M. Fukuda

Design of a focusing high-energy heavy ion microbeam system at the JAERI AVF cyclotron

Abstract For bio-medical applications of single-ion hit techniques such as radio-microsurgery, a focused high-energy heavy ion microbeam was designed and installed as a vertical beam line connected to the AVF cyclotron (K=110) facility at JAERI Takasaki. By extracting a heavy ion microbeam into atmosphere, living cells can be irradiated with an accuracy smaller than typical cellular sizes. In addition, a high-speed automatic targeting and single-ion irradiation system was combined with a two-dimensional microbeam scanning system allowing more than 1000 targets per minute to be hit within a set field of view. Such high speeds targeting is necessary when examining statistically significant trends in cell irradiation studies within feasible time constraints. A real-time single-ion hit position detecting system was also designed to further increase the reliability of such irradiations.

Performance and operation of a beam chopping system for a cyclotron with multiturn extraction

A beam chopping system for a cyclotron is in operation at the JAERI cyclotron facility. A combination of a pulse voltage chopper in the injection line and a sinusoidal voltage chopper after the exit of the cyclotron is adopted to produce beam pulses spaced at 1 μs–1 ms intervals from natural cyclotron beams. The chopping system was designed according to a simple formulation of the chopping process in which the multiturn extraction was taken into account. Performance of the chopping system was experimentally proved to satisfy requirements of the design. In actual operation to produce beam pulses at long intervals, however, the number of the multiturn extraction is usually larger than the assumed value in the design because of a large phase acceptance of the cyclotron. Careful tuning of the acceleration phase width or the base magnetic field of the cyclotron is necessary to reduce it. It is essential to strictly define the acceleration phase of injected beams in the central region of the cyclotron to improv...

Flat-top acceleration system for the variable-energy multiparticle AVF cyclotron

A flat-top acceleration system appropriate to minimization of energy spread in an ion beam was investigated for the JAERI AVF cyclotron. A combination of the fundamental- and the fifth-harmonic voltages to obtain a homogeneous energy gain distribution of accelerated particles is ideally suited to a variable-energy multiparticle cyclotron using acceleration harmonics of 1, 2, and 3. The flat topping of the energy gain distribution using the fifth harmonics has the advantages of minimizing amplifier power, reducing power dissipation in a resonator, and increasing the energy gain per turn. The flat-top acceleration system was designed to reduce the energy spread to 0.02%, which fulfills a beam focusing condition for production of a microbeam with a beam spot diameter of 1 μm. Tolerable fluctuations of acceleration voltages, required to achieve the energy spread of 0.02%, were 2.0×10−4 for the fundamental voltage and 1.0×10−3 for the fifth-harmonic voltage. Both fundamental- and fifth-harmonic phases were req...

Magnetic field stabilization by temperature control of an azimuthally varying field cyclotron magnet

A magnetic field drift, gradual decrease of the order of 10−4 in several tens of hours, was observed with the beam intensity decrease in an operation of an azimuthally varying field (AVF) cyclotron. From our experimental results, we show that the temperature increase of the magnet iron by the heat transfer from the excitation coils can induce such change of the magnetic field as to deteriorate the beam quality. The temperature control of the magnet iron was realized by thermal isolation between the main coil and the yoke and by precise control of the cooling water temperature of the trim coils attached to the pole surfaces in order to prevent temperature change of the magnet iron. The magnetic field stability of ±5×10−6 and the beam intensity stability of ±2% have been achieved by this temperature control.

An energy spread minimization system for microbeam generation in the JAERI AVF cyclotron

A heavy-ion microbeam with energy of hundreds of MeV is a significantly useful probe for research in biology and biotechnology. A single-ion hit technique using the heavy-ion microbeam is being developed at the JAERI AVF cyclotron facility for elucidation of biofunctions. For production of a microbeam with a spot size of one micro-meter in diameter, the energy spread in the beam is required to be reduced to 0.02% to minimize the effect of chromatic aberrations in the focusing lenses. The energy spread in the cyclotron beam depends on a waveform of the acceleration voltage and beam phase acceptance of the cyclotron. The typical energy spread of the cyclotron beam is around 0.1% in the ordinary acceleration mode using a sinusoidal voltage waveform. The energy spread can be reduced by superimposing a fifth-harmonic voltage waveform on the fundamental one to generate a flat-top waveform for uniform energy gain. The flat-top acceleration system has been designed for the variable-energy multi-particle AVF cyclotron with acceleration harmonic mode of 1, 2 and 3. An additional coaxial cavity has been installed to generate the fifth-harmonic voltage, coupled to the main resonator. The frequency range of the fifth harmonics, 55–110 MHz, was fully covered by the flat-top acceleration system.

