Minami Soma's "Black Dust" with Over 10 Million Bq/kg of Radioactive Cesium, Says Assemblyman Ooyama (Just Don't Multiply by 65!)

He keeps finding "black dust" in his city with ever higher radioactivity. That's extremely high, even though Mr. Ooyama hasn't given the details as to the exact measurement or the location in his blog post.

But one thing the readers had better keep in mind: YOU DO NOT MULTIPLY THIS NUMBER BY 65 TO CONVERT TO BQ/M2.

As I said in the previous post on Tokyo's "black dust", the multiplier of 65 is only applicable if:

• The soil sample is taken from the surface to 5 centimeter deep; and

• The soil's relative density is about 1.3 gram/cm3 (cubic centimeter).

In case of the samples that Mr. Ooyama has been collecting,

• The samples are taken from the surface, as in most cases the "black dust" is scattered on the paved road; and

• The soil's density is extremely light, 0.45 gram/cm3.

By automatically multiplying the Bq/kg numbers for Minami Soma's samples by 65 (or worse, 150), you would be grossly exaggerating the Bq/m2 numbers.

Japan Health Physics Society has a Q&A post detailing the step-by-step calculation to convert Bq/kg to Bq/m2. Let's figure out what kind of multiplier would be appropriate (if at all) to use for the Minami Soma "black dust" samples.

First, this is how the multiplier of 65 for the soil sample taken from surface to 5 centimeters and with the relative density of 1.3 gram/cm3 is calculated:

Calculate volume for 1 kg of a given soil sample using the relative density (or specific gravity). Then divide the volume by the depth to get the area. So,

Volume (cm3) = 1000 (g) / 1.3 (g/cm3) = 769 cm3
Area (cm2) = Volume (cm3) / Depth (cm) = 769 / 5 = 154 cm2
Area equivalent to Bq/kg = Bq/154 cm2
Converting this into Bq/m2 (which is 10000 cm2), with the multiplier "x",
Bq/154 cm2 = x Bq/10000 cm2
x = 10000/154 = 64.9 ≒ 65

Variables are "relative density" and "depth".

Now, in case of Minami Soma's "black dust", let's assume the sample is taken from surface to 1 centimeter deep (I suspect it's thinner than 1 centimeter). The density is 0.45 gram/cm3, as one of Mr. Ooyama's Youtube video states (100 ml for 45 gram):

Volume (cm3) = 1000 (g) / 0.45 (g/cm3) = 2222 cm3
Area (cm2) = Volume (cm3) / Depth (cm) = 2222 / 1 = 2222 cm2
Area equivalent to Bq/kg = Bq/2222 cm2
Converting this into Bq/m2, with the multiplier "x",
Bq/2222 cm2 = x Bq/10000 cm2
x = 10000/2222 ≒ 4.5

If it is possible to take the sample from 1 millimeter,

Volume (cm3) = 1000 (g) / 0.45 (g/cm3) = 2222 cm3
Area (cm2) = Volume (cm3) / Depth (cm) = 2222 / 0.1 = 22220 cm2
Area equivalent to Bq/kg = Bq/22220 cm2
Converting this into Bq/m2, with the multiplier "x",
Bq/22220 cm2 = x Bq/10000 cm2
x = 10000/22220 ≒ 0.45

So, if the "black dust" sample is taken from the surface to 1 centimeter deep, the appropriate multiplier would be NOT 65 BUT 4.5. If the sample is taken from 1 millimeter, the multiplier would be 0.45.

The numbers, whether they are in Bq/kg or Bq/m2, are extremely high, particularly in Minami Soma City. I don't understand why the municipal governments, whether it is in Minami Soma City or in Tokyo, aren't removing the substance, particularly from the school routes and parks.

But it is plain wrong to hype the already bad numbers by multiplying by 65. As the "black dust" is not uniformly spread over wide areas, I'm not sure if doing the conversion from Bq/kg to Bq/m2 makes any sense.

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