問題

バックグラウンドの線量率

以下の文章は正しいですか？
『原子力発電所事故の影響がないと仮定した場合のバックグラウンドの線量率を推定できる。』

参考資料等

EC

RADIATION PROTECTION NO 160. Technical Recommendations for Monitoring Individuals Occupationally Exposed to External Radiation
An important input quantity, in particular for the low dose performance of a dosemeter, is the subtraction of the dose due to natural background radiation.

The methods of natural background subtraction are to use either an average value based on customer geographic spread (usually a national average) or specific customer or location values. In Europe, the cosmic radiation photon and directly ionizing component is about 300 μSv per year. The terrestrial photon natural background ranges upwards from about 300 μSv per year, with considerable geographic variation (thus the total ranges upwards from about 600 μSv) [UNSCEAR 2000]. Nevertheless, for monthly issue, the use of a geographic spread average background between readouts, although adding to the total uncertainty of dose assessment, will for many services still enable the recommended accuracy requirement to be met. For example, where for whole body photon/electron dosemeters, the difference between a local and the geographic spread average natural background radiation dose is no greater than about 100 μSv per year, it would seem to be acceptable to simply use the average value. The value of 100 μSv is 10 % of the lower limit of 1 mSv for which the recommended uncertainty bound is 30%.

Obviously for those instances where a dosimetry service supplies customers in areas where the terrestrial natural background is significantly greater than, or less than (an extreme case is in submarines) the national average, the local natural background dose rate will need be taken into account. Local background variation can be taken into account by the use of control dosemeters which are supplied by an ADS to a customer, and stored at the location where workers’ dosemeters are kept when not being used. In some cases, subtraction of transit doses may be done. For dosemeters issued to customers in Europe but issued and processed in the USA, natural background radiation transit doses (return trip) for an electron/photon dosemeter may be about 30 to 50 μSv.

The contribution to photon/electron dosemeters of a few tens of μSv from the dosemeter wearer’s incorporated 40K can be neglected, as can, in general, the neutron cosmic radiation background. This is about 100 μSv per year, but about half is from neutrons of energy greater than 20 MeV, measured with a lower response by many detectors. A method to determine the natural background distribution is described [Stadtmann, 2007] and a method to estimate the uncertainty resulting from the variability of the natural background by [van Dijk, 1996], both methods using an analysis of the results for issued dosemeters. These methods are based on the assumption that the majority of issued dosemeters are only exposed to natural background radiation. Mean values and standard deviations can then be derived from an examination of the relationships of dose and number of days of exposure using regression analysis.

9.2 Recommendations

It is recommended that:

• Approval procedures for dosimetry services in relation to dose reporting should: a) state the dose information needed on the report, e.g. dose in measurement period, annual and/or 5-year accumulated dose; b) state detection limits of the dosimetry system; c) detail background subtraction methods; d) state the destination of the dose report; e) give details about the storage of monitoring records and reported dose values; f) state monitoring and reporting periods.

9.7 NDR links to other data sources and databases

9.7.1 Links

NDR may periodically share information with international organizations concerned with the characterization of occupational exposure such as ISOE, ESOREX and UNSCEAR. The sharing of dose results to perform occupational exposure studies reveals the need for harmonized procedures for generating, storing and reporting results. Different dose measurement methods may be used, however background dose subtraction methods should be considered, as well as the use of detection limits instead of recording levels. The use of notional doses, that is, a dose value on a record that does not correspond to an actual dose received by a worker, should also be avoided.

IAEA

IAEA Safety Standards Series No. RS-G-1.8

Environmental and Source Monitoring for Purposes of Radiation Protection

COVID-19の蔓延下でのBG評価用素子の設置期間の延長とBGの評価の質の関係も扱われている例

Individual monitoring with radiophotoluminescence (RPL) passive integrating dosimeters

5. PROGRAMMES FOR MONITORING IN PRACTICES AND INTERVENTIONS

MONITORING IN SITUATIONS OF CHRONIC (PROLONGED) EXPOSURE

External exposure

5.123. Monitoring of human exposure due to external sources of gamma radiation should be carried out by measurement of dose rates in air at locations accessible to the public. To evaluate the contribution of the radioactive contamination at the site to the effective dose, the background dose rate should be estimated and subtracted from the measurement data.

