香蕉等效剂量
香蕉等效剂量（Banana Equivalent Dose，BED）是一种非正式的电离辐射暴露剂量单位，旨在向大众比较通常的辐射剂量与吃下一个平均尺寸香蕉所受到的天然放射性核素辐射剂量。香蕉包含着天然放射性的钾-40 (40K)。
1香蕉等效剂量=0.0778微西弗。计算的依据主要是由于香蕉含有大量的钾元素，天然钾当中约有百万分之117(0.0117%)的天然放射性的同位素钾40。钾40由于半衰期长达12亿5千万年（地球寿命为45亿年）。每克天然钾的放射性活度，等于其中40K的原子数量除以40K原子以秒计量的平均寿命。每克天然钾中40K的原子数量是阿伏加德罗常数 6.022×1023除以天然钾的原子量39.0983克/摩尔再乘以40K的丰度0.000117，结果为1.80×1018每克。 对于指数衰减，平均寿命等于半衰期(3.94 × 1016秒)除以2的自然对数，结果为5.684×1016秒[1]因此每1克天然钾中，放射性活度约31贝克勒。而1根香蕉中，平均而言约含有0.5克的天然钾，[2]所以每根香蕉约含有放射性活度15.5贝克勒的放射性钾40。依据美国环境保护署提供的转换因数，每贝克的钾40平均对每个成年人在50年期间造成的等效剂量为5.02纳西弗。所以，1根香蕉的等效剂量=(15.5贝克勒)(5.02纳西弗/贝克勒)=0.0778微西弗。1根香蕉中包含约15克碳中的碳-14(14C)提供了3-5次贝塔衰变每秒。
由于人体对钾的新陈代谢保持了钾的总量平稳，因此从香蕉等食物摄入的钾并不会在人体内蓄积。因此摄入钾-40的放射性剂量也不会在人体内蓄积。[3][4] 因此任何从食品中过量吸收的钾都会有等量的钾被肾脏排出体外。[5][6]
1根香蕉的辐射剂量，相当于每日的天然本底辐射的百分之一。即100根香蕉等效剂量。核电站对周边环境的最大允许辐射暴露是每年2500根香蕉等效剂量(250 μSv)。胸部CT扫描是7万根香蕉等效剂量(7 mSv)。致死剂量是3千5百万根香蕉等效剂量(3500 mSv)。距离1979年三里岛核事故10英里的人承受了800根香蕉等效剂量。[7]
历史 编辑
有据可查的香蕉等效剂量最早出现在1995年RadSafe核安全邮件列表中， 劳伦斯利佛摩国家实验室的Gary Mansfield提到了香蕉等效剂量在向公众解释极小剂量带来极小风险时非常有用。[5] [8][9][10][11]
参考文献 编辑
- ^ Bin Samat, Supian; Green, Stuart; Beddoe, Alun H. The 40K activity of one gram of potassium. Physics in Medicine and Biology. 1997, 42 (2): 407. Bibcode:1997PMB....42..407S. doi:10.1088/0031-9155/42/2/012.
- ^ Bananas & Potassium. [2011-07-28]. （原始内容存档于2011-08-14）.
...the average banana contains about 422 mg of potassium...
- ^ U. S. Environmental Protection Agency (1999), Federal Guidance Report 13 （页面存档备份，存于互联网档案馆）, page 16: "For example, the ingestion coefficient risk for 40K would not be appropriate for an application to ingestion of 40K in conjunction with an elevated intake of natural potassiumm. This is because the biokinetic model for potassium used in this document represents the relatively slow removal of potassium (biological half-time 30 days) that is estimated to occur for typical intakes of potassium, whereas an elevated intake of potassium would result in excretion of a nearly equal mass of natural potassium, and hence of 40K, over a short period."
- ^ Eisenbud, Merril; Gesell, Thomas F. Environmental radioactivity: from natural, industrial, and military sources. Academic Press. 1997: 171–172. ISBN 978-0-12-235154-9.
It is important to recognize that the potassium content of the body is under strict homeostatic control and is not influenced by variations in environmental levels. For this reason, the dose from 40K in the body is constant.
- ^ 5.0 5.1 RadSafe mailing list: original posting （页面存档备份，存于互联网档案馆） and follow up thread （页面存档备份，存于互联网档案馆）. FGR11 discussed.
- ^ Maggie Koerth-Baker. Bananas are radioactive—But they aren't a good way to explain radiation exposure. Aug 27, 2010 [2017-09-26]. （原始内容存档于2011-05-28）. (Accessed 25 May 2011). Attributes the title statement to Geoff Meggitt, former UK Atomic Energy Authority.
- ^ Three Mile Island Accident. [2015-10-25]. （原始内容存档于2016-01-28）.
...The average radiation dose to people living within 10 miles of the plant was 0.08 millisieverts...
- ^ Federal Guidance Report #11 （页面存档备份，存于互联网档案馆） (table 2.2, page 156) Lists conversion factor of 5.02×10−9 Sv/Bq for committed effective dose equivalent of ingested pure potassium-40 (not of natural potassium).
- ^ Environmental and Background Radiation （页面存档备份，存于互联网档案馆）, Health Physics Society （页面存档备份，存于互联网档案馆）.
- ^ Radiation chart. [2017-09-26]. （原始内容存档于2011-07-05）.
- ^ Tom Watson. Radioactive Banana! Peeling Away the Mystery. Feb 26, 2012 [2017-09-26]. （原始内容存档于2012-04-08）. (Accessed 14 March 2012).