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Radiological Analysis of Namie Street Dust

Jun Ohnishi from Ibaraki Prefecture sent a sample of highly radioactive street dust from Namie, Fukushima, to Marco Kaltofen for analysis.

From Jun Ohnishi:
The sample was collected at the coordinate 37.4752, 140.9461.
The address is 102-1 Shimizu Onoda Namie-machi, Futaba-gun, Fukushima Prefecture.

Onoda district is designated as the "restricted zone," but anybody can enter without permission.  If you proceed westward about 50m past the collection point, you come to a barricade at the entrance to the "exclusion zone."

Here's the video showing the actual collection of the sample.
 "2013.4.6 Namie street dust 86.09 μSv/h at 1 cm above ground"

Truth we must face

By Jun Ohnishi


Radiological analysis of Namie street dust

May 31, 2013

Marco Kaltofen, MS, PE
Boston Chemical Data Corp.
2 Summer Street Suite 14
Natick, MA USA 01760

Department of Civil and Environmental Engineering,
Worcester Polytechnic Institute
Worcester, Massachusetts, USA

AbstractA sample of street dust was received from a location about 17 Km from the
Fukushima-Daiichi accident site. The street is in Namie-machi, Futaba-gun,
Fukushima Prefecture. This is in the restricted zone, close to, but is just outside
of the exclusion zone. The dust sample was analyzed by Scanning Electron
Microscopy with Energy Dispersive X-ray analysis and by sodium iodide gamma
spectrometry. An autoradiograph was prepared from the sample using bluesensitive
X-ray film. The sample contained 1,500 Bq/gram of combined Cs-134 +
Cs-137 as well as 0.3 Bq/gram of Co-60. The sample was uniformly radioactive
when analyzed by autoradiography. Analysis by SEM/EDS found widely
scattered particles of suspected fission products among larger aggregates of
mineral matter.

Introduction and Methods
Airborne dusts can transport radioactive materials in the form of isolated
individual particles containing high concentrations of radioisotopes. The specific
activity of an individual particle can be significantly higher than that of the
surrounding particles in a dust sample. These high activity particles, called hot
particles, are isolated and analyzed by scanning electron microscopy / energydispersive
X-ray analysis. (SEM/EDS).

A portion of the Namie dust sample was mounted on a glass slide with conductive
tape, and carbon coated and scanned by a LEO/Brucher SEM/EDS system, using
a lithium drifted silicon semiconductor X-ray detector for the electron
microscopy analyses. All SEM/EDS analyses were performed at Microvision
Labs of Chelmsford, MA, a commercial microscopy laboratory. The electron beam
current was 0.60 nAmperes, accelerated at a voltage of < 0.5 to 60 keV.
Backscattered electrons are detected and provide imaging contrast determined by
the atomic number of the nuclei with which it interacts. Characteristic X-rays are
emitted by ions in excited states created by interaction with the electron beam.
These characteristic X-rays are detected by the lithium drifted silicon detector.

SEM / EDS does not distinguish between stable and unstable, (radioactive),
nuclei of a given element. Additional information is required to determine
whether a particle contains radioactive materials. For certain elements, including
uranium, thorium, and plutonium, the known isotopes are radioactive. For other
elements, including lead, yttrium and many rare earths, the known isotopes are
both radioactive and stable. For elements with both stable and radioactive forms,
gamma spectrometry provides confirmation of the presence of radioactive
isotopes in bulk particulate samples. In this analysis, initial gamma spectrometry
analyses were performed with an Amptek CdTe gamma detector and MCA,
scanning the range from 10 to 2060 keV, equipped with a copper/lead multilayer
shield. Laboratory-based gamma spectrometry analyses were performed with an
Ortech 2 inch NaI gamma detector and lead shield.