ASSESSMENT OF DOSES FROM NORMAL DISCHARGES

External exposure

7.7. The assessment of external irradiation from the source is straightforward, at least in principle. When the source is discrete, the radiation fields in its vicinity may be measured (the natural background radiation should be estimated and subtracted from the results) or calculated using simple techniques.

7.8. External exposure due to radionuclides present in the plume or on the ground is generally difficult to assess from direct radiation measurements because variations in the natural background radiation are usually larger. Nevertheless, in many cases such external exposure due to radionuclides can be derived from spectrometric measurements of air contamination and ground deposition using established contamination to dose conversion models and the proportion of the year that a member of the critical group is likely to spend in the area. Reductions in exposure due to shielding by building structures as well as increases in exposure due to deposition on the walls and roofs of buildings can be taken into account if data on building structures are available or by using published default shielding factors.

Internal exposure

7.14. The calculation of doses from the results of environmental monitoring requires appropriate processing of the monitoring results. The background radiation, whether natural background radiation or that due to fallout from nuclear weapon tests, should be identified, generally by means of comparison with results from monitoring in an area that has not been contaminated (if such an area has been well characterized); for the calculation of doses due to releases from a source or a practice, these background radiation levels should be subtracted from the results for contamination. If the contamination is due to releases from several sources, the total dose should be calculated on the basis of environmental monitoring measurements, but it is generally difficult to attribute fractions of the dose to each source.

7. CONSIDERATIONS IN DOSE ASSESSMENT

DOSE ASSESSMENT IN EMERGENCIES

External exposure

7.25. External exposure due to the deposition of radionuclides on the ground can be derived from direct radiation measurements made after the plume has passed (when the dose rate is well above natural background levels) or when spectrometry has been performed. It can also be derived from measurements of radionuclide concentrations made on environmental samples (e.g. soil, grass and rain water). External doses due to the deposition of radioactive materials are generally calculated for a limited time period, typically of a day or a few days, that is consistent with the implementation of urgent protective actions (sheltering or evacuation). For such short periods, deposition can be assumed to be constant except for the radioactive decay of short lived radionuclides. The effect of shielding by building structures may be taken into account provided that data are available and that sheltering has been effective..

DOSE ASSESSMENT FOR SITUATIONS OF CHRONIC (PROLONGED) EXPOSURE

External exposure

7.34. The set of measurements of dose rates performed at various locations where members of the critical group usually reside, both outdoors and indoors, can be used directly to assess the existing external doses. To define the contribution of a particular radiation source to the external dose, methods of field gamma spectroscopy should be applied with subsequent assessment of the dose due to particular radionuclides or subtraction of the background radiation as determined in similar conditions.

UNCERTAINTIES IN DOSE ASSESSMENTS

7.48. In the conduct of practices, rates of release of radionuclides are generally low and the possibilities for a detailed analysis of exposure might be limited if, for example, the external dose rate attributed to releases is of the same order as the fluctuations in the dose rate due to background radiation. In this case, the dose can be assessed as a value less than the dose estimated with the minimum detectable activity for the measurement used as input data. This dose assessment can be assigned an estimated uncertainty that takes into account the uncertainties in the parameters of the dosimetric models.

8. INTERPRETATION OF MONITORING RESULTS

GENERAL CONSIDERATIONS

8.9. To avoid the misinterpretation of monitoring data, a thorough understanding of the conditions of sampling and measurement is necessary. The types of conditions include:
— The geographical location;
— The date and time;
— The duration of sampling;
— The procedures for sampling and measurement;
— A clear understanding of the physical quantities measured;
—The background radiation levels and radionuclide concentrations in the environment.

Compliance with criteria for public exposure

8.24. The dose received by individuals in the population should be derived from the results of environmental monitoring, with the natural background taken into account. The background levels should be subtracted from the results of the measurements so as to assess the doses due to practices only. Both statistically significant measurement data (above the detection limit) and measurements under the detection limits can be used for dose assessment purposes with the associated uncertainties taken into account.

有意でなくても、同等とは言えないこともあります。

8.25. The source related doses can also be derived from the results of environmental monitoring by removing the base line, including natural background radiation and other sources. Such source related doses should nevertheless be interpreted cautiously since the fractions of radiation or radionuclide concentrations that are attributable to other sources may be subject to large uncertainties.

