Evaluation of different methods for determining the magnitude of initial recombination in ionization chambers Klaus Derikum Physikalisch-Technische undesanstalt undesallee 100, 38116 raunschweig, Germany e-mail: Klaus.Derikum@ptb.de
Mechanisms of Ion Recombination initial recombination volume recombination positive ion negative ion recombined - + - + independent on dose rate depends on dose rate to be corrected for
Motivation misleading statement in IAEA TRS 398 (section 4.4.3.4) continuous beams: k S = ((V 11 /V /V 22 ) 22-1) 1) / ((V 11 /V /V 22 ) 22 -(M 11 /M /M 22 )) )) This relation is isbased on on a linear dependence of of 1/M on on 1/V 2.. The presence of of initial recombination disturbs this linearity but this is isnormally a small effect which may be beneglected. NE 2571 (200V) (200V) at at 1 Gy/min volume recombination ΔQ/Q = 7 10 10-5 -5 initial recombination ΔQ/Q = 1 10 10-3 -3
M(100V) / M(U) 1.020 1.015 1.010 1.005 M s / M = 1 + b/u + β j d 4 / U 2 0,1 Gy/min 0,4 Gy/min 1,1 Gy/min 1,6 Gy/min 3,0 Gy/min 1.000 NE 2571/977 0.995 0 5 10 15 20 25 30 kv U -1 kv -1-1
does not correct N D 60, w, Co for recombination losses N 60 Dw,, Co : absorbed dose to water calibration factor for 60 Co radiation N D,w accounts for ion loss independent on dose rate (i. e. ion diffusion and initial recombination)
Ion Loss in Pulsed eams Derikum, Roos, PM 38 755 (1993) Qs/Q = 1,001 + 0,15 d 2 q / U Seuntjents et al. MP 27 2763 (2000) measurements NE 2571 : 1,0021 1,0022 NE 2611A : 1,0022 1,0023 PR06-C : 1,0018 1,0015 Exradin A12 : 1,0020 1,0019 Formula describes result of of extrapolation within 0.1 %.. Constant term attributed to to initial recombination..
Measuring the amount of initial recombination Method 1: fitting M/ M s = 1 - b/u - β j d 4 / U 2 ( 60 Co) simultaneously for various dose rates Method 2: fitting M/ M s = 1 - b/u - β j d 4 / U 2 ( 60 Co) at dose rate < 0.1 Gy /min Method 3: 3.1: fitting M/ M s = 1 - a/u (Linac) for various doses per pulse 3.2: fitting M 1 / M s = 1 - b/v 1 - c q ( parameter - variable )
Results from fitting simultaneously M/ M s = 1 - b/u - β j j d 4 // U 2 chamber b β NE NE 2561/244 0,27 V 7,7 10 10 13 13 V 22 /(A m) m) NE NE 2561/293 0,27 V 7,9 10 10 13 13 V 22 /(A m) m) NE NE 2561/297 0,27 V 6,0 10 10 13 13 V 22 /(A m) m) 0,28 0,28 V urns, Rosser PM 35 (1990) NE NE 2571/977 0,24 V 9,3 10 10 13 13 V 22 /(A m) m) NE NE 2571/2906 0,22 V 7,6 10 10 13 13 V 22 /(A m) m) FC FC 65-G/771 0,17 V 7,3 10 10 13 13 V 22 /(A m) m) oag (1987) :: 6,7 6,7 10 10 13 13 V 22 /(A /(A m) m)
ΔM/M 0 1 % 0-1 -2-3 -200V -200V NE2571-2906 E 0 = 20 MeV FS = 15x15 cm 2 z = 22 mm -100V -50V -40V 0 10 20 30 40 50 60 t / min Run Nr 14277
M(200V) / M 1.06 1.04 1.02 1.00 NE 2561/244 D i = 0,25 mgy 0 10 20 30 40 50 U -1 / kv -1 ksne2561-244
charge deficit at 100 V 5 % 4 3 2 1 NE2571-2906 1.5 % 1.0 0.5 plane parallel d = 2 mm 0 0 1 2 10-5 C/m 3 3 pulse charge density 0.0 0.0 0.5 1.0 1.5 pulse charge density ksq 10-5 C/m 3 x = electron beam, = photon beam
Linac vs. 60 Co f i = 1 - b/u ----------------------------------------------------- b / V pulsed beam ----------------------------------------------------- NE 2561 0.30 0.27 (0.28) NE 2571 0.23 0.23 PR06C 0.3 - W23331/2 0.3 - Roos-type 0.12 - FC65-G - 0.17 60 Co -----------------------------------------------------
Conclusions Consistent results obtained by different methods. (uncertainties < 0.05 %, k = 2, prelim. guess) Initial recombination in high energy beams does not depend on beam quality. ( 60 Co, pulsed photon, pulsed electron) Charge loss is negligible if corrected appropriately. (needs coordinated practice)
thank you