Isotope Production Yield Optimization.

Envelope fits with Graphic Transport are a valuable tool for optimizing beam spot sizes on targets for isotope production. At PSI Isotope Production is done with the peeled off (parasitic) proton beam from the 72 MeV injector cyclotron for the HE production. At PSI every week several runs of isotope production are taking place. Longtime Isotope Production runs with around 60 µA of proton beam intensity and continuously stretched over several days during weekends are frequent.

Isotope Production Yield Optimization at PSI
by using Graphic Transport Envelope Fits

Isotope Production 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.
Isotope Production In order to prevent this by keeping the proton beam spot size at the isotope production target within desired values, a feedback loop with an online version of Graphic Transport (until 2005 running under VAX-openVMS, now under Linux) has been built. The used procedures for optimizing the yield for isotope production are

Isotope Production in a first step a (backward) fit to establish the initial beam parameters (projected emittance) via an envelope fit andIsotope Production
Isotope Production in a second step a beam size (forward) fit by varying some quadrupole gradients.

Isotope Production 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).
Isotope Production 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.
isotope production 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.
Isotope Production 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.

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Isotope Production Isotope Production Last updated by Urs Rohrer on 21-Dec-2006
visits since 14-06-2006