Yoshikazu Kumamoto

Spot scanning system for proton radiotherapy

In order to provide a uniform and desirable dose distribution over a large radiation field, spot beam scanning is one of the most useful methods. A new spot beam scanning system was constructed for a 70 MeV proton beam. The lateral dose distribution was uniform with +/- 2.5% for an 18 cm square field. It was possible to control the dose at each point in the radiation field by this spot scanning method. This system has been confirmed to be satisfactory for delivering a proton beam in the desired field shape and dose level.

Status Report on the Nirs-Chiba Isochronous Cyclotron Facility

A variable energy isochronous cyclotron has been installed for biomedical studies. Since the fall of 1975, the cyclotron has been used for the clinical trial of fast neutron therapy and the production of short-lived radionuclides. To improve the beam stability, a beam-phase stabilizer has been developed. Recently, a heavy ion source of self heated cathode PIG type has been developed and several kinds of ions have been successfully accelerated. The facility and some experiences in the application are also described.

Effects of fast neutrons and 60Co gamma-rays on Artemia.

The effects of a single exposure to 2-MeV fast neutrons were compared with the effects of exposure to60 Co γ-ray irradiation on dry eggs of Artemia. Comparison of the survival data showed that neutrons were 2.1, 2.8, and 4.8 times as effective as γ-irradiation in producing mortality of nauplii at 1, 2, and 3 days after hatching. Irradiation with fast neutrons produces severe chromosomal abnormalities and leads to a depression in mitotic activity. On the basis of the cytological observations, the relative biological effectiveness of fast neutrons was about 2 with reference to γ-rays.

Attenuation of 4-32 MeV X-rays in Ordinary Concrete, Heavy Concrete, Iron and Lead

Abstract The broad-beam attenuation coefficients for lead, iron, heavy concrete and ordinary concrete were evaluated using the X-ray beams from a 32 MeV betatron, a 31 MeV betatron and a 6 MeV linear accelerator for the purpose of the design of the protective shielding to be placed around high energy accelerators. Attenuations in these materials were determined by measuring the ionization in a cavity in a water phantom behind various thicknesses of materials. The variation in tenth-value thickness with field size for these materials is discussed on the basis of data obtained from the attenuation measurements. The tenth-value thickness for these materials is shown as a function of X-ray energy. The tenth-value thicknesses determined using the three X-ray generators are compared.

Spectra of primary x-ray and secondary photons from shielding materials for 4-30 MV x-rays

Abstract The spectra of primary X-rays and secondary photons from shielding materials (lead, iron, heavy concrete, ordinary concrete and water) have been studied experimentally over the megavoltage region 4–30 MV, using NaI(Tl) scintillation spectrometers, from the point of view of radiation protection against a medical betatron. Spectral measurements were made of both photons transmitted through and scattered from the shielding materials. The measured pulse-height distributions were converted to photon spectra, using inverse response matrices of the crystals. Resultant spectra are tabulated in the unit of the relative photon number per unit energy interval. The average energy of primary and secondary photons has been calculated from the spectrum. Spectra experimentally determined are compared with theoretically calculated spectra.

Genetically significant dose from beam therapy in Japan, 1971.

The genetically significant dose (GSD) from beam therapy in 1971 was estimated, based on a nationwide survey carried out for a 30 consecutive day period in June and July 1971. From 1480 hospitals which had therapy units, 348 hospitals were selected at random for the survey. Sixty-eight per cent of the questionnaires were answered. The contribution to the GSD from beam therapy in Japan was estimated by using the results of experiments on scattered and leakage radiation, designed to allow a reasonable projection of the dose; percentage depth dose tables; and determination of the expected number of children from vital statistics. The GSD estimated was 0.25, 0.70 and 0.03 mrad/person/yr for therapies with O0Co units, conventional X-ray units and high-energy accelerators, respectively.

Measurements of effective doses of natural background level of neutrons with etched-track detectors.

Abstract— An etched-track method was applied for a measurement of the effective dose of natural background neutrons. Ten assemblies of polycarbonate film and 10 B were placed at the center of a rem-counter type moderator. The assemblies were put in an air-tight container in order to avoid a continuous contribution from radon. Electrochemical etching was adopted. The sensitivity of the detector was 0.21 ± 0.02 &mgr;Sv pit −1 cm 2. After a period of storage of 4.3 y, the number of recorded artifact pits in the films corresponded to an effective dose rate of 0.8 ± 0.2 &mgr;Sv y −1. The effective dose rate of natural background neutrons was 18.1 ± 3.6 &mgr;Sv year −1 for the same period, with an artifact dose being subtracted. The total net number of etched-track pits of the present detector was 75 ± 15 when used for measuring an effective dose of natural background level neutrons for 1 mo, which gives enough counting statistics. The method can thus be applied for monitoring the effective dose of neutrons around nuclear sites and high-energy accelerator facilities.

The Effect of Oxygen on the Radiation Sensitivity of Artemia Eggs: A Preliminary Result of OER on Dry Eggs

The effects of oxygen on the radiation response of Artemia eggs in a dried state was examined by using a high-vacuum system. As a procedure to measure the oxygen effect, eggs were dried at 5× 10-5 Torr for 7 hr, exposed to gases, and left overnight. Under these conditions, an oxygen enhancement ratio of 17 was found for the reduction of hatchability to 50%.

ESTIMATION OF GENETICALLY SIGNIFICANT DOSE EQUIVALENT FROM OCCUPATIONAL EXPOSURE TO EXTERNAL SOURCES OF RADIATION IN JAPAN.

The genetically significant dose equivalent from occupational exposure in Japan has been estimated. The annual individual dose from external X-ray, gamma-ray and neutron sources in various types of radiation work has been evaluated from the personnel monitoring records. This report also presents information on the numbers of installations and radiation workers in Japan. The mean annual dose equivalent in medical, industrial, research and educational, and atomic work was 377,291, 88.6 and 125 mrem per yr, respectively. The genetically significant dose equivalent in medical, industrial, research and educational, and atomic energy work was 0.082, 0.032, 0.0053 and 0.0056 mrem per yr, respectively.