Hiromichi Ryuto

Effects of heavy-ion beams on chromosomes of common wheat, Triticum aestivum

To investigate the nature of plant chromosomes irradiated by heavy-ion beams, the effects of nitrogen (N) and neon (Ne) ion beams on hexaploid wheat chromosomes were compared with those of X-ray. Chromosome aberrations, such as short, ring and dicentric chromosomes appeared in high frequency. The average numbers of chromosome breaks at LD-50 by irradiation with X-ray, N and Ne ion beams were 32, 20 and 20, respectively. These values may be underestimated because chromosome rearrangement without change in chromosome morphology was not counted. Thus, we subsequently used a wheat line with a pair of extra chromosomes from an alien species (Leymus racemosus) and observed the fate of the irradiated marker chromosomes by genomic in situ hybridization. This analysis revealed that 50Gy of neon beam induced about eight times more breaks than those induced by X-ray. This result suggests that heavy-ion beams induce chromosome rearrangement in high frequency rather than loss of gene function. This suggests further that most of the novel mutations produced by ion beam irradiation, which have been used in plant breeding, may not be caused by ordinary gene disruption but by chromosome rearrangements.

Heavy-ion-induced mutants in sweet pepper isolated by M1 plant selection

We examined the effects of heavy-ion bombardment on mutagenesis in sweet pepper (Capsicum annuum L.). Dose–response studies indicated that 10 Gy irradiation of 12C or 20Ne ions on dry seeds is suitable for inducing mutations in plants. From 20Ne-irradiated M1 plants, putative mutants included two dwarf plants and one plant whose pericarp was yellow were isolated. Phenotypes of their M2 progeny were similar to those of the M1 plants and did not segregate. F1 plants resulting from reciprocal crosses between the mutants and wild-type plants showed the wild-type phenotype, but phenotypes of F2 and BC1F1 segregated at 1:3 (mutant:wild) and 1:1, respectively. These crossing experiments indicate that the three mutants have monogenic recessive mutations in nuclear genes. In light of these data, we discuss the effectiveness of using heavy-ion bombardment to mutate sweet peppers.

An effective method for detection and analysis of DNA damage induced by heavy-ion beams.

We have developed an efficient system to detect and analyze DNA mutations induced by heavy-ion beams in Arabiopsis thaliana. In this system, a stable transgenic Arabidopsis line that constitutively expresses a yellow fluorescent protein (YFP) by a single-copy gene at a genomic locus was constructed and irradiated with heavy-ion beams. The YFP gene is a target of mutagenesis, and its loss of function or expression can easily be detected by the disappearance of YFP signals in planta under microscopy. With this system, a 12C6+-induced mutant with single deletion and multiple base changes was isolated.

Low-Damage and High-Rate Sputtering of Silicon Surfaces by Ethanol Cluster Ion Beam

To realize the high-rate and low-damage sputtering of a Si surface, the effect of irradiating an ethanol cluster ion beam on a Si surface was investigated. The sputtering depths in Si substrates induced by the ethanol cluster ion beam irradiation were larger than those in SiO2 substrates, which was due to a chemical sputtering effect. The lattice disorder and the surface roughness of the Si substrates decreased with increasing retarding voltage.

Effects of Ion Irradiation on Seedlings Growth Monitored by Ultraweak Delayed Luminescence

