A note on the measurement of radiation dosage

I realized that the dosage depends on how you measure it. This also depends on the measurement location. The measurement depends on the height of the measurement device. (Thus, it is mandated to be placed at the height of 1.5m above ground level.) If you clean up the measurement point, the dosage at that point becomes lower. We should care about the hotspot (a relatively small spot that has high radiation), however, we cannot say that the whole region has high radiation just because of one hotspot. In an extreme example, if we can avoid all the hotspots and the rest of the place has no radiation, we can just avoid all the hotspots and live safely. The existence of hotspots is just telling us that there could be some dangerous spots. When we observe some hotspots we usually need more measurement points. It could be that the measurement points are too sparse and we might miss some of the high radiation points. We should care about how the radiation is measured and we need to think about the risk ourselves.

For example, if the measurement point is located near a station, it is good since a lot of people are passing by. If the location point can not be accessed by anybody, it usually doesn’t matter. But such a place is usually cleaned up frequently, thus the dosage may be lower, however only in the surroundings of the measurement point. There might be higher dosage 20 meters away from the measurement point. When we are told about the values of today’s radiation, we should also know where are the measurement points. If the whole area has not been cleaned up and only some spots were, we cannot use the cleaned up spot as a measurement point.

If someone decides to live in a low radiation place, they should think what is the important place for them. If you almost never go to a station, the radiation measured there is not so important. For you, the important place could be a school, your working place, and so on.

Radiation is not just one kind (What is Sievert?)

If you check an introductory text of chemistry, you notice that the radiation is not just one kind. There are many kinds of radiation: alpha radiation, beta radiation, gamma radiation, neutron radiation, and so on. The different kind of radiation affects differently the human body even at the same absorption dosage (gray in unit). For example, alpha radiation is more dangerous than beta radiation at the same energy level. Therefore, there is a unit called Sievert that adjusts for this difference (Figure 8). It seems scientists agreed that we need some adjustment, but how much adjustment is needed has raised some discussions. For example, how much more dangerous alpha radiation is compare to beta radiation is difficult to determine. This adjustment is called “radiation type weighting factor.” This currently differs from country to country. In Japan, this factor is 20 for alpha radiation, 5 for proton radiation, based on a gamma radiation is value of 1. According to IPRP report 103 (Wikipedia, Sievert), the factor is 20 for alpha radiation, 2 for proton radiation, based on a gamma radiation is value of 1. This coefficient is multiplied by gray to get the value as Sievert. Sievert considers the effect to the human body, but this weighting factor is hard to determine, because this depends on many other factors like person’s age, sex, health status. It is also difficult to make detailed experiments with the human body.

The following analogy is only understandable for someone who likes computer games. But I think this is a good analogy, so I will try to use it here. In many fantasy games, usually a character has some attribute, like fire or water. If a character has a water attribute, he or she can resist more to the water magic. When a character is hit by some fire magic, the damage this character gets depends on his/her fire attribute. At the end, how many health points the character lost is the most important effect. The damage has been adjusted according to the attribute. For radiation there are different types, and the damage to the human body depends on the type. In a similar way, Sievert is an adjusted value of the absorbed energy (gray).

If a human body gets the same amount of energy but in a different amount of time, the effect of radiation would be different. For example, whether a person gets 10mSv of dosage in a day or in a year it is a different thing, we cannot add yesterday’s radiation dosage and today’s radiation dosage. However, we still have no better measurement than Sievert. It seems Sievert is the best approximation to measure the effect of radiation on the human body. The effect of radiation is not linear, but the Sievert unit assumes that the effect is linear. If you are not familiar with the word `linear’, it is somewhat similar to say that `you can add that up’.

For example, a person cannot eat 500g of salt in an hour, that probably causes death. However, if the same person uses 10g salt per day for 50 days, the danger is drastically less (yet, it still is too much salt). We usually cannot simply add up the amount to difference the effect on a human body.

However, we assume the radiation dosage (Sievert) can be added as an approximation. We should remember that this is an assumption. The Sievert unit is not like the meter unit, which can be added up.

I would like to clarify make clear the difference between 1 mSv/y and 1 mSv since I read many news articles about them. 1 mSv/y means that if you stay at a location where your measured radiation is 1 mSv/y for one year, you get 1 mSv dosage. If you stay in such a location for two years, you get 2 mSv. The current criterion of the evacuation counsel for disasters is 20 mSv/y in Japan (2014). If you stay in a location where you have 20 mSv/y location for five years and assuming that the radiation stays the same, the dosage is 100 mSv. Please notice this difference. If you decide to take the 10 mSv risk, you can only stay at a 1 mSv/y location for 10 years. Especially young children usually have higher risk for the same dosage, so you also need to consider long term exposure. The difference between mSv/y and mSv is similar to you need to pay 1000 Euro every year and you need to pay 1000 Euro only once. Please do not confuse every year payment and one time payment.

