Posts Tagged ‘cs-137’

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Gamma activity measurements of Tokyo-area soil samples

November 4, 2011

Three nuclear reactors melted down at the Fukushima-I Nuclear Power Plant following the Tohoku Earthquake of March 11 this year, resulting in the release of volatile fission products in what is widely regarded as the worst nuclear accident since Chernobyl.  Radionuclides were carried by air currents across eastern Japan.  Areas closer to the stricken plant suffered heavier contamination, but even densely-populated Tokyo, some 150 miles distant, received significant fallout.  Last month, I received a set of six soil samples from the Tokyo region, and, using my HPGe gamma detector, I have attempted a quantitative analysis of the two predominant gamma activities in these samples, Cs-137 and Cs-134.  I am grateful to Jamie Morris for the specimens, and to Dr. Steven Myers, Los Alamos National Laboratory, for his helpful communications about technique and analysis.

Jamie collected six soil samples of about 5 fl. ounces apiece, three from roadside gutters and three from nearby garden areas in the greater Tokyo region, and sent them to me in Ziploc baggies by regular airmail declared as “soil samples.”  He documented his collecting spots with geotagged photos (below).

Upon receipt of Jamie’s samples, I packed them into 3-oz clear plastic wide-mouth jars (Uline S-17034), weighed the contents, and Superglued the lids on to prevent spills.

It is important to control the source-detector geometry in quantitative measurements.  To that end, I lathe-turned a holder for the jars out of acrylic that fits onto the HPGe detector’s cap.  The jars press-fit into this holder until the lip of the cap thread contacts the front face of the acrylic piece.  Held thusly, the bottom of the sample jar is nominally one inch from the end of the HPGe cap.

A standard source, consisting of a known quantity of Cs-137 in a matrix and geometry approximating those of the samples as closely as possible, will be used as a reference against which to compare the activity in the samples.  Although commercially available, such sources are astronomically expensive and companies making them are reluctant to sell to individuals who just want to fool around.  So I’ll produce my own from the following supplies, using the procedure recommended on Slide 23 of this IAEA presentation:

  • Play sand (Lowe’s)
  • Liquid Cs-137 source (25µl / 0.5 µCi nominal activity, ±5%) ordered from Spectrum Techniques
  • Sealed Cs-137 disk source (0.5 µCi nominal activity, ±5%) ordered from Spectrum Techniques
  • Nitric acid
  • Beakers, syringe, stirring rod
  • Geiger counter (or scintillator)
  • An oven

Basically, the Cs-137 is mixed with sand and put in a Uline jar.  Click any photo below for a caption describing relevant details from the process.

Gamma spectra are collected from each sample and from the standard in my Canberra NIM MCA, using Mark Rivers’ open-source “mca” application for EPICS and my own LabVIEW interface.  8192 channels of memory are used, with the gain set at about 0.2 keV per channel.  I process the spectra to subtract background and find peak areas in the free evaluation version of FitzPeaks (note: does not work on 64-bit Windows 7).  Spectra for each sample are displayed below (click any image for a full-size version).

Activities are estimated by comparing net counts in the relevant peaks in the sample spectra with net counts in the 662-keV peak of the standard source.  Count rates are scaled to account for gamma emission probability of each nuclide.  A simple exponential attenuation mode is used to correct for matrix density variations; better accuracy can be expected for samples that most closely resemble the standard (i.e. the gutter debris samples).  I use only the 605-keV peak to estimate Cs-134 activity, since it lies closer to the 662-keV calibration energy and the systematic errors involved with energy and matrix density corrections will be smaller than for the 796-keV peak.  Ultimately, the values of interest—specific activities, becquerel per kilogram—are obtained, along with uncertainty propagated through the calculations.  These values are illustrated below:

Download the data and analysis spreadsheet (Excel 2010 format) here.

In conclusion: The synthetic fission products CS-137 and Cs-134 dominate the natural gamma radioactivity (K-40 and U / Th daughters) in all six samples.   Cs-137 is present at levels at least 1-2 orders of magnitude above levels expected from older atmospheric weapons tests and the Chernobyl accident in every one of these samples.  Total activity is roughly evenly divided between Cs-137 and the shorter-lived Cs-134 at this time; the Cs-134 will decay to irrelevance in the span of 5-10 years.  Together, high concentrations of Cs-137 and Cs-134 point to the recent Fukushima accident as the source of virtually all of this activity. The gutter debris sample from Chiba (#C) has the highest activity, and depending on how representative this sample is of the surrounding soil, MAY be indicative of significant enough cancer risk to human residents to encourage alternate patterns of occupancy or land use.  More information would be needed to quantify the severity of this kind of risk from external exposure and various routes of possible internal exposure.   Sample #C is also easily detected with small consumer-grade and homebrew Geiger and scintillation counters.   It should be noted that various physical / chemical mechanisms (e.g., runoff of soluble Cs into road gutters) tend to increase the activity of some of these particular samples relative to the surroundings.