SIMULATION OF SPIRAL BEAM SCANNING FOR UNIFORM IRRADIATION ON A LARGE TARGET

Abstract A new circular beam scanning method for uniform irradiation over a large area has been developed. A beam follows a spiral trajectory on a target, which guarantees continuous circular irradiation. A scanning speed and a trajectory spacing in a radial direction are invariable to make a particle distribution uniform. A radial position of a beam spot and an angular frequency of the spiral scanning are expressed by an irrational function of time. Relative variation in a two-dimensional particle distribution has been investigated by using a simulation program. In case a radial pitch of the spiral trajectory is much smaller than a beam width, uniformity of the particle distribution depends completely on the ratio of the minimum radius of the trajectory to the beam width. Less than ±10% uniformity of the particle distribution can be obtained for the minimum radius to beam-width ratio of less than 0.3.

TEMPERATURE CONTROL OF A CYCLOTRON MAGNET FOR STABILIZATION OF THE JAERI AVF CYCLOTRON BEAM

Frequent corrections of the magnetic field of the JAERI AVF cyclotron were required for keeping a beam current constant during long time operation. We observed correlation between the magnetic field and the temperature of the cyclotron magnet yoke by measuring the magnetic field with an NMR probe and the temperature with platinum resistance thermometers. As a result, this instability of a cyclotron beam was induced by temperature-change of the magnet yoke caused mainly by thermal conduction from the main coil. To restrain the thermal conduction to the yoke, we have inserted temperature-controlled copper plates between the yoke and the main coil. In addition, a temperature control system for the cooling water of the trim coils has been installed, which is independent of the total cooling system for controlling the pole tip temperature. An optimum condition of the temperature control systems for stabilizing the magnetic field has been investigated.

Development of Cyclotron Beam Technology for Applications in Materials Science and Biotechnology at JAERI‐TIARA

Recent progress of cyclotron ion beam development for applications in materials science and biotechnology at the ion‐irradiation research facility TIARA of the Japan Atomic Energy Research Institute(JAERI) is overviewed. The AVF cyclotron in TIARA can accelerate protons and heavy ions up to 90 MeV and 27.5 MeV/n, respectively. In order to conform to the requirement of a reliable tuning of microbeam formation, the cyclotron beam current has been stabilized by controlling the temperature of the magnet yoke and pole within +/−0.5° and hence by decreasing the variation of the magnetic field ΔB/B below 10−5. A heavy ion microbeam with energy of hundreds MeV is a significantly useful probe for researches on biofunctional elucidation in biotechnology. Production of the microbeam with spot size as small as 1μm by quadrupole lenses requires the energy spread of the beam ΔE/E < 2 × 10−4. In order to minimize the energy spread of the cyclotron beam, the fifth‐harmonic voltage waveform has been successfully superpose...

DESIGN OF THE FLAT-TOP ACCELERATION SYSTEM FOR THE JAERI AVF CYCLOTRON

A flat-top acceleration system for the JAERI AVF cyclotron has been designed. The fifth harmonic of the fundamental frequency is used to obtain uniform energy gain. To determine optimum parameters of the flat-top system, a cold model test was carried out and flat-top waveforms of the voltages were observed successfully in the whole range of the fundamental frequency. An rf power required for generating a flat-top dee voltage of 30 kV was estimated to be about 1 kW. The design of the flat-top cavity is being modified using the MAFIA code.

The IRAC Code System to Calculate Activation and Transmutation in the TIARA Facility

A computer code system IRAC has been updated to calculate nuclide transmutation and induced radioactivity for incident particles of neutron, proton, deuteron, alpha up to 150 MeV, and l2C, l4N, l6O,20Ne, 40Ar up to 500 MeV in arbitrary multi-layer target system of three-dimensional geometry. The system provides four libraries that consist of ACSELA for isotope production cross-sections, DECAYLIB for decay data, GAMMALIB for photon emission data, and MASWPS for atomic mass data. Features and the system performance have been presented. To validate accuracy of the code system, radioactivities of residual isotopes for incident 260-MeV 20Ne ions on cobalt target were measured by using gamma-spectrometry. Comparison of radioactivity has been shown between calculated results and experimental data for each cooling time.