Fast Chopper Research Articles

Total Neutron Cross Section of 228Th for the 1.9- and 7.5-eV Resonances

The total neutron cross section of 228Th has been measured in the regions of 1.9 and 7.5 eV, with the Materials Testing Reactor (MTR) fast chopper. The 228Th occurred as a contaminant in several samples of the parent 232U, on which cross-section measurements were being made at various decay times. For the two resonances which grew in with enough prominence to allow analysis, the resonance parameters deduced are: 1. E0 = 1.896 ± 0.002 eV, Γγ = 36 ± 2 mV, = 0.57 ± 0.04 mV, 2. E0 = 7.55 ± 0.02 eV, Γγ = 70 ± 30 mV, = 0.44 ± 0.05 mV.

Total Neutron Cross Section of Lanthanum

The total neutron cross section of lanthanum has been measured from 0.1 to 1000 eV with the Argonne fast chopper. In order to explain its variation over the entire range, it is necessary to assume contributions by bound states together with contributions by the measured resonances at positive energies. The data could be fitted by assuming a bound level whose resonance parameters are the resonance energy ${E}_{0}=\ensuremath{-}37.5$ eV, the reduced neutron width $\ensuremath{\Gamma}_{n}^{}{}_{}{}^{0}=0.115$ eV, and the radiation width ${\ensuremath{\Gamma}}_{\ensuremath{\gamma}}=0.0565$ eV. The parameters for the resonances at 0.734, 2.994, 20.5, 72.3, 618, and 1182 eV have been measured. The state of ${\mathrm{La}}^{139}$+neutron at the neutron energy of 72.3 eV has been identified to be ${J}^{\ensuremath{\pi}}={3}^{+}$ on the basis of the observed $\ensuremath{\gamma}$-ray spectrum. The potential-scattering radius is measured to be 5.2\ifmmode\pm\else\textpm\fi{}1.0 F. Resonance at energies up to about 11 keV have been detected. On the basis of the measured positive-energy resonances and the negative-energy bound level, the thermal capture cross section has been calculated to be 9.6\ifmmode\pm\else\textpm\fi{}0.7 b. The resonance-capture integral is 12.1\ifmmode\pm\else\textpm\fi{}1.0 b. The $s$-wave neutron strength function ${S}_{0}$ for ${\mathrm{La}}^{139}$ is ($0.7_{\ensuremath{-}0.3}^{}{}_{}{}^{+1.0}$)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$.

The Brookhaven High Flux Beam Reactor fast chopper facility

A pulsed neutron time-of-flight system designed primarily for the investigation of radiative transitions induced by neutron capture in the epithermal region is described. The neutron burst is formed by a massive rotor fabricated from tungsten, nickel, and steel placed at the radial beam tube at the Brookhaven High Flux Beam Reactor (HFBR). A novel in-pile collimator is used to produce a relatively narrow spot size at the target, normally situated 22 m from the rotor, without the use of external collimation. Details of design, construction, and performance are given.

Total Neutron Cross Section of Protactinium-233

Transmission measurements on 233Pa have been taken with the Materials Testing Reactor (MTR) fast chopper. The total cross section has been calculated in the energy range from 0.01 to 10 000 eV. These measurements were made on 700 mg of chemically separated 233Pa in an oxide form. The protactinium was produced by irradiating 280 g of 232Th in the Engineering Test Reactor (ETR). The sample represented approximately 15 000 Ci of activity. The data were taken with a resolution of 0.08 to 2.0 μsec/m. The Breit-Wigner (B-W) resonance parameters have been obtained for the resonances below 18 eV. The average parameters give a value of 0.75 × 10 −4 for the s-wave neutron strength function . Weighting the level spacings inversely as 2J + 1 gives the average observed level spacings per spin state of 1.10 and 1.84 eV. A second-order polynomial least-squares fit to the data between 0.01 and 0.10 eV gives a 2200 m/sec total neutron cross section of 55 ± 3 b, superseding a value of 57 b given previously. The resonance-absorption integral for neutrons with energies above 0.4 eV was calculated to be 901 ± 45 b.

