H Bichsel

Neutron dosimetry with spherical ionisation chambers. I. Theory of the dose conversion factors r and Wn

A model for the calculation of the deposition of energy and the production of ionisation by neutrons in finite spherical cavity ionisation chambers is described. Contributions from particles entering the cavity from the wall (crossers and stoppers) and particles produced in the gas (insiders and starters) are included. Composition and stopping power of wall material and gas are arbitrary. The limiting cases, a Bragg-Gray (infinitesimal) and an infinite cavity, are also discussed. The calculation of the average energy needed to produce one ion pair, Wn, the dose conversion factor, r, and the average charge, q, produced by one neutron incident per unit area are discussed from monoenergetic neutrons as well as for neutron spectra. Earlier approximations of r and Wn are described.

Neutron dosimetry with spherical ionisation chambers. III. Calculated results for tissue-equivalent chambers

Values are given of the average energy required to produce an ion pair, Wn, and of the dose conversion factor, r, for tissue-equivalent chambers filled with either tissue-equivalent gas or air, irradiated by neutrons. The model for finite spherical cavities previously presented in part I was used for the calculations, which employed the W values and stopping powers of the charged particles produced in the materials described in part II. Neutron energies ranging from 0.4 to 14 MeV were considered: many of these energies were chosen because of their particularly large or small total cross-sections in order to explore the range of fluctuations of r and Wn. The results are therefore not very suitable for spectral averaging. Cavity sizes ranging from the infinitesimal Bragg-Gray to the infinite cavity were studied. It was found that the changes of r with cavity size and with neutron energy are smaller for the TE-TE chamber than for the TE-air chamber, but for Wn they are about equal; the TE-TE chamber should therefore be considered the ionisation chamber of choice but absolute doses cannot be determined with it to better than +/- 8%.

Lateral displacement in small angle multiple scattering

Discusses the average lateral displacement in small angle multiple scattering of protons with energies of several hundred MeV. Errors in work published by A.A. Mustafa and D.F. Jackson (Phys. Med. Biol., vol.26, p.461, 1981) are described. The results of calculations giving displacement values for 350 MeV protons in water are presented.

Neutron dosimetry with spherical ionisation chambers. IV. Neutron sensitivities for C/CO2 and tissue-equivalent chambers

For pt.III see ibid., vol.27, p.1231, 1982. Values of the dose conversion factor, r, and the average energy needed to produce an ion pair, Wn, for neutrons with energies from 0.4 to 14 MeV, in graphite chambers filled with carbon dioxide, calculated with the theory presented in pt.I (ibid., vol.27, p.893, 1982), are given. Neutron energies were chosen in part for their especially large or small nuclear cross-sections to examine the range of fluctuations possible. It was found that for cavity sizes of 1 cm3 and above r can be approximated by the ratio of kerma factors KC/K(CO2) to better than 10% and Wn is within 3% of the value infinity Wn for an infinite cavity. With the present information and that given previously (pt.III) on the TE/TE and the TE/air chambers the relative neutron sensitivities, ks, were calculated. The cavity size dependence and, more importantly, the strong fluctuations of ku with neutron energy appear to make the C/CO2 chamber unsuitable for the purpose of photon discrimination. Measurements with proportional counters should be considered superior to the dual chamber method.

Dosimetric properties of neutrons from 21-MeV deuteron bombardment of a deuterium gas target.

Spectra, yields, average energies, and kerma rates in tissue of neutrons from 21-MeV deuteron bombardment of deuterium gas targets have been calculated for target thicknesses of 1, 3.5, and 5 MeV. A high pressure gas cell was constructed and was filled with 33 atm of D2 gas (equivalent to an energy loss of 3.5 MeV for 21-MeV deuterons); dose rate, dose buildup, and depth-dose properties of neutrons produced by the D(d,n) reaction were measured. Dosimetric properties of these neutrons are superior to those of neutrons from a thick Be target bombarded by a deuteron beam of the same energy.

A physics cyclotron adapted for fast neutron beam therapy

The adaptation of the University of Washington physics cyclotron to fast neutron beam radiation therapy was described. The approaches to the problem were described, including acquisition of basic machine data. The target, main barrier assemblies, collimators, dose control and safety system design and installation were outlined. The performance, including dose distributions, photon contamination, collimation achieved, etc. were described.

Fast neutron beam dosimetry: comparison of the ion chamber and proportional counter approaches

Data were presented comparing the absorbed dose in tissue exposed to beams of fast neutrons as measured by ion chambers and tissue-equivalent proportional counters. The fast neutron beams studied include 14 MeV monoenergetic beams and those generated by high energy deuterons incident on beryllium. The ion chamber approaches include the tissue-equivalent plastic, tissue-equivalent gas and polyethylene wall-ethylene gas systems. The proportional counters were of the tissue-equivalent wall-tissue-equivalent gas type. Sources of uncertainty were discussed, including values of stopping power ratios, W for the gases, differences in atomic composition between the wall and the gases, and separation of the photon component of the total dose.

A possible mechanism for reduced radiation damage by relativistic charged particles, e.g. in electron microscopy

Contrary to the common belief that radiation damage varies slowly as the energy of the incident particle becomes relativistic, the authors argue that a strong energy dependence may occur, provided that the target material has very strong directional properties. In the case of a material with extremely strong directional properties, the damage may decrease as much as the inverse of the square of the energy. This effect arises from a general feature of the primary collision of the projectile with an atom, i.e. that the cross-section differential with respect to angle has a very strong energy dependence even at relativistic energies. Thus, the Rutherford differential cross-section, which gives a reasonable description of both elastic and inelastic processes over some important kinematic regions, decreases as the inverse of the square of the energy. The correlation between the energy dependence and directional properties of the target has been studied in the context of a simple model of radiation damage.