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Saturday, April 05, 2014

Nuclear Research Reactors - MK 1 - Neutron Activation Analysis

General Atomics owned and operated the very first TRIGA reactor between 1958 and 1997.  It was  called the MK 1, and was the brain-child of the awesome physicist Freeman Dyson (NOT the  vacuum-cleaner sales guy).  The reactor has since been decommissioned and I believe is now a technological historic site.

During my time at the facility, in the late 1980's, the larger Mark 4 reactor was tied up full-time with the thermionic cell experiment and the gemstone color enhancement "experiment" (or production).  While the activities on the MK 4 were interesting and paid the bills, there were a greater variety of experiments that took place on the MK 1 reactor, which was available for other uses.

The MK 1 was frequently in use by a radiochemist, who was doing a great deal of analyses for a class-action lawsuit over PCBs.  PCBs, or PolyChlorinated Biphenyls, are persistent toxic pollutants, and are carcinogenic.  The maximum allowable standard set by the EPA for drinking water is zero, but due to the limitations of water treatment technology, the de-facto action level is 0.5 parts per billion, or ppb.

Resolving PCBs down to 0.5 ppb is difficult when using wet chemistry however.  PCBs are difficult to extract from a sample and determine the quantity.  This is where radiochemistry helps out.  Because PolyChlorinated Biphenyls contain many atoms of chlorine, we can take advantage of chlorine's nuclear properties to make it stand out.  Here's how:

Neutron Activation Analysis:  The PCB-contaminated samples would be placed in the reactor, along with a blank (an uncontaminated sample of the same material).  These would be exposed to thermal neutrons for a period of time, allowing the chlorine atoms enough time to absorb neutrons, and become radioactive. 

25% of the naturally occuring Chlorine is Cl-37, which readily absorbs a neutron to become radioactive Cl-38.  Chlorine-38 decays with a very short half-life (37 minutes), by undergoing a beta (electron) decay to Argon-38, which is not radioactive.  While decaying, Chlorine-38 also emits gamma rays at energies of 1.64 MeV and 2.16 MeV (Million electron Volts).

Because the half-life of Chlorine-38 is so short, it's a bit like flash-powder:  It flares vastly brighter than its surroundings for a while, then drops off pretty quickly thereafter.  We can take advantage of that, because even a tiny trace of Chlorine will emit a massive amount of gamma radiation, with energies of 1.64 and 2.16 MeV.  We just need an instrument to look for those energies.  For this, we use a Gamma Ray Spectrometer.

A Gamma Ray Spectrometer is a device that is sensitive to the higher energy gamma rays.  Similar to how a prism can segregate light into its component colors (energy levels), the gamma ray spectrometer can determine the energy levels of gamma rays interacting in a gallium or silicon detector, and can segregate these interactions by energy levels. 

Below is a photo of an older gamma ray spectrometer, similar to what we used.  The detector is inside the box at the right.  The bricks are made of lead, to keep any other radioactive items in the lab from interfering with the measurement being made inside.  They are also useful for shielding the technician from hot samples that have just come out of the reactor :)  The little four-wheel cart appears to be how the sample is inserted and removed from the assembly.  These bricks are quite heavy, and you don't want to move them every time you run a sample!  The computer takes the signal from the detector and software generates a graph of the gamma ray spectrum.  The energy levels of gamma rays are like fingerprints.  Each radio-nuclide has its own unique gamma ray decay energies.

Below is the gamma ray spectrum generated by a Cobalt-60 gamma source.  The peaks from Cobalt-60 are at 1.17 and 1.33 MeV, and only Cobalt-60 can generate these particular gamma energies (as it decays to non-radioactive Nickel-60).

With this in mind, we can go back to our original samples of contaminated and uncontaminated material, and look at the 1.64 MeV and 2.16 MeV gamma peaks.  Depending on how much larger they are on the contaminated material, we can determine how badly contaminated with PCBs the material is, without ever the need to extract PCBs or process the sample material in any way. 

 "Yeah, Science!"

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