Neutron interactions with germanium isotopes and amorphous and crystalline GeO2

Coherent neutron scattering lengths and total cross sections have been measured on elemental and oxide samples of ordinary Ge and of isotopically enriched substances. From the experimental results the following values were obtained:

the coherent scattering lengths (in fm) of the bound atoms Ge(8.185±0.020);⁷⁰Ge(10.0±0.1);⁷²Ge(8.51±0.10);⁷³Ge(5.02±0.04);⁷⁴Ge(7.58±0.10) and⁷⁶Ge(8.2 ±1.5);

the absorption cross sections at 0.0253 eV (in barn) for Ge(2.20±0.04);⁷⁰Ge(2.9±0.2);⁷²Ge(0.8±0.2);⁷³Ge(14.4±0.4) and⁷⁴Ge(0.4±0.2);

the free cross sections for epithermal neutrons and the zero energy scattering cross sections.

On the basis of this data, the isotopic- and spin-incoherent cross sections and thes-wave resonance contributions to the coherent scattering lengths have been determined and discussed. Transmission measurements at 0.57 meV on amorphous and crystalline GeO₂ yielded for the amorphous sample an inelastic cross section eight times larger than for the crystalline samples. This effect corresponds to a clearly higher density of low energy states in the amorphous than in the crystalline substances.     

Interaction of slow neutrons with nuclides of scandium,titanium, vanadium and manganese

Total cross section measurements at various neutron energies were performed on metallic and oxide samples of natural Sc, Ti, V, and Mn. By means of the Christiansen filter technique the coherent scattering lengths were determined on natural and isotopically enriched samples of Ti-, V-, and Mn-compounds. From the measured values we deduced the following data:

? the absorption cross sections (at 0.0253 eV in barn) of^natTi (6.43±0.06) and of^natV (5.06±0.04);

? the bound coherent scattering lengths (in fm) of^natTi (?3.370±0.013),⁴⁶Ti (4.72±0.05),⁴⁷Ti (3.53±0.07),⁴⁸Ti (?5.86±0.02),⁴⁹Ti (0.98±0.05),⁵⁰Ti (5.88±0.10),^natV (?0.443±0.014), and Mn (?3.750±0.018);

? the free potential radii (in fm) of Sc (3.60 ±0.10),⁴⁶Ti (3.50±0.10),⁴⁷Ti (3.60±0.10),⁴⁸Ti (3.90±0.10),⁴⁹Ti (4.10 ±0.15),⁵⁰Ti (4.50±0.15),⁵¹V (5.20±0.20), and Mn (6.50±20).

On the basis of the well known resonance parameters in combination with new fits of bound levels the spin state scattering lengths, the spin incoherence, and the obtained potential radii are discussed. Total cross section calculations were performed by multi-level formalism and are compared with the measurements.     

Interaction of neutrons with zinc and its isotopes

Coherent neutron scattering lengths and total cross sections have been measured for elemental Zn,Zn-compounds and on isotopicly enriched samples for neutron energies from 0.5 meV up to 143 keV using different techniques. From the experimental data the following quantities were obtained:

the coherent scattering lengths (b in fm) of Zn (5.689±0.014);⁶⁴Zn (5.23±0.04);⁶⁶Zn (5.98±0.05);⁶⁷Zn (7.58±0.08;b ₊=9.4±0.5/5.8±0.5;b _?=5.0±0.7/10.1±0.7);⁶⁸Zn (6.04 ±0.03);

the potential scattering radii (R′ in fm) of Zn (6.2±0.2),⁶⁴Zn (6.0±0.3) and⁶⁶Zn (6.2 ±0.3);

the absorption cross sections (σ _γ at 0.025 eV in barn) of Zn (1.11±0.02);⁶⁴Zn (1.1 ±0.1) and⁶⁶Zn (0.62±0.06).

Derived quantities are the “zero energy” scattering cross sections (σ ₀ in barn) for Zn (4.128±0.010) and⁶⁷Zn (7.8±0.3) and the incoherent bound cross sections of Zn (0.061 ±0.023) and⁶⁷Zn (0.6±0.4). In the epithermal region the Zn-cross section can be described by introduction of two strong bound levels of⁶⁷Zn+n for which estimated parameters are given.     

Freezing of water and aqueous NaCl droplets coated by organic monolayers as a function of surfactant properties and water activity

This study presents heterogeneous ice nucleation from water and aqueous NaCl droplets coated by 1-nonadecanol and 1-nonadecanoic acid monolayers as a function of water activity (a(w)) from 0.8 to 1 accompanied by measurements of the corresponding pressure-area isotherms and equilibrium spreading pressures. For water and aqueous NaCl solutions of ~0-20 wt % in concentration, 1-nonadecanol exhibits a condensed phase, whereas the phase of 1-nonadecanoic acid changes from an expanded to a condensed state with increasing NaCl content of the aqueous subphase. 1-Nonadecanol-coated aqueous droplets exhibit the highest median freezing temperatures that can be described by a shift in a(w) of the ice melting curve by 0.098 according to the a(w)-based ice nucleation approach. This freezing curve represents a heterogeneous ice nucleation rate coefficient (J(het)) of 0.85 ± 0.30 cm(-2) s(-1). The median freezing temperatures of 1-nonadecanoic acid-coated aqueous droplets decrease less with increasing NaCl content compared to the homogeneous freezing temperatures. This trend in freezing temperature is best described by a linear function in a(w) and not by the a(w)-based ice nucleation approach most likely due to an increased ice nucleation efficiency of 1-nonadecanoic acid governed by the monolayer state. This freezing curve represents J(het) = 0.46 ± 0.16 cm(-2) s(-1). Contact angles (α) for 1-nonadecanol- and 1-nonadecanoic acid-coated aqueous droplets increase as temperature decreases for each droplet composition, but absolute values depend on employed water diffusivity and the interfacial energies of the ice embryo. A parametrization of log[J(het)(Δa(w))] is presented which allows prediction of freezing temperatures and heterogeneous ice nucleation rate coefficients for water and aqueous NaCl droplets coated by 1-nonadecanol without knowledge of the droplet's composition and α.     

