α-Particle Dosimetry and the Inhibition of Mitosis in the Grasshopper Neuroblast by Low Dosage α-Irradiation

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Abstract
Polonium [alpha]-particle sources and a microscope-adapted source holder were utilized for irradiating special mica hanging-drop living culture preparations of grasshopper embryos. A single-crystal zinc sulfide scintillation-photomultiplier technique was developed and employed for measuring a-particle traversal of single biological cell dimensions with improved dosimetry. Studies were made of polonium sources comparing scintillation, photographic, and mathematical techniques in measurement of the aberrant effects of non-uniform polonium deposition. The ranger of polonium [alpha] -particles in air and mica were determined and the a-particle energy delivered to biological material was graphically computed. The scatter of a-particles in mica, the spatial distribution of particles, the random emission, and the Poisson dose distribution were studied. The effects of 56 rads of [alpha]-radiation on the mitotic rate of grasshopper neuroblast cells were determined, only 75% of normal mitotic activity being observed 6 hours after irradiation. Comparison with similar studies of [beta]- and X-ray inhibition indicated that a-particles were most effective and [beta]-particles least effective in producing inhibition of mitosis, with X-rays intermediate. A low-dosage [alpha] -radiation study of five substages comprising midprophase stage in the neuroblast showed that inhibition of mitosis was related to the Poisson distribution of dosage in gross effect, the dose having been administered in 1 second with an average of 0.8 [alpha]-particle per second per neuroblast. The effects presumably attributable to a single a-particle were observed, with radiosensitivity decreasing with mitotic progress throughout the midprophase.

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