Slow Chopper Research Articles

Transient fluorescence spectroscopy of terbium doped dipicolinic acid: a fluorescence lifetime measurement technique

An innovative fluorescence lifetime technique, called transient fluorescence spectroscopy, has been developed and applied toward the determination of the fluorescence lifetimes (a value near 1 ms) of terbium doped dipicolinic acid (DPA-Tb). This different lifetime measurement technique uses a high-pulse-repetition-frequency (8 kHz) pulsed UV laser excitation source and a slow modulated chopper (50to1000 Hz) to produce the modulated emission intensity from the DPA-Tb, and compares these to a theoretical transient solution of the population rate equations for the DPA-Tb energy levels. Curve fitting of the transient solution of the modulated emission intensity to that obtained experimentally yielded a value for the fluorescence lifetime.

Stochastic and deterministic attenuation of one beam in the neutron interferometer

Experiments are reported, where one of the beams inside the neutron interferometer was attenuated in three different ways: by partially absorbing gold or indium foils, by a slow beam chopper and by a fine cadmium lattice. When the attenuation was the same the different methods still had a different effect on the amplitude of the interference pattern. This counter intuitive result can be formally explained in a quantum mechanical analysis which nicely illuminates how the history of a neutron affects its wave function.

An alternative method of determining the chopper transmission function

A method of determining the transmission function of a curved-slit slow neutron chopper by using monoenergetic neutron beams is described. The transmission function is determined directly from the time-of-flight spectra measurements, performed at several chopper speeds, of a monoenergetic neutron beam obtained by the use of a mica crystal as a sample. This method allows the experimental determination of the transmission function for the two chopper opening positions, 0° and 180°, simultaneously.

Multipurpose Beam Pulsing System for the 12UD Pelletron Tandem Accelerator at the University of Tsukuba

A beam pulsing system has been developed for a 12 MV tandem accelerator. The system consists of a pre-acceleration chopper, a klystron buncher and a post-acceleration chopper. The pre-acceleration chopper comprises a slow chopper and a fast travelling-wave chopper. Pulsed beams with widths in the range from 10 µs to ∼2 s are obtained with the slow chopper, and the repetition periods can be varied from 70 µs to 4 s. The fast chopper produces ion bursts having widths between 0.05 and 0.8 µs with a duty factor of ∼10%. The buncher is operated with the two choppers to obtain beam pulses as narrow as a few nanoseconds. Time-of-flight measurements yielded pulse widths 2–4 ns (FWHM) wide for ions in the mass range 1≦A≦28. The ratio of the dark to peak ion current was usually of the order of 10-4.

The accurate fission cross-section of 239Pu from 0·005 eV to 0·1 eV and its reference value at 2200 m/s

A precise measurement of the fission cross-section of 239Pu was performed with a slow chopper at the Belgian BR2-reactor (energy range 0·000 to 0·1 eV). Simultaneous recordings of pulse-height and time-of-flight spectra of successively compared 239Pu layers and standard elemental boron layers were made. The detection of the 10B( n, α) 7Li and 239 Pu(n, ƒ) reaction fragments is made with excellent pulse-height resolution in low geometry with a AuSi surface barrier detector. The separation between the pile-up tail produced by the α-particles of 239Pu and the fission fragment energy-distribution was very good. The amount of 10B in the elemental boron layers was determined by weighing in vacuum combined with careful chemical and isotopic analysis of witness foils. The relative amounts of 10B were checked by neutron-counting in the geometry of the experiment. The amounts of 239Pu on the fission foils were determined from precise low-geometry α-counting and the knowledge of the isotopic composition and α-half-life values. The α-activity was controlled before and after the measurements. The fission cross-section was calculated in the energy-range from 0·005 to 0·1 eV and its absolute value deduced at 2200 m/s was 741·9 ± 3·4b. These cross-section values were used to calculate the Westcott g / tf -factor for a 20·44°C Maxwellian neutron distribution with as results g ƒ = 1·0553 ± 0·0013 .