The optical technique based on the measurement of delayed luminescence emitted from the biological samples has demonstrated its ability to provide valid and predictive information on the functional status of various biological systems. We want to extend this technique to study the effect of ionizing radiation on biological systems. In particular we are interested in the action of ion beams, used for therapeutic purposes or to increase the biological diversity. In general, the assessment of the damage that radiation produces both in the target objects and in the surrounding tissues, requires considerable time because is based on biochemical analysis or on the examination of the evolution of the irradiated systems. The delayed luminescence technique could help to simplify this investigation. We have so started our studies performing irradiations of some relatively simple vegetable models. In this paper we report results obtained from mung bean (Vigna radiata) seeds submitted to a 12C ion beam at the energy of 62 MeV/nucleon. The dry seeds were irradiated at doses from 50 to 7000 Gy. The photoinduced delayed luminescence of each seed before and after ion irradiation was measured. The growth of seedlings after irradiation was compared with that of untreated seeds. A growth reduction on increasing the dose was registered. The results show strong correlations between the ion irradiation dose, seeds growth and delayed luminescence intensity. In particular, the delayed luminescence intensity is correlated by a logistic function to the seedlings elongation and, after performing a suitable measurement campaign based on blind tests, it could become a tool able to predict the growth of seeds after ion irradiation. Moreover these results demonstrate that measurements of delayed luminescence could be used as a fast and non-invasive technique to check the effects of ion beams on relatively simple biological systems.

Equipment to detect luminescence induced by cluster ion collisiona)

The possibility to analyze on-line the surface region of solid materials using the cluster ion beam luminescence spectroscopy has been examined. At this aim, the cluster ion beam apparatus for the processing of solid materials was modified. The neutral clusters were ionized by the electron impact ionization to obtain an intense cluster ion beam. The tungsten filament used in this ionization method was replaced with an oxide one to reduce the emission of the background light by decreasing the operating temperature of the filament. To further suppress this light, antireflection materials were used to cover the parts inside the vacuum chamber, such as walls and inner surfaces of the einzel lens. The signal to noise ratio was improved more than one order of magnitude. The emission of photons induced by the irradiation of cluster ion beams was detected.

Sources of polyatomic ions of organic liquids

We have developed two types of liquid ion sources, one of which was a polyatomic ion source using liquid organic materials with a high-vapor pressure. Liquid materials such as octane and ethanol could be heated up to a maximum temperature of 100 degrees C, and the vapors were introduced into an ion source. They were ionized by an electron bombardment method and extracted from the ionizer. The ion current obtained at an extraction voltage of 2 kV was 230 microA for octane and several fragment ions such as alkyl ions were produced. On the other hand, another type of polyatomic ion source using alkyl naphthalene mixed with ionic liquid such as imidazolium dicyanamide has been developed. Instead of the electron bombardment method, a high-electric field method was used for the ion-emission from a sharp tip, because the vapor pressure of the liquid materials was relatively low. The threshold voltage was approximately 4.5 kV and the ion current of approximately 250 nA was obtained at an extraction voltage of 9.5 kV.

Fabrication of Impregnated‐Electrode‐Type Polyatomic Ion Source with Ionic Liquid

Three types of ionic liquid ion source characterized by needle‐type, capillary‐needle‐type and impregnated‐electrode‐type emitters were fabricated and demonstrated. The ion emission of a pure ionic liquid was demonstrated for the capillary‐needle‐ and impregnated‐electrode‐type emitters. The emission stability of the impregnated‐electrode‐type emitter is currently insufficient for the shallow implantation of carbon due to occasional discharge. However, the impregnated‐electrode‐type emitter showed the largest emission current of 14 μA at 6 kV among these emitters. This was considered to be caused by the relatively high feedability of the ionic liquid obtained from the emitter tip made of porous copper.

Liquid Film Stripper for Intense Heavy-Ion Beams

A charge stripper system using a thin liquid film was developed for high-intensity heavy-ion beams at the RIKEN RI-beam factory. A stable free-standing film of silicone oil with a fairly large area was produced using a fast-rotating disk with a flat edge. A beam test was performed to measure the durability of the liquid film against ion beam bombardment.

Radiation effects on cryogenic temperature sensors of Cernox, CGR and PtCo

Irradiation tests have been carried out in view of selecting the cryogenic temperature sensors for use in the RIKEN Superconducting Ring Cyclotron (SRC). About 300 temperature sensors will be installed, and most of them will be exposed to high radiation doses during the operation of accelerator. Those cryogenic temperature sensors, Cernox/spl trade/, CGR and PtCo, at room temperature were bombarded by the intense 210 MeV proton beam at the RIKEN Accelerator Research Facility (RARF) of RIKEN.