Data from individual monitoring

10.11. Information on measurements of external doses for individuals should include personal information, dates and times of the issue and collection of the dosimeter, device readings, and procedures for calibration and for the determination of the radiation background. Information from in vivo measurements of radionuclide activity in the human body should include personal information, dates and times of the measurements, and the activity detected in the body.

Safety Reports Series No. 64

Programmes and Systems for Source and Environmental Radiation Monitoring

Document Preparation Profile DS505

Source Monitoring, Environmental Monitoring and Individual Monitoring for Protection of the Public and the Environment

3. JUSTIFICATION FOR THE PRODUCTION OF THE DOCUMENT

In addition, the update will also consider relevant sources of information from other organizations (e.g. UNEP, ICRU, UNSCEAR, ICRP), as well as lessons from experience (e.g. the IAEA Report on the Fukushima Daiichi Accident).

読み物

バックグラウンド評価の大切さ

庄司 美樹．数値の意味するもの

CsIでのエネルギー特性

厚み2cmの検出器に入射した光子の相互作用

30 keVの光子

同じ厚みの水の場合

Cs-137からの光子

2 MeVの光子

10 MeVの光子

600 MeVのミュー粒子

注意書きの例

コントロール用ガラスバッジの保管方法および取扱上の注意

【保管方法】

『コントロール用ガラスバッジは、除染作業以外の自然放射線(日常生活時)の値を差し引くために使用するものです。ご使用期間中は、汚染された土壌等の影響のない放射線量の低い場所(屋内で常温常湿)に保管してください。』

【注意】

『コントロールを除染作業場所の管理詰め所などに保管したまま、個人用のガラスバッジを自宅などにお持ち帰りした場合、差し引くコントロール値の方が高くなります。コントロールは、できるだけ放射線量の低い場所に保管してください。』

バックグラウンドの評価の問題で追加線量が評価できなかった例

参加医療機関の線量計の保管場所・返却遅延等の関係でバックグラウンドが高くなり解析不能となった。

校正場のバックグラウンド

JISZ4511:2018 X線及びγ線用線量（率）測定器の校正方法

8.6.8.3 環境バックグラウンド及び漏えい電流の補正

必要に応じて，環境バックグラウンド放射線による影響及び8.6.4に規定する漏えい電流の影響について補正する。

表A.1−放射線に関する量の基準条件及び標準試験条件

注a) 測定器のエネルギー定格範囲に137Csが含まれない場合は，他のエネルギーの標準場でもよい。

附属書JC

（参考）

低線量率における実用測定器の校正方法

JC.l 一般 一般的に線量（率），エネルギーなどの測定対象となる範囲は幅広く，さらに，測定条件も多岐にわたるため，測定器には広範囲での校正が求められる。言い換えると，全ての測定対象範囲がこの規格の適用範囲に収まるわけではなく，また，許容範囲を，満足できるわけではない。例えば，環境レベルに近い1 µSv･h以下の線量率測定に対するニーズは高く，据置形モニタのような測定器は，現実的に設置現場での校正しか可能でない場合もある。 この附属書は，この規格に基づいて校正した8.4の標準測定器を実用標準器として，箇条9の規定に基づき，この規格の適用範囲外である10 µGy･h−1未満の空気カーマ率，及び／又は散乱線が5 %を超える放射線場の校正方法について参考として記載するものであって，規定の一部ではない。
注記1 8.7.3に示す低線量率の空気カーマ率標準場の設定には，8.4の標準測定器から移行するために複数の仲介測定機器を用いることもある。
注記2 低バックグラウンド環境の標準場設定が必要な場合，遮蔽体を用いて，低バックグラウンド環境を設定できる（参考文献[1]，[2]及び[3]参照）。

議論例

日本保健物理学会2021年度企画シンポジウム国際対応委員会セッション「IAEA DS499（免除）及びDS500（クリアランス） の動向と論点―総合討論」．保健物理，56 (3), 156 ～159 (2021)
山岳地域における，原子力発電所事故由来の残留 放射線を議論する場合，自然界由来の放射線による影響を考慮しないと，測定された数値だけが独歩することが懸念される。

不適切な取り扱い例

施設定期検査における事業者検査記録の記載不備について

薬事行政での取り組み

生物学的同等性（BE）ガイドライン等

食品衛生分野

食品安全施策等に関する国際協調のあり方に関する研究

検出器汚染への対応

小林兼好．表面アルファ線検出器

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