The accurate fission cross-section of 235U from 0·002 to 0·15 eV and its reference value at 2200 m/s

A new precise measurement of the 2200 m/s fission cross-section of 235U was performed with a slow chopper at the BR2 reactor (energy-range from 0·002 to 0·15 eV). Simultaneous recordings of pulse-height and time-of-flight spectra of successively compared 235UF 4-layers and “standard” elemental boron-layers were made. The detection of the 10B( n, α) 7Li- and 235U( n, f) reaction fragments is made with excellent pulse-height resolution in low geometry with a surface-barrier detector. The absolute 10B contents were determined from direct weighing in vacuum combined with careful chemical and isotopic analysis of witness foils. The relative 10B contents of the standard boron-foils were checked by their neutron induced reaction rates in the geometry of the fission experiment. The amount of uranium on the fissile foils is determined by precise low geometry α-counting making use of the most recent and accurate α half-life values. The fission foils were also checked by α- and fission fragment counting in the geometry of the experiment. The fission cross-section was calculated in the energy-range from 0·002 to 0·15 eV and the absolute value deduced at 2200 m/s was 587·6 ± 2·6 barn. The value at 25 × 10 −3 eV is 591·8 ± 2·7 barn. These fission cross-section values were used to calculate the Westcott- g φ -factor for a 20·44°C Maxwellian-distribution with as result: g f = 0·9780 ± 0·0010.

Dielectric and Neutron Inelastic Scattering Measurements on Phenanthrene

The static dielectric constants and the linear coefficients of thermal expansion of high purity single crystals of phenanthrene were measured as a function of temperature in three mutually perpendicular directions and indicate a small increase in the dielectric constant in the a direction at about 72°C. Cold neutron inelastic scattering from polycrystalline phenanthrene was investigated at room temperature, 60, 75, and 85°C, using the Brookhaven Slow Chopper facility. Changes in the density of phonon states above and below 72°C were examined by difference analysis and loss of some low-lying states above 72°C was detected. Some implications of these dielectric measurements and the neutron spectra as well as earlier electrical and heat capacity measurements are discussed. The conclusion is that phenanthrene cannot at present be described as one of the known types of solids. It is suggested that the observed phenomena are not inconsistent with a model in which hindered vibration of the 4, 5 hydrogens above 72°C introduce a new degree of freedom.

Beta decay of 239Pu fission products at short times after fission

The beta activity of 239Pu fission products at short times after fission down to 1 msec has been measured using a slow neutron chopper and a time analyser. The paper describes the technique adopted for the experiment. The lower limit for the composite half-life of fission product beta decay has been estimated as 0·2 sec. The maximum beta energy of the fission product betas at 0·96 sec of decay has been found to be 5·7 MeV.

Certain aspects of the calibration and resolution of slow neutron spectrometers

The method of calibrating and determining the resolution of slow neutron spectrometers by the measurement of Bragg breaks is discussed. The folding of a Gaussian resolution function with a linear function presenting a sharp break is analytically studied. The ratio between the resulting “break width” and the resolution width is determined; in practical cases it is 1.0645. A relation between the mean abscissa, corresponding to the half-height of the resulting curve, and the true position of the break is given; they coincide only for symmetrical breaks. The effect of a Gaussian resolution in the transmission, inverse transmission, and total cross section, is theoretically evaluated for the iron (110) Bragg cut-off, for many resolution widths and several sample thicknesses. It is concluded that the analysis of the measured transmission curve gives the most direct and accurate result. The characteristics of a curved-slit slow neutron chopper and time-of-flight spectrometer built at the IEA are presented. The time resolution for the present conditions of geometry, with a very small detecting area, is 1% for a 3 m flight path and 4.046 Å neutrons. A calibration independent of the chopper speed, desirable when a wide energy range must be covered, has been attained within the precision of 4 μsec. The spectrometer can be used from 0.5 Å upto 12 Å for total cross section measurements.

Neutron cross sections of Pr, Yb, Lu, Er, Ho and Tm

The total neutron cross sections of praseodymium, ytterbium, lutetium, erbium, holmium and thulium have been measured by transmission within the neutron energy range 0.001 to 1.0 eV, with special attention given to obtain values at the thermal energy 0.025 eV. A crystal spectrometer coupled with a mechanical monochromator and a slow chopper were used at the swimming pool research reactor of the Instituto de Energia Atômica. Powder samples of the oxides were supplied in high purity with particular care to eliminate contamination by the rare earths which have high neutron cross section. The data were analysed to determine the nuclear absorption and the nuclear scattering cross sections. Effects of nuclear resonances and of the atomic paramagnetic scattering were considered. In the case of lutetium, the parameters of the first resonance are presented. In the cases of thulium and holmium, experimental values of the paramagnetic scattering cross sections are compared with those expected from calculated electronic wave functions.

