data that show that calculations and measurements of neutron transmission through thick iron (as in the case of the Hiroshima bomb) should be softer than is currently calculated. To follow that lead, the final experiment funded by the DNA program on Hiroshima neutrons was the measurement of neutron transmission through a cross section of the bomb case and tamper. Time-of-flight measurements were made with the LAMPF 800-MeV proton linear acceleration at LANL as a “white” neutron source to measure the transmission of neutrons 0.6–600 MeV.


Following the recommendations of this committee (see Chapter 8), a number of projects have been funded and are in progress in an attempt to refine the calculations of DS86. These include a total recalculation of the output of the Hiroshima bomb, the total reevaluation of shielding models for Hiroshima and the factory workers in Nagasaki, and an evaluation of Sn and Monte Carlo calculations as a method for an adjoint determination of a source for the Hiroshima bomb that will agree with the measurements.

The recalculation of the Hiroshima bomb will be the most comprehensive ever accomplished. It will include a late-time output spectrum that incorporates new iron cross sections and transmission through the bomb case, a new Monte Carlo source term as a function of energy and angle, and a Monte Carlo transport of the new source to the ground accounting for the tilt and heading of the bomb, and the transport of delayed neutrons over time through air created in a new spherical air blast calculation. The new calculations, which will use the latest ENDF/B-6 cross sections, will be compared with source and DORT calculations. If it is necessary for a satisfactory level of agreement, an adjoint-to-source and forward to free-in-air kerma Monte Carlo calculation will be performed.

The next kind of work recommended by this committee is an examination of the adequacy of shielding models in DS86. Two efforts to follow this recommendation are under way. The first is an examination of the nine-parameter, globe, and terrain shielding in Hiroshima. This effort will identify required changes in how shielding is handled in DS86. The second effort is the improved modeling of the shielding environment for the factory workers in Nagasaki. The biological dosimetry for these workers indicates that the shielding could be in error. With better accounting for the shielding provided by these structures and the heavy machinery they contained, the RERF dosimetry system can determine the dose for these workers better.

The work to improve the radiation-transport calculations should be completed as quickly as possible. Any improvements derived from the work and other improvements in radiation cross sections and transport methodology achieved since DS86 should be fully implemented in the RERF dosimetry system when they have been reviewed by US and Japanese senior review panels, fully documented, and approved.

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