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Videos from my recent trip to Chernobyl

September 17, 2011

Two videos from my most recent radioactive scavenger hunt in Ukraine’s Chernobyl exclusion zone are now on YouTube.  One features a pinhead-sized piece of spent nuclear fuel (pictured at left) that was carefully excavated from under about six inches of soil with the aid of a CDV-700 Geiger counter probe, taken back to our hotel through Checkpoint Lelev (where the scintillation portal monitor was conveniently out of service), and analyzed using a scintillation detector and Marek Dolleiser’s “PRA” software—a clever MCA emulator that uses one’s computer audio device as a nuclear ADC.  Check it out (I recommend selecting the HD format at the bottom of the window):

The second video illustrates some environmental radiochemistry at work, namely the affinity of the beta emitter Sr-90 for the phosphate matrix of deer antlers.  In this video I show that although the gamma activity (i.e. Cs-137 activity) in a pair of shed antlers is no different than local background, the beta activity is much higher.  The reasons for Sr-90’s notoriety are tangibly apparent: a decades-long half life that keeps it cracklin’ long after the accident, and alkaline-earth chemistry that favors uptake in bone.

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Nuclear Collection (Part VI)

March 13, 2011

Click any thumbnail image to view in full size. And, as always, if you have something radioactive and in need of a good home, contact me: I buy and trade all the time. Enjoy!

Lithograph by Leo Vartanian commemorating the CP-1 nuclear reactor.  In what has to be the winningest art idea ever,  moderator graphite from the historic reactor was actually ground up to make the ink in which the portraits of physicists Leo Szilard, Arthur Compton, Enrico Fermi, and Eugene Wigner were rendered.  Prints were distributed by Argonne National Laboratory to honor long and illustrious careers.  The ink is not detectably radioactive.  See my other mementos of CP-1 here. Size is 17″ by 22″(framed).

Though it is in many ways a modern and progressive nation, Japan steadfastly clings to certain curious anachronisms.  From the land of whaling and sailor-suit school uniforms come these examples of radioactive “quack cures”, modern instances of a fad phenomenon that, half a century ago, had largely been driven into extinction in the US and Europe.  Both items pictured—the Wellrich Co. Ltd. “Health Card” (top) and the “Mainasu ION” plaque (bottom)—contain natural thorium as verified by gamma spectrometry.    The “Health Card” claims to offer benefits that include denaturing nicotine in cigarettes.  The health benefits of the negative ion disk aren’t mentioned on it, but surely have no basis in sound science.  It is equipped with an adhesive surface on the back for mounting.  Dozens of varieties of negative ion quack products are peddled by Asian eBay sellers, and I have no idea how many of these items might be radioactive.  The Wellrich card and the ion disk measure 1400 CPM and 550 CPM respectively on a Ludlum 44-9 pancake Geiger tube.  (Donated to my collection by Bill Kolb.)

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More radioactive vacuum tubes. All the specimens in this batch were kindly donated anonymously, and all are receiver protection tubes for military radar sets.  In this application, gas breakdown, aided by deliberately-included radioactivity, dissipates any high-power RF energy that finds its way into the receiver waveguide.  From left to right in the top photo: Varian MA37002X with Co-60 (originally “0.7 microcuries max.”), date code 1995; Omni-Wave MPT-24 with (originally) 25.0 microcuries of Kr-85, date code 1984; Omni-Wave MPT-47-B with (originally) 25.0 microcuries Kr-85, date code 1976.  The gamma spectra of the two Kr-85 tubes clearly shows the residual 514-keV gamma activity of the 10.8-year fission product and even permits a coarse estimate of the quantity remaining (about 3 microcuries in the MPT-24, 0.2 microcuries in the MPT-47-B).  More radioactive tubes are described here and here.

Large receiver protection tube with tritium. The application is the same as the tubes mentioned above, but this one is a monster, measuring almost 16 inches in length.  The part number is MA3948L-12, the manufacturer is Varian, and the contents are mostly argon and a small amount of radioactive tritium (H-3), 10 mCi.  The second photo shows an electrodeless RF discharge established in the tube.
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Contaminated Geiger counter strap from Chernobyl trip. Last summer’s trip to Pripyat resulted in detectable radioactive contamination of my shoes (see description) as well as this shoulder strap.  Gamma spectrometry easily identifies Cs-137, one of the handful of long-lived fission products, in a hot spot on the strap.  The activity in the spot is small, only about one nanocurie (~35 Bq).  Some possible contribution from the synthetic transuranic americium-241 is also noted.

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