B.M. Oliver

Helium concentration in the Earth's lower atmosphere

Abstract Measurements in 1981 of the helium content of the Earths lower atmosphere have given a value of 5.222 ± 0.017 ppm by volume. This value, obtained by isotope dilution mass spectrometry, is 0.3% lower, but in essential agreement with the currently accepted value of 5.239 ± 0.004 ppm determined by Glueckauf in the 1930s. A consideration of processes that could have altered the helium concentration since the 1930s indicates that the concentration could have increased measurably due to release of helium by natural gas production. Possible net helium loss from the atmosphere is, however, not readily quantifiable.

Helium production in pure elements, isotopes, and alloy steels by 14. 8-MeV neutrons

The results of an extensive series of total helium production cross-section measurements for incident neutrons in the 14- to 15-MeV energy region are presented, and an experimental data base for the prediction of helium generation in candidate fusion reactor materials is provided. The measurements were made by isotope-dilution gas mass spectrometry. They include the pure elements Be, B, C, O, F, Al, Si, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Y, Zr, Nb, Mo, Ag, Sn, Ta, Pt, Au, and Pb; the separated isotopes of B, Fe, Ni, Cu, and Mo; and the alloy steels Type 316 stainless steel, HT-9, and 9 Cr-1 Mo. The results are in generally good agreement with other total helium production measurements in the literature, but comparisons with the ENDF/B-V nuclear data file indicate that the helium gas production files require revision for the structural elements vanadium, chromium, manganese, cobalt, and nickel. Comparisons with published cross sections for individual reaction channels indicate that reactions other than (n,..cap alpha..) contribute significantly to helium production in several materials.

Production of helium by medium energy (600 and 800 MeV) proton

Abstract There are no neutron sources that can produce simultaneously displacement cascade damage, gases and other transmutation product impurities, at levels significant to a fusion reactor such as the Next European Torus (NET). However, this same combination of damage is produced in materials bombarded by medium energy protons. Therefore, proton irradiation facilities have been added to the worlds most intense medium energy proton accelerators, SIN in Switzerland and LAMPF in the USA, to study the effects of these damage forms in combination. To confirm the accuracy of the computer codes used to model the nuclear reactions, especially regarding helium production, Fe, Cu, Mo, W, Ni, Al and Au were bombarded in the LAMPF proton beam. The buildup of 4 He and 3 He per proton was determined by vaporization of the samples and analysis of the gases released by isotope dilution mass spectrometry.

An assessment of the 59Ni isotopic tailoring technique to study the influence of heliumdpa ratio

Abstract The 59Ni isotopic tailoring experiment conducted in the Fast Flux Test Facility (FFTF) to study the effects of helium on the radiation-induced evolution of microstructure and mechanical properties of austenitic steels has been assessed. The experiment has been judged to be very successful in reaching its objectives, and guidelines are presented to aid in planning similar experiments. Conducting such experiments successfully requires etching the surface regions of specimens before measuring the helium. Attempts to calculate in advance the helium dpa ratio resulted in 20 to 40% underestimates of the measured levels. These underestimates appear to arise from uncertainties in the near-edge and out-of-core regions of both the neutron flux spectra and the neutron cross sections for 59Ni in the epithermal energy region.

Tritium half-life measured by helium-3 growth

Abstract Determinations of the tritium half life have been made by measuring the rate of 3He buildup in neutron-irradiated natural lithium. The measurement method, which is substantially different from those used previously, consisted of mass spectrometric measurements of the 4He/3He isotopic ratio in samples of lithium metal that had been irradiated in the core of a Triga reactor. Almost all of the 3He in the samples resulted from the decay of tritium produced by the 6Li(n, t)4He and the 7Li(n n′t)4He reactions, both of which yielded equal amounts of tritium and 4He. The helium isotopic ratio measurements, combined with the known decay times between irradiation and analysis, yielded an average tritium half life of 12.38 ± 0.03 mean solar years (1 solar year = 365.2422 days). This value is higher than several earlier half life measurements, but is consistent with a recent determination by the U.S. National Bureau of Standard.

Experimental helium generation cross sections for fast neutrons

Abstract Total helium generation cross sections of Al, V, Fe, Ni, Cu, Zr, Mo, Au, and the separated isotopes of Fe, Ni, Cu, and Mo have been measured for ∼14.8-MeV T(d,n) neutrons and for a ∼0–32 MeV Be(d,n) neutron field. The results, obtained using high-sensitivity gas mass spectrometry, are presented in this paper along with a summary of our previous T(d,n) helium generation cross section measurements.

