used extensively in the last two Gulf Wars, in the former Yugoslavia and on practice and test ranges. Each of these rounds bursts into flame upon impact forming clouds of uranium oxide dust that settles over the surrounding.

Domestic and overseas areas where uranium is mined and ore processing sites will give high readings because of spills and leaks.

Some areas have naturally occurring high levels of dissolved radium salts in well water. Gases purged from these wells, primarily radon, will give false positive readings.   

In spite of the occurrence of false positive responses, a detection methodology can be structured for any situation.

Equipping and training a large number of first responders with a small, rugged and low cost Geiger counters like the Model 500VBR would quickly identify hotspots and allow the greatest area to be covered. Contaminated areas can be taped off to prevent personnel from tracking radioactive debris over a larger area. Decontamination stations can be placed at checkpoints for emergency responders and victims.

PROGRESSIVE SYSTEMS COMPANY    1620 West Main Street    Houston, Texas 77006-4712
Tel:  713 523 0515

Back to Table of Contents