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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.

11 comments

  1. Thanks for taking the time to do this work. It has been difficult to get what I would call believable information about radionuclide distributions for this accident. Having done similar measurements some years ago, it’s easy to relate to what you have done and reported here.


    • Hi Bill, I have seen plenty of questionable measurements from Japan, mostly made by people who don’t understand their instrument’s limitations. CR-39 plastic detectors being used to proclaim the existence of plutonium in air filters…people reading a mSv or mR scale when detecting lots of betas…sloppy analyses of gamma spectra taken with scintillators. I think it’s great that people are taking this opportunity (however unfortunately it came about) to learn about radiation first-hand by getting some equipment, but scientific thinking, and the importance of learning to walk before running, need to catch up. Even my own experiment here doesn’t meet ANSI lab standards!


      • Well, even if they don’t meet ANSI lab standards, your procedure left not all that much to the imagination. Your standards preparation reminded me of the one and only time in my career that I had a budget to get some gamma-ray volumetric standards built. They were done by a very experienced guy at Los Alamos and cost what must be a real bargain price these days of around $5k each. Especially considering that they contained up to a gram of Pu. I had about a half dozen built, I can’t remember the details at the moment, but I have somewhere a nicely produced report describing their preparation and properties. I remember they went to great lengths to ensure adequate mixing of the matrix and Pu. The Pu started out in solution and was dropped over a pan of diatomaceous earth with a pipette, and then mixed, sampled, and so on until it was sufficiently uniform. Not a trivial step, that. They were shipped to Argonne-West singly-contained and I put the outer container on them. We used stainless medical containers and epoxied the lids on. Eventually they were disposed of as safeguards measurements, especially for waste, were kind of deemphasized. I can’t remember what I was going to use them to calibrate our detection system for anymore, but it was some kind of waste from our experimental fuel processes I believe.


    • Usually the cal source has to be made specifically for the application: if you’re going to measure Pu in water, you have to have a water-equivalent epoxy with Pu in it, if you’re going to measure I-131 in seaweed…well, I don’t know what they prefer to do about that one! A multistandard source is sold by Canberra and some other companies. You specify the density and they make it up. The components give you a range of energies to calibrate against, and it’s all very easy during the few months your source is hot enough to use. They obviously weren’t interested in selling one since they never responded to my phone calls or emails. Onto another subject…I noticed your post about this on Daily Kos. Thanks for your interest and perhaps some other folks will get interested as a result. The “bryfry” character seems to want to heckle me about my professionalism though. Take care- Carl


      • bryfry tends to attack whatever he sees as antinuclear. He’s not the only one, but I think joieau has well and truly chased off almost all the others from her diaries. She chased me off, too, for the most part. She’s about as rabid in the other direction, while I just enjoy a good conversation and the odd reminiscence.

        The safeguards nondestructive assay field that I spent my career in does not often have the “luxury” of having access to anything like specifically made calibration sources when it comes to gamma ray measurements. We were used to doing a lot of calibration correction calculations to justify using small sources for measurements of big items. Quart sized standards for 55-gallon drum measurements, for example. Not very rigorous, but the best we could do a lot of the time. Lots of papers were written in the early days about experimental verification of those methods. We were used to thinking in terms of biases of 10 or 20 percent, but even those were guesses. For manufactured items or most other cases besides soft waste, the situation was generally a lot better.


  2. As usual, excellent work Carl.

    Got a question… at the low end of the spectrum do you find a peak a 27.5 keV?

    Best regards,
    Jon


    • Hi Jon, I have a small and barely-statistically-significant peak in the hottest (C) near 26.5 keV. The others, probably not. My calibration probably isn’t great way down there. What are you thinking about? -Carl


  3. I just finished a 24-hr count on 10 g of Tokyo gutter dirt a friend sent me. (Collected in July) In addition to the Cs-134 / 137 lines there was a fairly strong peak at 27.5 keV. My best guess… it’s a Kα x-ray from the decay of 33.6-day Te-129m. I saw no gammas, but I think that’s understandable as Te-129m’s strongest gamma (696 keV) has an intensity of only 3% whereas its Kα x-ray has an intensity of 15%. I’ll have to run it again in a month to confirm.

    Jon


    • I would be really, really surprised for there to be any significant amount of Te-129m remaining now. It seems more likely that it’s an x-ray of unrelated origin. Could you post the spectrum somewhere or let me have a look at it?


  4. [...] is HPGe gamma-ray spectrometry.  I followed the same approach discussed in my earlier analysis of Japanese soils, involving comparison of the test specimen with an identically-shaped Cs-137 sand standard.  My [...]


  5. Hi,
    you wrote thats fitzpeaks dosent work with win7 64 bit
    …thats was the same experience i had make… but i have find a way to get it work.
    I have install another version of win7 but the 32bit on the same system. So i have in the boot menu the option for both versions. Then i have install fitzpeak at the 32 bit version.after that i can start it in the 64bit version. But remind that the pc is after that more than a little problematic with all software thats ibstalled before. Its not the best way/option to do that



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