Neutron diffraction by time-of-flight

A fast neutron chopper has been used to make crystal structure investigations by neutron time-of-flight technique. The ability to collect data simultaneously from different crystal planes affords a distinct advantage over the conventional method. Data are presented for aluminium, silicon, lead and copper powder samples. The measurements show that the technique is capable of giving crystal structure amplitude factors with a reliability of better than 5%.

Optical-Model Interpretation of Average Total Neutron Cross Sections in the keV Region

Total neutron cross sections for the elements Nb, Mo, Rh, Ag, Cd, and In in the range of 11 to 101 keV have been measured using the Brookhaven fast chopper at the NRU reactor, Chalk River, Canada. The data, after correction for sample thickness effects, are interpreted in terms of a spherical optical-model potential. The comparison of the data with the model indicates that the potential is more likely to be of the surface-absorption type rather than of the volume-absorption type. There is some evidence of fluctuations in $S$-wave strength functions from nuclide to nuclide, instead of the smooth variation predicted by the optical model. It seems that the parameters of the potential well, chiefly the imaginary part, may be different for $S$- and $P$-wave neutrons.

Time-Shared Time-of-Flight Analysis by Computer

A time-shared computer-centered data collection system has been placed into operation at the Brookhaven Graphite Research Reactor. The system provides simultaneous data collection and sorting for two physically unrelated neutron time-of-flight experiments in time-share with a general computation facility. A general purpose stored-program digital computer with a 4096 word, 24-bit memory is the heart of the system. The computer system provides, for the ``fast neutron chopper'' experiment, a 1024-channel time analyzer for total or partial neutron cross section experiments, three elapsed-time clocks, three total events scalers, two monitor scalers, and a chopper tachometer. At the same time, for the ``slow neutron chopper'' experiment, the system provides a 256-channel time analyzer, a clock, a total events scaler, a monitor scaler, and a chopper tachometer. The two experiments are independent of one another and are controlled by the typing of four-letter mnemonic codes on the system typewriter. A live cathode-ray tube display of either experiment is present at all times except when the time-shared computational facility is being used. Connections between the computer and the external analog-to-digital converters are made on a patchboard, allowing convenient control of data format. The system has been designed to facilitate expansion to multiparameter data handling.

Total Neutron Cross Section ofCm244

The total neutron cross section of ${\mathrm{Cm}}^{244}$ has been studied from 0.01 to about 900 eV with the Argonne fast chopper. Resonance parameters are given for many of the approximately 20 resonances observed below 500 eV. Three resonances have been assigned to ${\mathrm{Cm}}^{246}$, but none to ${\mathrm{Cm}}^{245}$ although this isotope was about as abundant as the ${\mathrm{Cm}}^{246}$ in one of the samples studied. The value of the strength functions of ${\mathrm{Cm}}^{244}$ and ${\mathrm{Cm}}^{246}$, when combined with other measured values, suggests that this quantity has a maximum near $A=239$.

Fluctuations in the Partial Radiation Widths ofU239

The Argonne fast chopper has been used to measure the distribution of the sum of the intensities of transitions having energies between 3.8 and 4.2 MeV in the neutron-capture spectra of 12 neutron resonances of ${\mathrm{U}}^{238}$. The pulse-height spectra were observed in a large (8\ifmmode\times\else\texttimes\fi{}6 in.) NaI(Tl) scintillator. The partial radiation widths exhibit large fluctuations. If the observed distribution is characterized by a number $\ensuremath{\nu}$ of degrees of freedom, as in the theoretical treatment by Porter and Thomas, a value of $\ensuremath{\nu}=5.8\ifmmode\pm\else\textpm\fi{}2.3$ is obtained. This is in contrast to the previously reported results which range from 11 to 90 and is consistent with the predictions of the simple statistical model for the distribution of the sum of four uncorrelated partial radiation widths.

Parameters of the 132 ev Neutron Resonance in Co59

An accurate determination of the parameters of the resonance excited by the interaction of 132 ev neutrons with the Co59 target nucleus has been made, using the fast choppers at Brookhaven National...

Measurement of the Neutron Total Cross Section of U233from 0.07 to 10,000 ev

The neutron total cross section of U233 was measured from 0.07 to 10,000 ev, using the ORNL fast chopper time-of-flight neutron spectrometer. Below 30 ev the neutron energy resolution varied from 0...

