At PSI an isotope production facility exists, which is utilizing some
72 MeV proton beam pealed off with an electrostatic septum from the 2 mA high intensity proton beam of the
transfer beam line
connecting the 72 MeV Injector-2 clyclotron
and the 590 MeV Ring Cycloton
for HE production. Because
the required proton beam intensity for isotope production is in the range of 10 to 70 µA, which
is only a small fraction (0.5 to 3.5 %) cut off from the edge of the main beam, the phase space of
the proton beam for isotope production may vary
from day to day. Therefore the spot size of the beam at the isotope production target may vary in size quite
considerably with a fixed optics. If not corrected this means either less isotope production yield or possibly some
overheating of the isotope production target cell.
In order to perform an envelope fit, the beam extents in horizontal and vertical transverse direction have to be measured with the help of profile monitors at several locations along the proton beam line to the isotope production facility. These data together with the actual quadrupole settings have to be inserted in a special Transport input file describing the beam line for isotope production with all its relevant parameters and its special fit conditions. This Transport input file describing the envelope fit of the 72 MeV isotope production beam at PSI is shown. The resulting beam envelope from Graphic Transport (for Windows, 30 kB) is also shown. The 'T's drawn in this graphic picture symbolize the measured 2 * sigma beam widths. The drawn curves connecting these 'T's represent the envelopes fitted with Transport by varying the 6 parameters x, x', r12, y, y' and r34. The quality of the fit is excellent. The size of the beam at the isotope production target is 18.6 mm in x-direction and 16.6 mm in y-direction (4 * sigma), which is close to the ideal value of 15 mm in both directions (round spot).
In order to achieve these 15 mm in both directions, a forward fit has to be done with the previously found beam parameters as starting values and by varying the field values of the last 4 quadrupoles in order to reach the desired beam diameters at the isotope production target location. This is done by loading a different (but corresponding) Transport input file and then by transferring the beam parameters with the command BOI (Beam from Output to Input) and the quadrupole fields with the command QOI (Quads from Output to Input) and finally with a special interactive procedure, which asks the operator to input the desired beam diameters at the isotope production target location (in this case 15 mm for both directions). This second Transport input file is also shown. The resulting beam envelope from Graphic Transport (for Windows, 30 kB) may also be seen. For comparison the red colored envelope curves show the envelope from the previous envelope fit. When setting the proposed new values of the last 4 quads, a newly started envelope fit with newly measured profile data shows that the desired beam diameter values at the location of the isotope production target usually are reached within an accuracy of ±0.5 mm.
The effect of the new settings in this example is rather minor (probably 10 % increase of the isotope production yield has been achieved in this case). But experience over time has shown that a fluctuation of the beam emittance may influence the isotope production yield up to a factor of 2 if not corrected. A skillful operation crew can also do the corrections without the help of Transport; but these efforts give less reproducible results and are much more time consuming. Besides the focusing adjustments the centering of the proton beam at the location of the isotope production target has also to be accomplished. This may also be more reproducible, if the spot size at the isotope production target-cell is corrected to the same values each time before the centering is done.
These online Graphic Transport procedures have demonstrated for the case of isotope production at PSI their usefulness and may also be used for other applications. Up to the end of the year 2003 they have also been used for the adjustment of the beam spot size at the target for the PIREX experiment. The downloadable version of 'Transport for Linux' or 'Transport for the Mac OS X' (investigate tpmenu's menu-item 'Special fit procedures') contains all the script files needed to be studied by interested people.
Other Proton Beam Therapy Application Examples:
Degrader Design for a 250 MeV Medical Cyclotron
Double Scattering Computations for Proton Beam Spreading
Medical Gantry Optical Design
Last updated by Urs Rohrer on 21-Dec-2006