Short Flight Path Research Articles

Texture analysis with the new HIPPO TOF diffractometer

The new neutron time-of-flight (TOF) diffractometer HIPPO (High-Pressure-Preferred Orientation) at LANSCE (Los Alamos Neutron Science Center) is described and results for quantitative texture analysis of a standard sample are discussed. HIPPO overcomes the problem of weak neutron scattering intensities by taking advantage of the improved source at LANSCE, a short flight path (9m) and a novel three-dimensional arrangement of detector banks with 1360 3He tubes, on five conical rings with scattering angles ranging from 2θ=10° to 150°. Flux at the sample is on the order of 107neutronscm−2s−1. A large sample chamber (75cm diameter well) can accommodate ancillary equipment such as an automatic sample changer/goniometer used in this study. This instrument was used to measure the texture of a round-robin limestone standard and extract orientation distribution data from TOF diffraction spectra with different methods (Rietveld harmonic method, Rietveld direct method, automatic fitting of individual peak intensities), and the results compare favorably. Also, there is good agreement with results obtained on the same sample measured at other facilities, but with greatly reduced measuring time for HIPPO.

The HADES time-of-flight wall

In the framework of the High Acceptance DiElectron Spectrometer collaboration a Time-of-Flight wall subdetector has been developed. In order to efficiently select rare events with dilepton pairs produced in the nuclear matter, in a high hadron multiplicity environment, such a wall must be able to resolve electrons from pions up to 0.5 GeV/ c and from protons up to 2 GeV/ c. The short flight path of about 2 m requires a time resolution better than 150 ps, while impact position and multiplicity measurements need adequate granularity. These requirements have been fulfilled by employing scintillator rods read-out at both ends by photomultipliers.

Measurement of 3H(d,n) 4He neutron spectrum by means of time-of-flight technique for a fast neutron generator

The energy spectrum of neutrons produced from the 3H(d,n) 4He reactions has been determined by using the double scintillator time-of-flight technique. Although the technique has limitations in determining the energy and its spread, due to the short flight path of the experimental setup, it has been shown that the determination of primary neutron energy spectrum down to 0.5 MeV, with the expected structures caused by multiple scattering and deuteron build-up effects, is easily achievable for a low energy accelerator operated in continues mode.

The time of flight wall for the HADES spectrometer

In the framework of the HADES (High Acceptance DiElectron Spectrometer) collaboration we are developing the Time Of Flight (TOF) wall subdetector. In order to efficiently select events with e/sup +/e/sup -/ pairs produced in the nuclear matter, the TOF wall must be able to resolve electrons from pions up to 0.5 GeV/c and from protons up to 2 GeV/c. Because the short flight path of about 2 m requires a 100-150 ps time resolution, we are building a granular system made of scintillator rods readout on both ends by means of photomultipliers, capable of such performance.

An old-new tool for nuclear analysis: Time-of-Flight spectrometry

The Time-of-Flight (ToF) technique can be used for mass identification, for separation of a specified mass or for measuring the energy of a given mass particle. The instrumentation required is simple and low in cost. The method features high yield, transmission efficiency is typically of 5 to 20%. Even with short flight paths (5 to 10 cm), ToF has adequate mass resolution (M/ΔM∼300 to 500) for identifying isotopic species. This paper examines the scope of ToF in nuclear science with examples in mass spectrometry, in mass separation and in kinetic energy measurements of fixed mass particles. An example of the latter is the energy determination of recoil nuclei. If a recoil is produced inside a solid, the residual recoil energy reveals the depth from which it originates. This approach is used for profiling nitrogen via14N(n, p)14C. The ToF measurement of the14C recoil energies reveals the depth distribution of nitrogen with better than 50 A resolution.

Moxon-Rae setup for the measurement of stellar (n, gamma ) rates and the example of 87Rb.

A setup with Moxon-Rae detectors was optimized for measurements of ({ital n},{gamma}) cross sections in the keV region. The experimental technique makes use of the quasi-Maxwellian spectrum that can be obtained via the {sup 7}Li({ital p},{ital n}){sup 7}Be reaction and is particularly suited for the determination of Maxwellian-averaged cross sections for nucleosynthesis studies related to {ital s}-process scenarios. It allows measurements at extremely short flight paths with the time-of-flight method as an option for background reduction. The experimental determination of the efficiency with calibrated {gamma} sources and with two-step cascades from selected resonances of the {sup 34}S({ital p},{gamma}) reaction revealed new properties of the response function. The reliability of the method was first demonstrated at the example of the well-known ratio of the ({ital n},{gamma}) cross sections of Ta and Au, and then used for a measurement of the stellar {sup 87}Rb cross section. With this new value {l_angle}{sigma}{ital v}{r_angle}/{ital v}{sub {ital T}}=15.5{plus_minus}1.5 mb, a discrepancy in the previously existing data could be resolved. {copyright} {ital 1996 The American Physical Society.}

Spallation Neutron Measurement by the Time-of-Flight Method with a Short Flight Path

Spallation Neutron Measurement by the Time-of-Flight Method with a Short Flight Path

Spallation Neutron Measurement by the Time-of-Flight Method with a Short Flight Path.