The rotating crystal time-of-flight spectrometer for scattering law measurements with slow neutrons: Theoretical analysis and design considerations

The application of a rotating mosaic crystal as slow neutron monochromator and chopper in connection with a time-of-flight spectrometer for inelastic neutron scattering measurements in condensed matter is discussed. Effects unique to this monochromating technique, e.g. the Doppler effect and neutron storage effects are described quantitatively. Several resolution functions describing the performance of this type of spectrometer are given. Using the derived relations finally a reference design of a possible high resolution double spectrometer is outlined.

A slow neutron chopper for energies between 0.1 and 0.001 eV

A slow neutron chopper has been constructed and used at the Reactor RC-1, of the Casaccia Nuclear Center, for neutron cross-section measurements. The mechanical and electronic features of the instrument are described. Calibration measurements of the time of flight scale are reported. The neutron total cross-section of gold in the energy range between 0.1 and 0.001 eV has been carried out as a check of the effectiveness of our instrument.

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.

Low-Frequency Motions of H2O Molecules in Crystals

A number of selected samples containing water of crystallization or adsorbed water have been systematically examined by neutron inelastic scattering using the Brookhaven slow chopper to study translational and librational motions of the H2O molecules in a region lying below 800 cm—1. To gain an understanding of these motions in various configurations, spectra were measured for polycrystalline samples of natrolite (isolated H2O molecules in a ``cavity''); chabazite (waterlike H2O molecules); analcime (two H2O molecules per ``cavity''); apophyllite (distorted H2O molecules); copper sulfate pentahydrate (H2O molecules forming hydrogen bonds of various lengths with the lattice); and beryl (adsorbed monomeric H2O molecules). Despite the large variety of lattice configurations and bonding forces, all these compounds, with the exception of beryl, show peaks corresponding to torsional motions of the H2O molecules in a narrow frequency region (480 to 600 cm—1). Beryl, however, shows only a broad spectrum centered approximately at kT, as expected for weakly bound H2O molecules. Besides indicating the frequencies of torsional motions and the degree of rotational freedom of the H2O molecules, these measurements provide information concerning: distribution of water molecules among various lattice sites, influence of the distortion of the H2O molecules on the observed frequencies, and the effects of its environment on the translational modes of the H2O molecules.

A slow neutron chopper time-of-flight spectrometer at the reactor R2 in Sweden

A slow neutron time-of-flight spectrometer for studies in the solid and liquid state physics has been constructed at the light water moderated reactor R2. The thermal spectrum emerging from the used channel has been measured and found to be a Maxwellian. The spectrometer has a semi-monochromatizing chopper working in conjunction with a beryllium filter. The transmitted neutron spectrum has a half value width of ≈0.3Å at 4 Å. The time resolution corresponds to 12μsec/meter or 1.2% at 4 Å. A full description of the equipment with the collimator plug and the shielding house of the chopper will be given. A comparison is made with the time-of-flight spectrometer at R1.

Time Dependent Neutron Spectra in Graphite

An experiment is described in which the 28-Mev electron linac at Harwell was used as a neutron source to produce 1 mu sec bursts of 10/sup 11/ electrons 160 times per second. A natural uranium target was placed centrally against one face of a 60 x 62.2 x 71.1 cm graphite block, and a neutron beam extracted from a channel running into the center of the block. This beam was interrupted by a slow chopper running in synchronism with the pulses from the accelerator. In this way 100 mu sec time samples'' of the neutrons in the block were taken and their spectrum determined by the time-of-the-flight method. The instant of the time sample relative to the fast neutron pulse could be varied by an electronic delay circuit. Neutron spectra are presented for the time varying between 300 and 1000 mu sec after the pulse. These spectra differ from the Maxwellian shape but approach asymptotically to a cooled Maxwellian'' from which they are indistinguishable after 1000 mu sec. Detailed calculations of the spectra were made using an IBM 7090 computer to obtain a numerical solution of the time and energy dependent diffusion equation using a scattering kernel based on themore » scattering measurements made at Chalk River. The experimental results are also compared with spectra using a scattering kernel based on the heavy gas model with a fictitious mass of 33. (auth)« less

