Mixed-powder Method Research Articles

Neutron excitation functions for 45Sc, 93Nb, and 58Ni in the energy range 13–19 MeV

Activation techniques have been used to measure the excitation functions for 45Sc( n, 2 n) 44 m + g Sc and Nb( n,2 n) 92 m Nb in the energy range 13–19 MeV and 58Ni( n, p) 58 m + g Co and 58Ni( n,2 n) 57Ni in the energy range 13–17 MeV. The neutrons were produced by the 3H( d, n) 4He reaction with the Auburn University 3 MV Dynamitron accelerator. Absolute cross-sections were measured using the mixed powder method with 27Al( n, α) 24Na as the monitor reaction. The experimental isomeric-yield ratios were determined by measuring the ground state gamma activity resulting from the reaction and making a least squares fit to the time behavior of the activity in which the cross-section ratio was treated as a variable parameter. Cross-sections calculated from the semi-empirical relation due to Pearlstein are given for qualitative comparisons with the present results.

The excitation function for the 89Y(n, 2n)88Y reaction

Activation cross sections for the 89Y(n, 2n)88Y reaction at neutron energies between 12.6 and 17.8 MeV have been measured by using the mixed-powder method and γ-ray detection by a Ge(Li) spectrometer. Using the 27Al(n, α)24Na reaction for monitoring the neutron flux, the measured cross-section values for the 89Y(n, 2n)88Y reaction were found to be 331±32 mb, 603±58 mb, 820±79 mb, 1040±100 mb, 1072±103 mb, 1172±112 mb, 1221±117 mb and 1218±117 mb at the respective incident neutron energies of 12.6±0.1 MeV, 13.3±0.1 MeV, 14.0±0.4 MeV, 14.9±0.3 MeV, 15.1±0.3 MeV, 16.0±0.4 MeV, 16.7±0.5 MeV and 17.8±0.7 MeV. The measured values are compared with the experimental values of others and with the theoretical values obtained from calculations based on the statistical model for the formation of a compound nucleus and its subsequent emission of neutrons.

Isomeric cross sections for the (n, 2n) reaction on 82Se, 81Br, and 45Sc at 15.1 MeV

Activation techniques have been used to measure the cross sections at 15.1 MeV neutron energy for the following reactions: 82Se(n, 2n) 81m+gSe, 81Br(n, 2n) 80m+gBr, and 45Sc(n, 2n) 44m+gSc. Isomeric cross-section ratios were evaluated by applying the method of least squares to the time behavior of γ-ray activity following the ground-state decay of each isomeric pair. The absolute cross section σ m for the formation of the metastable state was measured by the mixed-powder method with the 27Al(n, α) 24Na reaction as the monitor. The cross section σ g for the formation of the ground state was then determined by using the isomeric cross-section ratio. The sum of σ m and σ g for each reaction is compared with the theoretical value obtained from calculations based on the statistical model for the formation of a compound nucleus and its subsequent emission of neutrons.

The (n, 2n) cross sections of 85Rb, 87Rb and 144Sm

The (n, 2n) cross sections at neutron energies between 14.9 and 17.0 MeV have been measured for 85Rb, 87Rb and 144Sm by the mixed-powder method and γ-ray detection by a Ge(Li) spectrometer. Using the 27Al(n, α) 24Na reaction for monitoring, the measured cross sections were (in mb): 85 Rb( n, 2 n) 84( m+g) Rb, 1125±141, 1177±148 and 1235±162 at 15.0±0.4 MeV, 16.2±0.7 MeV and 17.0±0.9 MeV, respectively; 85 Rb( n, 2 n) 84 m Rb, 662±83, 688±87 and 765±99 at 15.0±0.4 MeV, 16.2±0.7 MeV and 17.0±0.9 MeV, respectively; 87 Rb( n, 2 n) 86( m+g) Rb, 1336±168 and 1301±162 at 15.0±0.4 MeV and 16.2±0.7 MeV respectively; 144 Sm( n, 2 n) 143( m+g) Sm, 1202±130, 1300±141, 1516±179 and 1514±179 at 14.9±0.3 MeV, 15.5±0.3 MeV, 16.4±0.5 MeV and 16.7±0.2 MeV, respectively . The measured values are compared with the statistical model calculations of Pearlstein.

