Chapter 6 External Photon Beams: Physical Aspects - IRSN

856.16 SHUTTER CORRECTION TIME The shutter correction time sis typically determined bymeasuring two doses (D 1 and D n ) at a given point Q in aphantom:• D 1 is measured with a relatively long exposure time T (of the orderof 5 min), contains one end effect and is governed by:= + DD =11D( Ts) or D T + • D n is measured cumulatively with n dose segments, each havingan exposure time T/n. The dose D n thus contains n end effects; thecumulative beam-on time is again equal to T, and D n is:= + DnDnD( T ns) or D = T + n ssIAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.16 Slide 3 (169/170)6.16 SHUTTER CORRECTION TIMESolving the equation for the true dose rateD1 DnD= = T + T + nsThe shutter correction time is:• For D n > D 1 , s> 0• For D n = D 1 , s= 0• For D n < D 1 , s< 0s s= (D n D 1 )T(nD 1 D n) Typical shutter correction times are of the order of 1 s.DIAEA Radiation Oncology Physics: A Handbook for Teachers and Students - 6.16 Slide 4 (170/170)