We have two criteria that define what is nuclear waste. One is based on the Sievert value, the other is based on the Becquerel value. If the waste consists of one kind of radioactive substance, it is reasonable to define the criterion based on the Becquerel value. Since the half life are depends on the radioactive substance, you cannot really estimate what the danger is if the substance are mixed. Therefore, there are criteria based on each nuclide. However, if many kinds of radioactive substances are mixed up, which is often the case, it is difficult to determine what is nuclear waste and what isn’t. If we can separate all the nuclides, we can still use the Becquerel value criterion separately, however, it is usually not easy to do. In that case, we use a criterion based on radioactive dosage — we use the Sievert value in this case.

If we read a newspaper, it seems that these criteria are used arbitrarily. Sometimes we see a Cesium 137 Becquerel value for cleaning up bi-product waste (e.g., [1]). I wonder if there are other nuclides, for example, is there Strontium 90 in the waste?

Another problem is what kind of radiation can the measurement devices measure. Most of the measurement devices can only measure gamma radiation, but there are other types of radiation.

If you review the periodic table in chemistry, you may notice there are mass numbers in it. For example, there are several kinds of the same element, e.g., Cesium 134, Cesium 137. They are isotopes. These numbers represent their mass number. For example, there are many kinds of radioactive Cesiums. If a newspaper mentions a radioactive Cesium, I would like to know which one it is. Because their half lives differ. The half life of Cesium 134 is around 2 years. The half life of Cesium 137 is around 30 years. This means, the Becquerel value of Cesium 134 becomes 1/1000 after 20 years. On the other hand, the Becquerel value of Cesium 137 becomes only less than half after 20 years. You could learn these things in high school. I found high school science is quite sufficient to know most of these things. I understand that the information sources have only limited time and amount of information they can give. If you know more about this basic chemistry knowledge, you can understand the provided information more.

This is maybe a small detail, the safety criteria of the radioactive waste depend on organizations, countries, and the year [2],[3]. According to Japanese prime minister office (首相官邸)[4], the criterion based on which you should leave an area is when there is more than 20mSv/y. This threshold for the Ukrainian government is 5mSv/y [5]. There is no right and wrong here. Risk is evaluated based on some assumptions and these assumptions differ from government to government. The governments determine these assumptions, therefore, the criteria depend on the country.

In the end, there is always a risk when there is radioactive waste. There is no absolute safety, but there is also an acceptable risk level. Governments usually provide documents about how they consider the risk and what are their assumptions. We need to determine whether we can accept the risk and the assumptions. (Although if a government forces people to accept the risk in some way, or if people cannot decide on their own, I believe this is violation of human rights.) To judge the risk, we first need to understand the information. Then we need to think on our own. The risk is usually probabilistic, we should think through it and decide whether we can accept it or not. If people cannot agree with the government decision, then they should change the government.

The message I’m sending here is to understand this information. This is a first step. It would be not so simple at the end, but the important thing is that we understand the information and we take our decisions on our own.

References

1. Kahoku-shinpou (河北新報), The governor of Miyagi-prefecture has accepted for the investigation to build the final disposal repository (宮城県知事、詳細調査受け入れ　最終処分場), http://www.kahoku.co.jp/tohokunews/201408/20140805_11016.html, (Online; accessed 2014-12-26)
2. Ministry of Health, Labour and Welfare of Japan (厚生労働省), How we handle the radioactive substance in food (食品中の放射性物質への対応), http://www.mhlw.go.jp/shinsai_jouhou/shokuhin.html, 2014, (Online; accessed 2014-12-21(Sun))
3. Ministry of Health, Labour and Welfare of Japan (厚生労働省), We updated the safety criterion of radioactive substances in food (食品中の放射性物質の新たな基準値を設定しました), http://www.mhlw.go.jp/shinsai_jouhou/dl/leaflet_120329_d.pdf, 2014, (Online; accessed 2014-12-21(Sun))
4. Prime Minister’s Official Residence (首相官邸), Keeping everyone’s safety (みなさまの安全確保) http://www.kantei.go.jp/saigai/anzen.html/, How we set up “the planned evacuation region” and “preparation necessary region when the emergency “(「計画的避難区域」及び「緊急時避難準備区域」の設定について), http://www.meti.go.jp/press/2011/04/20110422004/20110422004-2.pdf, 2011, (Online; accessed 2014-12-21(Sun))
5. オレグ・ナスビット, 今中哲二, ウクライナでの事故への法的取り組み, http://www.rri.kyoto-u.ac.jp/NSRG/Chernobyl/saigai/Nas95-J.html, 2011, (Online; accessed 2014-12-21(Sun))