希沃特Sievert

吸收剂量（单位戈瑞，Gy）表示每千克物质吸收的辐射能量。当量剂量 H = w_R·D，辐射权重因子 w_R 对 X 射线、γ 射线和电子为 1，对 α 粒子为 20，对中子随能量为 2.5–20。有效剂量 E = Σ w_T·H_T，组织权重因子 w_T 对红骨髓、肺、胃、乳腺等较高（0.12），对皮肤、骨表面较低（0.01），各组织之和为 1。希沃特用于防护评价，不用于描述高剂量急性损伤，后者用戈瑞。

Absorbed dose, in grays (Gy), is radiation energy absorbed per kilogram. Equivalent dose H = w_R·D, with radiation weighting factor w_R of 1 for X-rays, gamma rays and electrons, 20 for alpha particles and 2.5–20 for neutrons depending on energy. Effective dose E = Σ w_T·H_T, with tissue weighting factors higher for red bone marrow, lung, stomach and breast (0.12) and lower for skin and bone surface (0.01), summing to 1. The sievert is for protection assessment, not for high-dose acute injury, which is described in grays.

早期使用伦琴（照射量）和雷姆（rem）。1979 年国际计量大会采用希沃特作为剂量当量的 SI 单位，1 Sv = 100 rem，以纪念长期研究辐射剂量测量与生物效应的罗尔夫·希沃特。国际放射防护委员会（ICRP）在 1977、1990、2007 年的建议书中逐步完善了权重因子。

The roentgen (exposure) and the rem were used earlier. In 1979 the General Conference on Weights and Measures adopted the sievert as the SI unit of dose equivalent, with 1 Sv = 100 rem, honouring Rolf Sievert for his long work on dose measurement and biological effects. The International Commission on Radiological Protection refined the weighting factors in its 1977, 1990 and 2007 recommendations.

全球平均天然本底辐射约 2.4 mSv/年；一次胸部 X 光约 0.02–0.1 mSv；一次胸部 CT 约 5–7 mSv；北京至纽约往返航班约 0.1 mSv。公众年剂量限值为 1 mSv（不含天然本底和医疗），职业人员为 20 mSv（5 年平均）。短时间内全身受照约 1 Sv 以上会出现急性放射病，4–5 Sv 在无治疗时约半数死亡。

World average natural background is about 2.4 mSv a year; a chest X-ray gives about 0.02–0.1 mSv, a chest CT about 5–7 mSv and a Beijing–New York round trip about 0.1 mSv. The public limit is 1 mSv a year excluding background and medical exposure, and the occupational limit 20 mSv averaged over five years. A whole-body dose above about 1 Sv in a short time causes acute radiation syndrome, and 4–5 Sv kills about half of those exposed without treatment.