The scattering of slow neutrons by cobalt

Different techniques have been used to determine with slow neutrons the coherent scattering amplitude b_c and the total scattering cross-section σ_sc of Co. We found values for the bound atom b_c=2.78+0.04 fm and for the free nucleus σ_sc=5.95+0.05 barns.From these quantities, the spin state scattering amplitudes b₊ and b_? corresponding to the compound nucleus states of spin I+1/2 or I?1/2 have been determined as b₊=?2.78±0.04 fm and b_?=+9.91±0.06 fm. These results are not in agreement with previous data, but they are found to be consistent with data obtained from resonance parameters.     

Scattering and resonance properties of the interaction of slow neutrons with the isotopes of silicon and sulfur

In¹¹⁷Te,¹¹⁹Te and¹²¹Te isomeric states withJ ^π=5/2⁺, 5/2⁺ and 7/2⁺ and half-lives of 19.1(9)ns, 2.2(2) ns and 86(6) ns, respectively, have been identified at low excitation energies using (α, 2n) reactions on enriched^115–119Sn targets. Positive parityΔJ=1 bands built on these isomeric states have been observed up to 17/2⁺. The states are interpreted as members of rotational bands built on deformation driving 5/2⁺ [402] and 7/2⁺ [404] Nilsson orbitals which overcome theN=64 subshell gap. The irregular level spacings and electromagnetic properties of the bands are well explained in Coriolis calculations. The moment of inertia parameter as function of collective angular momentum has been derived from the doubly even Te cores. The hindrance of the band head deexcitation may be caused by shape fluctuations of these transitional nuclei.     

Do NAD and NAT form in liquid stratospheric aerosols by pseudoheterogeneous nucleation?

Laboratory data of the freezing of nitric acid hydrates (NAD, NAT) from HNO(3)/H(2)O and HNO(3)/H(2)SO(4)/H(2)O solution droplets have been evaluated with respect to a "pseudoheterogeneous" (surface-induced) nucleation mechanism of NAD and NAT, which has been argued to possibly lead to the formation of polar stratospheric clouds (PSCs). In addition, a parametrization of pseudoheterogeneous nucleation of NAD and NAT suggested recently (Tabazadeh et al. J. Phys. Chem. A 2002, 106, 10238-10246) has been analyzed, showing that this parametrization should not be used in stratospheric modeling studies. The analysis of several laboratory data sets yields an upper limit of the pseudoheterogeneous nucleation rate coefficient of NAD of 2.2 x 10(-5) cm(-2) s(-1). In contrast, the upper limit of the pseudoheterogeneous nucleation rate coefficient of NAT could not be constrained satisfactorily, since formation of NAT has not been observed at stratospheric conditions in laboratory experiments applying small droplets. Maximum NAD production rates of 9.6 x 10(-9) cm(-3) (air) h(-1) in the stratosphere have been estimated assuming a pseudoheterogeneous nucleation mechanism that is constrained by the experimental observations. If maximum NAD supersaturation persisted for 4 weeks in the polar stratosphere the corresponding NAD particle number densities are estimated to be about 6 x 10(-6) cm(-3). These particle number densities are 3 orders of magnitude lower than particle number densities recently observed in the stratosphere. In conclusion, on the basis of laboratory data it is found that a pseudoheterogeneous nucleation mechanism is not sufficient to explain recent observations of large nitric acid containing particles in the polar stratosphere.     

N2O5 reactive uptake on aqueous sulfuric acid solutions coated with branched and straight-chain insoluble organic surfactants

A flow reactor coupled to a chemical ionization mass spectrometer was used to study the reactive uptake coefficients at 273 K of N2O5 on aqueous 60 wt % sulfuric acid solutions coated with insoluble organic monolayers. Both straight-chain surfactants (1-hexadecanol, 1-octadecanol, and stearic acid) and a branched surfactant (phytanic acid) were studied. The reactive uptake coefficient decreased dramatically for straight-chain surfactants. The decrease ranged from a factor of 17 to a factor of 61 depending on the type of straight-chain surfactant. In contrast to the straight-chain data, the presence of phytanic acid did not have a significant effect on the N2O5 reactive uptake coefficient (the decrease was less than the uncertainty in the data) compared to the uncoated solution. In addition to measuring the reactive uptake coefficients, we also investigated the relationship between properties of the monolayers and the reactive uptake coefficients. The reactive uptake coefficients measured on aqueous sulfuric acid subphases showed a relationship to the surface area occupied by the surfactant molecules. However, data obtained with other subphases did not overlap with this trend.