Efficiency Enhancement of Indirect Transverse Laser Cooling with Synchro-Betatron Resonant Coupling by Suppression of Beam Intensity

The efficiency of indirect transverse laser cooling with synchro-betatron resonance coupling has been improved with the reduction in beam intensity by scraping the tail part of the beam. In order to measure the beam size at a low beam intensity, a new scheme to measure the beam profile by observation of the survival ratio with changing the scraper position has been established. With 104 particles, the transverse cooling time was reduced to 1.2 s, and the cooled horizontal and vertical beam sizes were 0.19 and 0.61 mm, corresponding to temperatures of 20 and 29 K, respectively, which is largely improved compared with that in our previous experiment

Estimations of relative biological effectiveness of secondary fragments in carbon ion irradiation using CR‐39 plastic detector and microdosimetric kinetic model

PurposeTo estimate relative biological effectiveness (RBE) ascribed to secondary fragments in a lateral distribution of carbon ion irradiation. The RBE was estimated with the microdosimetric kinetic (MK) model and measured linear energy transfer (LET) obtained with CR‐39 plastic detectors.MethodsA water phantom was irradiated by a 12C pencil beam with energy of 380 MeV/u at the Gunma University Heavy Ion Medical Center (GHMC), and CR‐39 detectors were exposed to secondary fragments. Because CR‐39 was insensitive to low LET, we conducted Monte Carlo simulations with Geant4 to calculate low LET particles. The spectra of low LET particles were combined with experimental spectra to calculate RBE. To estimate accuracy of RBE, we calculated RBE by changing yield of low LET particles by ±10% and ±40%.ResultsAt a small angle, maximum RBE by secondary fragments was 1.3 for 10% survival fractions. RBE values of fragments gradually decreased as the angle became larger. The shape of the LET spectra in the simulation reproduced the experimental spectra, but there was a discrepancy between the simulation and experiment for the relative yield of fragments. When the yield of low LET particles was changed by ±40%, the change of RBE was smaller than 10%.ConclusionsRBE of 1.3 was expected for secondary fragments emitted at a small angle. Though, we observed a discrepancy in the relative yield of secondary fragments between simulation and experiment, precision of RBE was not so sensitive to the yield of low LET particles

Linear energy transfer (LET) spectra and survival fraction distribution based on the CR-39 plastic charged-particle detector in a spread-out Bragg peak irradiation by a 12C beam

Facilities for heavy ion therapies are steadily increasing in number worldwide. One of the advantages of heavy ions is their high relative biological effect (RBE). In a model used at NIRS (National Institute of Radiological Sciences), linear energy transfer (LET) spectra are required to estimate biological dose (physical dose × RBE). The CR-39 plastic charged-particle detector (CR-39) is suitable for measurement of LET. For the present study, done at the Gunma University Heavy Ion Medical Center (GHMC), we measured LET spectra at 11 depths in spread-out Bragg peak (SOBP) irradiation by a 12C beam of 380 MeV/u. The lower threshold of the CR-39 to measure LET was about 5 keV μm−1 due to poor sensitivity for low LET. Then we calculated biological dose and survival fraction distributions and compared them with treatment planning results at GHMC. We used Monte Carlo simulation (Geant4) to calculate LET spectra. The simulation results were in good agreement with the experimental spectra. Moreover, the biological dose and survival fraction distributions estimated from the CR-39 reproduced the treatment planning. The CR-39 is suitable for estimating biological dose in carbon ion therapy

Electrostatic deflectors and dispersion suppressors: Their formulation and application to a storage ring

A single particle dynamics in beam bending elements including electrostatic fields is formulated. A general form of scalar potentials of electrostatic deflectors is obtained from solutions of the Maxwell equation having axial symmetry. Equations of motion of a charged particle in various types of the electrostatic deflectors are derived based on Hamiltonian formalism. The equation of motion in dispersion suppressors, which are a combination of the electrostatic deflectors and dipole magnets, are also formulated and generalized. Application of one of the dispersion suppressors to an existing heavy ion storage ring S-LSR provides the better condition for generation of a multidimensional crystalline beam. It is shown that this condition is achievable by real fabricated devices composed of a dipole magnet and an electrostatic deflector equipped with intermediate electrodes. The effectiveness of this dispersion suppressor for the real operation is shown by a particle tracking including the nonlinear field component

Energy compensation of slow extracted beams with RF acceleration

In a conventional carbon-ion radiotherapy facility, a carbon-ion beam is typically accelerated up to an optimum energy, slowly extracted from a synchrotron ring by a resonant slow extraction method, and ultimately delivered to a patient through a beam-delivery system. At Japan’s Gunma University, a method employing slow-beam extraction along with beam-acceleration has been adopted. This method slightly alters the extracted-beam’s energy owing to the acceleration component of the process, which subsequently results in a residual-range variation of approximately 2 mm in water-equivalent length. However, this range variation does not disturb a distal dose distribution with broad-beam methods such as the single beam-wobbling method. With the pencil-beam 3D scanning method, however, such a range variation disturbs a distal dose distribution because the variation is comparable to slice thickness. Therefore, for pencil-beam 3D scanning, an energy compensation method for a slow extracted beam is proposed in this paper. This method can compensate for the aforementioned energy variances by controlling net energy losses through a rotatable energy absorber set fixed between the synchrotron exit channel and the isocenter. Experimental results demonstrate that beam energies can be maintained constant, as originally hypothesized. Moreover, energy-absorber positions were found to be significantly enhanced by optimizing beam optics for reducing beam-size growth by implementation of the multiple-scattering effect option

Possible observation of transverse laser cooled ultimate cold ion beam in S-LSR

Transverse laser cooling is pursued at an ion storage/cooler ring, S-LSR, Kyoto University. An RF bunched Mg+ beam was cooled transversely through synchro-betatron resonance coupling by a co-propagating laser. We investigated peaks of synchrotron oscillation spectroscopically so that we can observe them stably. Oscillation signals from a parallel-plate pickup were observed by a spectrum analyzer. We also observed the coherent synchrotron modes. As the beam temperature decreases, the ion beam would be in space charge limited region. According to the computer simulation, in the case the beam turns to be space charge limited, the disappearance of peaks of synchrotron oscillation is expected. We would like to propose a capability of detecting space charge limited region by observing such a frozen synchrotron oscillation