A small capsule containing radium that was buried in the ground has been identified as the source of high levels of radiation detected in a park in the Ikebukuro-Honcho area of Toshima Ward, Tokyo.
The surrounding soil was not contaminated and radiation levels have sharply declined, leading the ward to issue a declaration of safety, but many parents who used to let their children play in the park are still worried.
Located near Shimo-Itabashi Station on the Tobu Tojo Line, Ikebukuro Honcho Densha no Mieru Koen park is a recreational area in the corner of a residential district, once frequented by neighborhood parents and children.
Despite the lovely weather on April 28, however, few visitors were seen.
A 73-year-old woman visiting with her 2-year-old grandchild said: "There are usually about 10 children playing here with their mothers... I guess a lot of people must be worried." The parent of another child spoke of not being able to trust that the radioactive material was really gone.
On April 20, the ward received an e-mail saying, "There's a place in the park where the radiation level is high." Upon investigation, radiation levels of 0.50 to 2.35 microsieverts per hour - in excess of the ward's baseline level of 0.23 microsieverts per hour - were detected near the stairs of playground equipment that includes a slide.
When the stairs were removed, the readings at the ground surface rocketed to 480 microsieverts. Entry to the park was banned from April 23.
Suspecting radioactive material in the ground, the ward made an excavation and uncovered a stainless steel cylindrical capsule 3 millimeters in diameter and 3 millimeters long. According to the Japan Radioisotope Association in Bunkyo Ward, the radioactive material radium 226 was sealed within.
The upper portion had a "1" inscribed on it, and the organisation says it is possible that the capsule was a device used for testing the proper functioning of equipment that measures radiation.
A garage for city garbage trucks used to stand on the site of the park, but Toshima Ward purchased the land and opened a park there in 2013.
The city had tested for soil contamination caused by organic solvents and lead, but did not look for radiation.
No soil was brought from other locations, so the capsule is believed to have been buried there before reclamation work was done.
On April 26 and in the presence of resident representatives, radiation levels were confirmed to be below the baseline levels, and the park was reopened.
Measurements began from April 27 at all of the more than 200 public facilities in the ward used by children, such as parks and primary and middle schools.
Health consultation services were made available at Ikebukuro Public Health Center for individual consultations and explanatory meetings by experts.
A spokesperson for the National Institute of Radiological Sciences based in Chiba said: "The high levels of radiation detected were found in a very small area.
It's difficult to imagine that children playing around the park would've been exposed for a long time. For example, even if a child had played with the soil and put it in their mouth, there is no concern regarding internal radiation exposure."
Radium used in luminous paint
Radium was once used as luminous paint on clock dials, and for radiotherapy in the field of medicine.
In October 2011, high levels of radiation were detected in Setagaya Ward, on a ward road and around a supermarket entrance. Investigations revealed bottles containing radium and radioactive substances beneath the floors of a local house and on the supermarket premises.
"As the number of people walking around with equipment for detecting radiation increases, it's possible there will be more discoveries like this in familiar places," a spokesperson for the Japan Radioisotope Association said.