Inhomogeneity of high melting temperature elements in titanium alloys

With the object of obtaining homogeneous ingots of titanium alloys which contain more than a few percent of high melting temperature elements such as W, Ta, Mo and Nb, the consumable electrode of vacuum arc melting was improved by using a mixed powder method. The macrosegregation of these elements in the commercial scale ingot was studied with a chemical analysis and a radiograph technique.The optimum volume percentage of Mo in the above mentioned mixed powder was 40%, in order to get the homogeneous ingot of Ti-15Mo-5Zr alloys. The estimated melting temperature of the mixed powder was about 2, 170°CTa content was minimum at the center portion in the top of the Ti-5Ta ingot, because the partition coefficient of Ta was larger than 1. However, the degree of the Ta segregation in the above alloy was much smaller than the Co segregation in Ti-8Al-4Co alloys.The ring pattern detected by radiography in Ti-5Ta billet was identified to be the indication of the slight concentration difference of Ta, which was caused by turbulence in the liquid-solid interface during solidification.

Neutron activation cross sections at 14.4 MeV For some naturally occurring heavy elements in the region 76 ≦ Z ≦ 82

Abstract Neutron activation cross sections at 14.4 MeV were determined for isotopes of the following naturally occurring heavy elements: Os, Pt, Au, Hg, Tl and Pb. The mixed-powder method with Ge(Li) γ-detection was used, and the cross sections in mb are as follows. For (n, 2n) reactions ( m = metastable ; g = ground state ); 192 Os, 1993±200 ( m+g ); 192 Pt , 2035±150; 198 Pt , 1009±100 ( m ); 198 Pt , 707±85 ( g ); 197 Au , 151 ±18 ( m ); 197 Au , 1986±150( m+g ); 196 Hg , 1617±160( m ); 196 Hg , 363±54( g ); 198 Hg , 885±80( m ); 198 Hg , 1125±100( g ); 200Hg, 789±120( m ); 204 Hg , 2077±166; 203 TI , 1950±200; 204 Pb , 1737±140 . For (n, n′) reactions: 199Hg, 128±20( m ); 204 Pb , 51±10( m ) . For (n, 3n) reactions: 197Au, 61±20. For (n, p) reactions: 194Pt, 4.3±0.6; 196 Pt , 1.1±0.2; 198 Hg , 4.5±0.5; 199 Hg , 2.3±0.3; 208 Pb , 0.46±0.06 . For the (n, α) reaction; 203Tl, 2.2±0.4. For the [(n, np)+(n, pn)+(n, d)] reaction: 199Hg, 0.40±0.06. Cross sections calculated with the statistical-model equation of Pearlstein and an optimized level-density parameter equation are compared with a consistent set of measured (n, 2n) values over the region 30 ≦ Z ≦ 82; it is shown that deviations of the calculated cross sections from the respective measured values become larger as the energy needed to emit the second neutron increases, probably because fewer exit channels are available and the continuum assumption in the statistical model of the compound nucleus becomes invalid.

Neutron Activation Cross Sections for As, Br, Rb, and Sr Isotopes at 14.4 MeV

Activation cross sections at an incident neutron energy of 14.4\ifmmode\pm\else\textpm\fi{}0.3 MeV are measured for the ($n,2n$), ($n,p$), and ($n,\ensuremath{\alpha}$) reactions on isotopes of As, Br, Rb, and Sr, by using the mixed-powder method and $\ensuremath{\gamma}$ detection by Ge(Li) spectrometer. The use of a stoichiometric chemical compound as an alternative to the mixed-powder technique, where one of the constituents serves as the neutron flux monitor, is investigated and is recommended, whenever applicable, over all other methods for measurement of relative activation cross sections.

A comparison of the mixed-powder and substitution methods in the quantitative evaluation of X-ray reflections from crystalline powders

The conditions determining the quantitative measurement of X-ray reflections by the substitution and by the mixed powder methods are compared with regard to the evaluation of atomic scattering factors. The paper is supplementary to a paper by Brindley and Spiers; it contributes particularly to the discussion of particle-size, of extinction, and of differential absorption. A method is indicated for determining the presence of differential absorption and so avoiding errors which may arise from that cause when the mixed-powder method is applied to coarse particles. Techniques are described relating to the substitution and to the mixed-powder methods. Attention is drawn to a technique in which the mixed-powder and substitution methods are used conjointly.