As an aid to accurate neutron dosimetry the mean W values, W , have been calculated for a tissue-equivalent gas mixture, ethylene, acetylene and carbon dioxide at twenty-one different neutron energies in the range 0.1-15.6 MeV.The calculations are based on a previously derived relationship for the mean value of the energy, W ( E ) , expended in forming an ion pair by a heavy ion of mass M , atomic number 2 and initial energy E. For energies below 1.0 MeV per a.m.u. this relation takes the formand at higher energies W ( E ) = W6 E / ( Econstant) where W , is the mean value required to form an ion pair by an electron. in the elements hydrogen, carbon, nitrogen and oxygen are given. each neutron energy from the differential secondary particle spectra. Details of the cross-sections and kinetics of all the possible neutron induced reactions Kerma and the energy expended as elastic nuclear scattering are also evaluated at
As a preliminary step to evaluating recent theories concerning the biological effect of ionizing radiation, the charged particle fluence distributions and the dose distribution in linear energy transfer have been computed analytically for targets of biological and dosimetric interest irradiated by neutrons. Specifically, 14.7 MeV neutrons, the 252Cf neutron spectrum and a cyclotron generated neutron spectrum are considered to irradiate water, tissue-equivalent plastic and standard man tissue and results are given for all these cases. From a knowledge of the target composition, and the cross-sections and kinetics of all the possible neutron induced reactions in the elements hydrogen, carbon, nitrogen and oxygen, the secondary particle spectrum is calculated. By combining with stopping power data for the ions in the target, the charged particle fluence spectra and dose distribution in linear energy transfer are derived. Secondary quantities computed are kerma, energy expended as nuclear elastic scattering, specific ionization and mean quality factor. Stopping powers have been derived from published atomic stopping powers by summing according to the Bragg rule. A comparison between tissue-equivalent plastic and standard man tissue had been made for each of the neutron spectra.
A collimated beam of neutrons from a reaotor was used to irradiate the thyroid, inducing lZ8I which can be detected by external counting. Neutron dosimetry studies have been carried out with the unmodified beam and also using a boron liltem to absorb incident thermal neutrons.Five methods were used to provide dose estimates and the results were in reasonable agreement, giving for a clinical examinstion at 20 kw reactor power a mean dose-equivalent and standard error of 20.4k 1.1 rem for the filtered beam, and 24.1 & 1.2 rem for the unmodified beam due to neutrons a t the thyroid. The concurrent y-ray dose is 2.1 kO.2 rem. The dose to the eyes and gonads is less than 1 yo of the thyroid dose.
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