It looks the glass badge worn by a student who lives in the apartment helped identify the "hot" concrete.
A kind of perverse way to find out about radiation contamination...
From Fukushima TV News (1/15/2012):
Relatively high levels of radiation have been detected from the concrete in an apartment building in Nihonmatsu City [in Fukushima Prefecture]. The Ministry of the Environment suspects that radioactive materials were in the concrete, and is investigating.
The apartment was built in September last year in Nihonmatsu City. According to the Ministry of the Environment, the glass badge worn by a student who lives in the apartment registered 3-month cumulative radiation exposure of 1.5 millisievert on December 27, prompting the city to survey the surrounding area.
The survey found 1.4 microsievert/hour from the concrete foundation of the apartment, and 1.2 microsievert/hour inside the apartment. The Ministry of the Environment thinks the apartment was built using concrete with radioactive materials, and is asking the building contractor for details.
I wonder if it is just concrete.
If you recall, 446,000 becquerels/kg of radioactive cesium was detected from the sewage sludge in Fukushima City on May 8, 2011. The sludge had been sold to cement companies until the detection at Koriyama City on April 30, when 26,400 becquerels/kg of radioactive cesium was detected from their sewage sludge. In the case of Koriyama City, over 900 tonnes of this radioactive sludge had been sold to one cement company (Sumitomo Osaka Cement) alone.
If the surface radiation level of the concrete is 1.4 microsievert/hour, I believe the concrete has at least 100,000 becquerels/kg of radioactive cesium in it, if the radiation measurements of sludge and rooftop sediment are any indication.
(UPDATE: I tweeted this news and the information about the radioactive sludge from last year as above in Japanese, and judging by the number of retweets, many Japanese weren't even aware of the highly radioactive sewage sludge was being sold until April 30. Retweets exceeded 100 in probably less than 15 minutes. The news was in the major newspapers then, but I remember people were still sleepwalking, in a way, back then.)