Space radiation dose measurement result in the city

In the wake of the Great East Japan Earthquake, since June 2011, city elementary and junior high schools, nursery schools, and kindergartens have continued to measure air radiation doses at fixed points 5 cm and 100 cm above the ground. The measured value is below the standard value (0.23 microsieverts per hour) indicated by the Ministry of the Environment, and in Tokyo, the value of the radiation monitoring post in Shinjuku Ward, which was installed before the Great East Japan Earthquake, is below the level before the earthquake. Due to reasons such as the fact that it has decreased to the numerical value, we will suspend the measurement of the air radiation dose in the city after 2021. However, if any concerns arise, the measurement will be resumed immediately.

For more information, please visit the link below.

Please see the link below for the results of monitoring posts in Shinjuku Ward.

After changing the measurement frequency, we have measured the air radiation dose in the city, so we will announce the results.

The measurement results were below the index values recommended by the International Commission on Radiological Protection.

Note: If 0.23 microsieverts per hour is converted to 0.99864 millisieverts per year, the International Commission on Radiological Protection (ICRP) recommends that this is an indicator of normal radiation doses received by ordinary people (1 millisieverts or less per year (natural radiation) However, when the annual conversion value was calculated based on the measured value, it was less than 0.526 mSv per year (including natural radiation dose, etc.). , was less than 1 millisievert per year (excluding natural radiation dose, etc.).

○Measurement locations Municipal elementary and junior high schools, municipal/private nursery schools, private kindergartens

○ Measurement point schoolyard or garden ○ Measurement height 5 cm and 100 cm from the ground surface

○Measurer city staff

○Measurement equipment PA-1000 (elementary and junior high school)

・TCS-172B (nursery school/kindergarten)
Note: The number of digits in the measured value varies depending on the measuring instrument used. (PA-1000 is displayed with 3 digits after the decimal point, TCS-172B is displayed with 2 digits after the decimal point.)
For reference: The Tokyo Metropolitan Health Safety and Health Center is comparing the values ​​measured by multiple measuring instruments.
For more information, please see "We measured the premises of the Health and Safety Research Center with various types of radiation detectors" (external link) .

○Measurement method ・Average of 5 repeated measurements every 60 seconds (PA-1000)

・Average of 5 repeated measurements every 30 seconds (TCS-172B)

○ Measurement value unit: microsievert per hour

○ Future measurements

• The first measurement, which was conducted from June 28, 2011, showed that there was no significant change in the values due to height, so subsequent measurements were carried out at the ground surface of 5 cm and 100 cm.

About annual conversion value estimate of space radiation dose in the city, we did trial calculation under the condition that "the space radiation dose measurement result in the city" continued for one year.

○ Annual conversion value estimation method Annual conversion value (millisieverts per year) = (measured value - natural radiation dose) x (8/24 [time spent outdoors] + 16/24 [time spent indoors] x 0.4 [indoors] radiation reduction rate]) × 24 hours × 365 days ÷ 1000 [mm conversion]

《In the case of 0.090 microsieverts per hour》
⇒ 0.090 x (August 24 [time spent outdoors] + 16/24 [time spent indoors] x 0.4 [rate of radiation reduction indoors]) x 24 hours x 365 days ÷ 1000 ≒ 0.473 millisievert notes per year : This conversion value does not subtract the natural radiation dose.
[Reference] The average natural radiation dose in Japan is said to be about 0.04 microsieverts per hour.

Conditions - Assuming 8 hours outdoors and 16 hours indoors

● Reduction effect (coefficient 0.4) for staying in a wooden house (16 hours)

Note: Estimates are based on the assumption that the measured values at the locations where measurements were taken this time will continue for one year.

○ Glossary Sievert (Sv) Unit millisievert (mSv) that expresses the effects of radiation on the body Microsievert (μSv), a unit that is 1/1,000th of a sievert Microsievert (μSv) 1) unit of

○ICRP (International Commission on Radiological Protection) standards
During an emergency: 20 mSv to 100 mSv per year During recovery after an emergency accident: 1 mSv to 20 mSv per year Normal times: 1 mSv or less per year

Note: The above ICRP standards do not include natural radiation doses or radiation doses received from medical procedures.