Mass spectrometric determinations of the absolute tritium activities of NBS tritiated water standards

Abstract The tritium content of U.S. National Bureau of Standards tritiated water Standard Reference Materials SRM-4926, -4926C and -4927B have been measured by mass spectrometry, where the accumulation of 3 He from tritium decay is measured as a function of time. The averaged results, which are compared with other independent calibrations, are 0.7% higher than the NBS certified values and have an uncertainty (1σ) of 0.4%. Additionally, the half life of tritium has been determined from new calculations of the average energy of tritium decay, combined with existing measurements of the tritium decay energy emission rate. The new half life value, 12.38 ± 0.04 y, is essentially the same as the 12.38 ± 0.03 y value recently obtained mass spectrometrically with the same analysis system used in the present measurements.

Helium generation rates in isotopically tailored Fe-Cr-Ni alloys irradiated in FFTF/MOTA

Abstract Three FeCrNi alloys have been doped with 0.4% 59 Ni for side-by-side irradiations of doped and undoped materials in order to determine the effects of fusion-relevant levels of helium production on microstructural development and mechanical properties. The alloys were irradiated in three successive cycles of the Materials Open Test Assembly (MOTA) located in the Fast Flux Test Facility (FFTF). Following irradiation, helium levels were measured by isotope dilution mass spectrometry. The highest level of helium achieved in doped alloys was 172 appm at 9.1 dpa for a helium(appm)-to-dpa ratio of 18.9. The overall pattern of predicted helium generation rates in doped and undoped alloys is in good agreement with the helium measurements.

Helium Production by 9.85-MeV Neutrons in Elemental Iron, Nickel, and Copper and in 56Fe and 58,60,61Ni

Neutron-induced helium production, which can contribute substantially to radiation damage, is an important parameter in the choice of structural materials and other component materials for both fusion and fission reactors. Here, helium production cross sections for the elements iron, nickel, and copper and for the isotopes {sup 56}Fe, {sup 58}Ni, {sup 60}Ni, and {sup 61}Ni for 9.85-MeV neutrons have been measured by irradiation with an intense, quasi-monoenergetic neutron source followed by helium analysis with isotope dilution gas mass spectrometry. The results are in fair agreement with (n, {alpha}) cross sections measured by alpha-particle detection and integration over the alpha-particle energies and angular distributions.

Measurement of the 9Be(n,2n)8Be reaction cross section in the 9Be(d,n) thick-target neutron spectrum

Abstract 4 He production has been measured by an isotopic dilution mass spectrometry method, for the irradiation of Be-metal samples by fast neutrons from the 9 Be(d,n) source reaction, corresponding to 7-MeV deuterons incident on a thick Be-metal target. The 58 Ni(n,p) 58g+m Co dosimetry reaction was employed as a reference standard for measurement of the neutron flux. The integral cross section for the 9 Be(n,2n) 8 Be(2 4 He) reaction ( 8 Be breaks up very quickly into two α-particles) was deduced by applying a correction of 9.4% to account for the 9 Be(n,α) 6 He reaction contribution to the total 4 He yield from neutrons on Be. The experimental integral reaction cross-section ratio of 9 Be(n,2n)2 4 He to 58 Ni(n,p) 58g+m Co in this spectrum was found to be 1.217, with an error of 3.5%. This is in good agreement with the corresponding calculated ratio of 1.161 which is based on an average of results obtained by considering four distinct representations of the 9 Be(d,n) neutron spectrum and ENDF/B-VI values for the reaction cross sections. The scatter of the calculated results amounts to 0.5%. It is small because the shapes of the response functions, R ( E ) = o ( E ) σ ( E ), for these two reactions in this neutron spectrum are very similar. The observed C/E is 0.954. This is very good agreement, considering the various uncertainties involved, including those for the differential cross sections from ENDF/B-VI. This integral measurement is sensitive to the differential reaction cross section in the range 3–6 MeV. The uncertainty attributable to the 58 Ni(n,p) 58g+m Co reaction differential cross section in this range is estimated to be 3.5%. Therefore, the results of the present investigation indicate that ENDF/B-VI represents the 9 Be(n,2n)2 4 He reaction differential cross section quite well over this energy range.