Fig. 1. Structure diagram of the short-range particle source water absorbed dose absolute measurement ionization chamber

Fig. 2. Internal structure of the ionization chamber under Maxwell software

Fig. 3. Electric field diagram with and without guard ring(a) Without protective electrode, (b) With protective electrode

Fig. 4. Variation of electric field change with or without protective electrode

Fig. 5. Electric field diagrams of different guard electrode widths (high voltage electrode remains unchanged)(a) 9 mm, (b) 29 mm, (c) 49 mm, (d) 69 mm, (e) 89 mm, (f) 109 mm

Fig. 6. Electric field changes with different guard electrode widths under invariant high voltage electrodes (color online)

Fig. 7. Electric field diagrams of different guard electrode widths (high voltage electrode keeps the same outer diameter as guard electrode) (a) 9 mm, (b) 29 mm, (c) 49 mm, (d) 69 mm, (e) 89 mm, (f) 109 mm

Fig. 8. Electric field changes with different guard electrode widths while high voltage electrode keeps the same outer diameter as the guard electrode (color online)

Fig. 9. Electric field diagrams of different insulating ring widths (a) 1 mm, (b) 3 mm, (c) 5 mm, (d) 7 mm, (e) 9 mm

Fig. 10. Electric field variation with different insulating ring widths (color online)

Fig. 11. Electric field diagrams with different numbers of grid electrode (a) 0, (b) 1, (c) 3, (d) 7, (e) 15

Fig. 12. Electric field variation with different grid electrode numbers (color online)

Fig. 13. Schematic diagram of different grid shapes

Fig. 14. Electric field diagrams for different grid shapes(a) Rectangle, (b) Round, (c) Triangle

Fig. 15. Electric field variation for different grid shapes (color online)

Fig. 16. Electric field diagrams for different gate widths (a) 2 mm, (b) 5 mm, (c) 10 mm, (d) 20 mm

Fig. 17. Electric field variation for different gate widths (color online)