Showing papers on "Isotopes of boron published in 2004"

Release and Diffusion Rate of Helium in Neutron-Irradiated SiC

Abstract: Helium release measurements and diffusion coefficient calculations of helium in a neutron-irradiated α-SiC have been carried out in the temperature range of 750 1,260°C. The powder was obtained from SiC ceramics containing B4C, of which 10B concentration was 4.2×1016atom/mg. Neutron irradiation was conducted up to a fluence of 2.8×1024n/m2 (E>0.1 MeV) ~300°C. The diffusion coefficients were calculated for the temperature region of 750-1,060°C with time range of 0-10 min. The diffusion coefficient of helium for SiC containing B4C was D (cm2/s)=1.38×1010exp[—0.91±0.07 (eV/atom)/kT]. Dissociative and interstitial diffusion mechanisms were supposed. Diffusion coefficients were reduced above ~1,160°C probably due to accelerated migration of vacancies.

Energy loss of He, Li and B isotopes with MeV energies in Au

Abstract: The objective of this study was to experimentally investigate the stopping power and energy straggling of helium, lithium and boron isotopes with MeV energies in Au. A 9SDH-2 tandem accelerator was employed to perform ion bombardment experiments. The detection measurements were carried out using partially-coated silicon detectors. Furthermore, both the Monte-Carlo simulations and experimental results yielded by other findings were also conducted in this study in order to make comparisons. The uncertainty in this experiment was estimated to be less than 1%. The results indicated that our experimental results were in good agreements with both the theoretical and previous experimental results.

Boron isotopes in DSDP cherts: Fractionation and diagenesis

Abstract: Twenty samples of cherts from the Central Pacific (18 of them from DSDP, Leg17, Hole 167) were analyzed for B content and δ11B by ion-probe. In all samples δ18O, 87Sr/P86Sr and Ge have been previously measured. δ11B of DSDP cherts varies between — 9.3 and + 8%, indicating a “bulk” fractionation of 32–45% with respect to present day seawater B (39.5%), the highest fractionation of any marine phase. This fractionation is similar to the previously observed values in opal-A radiolarians (having δ11B of +2 to + 4.5%, Earth Planet. Sci. Lett.117, 1993, 567–580). Siliceous sediments are a significant sink of boron in the ocean, an even more significant sink of 10B. Assuming no secular variation in δ11Bsw our results can be the reflection of a three-stage precipitation-dissolution process of DSDP cherts: the first leading to opal-A and involving a large fractionation of B isotopes between tetrahedral boron in solution and opal-A, the second and third resulting in quartz via opal-CT. The dissolution-reprecipitation occurred at variable temperatures and water/rock ratios. The deepest cherts underwent diagenesis at higher temperature (low δ18O values) and in closed system (low δ11B values). Boron isotopic composition of marine cherts cannot be a clue to the paleochemistry (specifically paleo-pH) of seawater but indicates that some mineral phases might fractionate B isotopes differently than carbonates and thus be used in conjunction with carbonates to reconstruct paleo-pH and δ11B values of seawater.

First MeV ion loss measurements using activation technique in reversed B experiments at JET

Abstract: Fast ions measurements become increasingly important as we move closer to the International Thermonuclear Experimental Reactor (ITER) or burning plasma experiments and further R&D is especially required in the field of fast ion loss measurements [Bonheure et al., Rev. Sci. Instrum. 74, 1726 (2003).] in the perspective of ITER. A boron target mounted on a manipulator was exposed to deuterium plasmas during reversed field experiments at the Joint European Torus (JET). Due to the 10B(p,α) 7Be reaction, 7Be is produced in the boron target by MeV protons escaping the plasma. The proton losses are deduced from measurements of the intensity of decay emission of 7Be. Excellent agreement is found between measurements and prediction from fast ion classical drift losses. The MCORBIT Monte Carlo Simulation code was used to predict unconfined orbits fraction. Further developments are presented, including a new dedicated instrument to be used in experiments already planned at JET with ion cyclotron range of frequency a...

Thermal Ionization Mass Spectrometry Techniques for Boron Isotopic Analysis: A Review

Abstract: Publisher Summary Thermal ionization mass spectrometry (TIMS) produces the most reliable results and is the method of choice for precise boron isotopic determination. This chapter reviews the thermal ionization mass spectrometry techniques for Boron isotopic analysis. Boron is a quintessential crustal element that is widely distributed in surface rocks and aqueous fluids on Earth. It has two natural stable isotopes, 10B and 11B. The application of the boron isotopes as a geochemical tracer in earth sciences is considerably increasing. According to the charge condition of ionized species inside mass spectrometer, the TIMS methods are sub-divided into two categories namely the positive-TIMS and the negative-TIMS. The Cs-borate positive-TIMS and the BO2 negative-TIMS are the most commonly used techniques today. The Cs-borate technique achieves a better precision, but with low ionization efficiency and lengthy chemical preparation. The modified BO2 negative-TIMS applying the oxygen normalization attains a similar precision level as Cs-borate, but can be applied to low abundance natural samples with simple chemistry. The new BO2 negative-TIMS technique is crucial for re-evaluation of small change of 11B or 10B in foraminferal shells and to check possible oceanic pH variations.

Isotope effects on the glass transition phenomena and thermal properties of B2O3 glass

Abstract: Boron isotope effects on the glass transition phenomena and low-temperature heat capacity of B2O3 glass with various 10B to 11B ratios were examined. The apparent activation enthalpy (H*) of the glass transition shows a large isotope mixing effect. H* of the glass with 10B:11B = 0.5:0.5 is 451 kJ/mol, while that of 10B-glass is 404 kJ/mol and that of 11B-glass is 348 kJ/mol. This is attributed to the fact that the glass transition is a cooperative phenomenon; mixing the isotopes should diminish the cooperative behavior. The isotope effect below the glass transition region is small. The low temperature heat capacity, from 160 to 300 K, of the glass with 10B:11B = 0.5:0.5 is slightly (

Nuclear reaction studies with exotic boron beams

Abstract: Spectroscopic investigation, on exotic nuclei, performed in two different experiments with secondary radioactive boron beams are discussed. Spectroscopic factors for the 13 B+n bound-states are obtained by measuring momentum distribution of the 13 B residual nucleus from knockout reaction of the 14 B beam at intermediate energy. The momentum distributions are measured in coincidence with the emited∞-rays from the excited residual 13 B nucleus allowing spin-parity assignment and partial cross section determination. In the other experiment, a deuteron transfer, 2 H( 8 B,fi) 6 Be, reaction with low energy 8 B beam are used to search for resonances in the unbound 6 Be nucleus. The ANC - Asymptotic Normalization Coefficient method are presented and astrophysical implications of these two experiments are discussed.