Ralph H. Thomas

Ames collaborative study of cosmic ray neutrons - Mid-latitude flights

Three aircraft flights at 12.5 km have yielded data on cosmic-ray neutron-flux densities, neutron dose equivalent rates, and exposure rates for ionizing radiation. The data, collected at 45, 38, and 48 degrees N, indicate: (1) neutron dose equivalent rates of 0.20, 0.14, and 0.22 mrem/hour, (2) exposure rates from the ionizing component of 0.37, 0.31, and 0.41 mR/hour, (3) that the dose equivalent from 3-13-MeV neutrons is 30% of the total for all neutrons, (4) that exposure rates decrease exponentially with decreasing pressure altitude with a 140 g/sq cm attenuation length at 43 degrees N geomagnetic latitude, and (5) that the ionizing component of cosmic ray secondaries is less sensitive to latitude changes than the neutron component.

A MEASUREMENT OF THE AVERAGE ENERGY REQUIRED TO CREATE AN ION PAIR IN NITROGEN BY HIGH-ENERGY IONS

At energies above approx. 10 MeV/amu, it is expected that the average energy required to create an ion pair, anti w, in gases will be independent of mass or charge state of the ion and with increasing energy will tend toward the value for electrons, which in nitrogen is 34.6 eV. Twenty-two measurements of anti w using 250 MeV/amu /sup 12/C/sup 6 +/ ions, 375 MeV/amu /sup 20/Ne/sup 10 +/ ions, and 479 MeV/amu /sup 40/Ar/sup 18 +/ ions were made and values of 36.4 +- 0.6, 35.4 +- 0.8, and 34.7 +- 0.5 eV, respectively, were obtained for nitrogen gas.

Neutron Production by Ne and Si Ions on a Thick Cu Target at 670 MeV · a with Application to Radiation Protection

Beams of neon and silicon at 670 MeV A from the Bevalac were stopped in a copper target. Neutron yields and angular distributions were measured with activation detectors. Attenuation of neutrons through a concrete shield was measured at 7°, 54° and 72° to the beam direction. Dose equivalent estimates were made in adjoining areas by using moderated BF3 proportional counters and carbon and aluminum activation detectors. Data are presented and radiation protection aspects discussed.

MOYER MODEL APPROXIMATIONS FOR POINT AND EXTENDED BEAM LOSSES

Abstract The use of the empirical Moyer model for the determination of transverse neutron shielding for high-energy proton accelerators is described and discussed. It is shown that an important advantage of the Moyer Model is the physical insight it offers towards understanding the complex interactions that comprise the shielding processes. Calculations for pointlike and extended uniform beam loss distributions are discussed and their relationship to practical shielding conditions developed. The calculations required by the model are readily performed on small programmable calculators and thus are widely accessible. Program listings for practical calculations using a Hewlett-Packard HP-97 calculator are available on request.

An Intercomparison of Dosimetry Techniques in Radiation Fields at Two High-energy Accelerators

A COmParison of radiation intensity measurements outside the shielding of the Bevatron and the 20 GeV Electron Linear Accelerator at Stanford is described. Measurements were made using several different techniques by workers from Brookhaven National Laboratory, Lawrence Berkeley Laboratory, and the Stanford Linear Accelerator Center. The measurements indicate large discrepancies (factors of two or more) between different techniques of measurement and illustrate the need for improvement in high-energy accelerator dosimetric techniques.

An Analytical Approach to Environmental Radiation Measurements

Environmental radiation measurements have been made and the results reported for the past 15 years. These measurements have included both the neutron and gamma contribution to the environment due to Laboratory operations. In order to determine these values an understanding of the natural environmental radiation must be known. The techniques for measuring environmental gamma radiation have been improved upon during this time and the latest instrumentation and interpretation methods commonly used at this laboratory are reported. Not only is the natural radiation dose rate important but the isotopes responsible need to be identified for a complete understanding of the observed radiation. The techniques for this determination are described.

The design of an experiment to study leukemogenesis in mice irradiated by energetic heavy ions.

The design of an experiment to study the incidence of cancer and hematological effects in mice irradiated by heavy ions is described. A beam of fully stripped

“povera E Nuda Vai, Dosimetria”

{\rm C}^{6+}

THE VALUE OF W(E) IN NITROGEN FOR HIGH ENERGY IONS

ions of energy 250 MeV/amu was produced by the Bevatron. Mice were irradiated in groups of 12 by rotating them in a wide beam (11.4 cm FWHM) once per minute. At a beam intensity of 108 ions per pulse irradiations of 250 animals to absorbed doses of 200 rad were completed in a few hours, an efficient use of accelerator beam time. The average LET of the radiation was 16.6 keV/μm with a spread in the animal of ±2 keV/μm. Radial and longitudinal variations in absorbed dose were less than 30% and 15%, respectively. Estimates of tissue-entrance absorbed doses with an ionization chamber and with thermoluminescent dosimeters differed by less than 3%.

Neutron flux‐density and secondary‐particle energy spectra at the 184‐inch synchrocyclotron medical facility

Abstract The concepts used in radiation protection are critically reviewed. It is concloaed that primary attention should be given to the specification of radiation fields in terms of particle flux density and energy spectra, from which all other parameters needed in health physics may be derived.