Results and Discussion
This analysis focused on fission products that are released from damaged nuclear
fuels. The most common fission products found in radioactively-contaminated
dusts from Fukushima Prefecture include Cs-134 and Cs-137. Nuclear reactors
tend to produce both heavy (atomic weight 125 to 155) and light (atomic weight
80 to 110) byproducts. These include light radioactive isotopes of the elements
yttrium and silver, plus the heavier isotopes tin, antimony, cesium, cerium,
neodymium, and lanthanum. All of these were detected in this dust sample by
SEM/EDS, in the form of tiny particles on the order of 10 microns in size.
Examples of the SEM/EDS-detected particles in the small, (100 milligram), dust
sample included thorium-containing rare earth particles, lead titanate, and yttrium
lanthanide particles. These were in the 2 micron to 10 micron size range.
The sample of street dust was also analyzed by sodium iodide gamma
spectrometry. (See Figure 1) An autoradiograph was prepared from the sample.
(See Figure 2) Gamma spectroscopy detected 153 Bq total of radioactive cesium,
(Cs-134 + Cs-137), and uranium daughter isotopes in the 100 milligram sample.
This is equivalent to 1530 Bq, per gram or 1.5 MBq per kg. Cobalt-60 was present
at 0.3 Bq per gram. The most active uranium daughter isotope found by gamma
spectroscopy was radium-226. (See Figure 1)

The dust sample had numerous particles containing mostly lead, yttrium, various
rare earths, and thorium. Some of these lead and rare earth particles were in the
respirable size range, measuring only 1 or 2 microns in size. (See examples in
Figures 3, 4, and 5)

This dust was collected just a few hundred feet outside the exclusion zone around
Fukushima-Daiichi. Occasionally observers have reported small deposits of
windblown black sediment which measures higher than normal for radioactive
forms of cesium and other radioisotopes. This is the first time we have examined
a sample that was clearly distinct from surrounding soils and dusts, by virtue of
its high radioactivity. The sample had the highest radium-226 levels of the
approximately 200 dust and soil samples analyzed by this laboratory.
This analysis is a limited one, since the subject is a single (and small) dust
sample. This sample is not representative of the Namie region as a whole. This
data demonstrates that isolated street dusts can reach radiation levels well in
excess of their general surroundings.

There is not enough data in a single sample to explain why a small street dust
sample was so contaminated with radioactive substances compared to
surrounding materials. Clearly some environmental mechanism has allowed this
more radioactive dust to remain segregated rather than dispersing into the soils
or being washed away by rains. Given the resistance to dispersion of this
radioactive dust, this analysis suggests that small localized radioactive hot spots
can persist despite the passage of months and years since the Great Northern
Japan Earthquake and subsequent radiation releases.

Author Disclosure Statement
The author declares that no competing financial interests exist. The author
gratefully acknowledges the efforts of Mr. Jun Ohnishi, who provided the sample
for this analysis.

Following pages: Figures 1 to 5

Figure 1: Sodium Iodide gamma spectrum of Namie street dust

Figure 2: Namie dust sample X-ray film autoradiograph (right) and scaled true color scan (left).

 Figure 3: Scanning Electron Micrograph w/ Robinson Detector image of a lead particle imbedded in a larger aggregate with a chart showing percent elemental composition of the particle.

Figure 4: Scanning Electron Micrograph w/ Robinson Detector image of a thorium-containing particle imbedded in a larger aggregate with a chart showing percent elemental composition of the particle.

 Figure 5: Scanning Electron Micrograph w/ Robinson Detector image of a yttrium-lanthanide particle imbedded in a larger aggregate with a chart showing percent elemental composition of the particle.

Kaltofen report PDF
Japanese translation by Yuri Hiranuma
Article in Japanese


  1. Thank you. This is exactly the kind of expert, detailed analysis that has been needed. It demonstrates why people should not be forced to return to these areas, regardless of what one believes about the risks of 20mSv/year background radiation. These hot spots are reason enough to stay away.
    In contrast to Namie,I've found hot spots in Narita, Chiba that give off 1~2 microSv/hour.
    My blog post from 2011:

  2. Thank you for sharing a very detailed analysis on this matter. I have relatives living there but I don't really know if it is near the area. I am still going to tell them about this though. It is better for them to be aware of this matter.



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