The experimental method of measuring neutron production double-differential cross sections was tested at incident proton energies of 0.8 and 1.5 GeV with C and Pb targets. Neutrons were measured with liquid scintillators by the time-of-flight method. The use of very weak secondary beam forced us to take a typical flight path length as short as 1 m. Preliminary experimental consideration led to the results that the amount of back ground neutrons were made low by the short flight path measurement, and a two-gate integration method was suited for the pulse shape discrimination between high energy neutrons and gamma-rays. The cross sections were obtained in the neutron energy range up to 100 MeV with acceptable energy resolution. The experimental results obtained at the incident proton energy of 0.8 GeV were consistent with the data taken with much longer flight paths. It was confirmed that the time-of-flight method with bare detectors at the short flight path was reliably applicable to the spallation neutron measurement.

The kaon spectrometer at SIS

A double-focussing QD magnetic spectrometer has been developed and installed at the SIS heavy-ion facility at GSI in Darmstadt. The spectrometer's primary purpose is to study meson production in energetic nucleus-nucleus collisions in detail. Its compact design is matched to the requirements of kaon detection with short flight path (5–6.5 m), large solid angle (up to 35 msr), wide momentum acceptance ( p max p min ⋍ 2) , maximum momentum 1.6 GeV/ c (1.9 GeV/ c at reduced solid angle) and reasonable momentum resolution (⋍ 1% without and ⋍ 10 −3 with ray tracing). A focal-plane length of about 1.5 m allows the efficient use of the detectors necessary for particle identification and ray-tracing, involving wire chambers, time-of-flight scintillators and Cherenkov detectors. Collisions can be characterized by two multiple-module plastic-scintillator hodoscopes detecting reaction fragments in the forward hemisphere. While the primary purpose for the construction of the spectrometer is the measurement of kaons, it can serve as a general purpose magnetic spectrometer. Its large solid angle also allows the study of two-particle correlations.

1990 C R Burch prize—joint award: The position-sensitive atom probe

An instrument has been developed which is capable of determining the chemical identity and position of atoms within a volume of order (20 nm) 3 in a material. Called the position-sensitive atom probe (POSAP), the instrument combines time-of-flight mass spectrometry and position-sensitive detection, in the configuration of a short flight path atom probe field-ion microscope. The POSAP makes it possible to reconstruct the three-dimensional nanometre-scale compositional variations in materials, and look at interfaces with sub-nanometre resolution. Apart from the gain in spatial resolution over atom probe analysis in previous designs, the POSAP opens up the possibility of totally new methods of characterisation, based on the morphology of phase microstructure on this scale. The three-dimensional data can also be compared directly with the results of modelling the atomic-scale processes occurring during the ageing of alloys.

Measurements of keV neutron capture cross sections with a 4 pi barium fluoride detector: Examples of 93Nb, 103Rh, and 181Ta.

A new experimental method has been implemented for precise measurements of neutron capture cross sections in the energy range from 3 to 200 keV. Neutrons are produced via the $^{7}\mathrm{Li}$(p,n${)}^{7}$Be reaction using a pulsed 3-MV Van de Graaff accelerator. The neutron energy is determined by the time-of-flight (TOF) technique using flight paths of less than 1 m. Capture events are detected with a 4\ensuremath{\pi} barium fluoride detector. This detector is characterized by a resolution in gamma-ray energy of 14% at 662 keV and 7% at 2.5 MeV, a time resolution of 500 ps, and a peak efficiency of 90% at 1 MeV. Capture events are registered with \ensuremath{\sim}95% probability above a gamma-ray threshold of 2.5 MeV. The combined effect of the relatively short primary flight path, the 10-cm inner radius of the detector sphere, and of the low capture cross section of ${\mathrm{BaF}}_{2}$ shifts the background due to capture of sample scattered neutrons in the scintillator to later TOF and therefore leaves the high-energy portion of the TOF spectrum undisturbed. The high efficiency and good energy resolution for capture gamma-rays yields a further reduction of this background by using only the relevant energy channels for data evaluation. In the first measurements with the new detector, the neutron capture cross sections of $^{93}\mathrm{Nb}$, $^{103}\mathrm{Rh}$, and $^{181}\mathrm{Ta}$ were determined in the energy range from 3 to 200 keV relative to gold as a standard. The cross-section ratios could be determined with overall systematic uncertainties of 0.7 to 0.8%; statistical uncertainties were less than 1% in the energy range from 20 to 100 keV, if the data are combined in 20-keV wide bins. This represents an improvement of factors 5--10 compared to existing experimental methods. The necessary sample masses were of the order of one gram. Maxwellian averaged capture cross sections were calculated in the temperature range relevant for s-process studies. Severe discrepancies were found compared to the data reported in literature.