An investigation of the energy levels in alkaline earth hydroxides by inelastic scattering of slow neutrons∗

The Brookhaven slow chopper was used to study the differential inelastic cross section for 5 meV incident neutrons scattered at 90° from polycrystalline samples of Mg(OH) 2 and Ca(OH) 2. In the energy interval covered (3–163 meV) corresponding to wave numbers of 25–1300 cm −1, four distinct peaks were observed in both compounds at 293°K. They occur at 633 ± 30 cm −1 ( A), 377 ± 8 cm −1 ( B), 281 ± 9 cm −1 ( C), 135 ± 10 cm −1 ( D), respectively, in Mg(OH) 2 and at 540 ± 30 cm −1 ( A), 316 ± 7 cm − ( B), 247 ± 15 cm −1 ( C), and 83 ± 7 cm −1 ( D) in Ca(OH) 2. A spectrum taken at 103°K for Mg(OH) 2 shows that peaks A, B, and C appear unshifted in energy while D increases in intensity relative to A, B, and C and appears at 120 ± 7 cm −1. The intensities of the individual peaks in Mg(OH) 2 are observed to vary with temperature in accordance with the temperature dependence of one-phonon differential scattering cross section, if Θ Debye ⪢ T is assumed. The low energy peaks D in both Mg(OH) 2 and Ca(OH) 2 are too broad to be due to a single transition and have shapes characteristic of an acoustic branch of normal mode. It is shown that the observed spectrum cannot be explained by a highly localized rotational motion of the OH-ions, as suggested by Hexter and some of the transitions observed are in qualitative agreement with an interpretation in terms of low-frequency lattice modes of the crystal.

Low Frequency Hydrogen Vibrations in KH2PO4

The Brookhaven National Laboratory's slow chopper was used to investigate the low frequency motions of hydrogen in KH 2 PO 4 and KD 2 PO 4 at room temperature and below the ferroelectric Curie temperature. The energy interval covered (8∼170×10 -3 ev) corresponds to wave numbers of 25∼1300 cm -1 , a region which is difficult to be investigated by infrared absorption technique. The excitation modes associated with hydrogen are identified by the change in scattering intensity upon deuteration. It is found that a band of frequencies centered around 180 cm -1 is associated with the hydrogen vibrations. A comparison with Reid's semi-empirical theory for KH 2 PO 4 is made.

The fission cross section of 235U from 0·01 eV to 0·1 eV and its absolute value at 0·0253 eV

The absolute fission cross section of 235U at the reference neutron velocity of 2200 m/sec was measured. The counting rate of a 4π-fission chamber was compared with that of a 10B ionization chamber by alternate measurements in a neutron beam emergent from the thermal column of BRI. The energy selection was done with a slow chopper previously calibrated with respect to the sharp breaks of σ t for α Fe, Be, Bi, Al, C (graphite). The efficiency of the 1B chamber was compared to the well-known absorption cross section of 197Au by activation measurements with a (β-γ) coincidence technique. The thickness of the 235U layer was determined by comparison with α-standards of the same isotopic composition, and controlled by a mass-spectrometric method. Our determination yields: σ f 0 = (587 ± 6) barns Some information is also obtained about the nearby negative energy levels.

A neutron time-of-flight spectrometer with a semi-monochromatizing chopper

A new type of slow chopper intended for use in neutron inelastic scattering measurements has been constructed, which has a transmission curve much narrower than what is usual for slow choppers. This chopper is operated in conjunction with a beryllium filter to give a transmitted neutron spectrum of a half value width of ≈0.3 Å at 4 Å which is a considerable improvement over the usual width of the cold neutron spectrum of more than 1 Å. The time resolution of the new instrument corresponds to 12 μsec/meter. A full description is given of the construction and operation of the instrument.