M. Breinig

Observation of high-order electric multipoles in convoy electron angular distributions

The authors have discovered multipoles of even order to about 10 in the multipole moments characterising angular distributions of convoy electrons ejected by swift ions during penetration of thin solids. They suggest the strongly transverse distributions observed reflect electron loss from excited n, l states inside the solid, which, though rapidly created and destroyed, progress toward finite dynamic equilibrium populations. Alternative interpretations are also discussed.

Convoy electron production in heavy ion-solid collisions

Abstract The properties of the sharp ν c ≌ ν cusps observed in the velocity spectrum of convoy electrons (υe) ejected in heavy ion-solid collisions in the ion velocity range (ν) 6–18 a.u. are compared to the properties of analogous cusps observed in binary electron capture to the continuum (ECC) and electron loss to the continuum (ELC) collisions in gases. Apart from a skew toward νe >ν, the ν-independent convoy distributions observed are very similar to those for ELC and the cusp widths are the same in both cases. While the shape of convoy peaks is approximately independent of projectile Z, ν, and of target material, yields in polycrystalline targets (C, Al, Ag, Au) exhibit a strong dependence on Z and ν. Coincidence experiments in which convoy electrons are allocated according to emergent ion charge-state qe show a surprising independence of qe, mirroring the unweighted statistical emergent charge-state fraction Coincidence experiments on O6+ ,7+ ,8+ ions traversing 〈 110 〉 and 〈 100 〉 channels in Au show a strong yield suppression and a dependence of yield on the channel chosen. Interpretation of these observations, comparisons to convoy production studies using protons, and a discussion of remaining puzzles is given. The history of ECC, ELC, and “wake-riding” models of convoy electron production is also reviewed.

Electron Capture to the Continuum at Asymptotically High Velocities

The velocity distrubutions of electrons ejected from gaseous He targets into the forward direction by 15-18 au velocity bare O, Ne and Ar nuclei are measured. The shape of the resulting ECC (electron capture to the continuum) cusps is studied in detail by three fitting methods and compared to theory. The results favor theoretical treatments which include higher-order ejected electron partial waves to describe the observed asymmetry.

Velocity dependence of the widths of electron-loss cusps in the range 7-12.5 au

The velocity dependence of the full width at half maximum of projectile electron-loss cusps is investigated. For Siq+, Oq+ and Cq+ projectiles traversing He, Ne and Ar targets in the velocity range 7-12.5 au, the FWHM is found to be independent of the ion velocity. These results disagree with theoretical predictions predating those of Day and restrict the velocity dependence of the anisotropy parameter beta in Days theory (1980).

Anomalous mean free paths for scattering of convoy electrons generated by fast, highly ionized ions in thin solid targets

We have measured the yields of convoy electrons associated with 15.2-MeV/amu Ni/sup 24 +/ and Ni/sup 26 +/ ions transversing thin, poly-crystalline foils of C and Al. The thicknesses used were in the range 3 to 500 ..mu..g/cm/sup 2/. By observing how the yield saturates with increasing thickness, we obtained effective mean free paths (MFPs) for the scattering of convoy electrons out of the collector half angle (1.54/sup 0/).

Comparison of the target-thickness dependence of the convoy electron yield and the Rydberg electron yield measured in coincidence with exit charge states in fast ion-solid collisions

Abstract We have simultaneously measured the yield of convoy electrons and the yield of electrons in high Rydberg states of the projectile (n ≳ 70), produced by 2 MeV/u C projectiles passing through C foils, whose thicknesses range from 4–10 μg/cm2, for incident charge states qi = 4–6 and exit charge states qe = 4–6. We have found that these yields exhibit similar trends as a function of foil thickness, but that, nevertheless, the ratio of the number of convoy electrons detected in coincidence with ions of exit charge state q to the number of electrons detected in high Rydberg states of ions with the same exit charge state is a function of foil thickness. This may be due to a broadening of the convoy electron energy spectrum with increasing foil thickness.