The Karlsruhe 4π barium fluoride detector

A new experimental approach has been implemented for accurate measurements of neutron capture cross sections in the energy range from 5 to 200 keV. The Karlsruhe 4 π barium fluoride detector consists of 42 crystals shaped as hexagonal and pentagonal truncated pyramids forming a spherical shell with 10 cm inner radius and 15 cm thickness. All crystals are supplied with reflector and photomultiplier, thus representing independent gamma-ray detectors. The energy resolution of the 4 π detector is 14% at 662 keV and 7% at 2.5 MeV gamma-ray energy, the overall time resolution is 500 ps and the peak efficiency 90% at 1 MeV. The detector allows to register capture cascades with 95% probability above a threshold energy of 2.5 MeV. Neutrons are produced via the 7Li(p, n) 7Be reaction using the pulsed proton beam of a Van de Graaff accelerator. A collimated neutron beam is passing through the detector and hits the sample in the centre. The energy of captured neutrons is determined via time of flight, the primary flight path being 77 cm. The combination of short primary flight path, a 10 cm inner radius of the spherical BaF 2 shell, and the low capture cross section of barium allows to discriminate background due to capture of sample-scattered neutrons in the scintillator by time of flight, leaving part of the neutron energy range completely undisturbed. This feature, together with the high efficiency and good energy resolution for capture gamma-rays, allows to separate the various background components reliably enough, that the capture cross section ratio of two isotopes can be determined with an accuracy of ≤ 1.0%. The detector will be used for nuclear astrophysics to investigate the origin of the heavy elements in the slow neutron capture process.

A broad range spectrometer PIK for medium-energy meson spectroscopy

Abstract A broad range spectrometer, PIK, has been constructed for the study of Λ hypernuclei via the ( π + , K + ) reaction. The spectrometer can also be used for nuclear spectroscopy with medium-energy meson beams. It consists of two independent spectrometers: one analyzes the incident beam momentum, and the other determines the momentum of the scattered particle. The latter spectrometer is characterized by a large solid angle of 15–22 msr, a broad range momentum acceptance of 0.55–1.1 GeV/ c , a wide angular acceptance of 11°, and a short flight path length of 4.5 m. The scattered particle is identified by the use of silica aerogel Cherenkov counters and Lucite Cherenkov counters and by measuring its time-of-flight. The performance of the spectrometer is examined by the ( π + , K + ) reaction on a 12 C target at P π = 1.05 GeV/ c and by the ( π + , p) reaction on a CH 2 target.

Tailoring beams for small-angle neutron diffractometers

Small-angle neutron scattering instruments can be built to use either steady-state or time-of-flight techniques, although only the latter are practical at pulsed neutron sources. The techniques used to provide beams of suitable quality, wavelength range and angular collimation are considered in detail for steady-state and time-of-flight instruments at reactor neutron sources, and for time-of-flight instruments at pulsed neutron sources. For both instrument types a cold neutron source provides a definite advantage. Most, but not all, steady-state instruments use long flight paths, which can be shown to provide conditions which are optimum in many ways. However, frame overlap considerations force the use of a short flight path for time-of-flight instruments, and this in turn forces these instruments to use different collimation and beam-quality techniques from those that are usually used for steady-state instruments. Although adequate techniques now exist for building short-flight-path small-angle neutron scattering instruments, some of these short-path techniques are still developing, and can be expected to improve in the future. At present the time-of-flight instruments are more difficult to build and use, but for many experiments this difficulty is more than compensated by the large wave-vector range covered in a single measurement with such instruments.

