For civilians to measure the radiation accurately
Generally, the types of radiations measured in civilian’s radiation measuring laboratories are Gamma ray cesium 134 and cesium 137.
Universities or laboratories can analyze varieties of radiation because they use “Germanium Semiconductor Detector (Germa)”
to detect gamma ray.
The machine cost more than ten million yen and it needs to be kept cool by liquid nitrogen so the maintenance cost is also expensive.
Most of the civilian radiation measuring laboratories use Sodium iodide scintillation spectrometer (Nal) as their detectors.
Compared to Germa, ability to identify and analyze the radioactive materials of the near band channel is low.
However, it costs only around a few million yen to five million yen, and the only maintain cost is the electricity bill.
Those specialists at universities or laboratories who have experiences of measuring gamma ray using Germa
have preconception that Nal cannot conduct accurate detection.
However, after the Fukushima Daiichi nuclear power plant’s accident, it was citizen’s radiation measuring laboratories
task to increase the quality of measuring the radiation with affordable price, and to check the safety of foods and soils
that are part of our daily life. Various measurement laboratories had made much effort to pursue better measurement result.
Since we could gain reliable measurement results that are below the government’s standard limitation with NaI, when
appropriate effort is made. “Research Exchange Association” started since March 2012 organized by Takagi Jinzaburo
Citizen Science Foundation (Takagi Foundation) as a way to support citizen’s radiation measuring laboratories to exchange i
nformation and connect with each other. Below is the introduction of how “Research Exchange Association” and
“Minna no Data Site” have increased the quality of radiation measurement.
Nal scintillation spectrum meter which are used in many of the citizen’s radiation measuring laboratories.
How to decrease background radiation
The first task of “Research Exchange Association” was to compare the function and use of Nal from different manufacturer
as well as to share the information of how to decrease “background radiation”.
In our daily life, there are naturally derived radiation in air, soil or from the wall of buildings known as background radiation.
Usually, radiation detectors block out those background radiations by covering the sample (for example, food) with substance
like lead. This is to measure only radiation from the sample. However, when this is not enough, covering the detector itself
with lead or putting them into a cardboard box with mineral water in it are very useful. To connect onto future
Tokai net citizen’s radiation measuring laboratory (C-lab) uses affordable old lead water pipe to block the background radiation.
Those ideas are very useful for lowing the minimum detection rate. Generally, minimum detection rate for Nal is about 10 Bq/kg,
but by blocking out the background radiation, it is possible to lower it down to 1 Bq/kg.
Quality check by the original reference radiation source “standardized brown rice kit”
The accuracy of the detectors can be verified by measuring a reference radiation source that has been made to have certain
contamination and check if the detector comes up with same results as the original contamination. This is the one of
the reliable ways. Those sources are being sold, but they usually cost around one hundred to two hundred thousand yen.
It is not so affordable for citizen’s radiation measuring laboratories. Brown rice was chosen as substitute material in those
laboratories because it is easy to handle. We developed the source that are 100 Bq/kg, 50 Bq/kg, 10 Bq/kg and 5 Bq/kg
(they are cesium 137 and cesium 134 combined results) by mixing contaminated and non-contaminated brown rice.
We measured them with Germa to give them accurate contamination rate in the beginning.
Biggest challenge for this brown rice was rice weevil. To solve this, we put disposable hot pats and oxygen scavenger
into the box with brown rice to reduce the oxygen to prevent rice weevil to reproduce.
We measured this standard brown rice in seven laboratories as trials and we were confirmed that there is no big difference
in the results. We decided the average deviation value of the results and to estimate the difference between the original value
and results from the measurement statically by En value in the proficiency test.
Japanese Government also adopt this method to investigate on natural radiation and control the accuracy since 2006.
We created manual for those processes and created excel spreadsheet to enter measured date and results.
It enabled us to carry out automatic estimation with En value with taking in consideration of natural decrease of cesium 137
and cesium 134. It has been a long tough process to get here, but we have achieved to establish a reliable tool to verify
the accuracy of measurement of radiation by citizen’s radiation measuring laboratories objectively.
The condition to join Minna no Data Site is to pass the brown rice accuracy test.
There has been laboratories that had to take this test for several months until they could pass it.
Improving measurement accuracy by support visit
Members of C-lab had visited citizen’s radiation measuring laboratory “Hakarucha” in Toyama to investigate the reason
why the result of the brown rice exceeded the margin of error. As a result, they suspected that the samples and the container
of the detector had been contaminated by Radon, which belongs to same radioactive family with natural derived uranium
and radium from the substance used in the walls of the building.
Radon is gas form and has half life is short (3.8 days), but its daughter nuclides such as Lead-214 and Bi-214 are solid form
so they stick to objects.
As measures, they ventilate the room well to let radon out and washed the container well before measuring the sample.
The results of brown rice measurement became within the margin of error.
Set of standard brown rice.
They are contaminated by the nuclear power plant’s accident in Fukushima.
There are four levels of contamination; high-100Bq/kg, medium-50Bq/kg, low-10Bq/kg and very low-5Bq/kg.
Each of them is stored in a plastic container. Each citizen’s radiation measuring laboratory measures them for confirming
the detector’s accuracy by analyzing the results statistically taking the half-life in consideration.
Here is an example of a result of standard brown rice measurement.
When a citizen’s radiation measuring laboratory fill in their result… The result of margin of error is presented
with En value examination.
When the “En value“ is within ±1, then the accuracy of the detector is good.
Request to the manufacturer of the detectors to improve the quality of analyzing software
The accuracy tests on the detectors by citizen evidenced the quality differences in the manufacturers.
A result by a detector from one Japanese manufacturer showed cesium 134 higher than cesium 137, where it should be
the opposite by the fact and when the results of samples were around 10Bq/kg, they did not stabilize.
Those were obviously strange results and they were found in many citizen’s radiation measuring laboratories.
To investigate what was happening, we divided a brown rice sample into two samples and measured them
with two different manufacturer’s detector.
We compared the results and found that THE manufacturer’s detector conducts unreliable results.
We made a detailed document with the results of those two samples from two detectors and sent them to
THE manufacturer to ask for their explanation on how to analyze the data as well as we requested them
to improve their program.
After some long negotiation with the manufacturer, they responded “Based on the request from citizen’s radiation
measuring laboratories, we will revise our analyzing programs and provide new program to those who wish”.
However, on the down side, the manufacturer thinks that measuring down to 100 Bq/kg is enough.
So they are not willing to improve accuracy of the detectors despite actually citizen’s radiation measuring
laboratories are measuring down to 10 Bq/kg or even below.
At the end, the improved analyzing program also did not show stabilized results.
Unfortunately, our negotiation failed to increase the accuracy of the manufacturer's measurement as expected.
But, we are proud that this negotiation between manufacturer and us showed that citizen could be able to
meet equally with them. In areas other than improving software by manufacurer,
our efforts to improve accuracy are still continuing, and we have achieved certain results.
Integrate the measurement data
Numbers of food data on “Minna no Data Site” exceeded 10,000 with the work of more than 30 registered
citizen’s radiation measuring laboratories.
With the establishment of the website, it allows everyone from citizens as well as researchers to access
all the data from each laboratories at once.
Now “Minna no Data Site” became a data base of all the results of the measurements as well as
important platform for laboratories around Japan to cooperate.