When I saw the tweet, I thought it was another prank. The tweet had a link to a PDF file about the high levels of radioactive cesium in one tree in the yard of a resident in Abiko City located on the west corridor of Chiba with relatively high radiation contamination. The web address of the link indicated it was from the city government, but there was no mention of the city in the document. So I went to the homepage of Abiko City, and see if I could find the same document from the links at the homepage.
Well I could. After 4 clicks, I landed on this particular page which has a link to the PDF: http://www.city.abiko.chiba.jp/index.cfm/18,101468,241,1019,html
The apple in question was brought by a city resident and tested on August 20, 2012 using the city's NaI scintillation survey meter. As the number was extraordinarily high, the city sent officials to the resident's home to collect more samples and tested them using the germanium semiconductor detector to be more precise. The result using the germanium detector was even higher.
The city says it was cautious in releasing the information, for fear that it might generate "baseless rumors". The amount of radioactive cesium in the apples, leaves and branches from the particular tree was extremely high, and couldn't be explained by comparing it to the samples taken in the same yard and in the neighborhood.
Here's what the city's undated document says:
The apple tree is an ornamental apple tree, though the fruit is edible. It was planted by the resident of the house about 6 years ago when the resident moved in. The resident brought the apple to the city's testing lab, and the test was conducted on August 20, 2012.
The test results (radioactive cesium total):
Using the city's NaI scintillation survey meter: 1,500 Bq/kg
Using the survey meter at the Board of Education: 1,300 Bq/kg
Using the germanium semiconductor detector: 1,660 Bq/kg
Soil where the apple tree was planted
Using the survey meter at the Board of Education: 2,900 Bq/kg
Leaves and branches of the particular apple tree
City's test: over 10,000 Bq/kg
Leaves and branches of other trees in the same yard, including a different variety of apple tree right next to the highly contaminated apple tree
City's test: 48 to 111 Bq/kg
Apples from the same apple variety that neighbors grow
City's test: less than 100 Bq/kg
The city asked the Ministry of Agriculture's Horticulture Bureau and a government laboratory to investigate, but they couldn't come up with a plausible explanation why this peculiar contamination happened. The city also asked the National Institute of Radiological Sciences and the Central Research Institute of Electric Power Industry
The city decided to release the information with a caveat that the apple was from a home garden and not commercially sold, so that people wouldn't panic unnecessarily.
So no one knows why this one particular apple tree ended up with such a high level of contamination and not the tree right next to it. But aren't scientists curious?