Estimating aluminium particle combustion kinetics☆

Combustion of aluminium particles was observed with fast tine camera and light chopper techniques. The burning times, appearance of the flame, oxide particle size and oxide shape, including hollow oxide spheres, did not support the diffusion flame theory. An alternative combustion model, which assumes that diffusion through the oxide layer surrounding the metal droplet is the rate-controlling step, gave reasonably good agreement with experiment. Inadequate knowledge about diffusion coefficients in liquid alumina is the principal uncertainty involved. Metal boiling causes expansion of the oxide shell and accounts for the prevalence of hollow oxide spheres in the combustion products.

Total radiation widths of levels in antimony, mercury, ruthenium and iridium

An extension and elaboration of the “thin-thick” area method for the determination of the total widths of neutron resonances using transmission studies is described. Such determinations were made for a variety of levels in various isotopes of antimony, mercury, ruthenium and iridium, using the Brookhaven fast chopper and 1024-channel time-of-flight analyser. Using a broad range of sample thicknesses, these data, in conjunction with the neutron widths obtained for these levels, resulted in a determination of the total radiation widths of a total of 18 resonances in these nucleides. Of particular importance were the total radiation widths obtained for the 6.23, 15.4 and 29.7 eV levels in Sb 121 and the 43.3 and 71.8 eV levels in Hg 201. Previously reported experiments found these widths to differ by approximately a factor of 2 for levels in the same nucleide, contrary to what has been found in almost all medium and heavy nuclei investigated. The results presented here find these widths to be the same, within experimental error, for the levels in Sb 121, in disagreement with previous work, and confirm the wide fluctuations found for the resonances in Hg 201. A total of five radiation widths were obtained for levels in Ru, and in conjunction with data obtained using enriched isotopes, these results served to isotopically identify a level in Ru 99 and one in Ru 102, which were not uniquely assigned on the basis of data from separated isotopes only. Total radiation widths were also obtained for three levels in Ir 191 and one level in Ir 193. A discussion of these results is presented.

Estimating aluminium particle combustion kinetics

Combustion of aluminium particles was observed with fast tine camera and light chopper techniques. The burning times, appearance of the flame, oxide particle size and oxide shape, including hollow oxide spheres, did not support the diffusion flame theory. An alternative combustion model, which assumes that diffusion through the oxide layer surrounding the metal droplet is the rate-controlling step, gave reasonably good agreement with experiment. Inadequate knowledge about diffusion coefficients in liquid alumina is the principal uncertainty involved. Metal boiling causes expansion of the oxide shell and accounts for the prevalence of hollow oxide spheres in the combustion products.

Resonance CaptureγRays from Platinum

The high-energy gamma rays following neutron capture in a platinum target have been studied at the Brookhaven-AECL fast chopper facility at Chalk River. The transitions to the ${0}^{+}$ ground state and the first two excited ${2}^{+}$ states in ${\mathrm{Pt}}^{196}$ have been measured for 15 resonances from 11.9 to 296 eV. Relative transition probabilities for these cases have been determined and are compared to the chi-squared class of probability distribution functions.

Resonant Absorption of Neutrons by Crystals

A study has been made of the Doppler broadening of resonances in the total cross section for interaction of slow neutrons with nuclei bound in crystals. The resonance at 6.71 eV in Os metal and the resonance at 6.65 eV in U metal were chosen as examples of moderate crystal binding, while the 6.65-eV resonance in ${\mathrm{U}}_{3}$${\mathrm{O}}_{8}$, for which $k{\ensuremath{\theta}}_{0}=0.043$ eV, was chosen as an example of strong binding. The shapes of the resonance lines were determined as a function of temperature by measuring the neutron transmission of thin and thick samples by means of the Argonne fast chopper. These shapes were compared with theoretical line shapes calculated by means of Lamb's theory of Doppler broadening as applied to the spectra of lattice frequencies implied by simple models of the crystal lattices. For moderate binding, a simple Einstein model which reproduces the observed specific-heat behavior of the crystal above 40\ifmmode^\circ\else\textdegree\fi{}K gives accurate resonance-line shapes at all temperatures. However, for ${\mathrm{U}}_{3}$${\mathrm{O}}_{8}$ the line shape implies a generalized Nernst-Lindemann model of the lattice with a frequency spectrum $g(\ensuremath{\nu})=0.9\ensuremath{\delta}(h{\ensuremath{\nu}}_{1}\ensuremath{-}0.013 \mathrm{eV})+0.1\ensuremath{\delta}(h{\ensuremath{\nu}}_{2}\ensuremath{-}0.052 \mathrm{eV})$. The proportions of the high- and low-frequency components are quite different from the values resulting from specific-heat data. Possible interpretations of the ${\mathrm{U}}_{3}$${\mathrm{O}}_{8}$ results in terms of simple lattice models are presented.

