W. S. Snyder

CALCULATION OF RADIATION DOSE FROM PROTONS AND NEUTRONS TO 400 Mev

1. THIS report is a part of a continuing reappraisal of the meaning and calculation of the dose due to high energy neutrons and protons in tissue. Estimates of such dose are given in the report of Committee IV (1953-1959) of the International Commission on Radiological Protection.(l) The estimates of Committee IV, based on the work of NEARY and MULVEY,@) utilized the best experimental and theoretical data which were available at that time. Within the last few years, however, a considerable amount of new information has been obtained which permits a more detailed study of interactions of high-energy neutrons or protons with tissue. Also, new techniques have been developed to describe typical histories of neutrons or protons within a tissue phantom. In this report, revised estimates of dose and dose equivalent in tissue are given for neutrons and protons with energies up to 400 MeV.

Calculation of radiation dose due to protons and neutrons with energies from 0.4 to 2.4 GeV.

Estimates of the distribution of absorbed dose and dose equivalent with depth in a tissue slab have been made for irradiation by normally incident and also isotropically incident protons and neutrons with energies up to 2.0 GeV. The Monte Carlo calculations, based on a simplified model for nuclear interactions, take into account the production of pions as well as nucleons in nuclear interactions. The energy deposited due to ionization by charged particles is separated into LET ranges permitting different quality factors to be used. The total dose is broken down to show the contribution due to ionization by primary particles (in the case of incident protons), ionization by secondary particles, ionization by pions, excitation of residual nuclei following cascades, and the contribution from pions that stop within the tissue. Results are presented in the form of graphs showing the distribution of absorbed dose and dose equivalent with depth within the phantom for incident energies 0.6, 1.0, and 2.0 GeV. The results appear to be in reasonable agreement with experimental results. There is a rapid buildup of dose near the surface and then a more gradual buildup in the remainder of the 30 cm thick tissue slab. Pions are found to contribute less than 10% of the total dose. For normally incident neutrons, the quality factor decreases from near 10 at the surface from which the neutrons are incident to approximately 2.5 near the back of the slab.

Radiation dose from high-energy nucleons in targets containing soft tissue and bone.

Previous calculations for nucleons with energies up through 400 MeV incident on homogeneous soft tissue targets have been extended to phantoms in which bone is also present. Estimates of absorbed dose and dose equivalent are given for targets of dimensions approximating those of the human torso and a small primate. With incident protons, the presence of bone in the soft tissue is apparently of little consequence-the absorbed dose and dose equivalent are nearly the same with and without bone. With incident 400-MeV neutrons, however, the ratio of the dose equivalent and absorbed dose in the bone portions is found to be about 20% smaller than when the bone region is occupied by soft tissue. The reduction in the quality factor in bone for neutrons is due to the relatively smaller contribution of high-LET particles to the absorbed dose there. High-LET particles arise in the calculation as (1) recoil nuclei following an intranuclear cascade and (2) low-energy evaporated heavy nuclear fragments (for example, alp...

EFFECTS OF PHANTOM GEOMETRY ON DOSE DISTRIBUTION

Calculations have been made of the spatial distribution of dose and dose equivalent due to protons of energies 250 MeV and 400 MeV incident on a tissue phantom which has dimensions approximating those of a human torso and has the form of a right elliptical cylinder with a height of 70 cm and with semiaxes of the elliptical base as 20 cm and 10 cm. The results obtained have been compared with the corresponding data previously obtained for a tissue slab having thickness of 30 cm. It is shown that within the limits of accuracy due to statistical fluctuations the distribution of dose and dose equivalent are not substantially different in a cylindrical phantom from the corresponding values in a slab phantom.

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.