It had been developed by G. Failla in 1924 that radon seed was a perm anent implant source for brachytherapy sealed 222Rn gas in thin gold tube. In Japan, the first radon plant was installed at Cancer Institute Hospital, Tokyo, by H. Yamagiwa and M. Miwa in 1935. This plant broken off the war was resumed in 1947, and radon seeds of 231,279 were supplied from Cancer Institute Hospital between 1947 and 1975. The gamma ray output of radon seed in the supply was equivalent to 1 mg radium with Platinum wall thickness of 0.5 mm. But, the dose rate within 5 mm from source measured with TLD was 45% greater than that calculated only gamma ray because of leakage of beta rays. Technicians in the radon seed production received very much radiation exposure exceed dose limit. However, they do not have on radiation injury at the present time.
In the first few years following the medical use of x rays together with radium, there was almost a complete lack of radiation protection, due to an ignorance of the hazards involved. The history of their development are reviewed. In this paper, we describe the sealed-source lead safe for storage, drawer assembly, lead bed shielding for the protection of radium sources, telecurie unit and occupational exposure of physicians and nurses in radium-loading procedures, especially of radium nurses exposed to fingers, back, stomach, breasts and eyes of radium nurses of the Cancer Institute Hospital, Tokyo.
With increasing use of linear accelerators for photon beam radiosurgery, many kinds of very small ionization chambers have been commercially developed. Ion recombination loss and polarity effect are the most important factors to express chamber characteristics. To investigate the characteristics, one parallel plate chamber, six cylindrical and two hemispherical ionization chambers, which had nominal volumes between 0.1m1 and 3μl, were used. Measurements were made using 10MV X-rays and 70MeV proton beams for saturation characteristics and the polarity effect. The polarity effect as a function of applied voltage was evaluated as the charge ratio for vertical irradiation of the 10MV X-ray beam. All the chambers showed that the effect was less than 5%, except for one chamber which was larger than 50%. The experimental points of ion collection efficiency measured for all ionization chambers were plotted as a function of d2/V, where is the gap length and V, the applied voltage. If the ionization density generated in each chamber gas is identical, the experimental values are ideally expressed as one line. The actual values differed for the chambers because their shapes deviated from ideal cylindrical or spherical shapes. It was concluded that the ion recombination correction factor for very small ionization chambers must be determined experimentally for the actual beam to be used.
The cavity-gas calibration factor Ngas for a Markus parallel-plate ionization chamber was determined by comparing with a calibrated Farmer chamber using three different calibration methods as recommended in the AAPM's TG-39 protocol. These methods are: (a) high-energy electron beam calibration, (b) photon beam in-phantom calibration, and (c) 60Co in-air calibration. Ngas with the photon beam in-phantom calibration was determined by 60Co,4, and 10 MV photons using solid water and MixDp phantoms. The Ngas values determined by the photon beam in -phantom and 60Co in-air calibrations were about 1% higher than those determined by the high-energy electron beam, while the difference in Ngas values between the in-air and in pha ntom calibrations with a 60Co beam was not observed. The replacement correction factor Prepl,pp for the Markus chamber was also measured as a function of nominal electron energies from 4 to 15MeV by comparing with the Farmer chamber whose Prepl,cyl was obtained from data in the JARP protocol. Resultant Prepl,pp value decreased in the energy region below 10 MeV.