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Multiple ionisation of the CO molecule following the impact of 100-700 keV He 2+ ions was studied. Fragments arising from the dissociation of n CO 5)were recorded using the coincidence momentum imaging technique. By measuring the angular distribution of the fragment ions in coincidence, and applying the axial recoil approximation, the angle between the axis of the molecule and the direction of the projectile was determined. Strong anisotropy was observed for high degree of ionisation. The dependence of the anisotropy on the interaction strength and the projectile energy was studied and the results were compared with the predicted values in the literature for diatomic molecules. Furthermore, an asymmetry in the angular distribution owing to the heteronuclear nature of the molecule was observed and quantified, showing that the anisotropy is not entirely a geometric effect as has been presented in earlier works.

Fragmentation of multiply ionised molecular ions is investigated using the coincidence momentum imaging technique in an experiment involving proton–CO collisions over the energy range 25–200 keV. From the measured momentum vectors of the fragment ions, the orientation angle—that is the angle between the axis of the molecule and the direction of the projectile at the instant of collision—is determined under the axial recoil approximation. The multiple ionisation yield is found to strongly depend on the orientation angle, giving rise to an anisotropy as well as a forward–backward asymmetry, reflecting the heteronuclear nature of the CO target. A simple model is used to calculate the probability of multiple ionisation, involving Rutherford-like trajectories with an orientation dependent distance of closest approach and an impact parameter dependent single ionisation probability. The computed probabilities show an orientation dependence which agrees closely with experimental observations.

Knowledge about the type of mutagen used and its optimized dose are of paramount importance to design and implement any plant mutation breeding programme. Present study was first time carried out to evaluate the comparative effectiveness, radio-sensitivity behavior and relative biological effectiveness of four physical mutagens viz., gamma rays, X-rays, electron beam and proton beam on two short grain aromatic rice landraces viz., Samundchini and Vishnubhog. The seeds of these two varieties were treated with 15 different doses of all four mutagens, ranging from 50Gy to 750Gy with an interval of 50Gy. Germination percentage and seedling growth parameters were recorded at seven and 15 days after sowing, respectively in two replications. It was observed that germination percentage, shoot and root length of the seedling gradually declined with the increase in doses of all the physical mutagens. On the basis of these observations, LD50 and GR50 doses were calculated. The present study reports the optimum range of doses for gamma ray (280 to 350 Gy); electron beam (290 to 330Gy); X-ray (200 to 250 Gy) and proton beam (150 to 200Gy). GR50 doses were observed higher than LD50 doses for all the mutagens in both landraces. However, Samundchini showed higher LD50 and GR50 doses than Vishnubhog indicating later to be more radio-sensitive. Furthermore, both the genotypes were highly radio-sensitive for proton beam and least for gamma rays. Similarly, high relative biological effectiveness was observed for proton beam followed by X-ray, electron beam and gamma rays indicating their decreasing trend of penetration capacity and lethality. Results of present study will be useful for plant breeders to use the above mutagens in an appropriate dose for mutation breeding in rice.

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