Gamma activity measurements of Tokyo-area soil samplesNovember 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).
- Sample A: Roadside gutter debris, Nakano (Google Maps)
- Sample 1: Garden soil, Nakano (Google Maps)
- Sample B: Roadside gutter debris, Adachi (Google Maps)
- Sample 2: Garden soil, Adachi (Google Maps)
- Sample C: Roadside gutter debris, Minami-Nagareyama (Google Maps)
- Sampe 3: Playground soil, Minami-Nagareyama (Google Maps)
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:
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.