Turtle is a well-suited program for studying - with the help of Monte-Carlo methods - the effects
degrader material has on the phase space of a fixed energy proton beam extracted from a cyclotron.
A Turtle input file (FOR001.DAT) for this degrader design problem is shown below.
Fig. 1 (21 kB) presents the HPLOT output of the 6 computed histograms. Histogram 1 shows the (x/x')
contour plot of the unscattered initial beam at the location of the waist (Corr = r12 = 0, inside the degrader)
Histogram 2 shows the same for the scattered beam at the back-projected virtual waist position
(see Fig. 2 (34 kB)) or at the effective origin
(see Fig. 3 (22 kB)). The computed correlation coefficient
Corr (r12) is -0.01 or nearly zero. The (x/x') projected emittance
has increased by a factor of 150. Histograms 3 and 4
show the (x/y) spots at the virtual waist position and at the exit of the degrader material.
Histograms 5 and 6 are showing the momentum and the kinetic energy distribution of the protons
leaving the degrader. The FWHM of the dp/p-distribution is about 6.8 %. Because of the large angular straggling and
considerable momentum spread, a conventional beam line after the degrader can only transport about
1.5 to 2 % of the 70 MeV protons leaving the degrader. (For less degraded protons the transmission
will be higher.) For a typical proton beam line design (see
Proscan area layout) {Quadrupole aperture radius = 45 mm, dp/p (2) = 1.2 %, see also article from PSI Large Research Facilities Scientific and Technical
Report 2002, Volume VI, Optical Design of the Proton Beam Lines
for the Proscan Project (119 kB) [1]} the computed energy dependence of the transmission is shown in
Fig. 4 (10 kB). The relatively high
beam losses {for the loss-distribution along a typical proton beam line design see
Fig. 5 (8 kB)} induced by the usage of a fixed energy cyclotron together with a degrader are a
disadvantage compared to the usage of a variable energy synchrotron. On the other side modern
magnet and power supply design for a beam line together with a fast moving wedge degrader will
make it possible to perform an energy step equivalent to 4 mm range-change in water within
50-150 milliseconds (required for the Dynamic Spot Scanning).
/Degrader Design, 250->71 MeV/ 100000 15. 11.0 /MeVc/ .001 ; 15. 12.0 /MeV/ .001 ; 15. 6.0 /PM/ .1 ; 15. 1.0 /MM/ .1 ; 16. 190. 0. 100. /FILE/ ; (Metafile for graphics = FOR100.DAT) 1.0 1.0 2.0 1.0 2.0 0.0 1.0 729.0 /BEAM/ ; (ex = ey = 2 * Pi mmmrad, Ekin = 250 MeV) 16. 3.0 1836.7 /MASS/ ; (Protons) 16. 165.0 /MULT/ ; (enable multiple scattering) 16. 160. /INVS/ ; (take inverse slits-->degrader or target) 16. 198. 100.0 ; 13. 10. ; 30. 250. 3.55 200. 4.05 100. 6.60 10.0 41.5 1.0 240.0 0.0 /dEdx/; 51. 1.0 -2.5 2.5 .25 ; (Histogram 1, x-axis) 52. 2.0 -5.0 5.0 .50 ; (Histogram 1, x'-axis) 3. -0.1194 ; ( -d1 ) 9. 892. ; (loop over 892 degrader segments) 6. 1.0 50.0 3.0 50.0 /SLIT/ ; 1. 0.879 9.30 295.5 .630 144.2 0.0315 0.1261 0.0 /C/ ; 3. 0.0002 ; ( 892 * 0.0002 = 0.1784 = d1 + d2 ) 9. 0. ; (end of loop) 3. -0.059 ; ( -d2 , back-projection to virtual source) 51. 1.0 -5.0 5.0 .5 ; (Histogram 2, x-axis) 52. 2.0 -100. 100. 10. ; (Histogram 2, x'-axis) 51. 1.0 -5.0 5.0 .5 ; (Histogram 3, x-axis) 52. 3.0 -5.0 5.0 .5 ; (Histogram 3, y-axis) 3. 0.059 ; ( d2 ) 51. 1.0 -10.0 10.0 1. ; (Histogram 4, x-axis) 52. 3.0 -10.0 10.0 1. ; (Histogram 4, y-axis) 50. 11.0 345.0 395. 2. ; (Histogram 5, momentum-axis) 50. 12.0 62.0 80. 1. ; (Histogram 6, energy-axis) 16. 11. 371.85 /p0/ ; (new central momentum, e.g. for following bends) SENTINEL SENTINELOther Proton Beam Therapy Application Examples: Isotope Production Yield Optimization at PSI Double Scattering Computations for Proton Beam Spreading Medical Gantry Optical Design
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