The use of wire chambers in structural biology

Position-sensitive neutron detectors with high detection efficiency and high spatial resolution are essential for neutron protein crystallography as well as for small-angle neutron scattering on large biological assemblies. Here we report on the performance of two wire chambers recently developed by the Brookhaven Instrumentation Division. Both detectors have a spatial resolution of about 1.3 mm, a detection efficiency of about 80%, and excellent differential and integral linearities. Using small-angle data as an example, we show that the new detectors, furthermore, have excellent temporal stability and can provide difference spectra of unparalleled accuracy. The Biology small-angle spectrometer, because of its location on the constrained reactor floor, has a short secondary flight path of 2 m. High detector resolution is therefore essential to preserve the ability to measure at small angles and to record narrowly spaced diffraction fringes. In addition, we demonstrate in detail that the state-of-the-art detector makes it possible to extend the precision and resolution of a protein single-crystal data set. By measuring beam divergence, wavelength spread, and the crystal mosaic in auxilliary experiments, a set of parameters is obtained for the calculation of a spatial filter for each diffraction spot.

Neutron Capture ins-Wave Resonances of Nickel-64

The neutron capture widths of the s-wave resonances at 13.9 and 33.8 keV in /sup 64/Ni have been determined using a setup with extremely low neutron sensitivity completely different from all previous experiments on this isotope. This feature is important because these resonances exhibit a very large scattering-to-capture ratio. A pulsed 3-MV Van de Graaff accelerator and a kinematically collimated neutron beam, produced via the /sup 7/Li(p,n) reaction, was used in the experiments. Capture gamma rays were observed by three Moxon-Rae detectors with a graphite, a bismuth-graphite, and a bismuth converter, respectively. The samples were positioned at a neutron flight path of only 6 to 8 cm. Thus, events due to capture of resonance-scattered neutrons in the detectors or in surrounding materials are completely discriminated by their additional time of flight. The short flight path and the high neutron flux at the sample position allowed for a signal-to-background ratio of approximately unity even for the broad resonance at 33.8 keV. The data obtained with the individual detectors were corrected for the efficiency of the different converter materials. For that purpose, detailed theoretical calculations of the capture gamma-ray spectra of the measured isotope and of gold, which was used as amore » standard, were performed. The final radiative widths are GAMMA..gamma.. (13.9 keV) = 1.01 + or - 0.07 eV and GAMMA..gamma.. (33.8 keV) = 1.16 + or - 0.08 eV, considerably smaller than the rough estimates obtained in previous work.« less

Detection and measurement of very short flight paths in nuclear emulsions

The application of the nuclear emulsion technique to detect and measure flight paths of unstable particles shorter than a few microns is given. Besides the description of the method of measurement and of the analysis, particular care is paid to the possible sources of biases. It is concluded that the transverse distances of the decay prongs to the primary vertex can be determined with a mean accuracy ∼0.05 μm.

Neutron-gamma discrimination using NaI(Tl) scintillators with application for measurement of the gamma-ray multiplicity distribution in heavy-ion reactions

An intrinsic time resolution of (0.99 ± 0.08) ns fwhm has been measured for commercially available 7.6 cm × 7.6 cm NaI(Tl) scintillator detectors with fast photomultiplier tubes. This time resolution allows almost complete discrimination between neutrons and γ-rays over a short flight path. Application of this technique to the reaction 690 MeV 84Kr + Ag is discussed.

Neutron capture cross section measurements of Nb-93, I-127, Ho-165, Ta-181 and U-238 between 3.2 and 80keV.

The neutron capture cross sections of 93Nb, 127I, 165Ho, 181Ta and 238U were measured from 3.2 to 80 keV using the time-of-flight method. The neutron beam was produced by the KURRI 46-MeV linear accelerator and capture γ-rays were detected by a C6F6 or C6D6 liquid scintillation detector. As the result of the use of a direct-coupled operational preamplifier, the detector recovered rapidly from the γ-flash such that data for higher energy neutrons can be obtained in spite of the short flight path of 11.7 m. The neutron sensitivities of both detectors are compared and the superiority of the C6D6 detector is shown. The experimental results of capture cross sections, corrected for resonance self-shield- ing and multiple scattering and normalized to the 24-keV absolute values, are shown numerically and graphically, and compared with other data. The agreement between the data is fairly good for 165Ho and 238U, but not for 93Nb, 127I and 181Ta. The discrepancy between the data may be caused by the difference of t...

A facility for measuring electro and photo-pion spectra in a high background environment

A QD spectrometer for use in high background (γ, π) experiments is described. The spectrometer has a large solid angle (17 msr) and a short flight path (2.8 m). The experimental techniques employed are described, particularly those used in coping with the high background levels. These include a novel approach to dead time corrections.