CALCULATION OF RADIATION DOSE DUE TO HIGH-ENERGY PROTONS

Abstract The work reported here represents some of the results obtained in a continuing program of the study of the dosimetry of high-energy protons and neutrons. Studies in high-energy proton dosimetry are of interest in assessing the potential radiation hazards for manned space flight and are also of interest for radiological protection in the vicinity of high-energy accelerators and for biological irradiation experiments. The present calculations consider the dose deposited in tissue by protons with energies from 400 MeV to 2 GeV. Some of the results dealing with the dose due to protons with energies up to 400 MeV have been reported previously. (1,2) For proton energies below 400 MeV, the production of pions in the cascade process was not sufficiently common to be considered significant. However, in the present calculations it is necessary to take pions into account. The basic quantity of interest in dosimetry is the absorbed dose, defined in units of the rad where 1 rad is equal to 100 ergs/gram. In order to calculate the dose equivalent, one specifies a quality factor (QF) which is often related to the values of linear energy transfer (LET) at which the energy deposition takes place. This QF-LET relationship is then used to determine the dose equivalent. The tissue phantom is the same as used previously and has the form of a slab 30 cm thick and infinite in lateral extent. The protons are assumed to be normally incident on one surface of the tissue slab. The slab is divided into 30 sub-slabs, each of which is 1 cm thick and the dose and dose equivalent in each of the sub-slabs is calculated. The Monte Carlo technique has been used in writing a code for the CDC-1604 computer to obtain the present estimates of absorbed dose and dose equivalent. The calculations have been simplified considerably by using a simpler model for the nuclear interactions than was used in the calculations up to 400 MeV. The straight-ahead approximation is used in this report (i.e. the nucleons emitted in the cascade process are assumed to have the same direction as the incident nucleon which initiates the cascade). The validity of the straight-ahead approximation has been considered at 400 MeV by Alsmiller et al. (3) and found to give good agreement with more refined methods. It is expected that the straight-ahead approximation is even better at higher energies. Much of the statistical information concerning products of the cascade such as the average number and energy distribution of emitted protons, neutrons, and pions, excitation of the residual nucleus, etc. is obtained from Metropolis et al. (4) The details of the calculations will be described further in Part II of this investigation. As new statistical information becomes available, it can easily be incorporated into the program. The estimates of dose equivalent are made on the basis of the QF-LET relationship endorsed by the International Commission on Radiological Protection (ICRP) for long-term occupational exposure. The information regarding nuclear interactions that is currently available is very incomplete and it has been necessary to make several assumptions and compromises due to the lack of sufficient experimental data. Therefore, the results presented here are preliminary and are expected to be refined when new data are available. The program runs rapidly and can process 1000 incident particles in approximately 3 min on the CDC-1604 computer.