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.

The gamma-ray dose from the radioactivity induced in soil and building materials by neutrons was estimated on the basis of the experimental data. l'he chemical compositions of soil and building materials collected from Hiroshima and Iiagasaki were determined by an activation analysis. It was found that 24Xa and 5"Mn contributed mainly to the gamma-ray dose from the radioactivity induced in soil. The dose received by a person who entered the hypocenter area in Hiroshima one day after the bombing and remained there 8 fir would have been 3 rads. The doses at distances of 500 m and I000 m from the hypocenter in Hiroshima were 18% and 0.07% respectively, of that at the hypocenter. The accumulated gamma-ray dose from immediately after the explosion of the A-bombs to infinity was about 80 rads at the hypocenter in Hiroshima, and about 30 rads in Nagasaki.

Reassessment of gamma doses from the atomic bombs in Hiroshima and Nagasaki has been carried out with thermoluminescent measurements of ceramic materials, such as bricks and decorative tiles, which were collected from buildings that remain as they were at the time of the explosions. The thermoluminescent measurements were performed using thermoluminescent dating techniques generally used in archaeology. Annual background dose rates from natural radionuclides in the ceramic materials and from environmental radiation including cosmic rays were determined with commercially available thermoluminescent detectors. A time-zero point at the original firing of the ceramic materials was estimated from the age of the buildings given in "the register book." Total background dose was evaluated by multiplying the period between the time-zero point and the time of measurement by the annual dose rate. The resultant gamma doses in Hiroshima and Nagasaki are given as a function of distance from ground zero and are compared with the DS86 (Dosimetry System 1986) and the T65D (Tentative 1965 Dose) gamma doses.

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 3 1 iMeV betatron and a 6 lMeV linear accelerator for the purpose of the design of the protective shielding to be placed around high enexgy 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.

No abstract