Development of a water calorimeter for medium energy x-rays

Transcription

1 Development of a water calorimeter for medium energy x-rays Leon de Prez Eduard van Dijk Patricia Damen Nederlands Meetinstituut

2 Introduction Introduction Design and construction Measurement assembly Correction factors Estimate of the uncertainties Preliminary results

3 Introduction - Aim Why medium energy x-rays? medium energy x-rays are still used in the Netherlands and for certain treatments an alternative to electron therapy What is the aim? determination of N Dw for a reference chamber with an uncertainty smaller or equal to the current Dutch dosimetry protocol NCS-10 i.e. 2 3 % (1 SD) How is it achieved? feasibility study for one NE 2571 chamber 4 x-ray qualities ( kv; HVL mm Cu)

4 Introduction - Beam qualities HVL and E mean behind insulation materials (no correction applied for attenuation of calorimeter walls) kvp HVL / mm Cu E mean in air 250 kv kv kv kv

5 Introduction - Principle D w = ΔT C k w Sealed water calorimeter operated at 4 C Horizontal beam (SDD 620 mm; depth 3.5 g cm -2 ) ΔT with 2 thermistors in a Wheatstone bridge Correction factors: attenuation and scatter: (PENELOPE) excess heat: (Comsol Multiphysics) heat defect: ~ (published data)

6 Design and construction

7 Design and construction

8 Design and construction

9 Design and construction

10 Design and construction Glass cell diameter 3.9 mm, glass thickness 0.8 mm Two 20 kω thermistors in glass probes (diameter 0.5 mm) Ultra pure water (Millipore: 3 ppb TOC; 18.2 MΩ cm) Saturated with Argon

11 Measurement assembly Optimization of signal SDD as small as practically possible (620 mm) Optimization of electrical circuit: guarding in star shape DC Wheatstone bridge in stead of direct resistance measurement ability to compare 4-wire resistance with a Wheatstone configuration Result: SD 1.7 % %

12 Measurement assembly

13 Measurement assembly

14 Measurements 5 Example water calorimeter run 135 kv; mm Cu dv [µv] µk min -1 1 Gy time [s]

15 Correction factors k HD Heat defect (~ 1.000) k GLASS Presence of non-water materials k XS Temperature gradients / heat transport

16 Glass correction Calculated with the PENELOPE MC code based on photon fluence spectra measured with HP Gespectrometer and verified with measured depth dose distributions.

17 Glass correction - PDD Relavitive depth dose [-] Relative depth dose normalized to 3.5 g cm kv; mm Cu Measured Calculated (PENELOPE) depth [cm] Photon fluence spectrum in air

18 Glass correction - Results Correction factor [-] Correction for presence of glass kvp k glass 250 kv kv kv kv Estimated type B uncertainty 1 % HVL [mm Cu]

19 Excess heat correction Calculated with Comsol Multiphysics 3.2 based on MC calculations for the absorption of energy in the glass compared to water model parameters heat conduction (assuming no convection)

20 Excess heat correction Relative E deposited [-] Relative energy deposition in water and cel 135 kv; mm Cu Cell wall Probe kvp E cell /E E probe /E w w 250 kv kv kv kv depth [g cm -2 ] Used as input for the heat transport calculations

21 Excess heat correction Heat transport model Comsol Multiphysics 3.2 Heat transport by conduction PDD from MC calculations Relative energy absorption in glass compared to water from MC calculations

22 Excess heat correction Beam Cell Probe

23 Excess heat correction

24 Excess heat correction 45 Excess heat correction 135 kv; mm Cu 35 time [s] "ideal run" excess heat run time [s]

25 Excess heat correction 1.07 Correction for excess heat kvp k XS Correction factor [-] kv kv kv kv Estimated type B uncertainty 1 % HVL [mm Cu]

26 Correction factors - Results kvp k glass k XS k tot 250 kv kv kv kv

27 Preliminary results Comparison with Dutch dosimetry protocol: NCS-10 Water calorimeter N D w = Dw monitor Mstp monitor NCS-10 protocol ND w = N K μen ρ d w / air k ch

28 Preliminary results WCM NCS-10 kvp N Dw u (k = 1) u (k = 1) N Dw 250 kv % % -2.4 % 180 kv % % -1.8 % 135 kv % % -0.9 % 100 kv % % %

29 Preliminary results N Dw N Dw for for NE a NE N Dw [mgy pc -1 Dw [µgy pc -1 ] ] NCS-10 NCS-10 Water calorimeter Water calorimeter HVL HVL [mm [mm Cu] Cu]

30 Conclusion and future work Conclusion: Water calorimetry for medium energy x-rays is possible within 2 3 % uncertainty Future work: Further verifying calculations with measurements Improving the uncertainty budget

31 Thank you for your attention.