西弗積分

西弗積分(Sievert Integral)是用於模擬放射性劑量的一種特殊函數，為瑞典醫學物理師羅爾夫·馬克西米連·西弗（瑞典語：Rolf Maximilian Sievert）所推得，輻射轉移定義如下：^[1][2]

Sievert Integral

${\displaystyle S(x,\theta )=\int _{0}^{\theta }e^{-xsec(\phi )}d\phi }$

關係式

${\displaystyle S(x,\theta )\approx {\sqrt {\frac {\pi }{2x}}}e^{-x}erf({\sqrt {\frac {x}{2}}}\theta )}$

${\displaystyle S(x,\pi /2)=BesselK(1,x)}$

級數展開

• ${\displaystyle S(x,{\frac {\pi }{5}})\approx {.628-.674*x+.364*x^{2}-.131*x^{3}+0.357e-1*x^{4}-0.780e-2*x^{5}+0.143e-2*x^{6}-0.225e-3*x^{7}+0.310e-4*x^{8}-0.368e-5*x^{9}+O(x^{1}0)}}$

• ${\displaystyle S(3,\theta )\approx {0.498e-1*\theta -0.249e-1*\theta ^{3}+0.498e-2*\theta ^{5}+0.862e-3*\theta ^{7}-0.104e-3*\theta ^{9}-0.857e-4*\theta ^{1}1-0.222e-4*\theta ^{1}3-0.155e-5*\theta ^{1}5+O(\theta ^{1}7)}}$