A fission fragment detector for correlated fission output studies Mosby, S.; Tovesson, F.; Couture, A. ...
Nuclear instruments & methods in physics research. Section A, Accelerators, spectrometers, detectors and associated equipment,
09/2014, Letnik:
757
Journal Article
Recenzirano
A digital data acquisition system has been combined with a double Frisch gridded ionization chamber for use at both moderated and unmoderated neutron sources at the Los Alamos Neutron Science ...(LANSCE) facility. The high efficiency of the instrument combined with intense LANSCE beams and new acquisition system permits fission output measurements across 11 orders of magnitude incident neutron energy. The acquisition and analysis system is presented along with the first in-beam performance tests of the setup.
The Low Energy Neutron Detector Array (LENDA) is a neutron time-of-flight (TOF) spectrometer developed at the National Superconducting Cyclotron Laboratory (NSCL) for use in inverse kinematics ...experiments with rare isotope beams. Its design has been motivated by the need to study the spin–isospin response of unstable nuclei using (p,n) charge-exchange reactions at intermediate energies (>100MeV/u). It can be used, however, for any reaction study that involves emission of low energy neutrons (150keV to 10MeV). The array consists of 24 plastic scintillator bars and is capable of registering the recoiling neutron energy and angle with high detection efficiency. The neutron energy is determined by the time-of-flight technique, while the position of interaction is deduced using the timing and energy information from the two photomultipliers of each bar. A simple test setup utilizing radioactive sources has been used to characterize the array. Results of test measurements are compared with simulations. A neutron energy threshold of <150keV, an intrinsic time (position) resolution of ∼ 400 ps (∼ 6cm) and an efficiency >20% for neutrons below 4MeV have been obtained.
We report laboratory experiments and modeling calculations investigating the effect of a hydrocarbon coating on ammonia ice spectral signatures. Observational evidence and thermochemical models ...indicate an abundance of ammonia ice clouds in Jupiter's atmosphere. However, spectrally identifiable ammonia ice clouds are found covering less than 1% of Jupiter's atmosphere, notably in areas of strong vertical transport, indicating a short lifetime for the signature of ammonia absorption on condensed ammonia particles Baines, K.H., Carlson, R.W., Kamp, L.W., 2002. Icarus 159, 74–94. Current literature has suggested coating of ammonia ice particles by a hydrocarbon haze as a possible explanation for this paradox. The work presented here supports the inference of a coating effect that can alter or suppress ammonia absorption features. In the experiments, thin films of ammonia ices are deposited in a cryogenic apparatus, coated with hydrocarbons, and characterized by reflection-absorption infrared spectroscopy. We have observed the effects on the ammonia ice absorption features near 3 and 9 μm with coverage by thin layers of hydrocarbons. Modeling calculations of these multilayer thin films assist in the interpretation of the experimental results and reveal the important role of optical interference in altering the aforementioned ammonia spectral features. Mie and
T-matrix scattering calculations demonstrate analogous effects for ammonia ice particles and investigate the relative effects of ammonia ice particle size, shape, and coating layer thickness on the ice particle spectral signatures.
The average Total Kinetic Energy (TKE) release and fission-fragment yields in neutron-induced fission of 235U and 238U was measured using a Frisch-gridded ionization chamber. These observables are ...important nuclear data quantites that are relevant to applications and for informing the next generation of fission models. The measurements were performed a the Los Alamos Neutron Science Center and cover En = 200 keV – 30 MeV. The double-energy (2E) method was used to determine the fission-fragment yields and two methods of correcting for prompt-neutron emission were explored. The results of this study are correlated mass and TKE data.
Charge-exchange reactions at intermediate energies have been used extensively in nuclear structure studies as a sensitive probe of the spin-isospin response of nuclei. Experimental investigations ...have been mostly limited to stable targets. There are, however, compelling scientific arguments to extend these studies to unstable nuclei. The Low Energy Neutron Detection Array (LENDA) is designed to facilitate the study of (p,n) charge exchange reactions in inverse kinematics using unstable beams. The array, which is designed to measure neutron energies and angles with high detection efficiencies, is currently under development at the National Superconducting Cyclotron Laboratory (NSCL). The final array will consist of 24 plastic scintillator bars each with dimensions of 300 times 45 times 25 mm. The neutron energy will be determined by the time-of-flight technique, while the position of interaction will be deduced using the timing and energy information from photomultipliers attached to both ends of each bar. A prototype of the final array has been constructed and characterized in a simple test setup. Results of test measurements and simulations have demonstrated a neutron energy threshold of <130 keV and overall time (position) resolution of les 1 ns (~ 4 cm).
First experimental evidence for a high-spin isomer (25/2+) in 97Cd, a waiting point in the astrophysical rapid proton capture process, is presented. The data were obtained in β-decay studies at NSCL ...using the new RF Fragment Separator system and detecting β-delayed protons and β-delayed γ rays. Decays from ground and isomeric states were disentangled, and proton emission branches were determined for the first time. We find half-lives of 1.10(8) s and 3.8(2) s, and β-delayed proton emission branches of 12(2)% and 25(4)% were deduced for the ground and isomeric states, respectively. With these results, the nuclear data needed to determine an rp-process contribution to the unknown origin of solar 96Ru are in place. When the new data are included in astrophysical rp-process calculations, one finds that an rp-process origin of 96Ru is unlikely.
We present a novel technique for studying the quenching of shell gaps in exotic isotopes. The method is based on extracting Gamow-Teller (ΔL=0, ΔS=1) transition strengths B(GT) to low-lying states ...from charge-exchange reactions at intermediate beam energies. These Gamow-Teller strengths are very sensitive to configuration mixing between cross-shell orbitals, and this technique thus provides an important complement to other tools currently used to study cross-shell mixing. This work focuses on the N=8 shell gap. We populated the ground and 2.24 MeV 0+ states in 12Be using the 12B(1+) (7Li, 7Be) reaction at 80 MeV/u in inverse kinematics. Using the ground-state B(GT) value from β-decay measurements (0.184±0.007) as a calibration, the B(GT) for the transition to the second 0+ state was determined to be 0.214±0.051. Comparing the extracted Gamow-Teller strengths with shell-model calculations, it was determined that the wave functions of the first and second 0+ states in 12Be are composed of 25±5% and 60±5% (0s)4(0p)8 configurations, respectively.
A new technique to measure (p,n) charge-exchange reactions in inverse kinematics at intermediate energies on unstable isotopes was successfully developed and used to study the (56)Ni(p,n) reaction at ...110 MeV/u. Gamow-Teller transition strengths from (56)Ni leading to (56)Cu were obtained and compared with shell-model predictions in the pf shell using the KB3G and GXPF1A interactions. The calculations with the GXPF1A interaction reproduce the experimental strength distribution much better than the calculations that employed the KB3G interaction, indicating deficiencies in the spin-orbit and proton-neutron residual potentials for the latter. The results are important for improving the description of electron-capture rates on nuclei in the iron region, which are important for modeling the late evolution of core-collapse and thermonuclear supernovae.
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