TH‐E‐ValA‐03: Topographic Leaf‐Sequencing Using a Genetic Algorithm

Purpose: To develop a leaf‐sequencing algorithm for fixed‐gantry (nonrotational) treatment delivery on a commercial helical tomotherapy system (HI‐ART, TomoTherapy, Inc., Madison, WI). Method and Materials: A genetic algorithm was used to determine the multileaf collimator(MLC) leaf open times for a series of fluence test maps generated from a tomotherapy machine with a fixed gantry angle of 0 degrees (IEC scale). A series of wedge shapes (15, 30, 45, and 60‐degree) were mathematically created to test the algorithms ability to produce simple modulations, similar to those which would be encountered in breast radiation therapy.Results: In general, the topographic treatment delivery yielded reasonable dose distributions. The agreement for the wedge cases was within ±2%, or 2‐mm distance‐to‐agreement (DTA) in the high dose gradient regions. The central axis measured dose was between 3.6 and 4.2 percent higher than the expected dose for the wedge cases. For double peaks, the agreement was within ±2%, or 2‐mm DTA across the entire measured film. For quadruple peaks, the agreement was within ±2%, or 2‐mm DTA in the high dose gradient regions. At the first peak, calculated and measured agreed to within ±0.5%. The dose gradient between the first peak and the first valley was 5 percent per centimeter. The dose in the first valley agreed to within ±1.6% of the prescribed dose (at the first peak). The maximum error in the quadruple peaks occurred at the second peak, where the measured dose was 3.8% low (relative to the prescribed dose at the first peak). Conclusions: The developed algorithm produced calculated deliverable distributions that agreed well with the artificially constructed distributions. This delivery technique could be used for treatment of a whole intact breast. Additional work is needed to optimize the algorithm to improve agreement between the calculated doses and deliverable dose distributions.

WE-E-T-617-07: Topographic Treatment Planning and Delivery

Purpose: The purpose of this work was to develop a leaf‐sequencing algorithm for fixed‐gantry (non‐rotational) treatment delivery on a helical tomotherapy system. Topographic delivery creates intensity‐modulated fields by moving the couch at a constant velocity relative to 10‐mm wide fan‐beam that is modulated with a 64‐leaf binary MLC.Method and Materials: Inverse treatment planning was performed using Pinnacle 7.4. The calculated intensity maps were exported to a custom leaf‐sequencing program that modeled the leaf sequences as a tap moving over a collection of bottles. The leaf‐sequencing algorithm was developed using a tap analogy in a step‐wise process. The initial back edge and final front edge of each step were determined from Newtons Equations of Motion. Once the edges of the steps were established, the number of covered bottles was calculated. The final step was the determination of the time required to fill the each bottle with the prescribed dose from Fouriers Convolution Theorem. Results: The leaf‐sequencing algorithm was initially tested using artificially constructed dose distributions that were compared with the calculated deliverable dose distributions based on the algorithm output. The difference between the theoretical doses and the deliverable doses was much less than 1 percent. The agreement between the Pinnacle intensity maps and the deliverable doses were generally less than 1 percent, with the exception of near the field edge where the intensity map values decreased by 80% in one pixel width. Conclusion: A leaf‐sequencing algorithm was developed for fixed‐gantry treatment delivery on a helical tomotherapy system. The developed algorithm produced calculated deliverable distributions that agreed remarkably with the artificially constructed distributions. Accuracy of the leaf sequencing algorithm will be verified by the film dosimetry method and will be presented at the meeting.

Production and transport of convoy electrons in amorphous carbon foils

Abstract The production of free convoy electrons, emitted with velocities near the ion velocity in ion-solid collisions, is not well understood. Experiments concerning thickness-dependent yields have suggested the dominant mechanism for convoy production is electron loss to the continuum (ELC) in the bulk of the solid. Free electrons created in the bulk are subject to multiple elastic and inelastic scattering during transport through remaining layers of the solid. We discuss double-differential measurements of convoy electrons as a function of target thickness for fast O 5+ ion projectiles incident on carbon foils of varied thicknesses. Angular distributions confirm the ELC model for convoy production. From the radial broadening of the convoy cusps we have determined energy and angular spreading parameters due to postcollisional multiple scattering.

Doubly differential emission distributions for electron loss to the continuum from fast heavy projectiles in gas targets

We have constructed an apparatus which utilizes position-sensitive detector technology for the measurement of doubly differential electron emission cross-sections in ion-atom and ion solid collisions at emission angles near the forward direction. The properties of the instrument make it especially well suited for the simultaneous doubly differential measurement of electron emission processes which are centered in the frame of the projectile, and so it is of value for investigations of the well known electron capture and loss to projectile-centered continuum state processes (ECC and ELC) which have received much attention at this symposium. Further, the multichannel nature of the system leads to improved data acquisition efficiency, sometimes an important advantage for the observation of processes which require the use of heavily-subscribed accelerator user facilities. We describe here results of some of the first measurements to be made with this apparatus, specifically for the doubly differential (in velocity and angle) cross s~tion (DDCS) for continuum transfer emission from collisions of 41, 82, and 105 MeV O ~ in helium and argon gas targets. Because the projectiles carry a relatively loosely bound 2s electron into the collision, ELC processes are expected to dominate the emission characteristics.