Measurements of Neutron Spectra with a Fast Chopper

Spectra of neutrons scattered from various materials placed in the central vertical channel of the reactor Rl are studred. Interest rs given to the epithermal region, in particular to the distortion of the spectrum caused by the fuel. For scatterers with heavy atoms it is thus possrble to observe steps'' in E - M(E) at each of the three lowest uranium resonances. The heights of these steps compare favorably with calculations. The upper energy limit for the measurements made is about 10,000 ev. Some experiments using a heated scatterer are also penformed. (auth)

The Neutron Elastic Scattering Cross Section of U233 below 20 ev

The neutron scattering cross section of U233 has been measured relative to that of copper below 20 ev on the Materials Testing Reactor fast chopper. The observed scattering cross section is adequately described by previously obtained multilevel parameters, which give a value for the potential scattering of 12.6 b. The data are not of high enough quality to permit the assigning of spins to the resonances.

Anomalous Neutron Scattering ofAr36

The thermal neutron scattering of argon is unusual in that it shows a large amount of incoherent scattering, even though it is even-even practically monoisotopic. The incoherence has been ascribed by Henshaw to a large scattering cross section for ${\mathrm{Ar}}^{36}$. Measurements with the BNL fast chopper for a gas sample enriched to 63% ${\mathrm{Ar}}^{36}$ have shown that the total cross section from 0.1 ev to 6 kev varies in such a way as to reveal the presence of a negative energy level. The parameters ${{\ensuremath{\Gamma}}_{n}}^{0}=82$ ev, ${E}_{0}=\ensuremath{-}9.8$ kev, and ${\ensuremath{\Gamma}}_{\ensuremath{\gamma}}=1.85$ ev have been deduced for this level. The consequences of this anomalous scattering for neutron studies of atomic motions in a liquid are discussed.

Isotopic Identification of Neutron Resonances of Cd, Sb, Os, and Pt from Capture Gamma-Ray Spectra

The Argonne fast chopper has been used in conjunction with a 3-variable magnetic-tape recording system to demonstrate the high efficiency and sensitivity of isotopic identification of neutron resonances by observation of capture $\ensuremath{\gamma}$ rays. A discussion of features of capture spectra which can be useful in isotopic assignment is presented as well as results for resonances in natural Cd, Sb, Os, and Pt. The experimental study reported emphasizes primarily the observation of coincident $\ensuremath{\gamma}$ rays as a means of measuring binding energies and characteristic low-energy lines of nuclides giving rise to resonant capture. The isotopic assignments include 23 resonances in Cd between 18 ev and 235 ev, 6 of which were previously unassigned; 20 resonances in Sb between 5 and 200 ev; 23 resonances in Os in the region between 6 ev and 61 ev; and two previously unresolved resonances at 67 ev assigned to ${\mathrm{Pt}}^{195}$. The data lead to observed level spacings of 33 ev in ${\mathrm{Cd}}^{111}$, 80 ev in ${\mathrm{Cd}}^{112}$, 27 ev in ${\mathrm{Cd}}^{113}$, 27 ev in ${\mathrm{Sb}}^{121}$, 12 ev in ${\mathrm{Sb}}^{123}$, and 5.1 ev in ${\mathrm{Os}}^{189}$. Strength functions for ${\mathrm{Cd}}^{111}$ and ${\mathrm{Cd}}^{113}$ based on the identifications are in agreement with previous reported values. For ${\mathrm{Os}}^{189}$, from transmission data previously unreported, the value $\frac{{{\overline{\ensuremath{\Gamma}}}_{n}}^{0}}{D}=({2.0}_{\ensuremath{-}0.7}^{+2.2})\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$ is obtained.