R. B. Theus

Quasi-free scattering in the D(p, pn)p reaction from 15 to 50 MeV

Abstract The D(p, pn)p reaction was studied in a kinematically complete experiment. Comparison with the kinematically equivalent D(p, pp)n reaction shows the surprising result that the measured cross section for the (p, pn) reaction is a factor of seven larger than that of the (p, pp) reaction at the lower energies. The impulse approximation describes the shape of the observed energy spectra, but does not predict the measured magnitude.

Dosimetry intercomparisons between fast-neutron radiotherapy facilities.

Neutron dosimetry intercomparison visits have been made by physicists from the M. D. Anderson Hospital-Texas A&M University Project to the Naval Research Laboratory, the University of Washington, and the MRC Cyclotron at Hammersmith Hospital. The Naval Research Laboratory and University of Washington physicists have made dosimetry intercomparisons at the Texas A&M Variable-Energy Cyclotron (TAMVEC). The parameters that are usually measured during these visits are tissue kerma in air, tissue dose at depth of dose maximum, relative central-axis depth dose, neutron/gamma ratios in air and in phantom, and photon calibrations of ionization chambers. In addition, beam profiles and dose buildup curves are sometimes measured. Other parameters that are compared are values of W, stopping power ratios, kerma corrections, and calculations that lead to the statement of tumor doses for patients. This paper presents some of the results of the intercomparisons and discusses the implications of the findings.

Physical characteristics of the NRL fast neutron beam for radiation therapy

A consortium of therapeutic radiologists in the middle‐Atlantic states and physicists at the Naval Research Laboratory has been established to investigate the use of fast neutron beams in the control of some cancerous tumors. Many radiobiology experiments have indicated that neutron beams may have an advantage in the control of local tumors over that of conventional forms of radiotherapy. In preparation for clinical radiotherapy trials, extensive measurements have quantified the various physical characteristics of the NRL cyclotron‐produced neutron beam. Techniques have been developed for the absolute determination of delivered dose at depth in tissue for this beam, accounting for the relatively small component of dose delivered by gamma rays, as well as that by the neutrons. A collimator system has been designed to allow the precise field definition necessary for optimum therapy treatment planning. A dose control and monitor unit has been engineered and has demonstrated a reproducibility of 0.2%. New techniques common to nuclear physics experiments have been utilized to obtain needed neutron beam dosimetry information. The relative biological effectiveness of this neutron beam has been studied with several biological systems to aid in determining proper radiotherapeutic dose levels. The objective of these studies is a full‐scale clinical radiotherapy trial to test neutron effectiveness as compared to that of conventional radiotherapy.

Pulse-shape discrimination of high-energy neutrons and gamma rays in NaI(Tl)☆

Abstract Pulse-shape discrimination can be used to separate neutron and gamma-ray interactions depositing energies up to in excess of 50 MeV in NaI(Tl) crystals. The secondary alpha particles, deuterons and protons produced in the neutron interactions are also resolvable.

Neutron beam dosimetry at the NRL cyclotron

A 35 MeV deuteron beam impinging upon a thick Be target is being employed to generate a neutron beam for radiobiological experiments of relevance to later possible fast neutron therapy trials. The primary calibration of the beam has been based upon tissue-equivalent plastic ionization chambers, calibrated in turn with 60Co gamma -rays. CaF2:Mn and 7LiF (TLD-700) thermoluminescent dosemeters have been employed to determine the gamma -ray dose component in the neutron beam, by a method depending upon the ratio of fast neutron sensitivities of the two phosphors.

Comparison of the effects of various cyclotron-produced fast neutrons on the reproductive capacity of cultured human kidney (T-1) cells

Abstract A comparison of the relative biological effectiveness (RBE) of neutron radiotherapy beams was performed using the reproductive capacity of cultured human kidney T -1 cells as end-point. RBE and relative RBE values are presented for the neutron beams produced by deuterons of energy E d on beryllium targets at cyclotrons at the Naval Research Laboratory (NRL) ( E d = 35 MeV), the National Institute of Radiological Sciences (NIRS), Chiba, Japan ( E d = 30 MeV), the University of Washington (UW) ( E d = 22 MeV), and the Institute for Medical Sciences (IMS), Tokyo ( E d = 16 MeV). An objective method of determining RBE values and their errors is outlined. This method accounts for factors such as cell mean recovery time, dose rate, and the cell multiplicity at the time of irradiation. The summarized RBE values thus determined, when irradiations were performed under identical conditions at the four cyclotrons were 2.43, 2.28, 2.60 and 2.96, respectively.

Use of Li(d,n) Neutrons for Simulation of Radiation Effects in Fusion Reactors

In this paper we show that the neutron spectrum from high-energy deuteron bombardment of a thick Li target is suitable for simulation of radiation effects in a fusion reactor. Neutron spectra from 15, 20, 25, 30 and 35-MeV deuterons incident, respectively, on a 2-cm thick Li target are reported. For these spectra, a recently-developed computer code was used to evaluate damage-energy cross sections, primary recoil energy distributions, and spectrum-averaged reaction cross sections in several metals. The results indicate that a (d,n) source can simulate the energy dependence of the recoil spectra, and the rate of helium production anticipated in a real fusion reactor

Displacement correction factor for fast‐neutron dosimetry in a tissue‐equivalent phantom

The displacement correction factor to be used for analysis of fast-neutron dosimetric measurements using air-filled EG and G tissue-equivalent ion chambers in a tissue-equivalent phantom has been investigated using the MANTA neutron radiotherapy beam generated by 35-MeV deuterons on a thick Be target. The displacement correction factor inferred from these measurements is 0.970 for the EG and G IC-17 (1.0-cm3) ion chamber, and is 0.989 for the EG and G IC-18 (0.1-cm3 ion chamber. This multiplicative displacement correction factor has no significant dependence on depth in the phantom or on neutron beam size.

Stripping-theory analysis of thick-target neutron production for D+Be (and tissue dose calculation)

The Serber theory for deuteron stripping is employed to predict the shape of the neutron energy spectrum produced by 35 MeV deuterons (D+) on a thick beryllium target. In particular, the observation that the maximum of the neutron energy spectrum (at 0degrees relative to the deuteron beam direction) occurs at approximately 0-4Ed, where Ed is the incident deuteron energy, is explained reasonably well by the calculations. The explanation stems mainly from the fact that the stripping theory for thin targets predicts a narrow maximum at 0-5Ed, and thick target effects shift the maximum downward in energy to approximately 0-4Ed. A number of recent spectral measurements are in agreement with these predictions for a wide range of target materials and incident deuteron energies. The application of this theory also accounts for the previously observed Dd2-99 dependence of the absorbed dose in tissue,per unit charge of D+ ions on target, in the direction of the incident beam. This approximate Ed3 dependence is shown to be a characteristic property of deuteron stripping in a thick target and follows directly from the calculations that predict the neutron energy spectrum.

Fast neutron dose rate as a function of incident deuteron energy for D+9Be

The apparatus and techniques employed in measuring the thick target fast neutron dose rate as a function of incident deuteron energy for D+9Be over the energy range 15<or=ED<or=40 MeV are discussed. The resulting data are compared with a summary of previous measurements made at other laboratories. There are significant discrepancies between the two sets of data at the higher energies.