R. Madey

Improved Predictions of Neutron Detection Efficiency for Hydrocarbon Scintillators from 1 MeV to About 300 MeV

Abstract Several improvements have been made to the Monte-Carlo neutron detector efficiency code of Stanton to provide improved agreement with several different detector efficiency measurements.The impovements include a re-adjustment of the inelastic cross sections for neutron-induced reactions on carbon, adoption of new light-response functions, use of relativistic kinematics, and exact determination of light deposited by escaping charged particles. The improved calculations agree with measured efficiences for both plastic and liquid hydrocarbon scintillators for neutron energies from 1 MeV to about 300 MeV and for detector thresholds from about 0.1 MeV to 22 MeV equivalent-electron energies; in most cases the agreement is good to within a few percent.

Measurement of the Neutral Weak Form Factors of the Proton

We have measured the parity-violating electroweak asymmetry in the elastic scattering of polarized electrons from the proton. The kinematic point [{l_angle}{theta}{sub lab }{r_angle}=12.3{degree} and {l_angle}Q{sup 2}{r_angle}=0.48 (GeV /c){sup 2} ] is chosen to provide sensitivity, at a level that is of theoretical interest, to the strange electric form factor G{sup s}{sub E} . The result, A={minus}14.5{plus_minus}2.2 ppm , is consistent with the electroweak standard model and no additional contributions from strange quarks. In particular, the measurement implies G{sup s}{sub E}+0.39G{sup s}{sub M}=0.023 {plus_minus}0.034(stat){plus_minus}0.022( syst){plus_minus}0.026({delta}G{sup n}{sub E}) , where the last uncertainty arises from the estimated uncertainty in the neutron electric form factor. {copyright} {ital 1999} {ital The American Physical Society}

Comparison of adsorption methods for characterizing the microporosity of activated carbons

Adsorption methods for characterizing the microporosity of activated carbons are discussed critically. Three methods—the αs-method and those based on the Dubinin-Radushkevich and Jaroniec-Choma isotherm equations—are compared with respect to the parameters that characterize the microporous structure of a solid. It is shown that the isotherm equations that account for the structural heterogeneity of activated carbons give values of the micropore volume similar to that obtained by the αs-method.

The response of NE-228A, NE-228, NE-224, and NE-102 scintillators to protons from 2.43 to 19.55 MeV

Abstract The response of NE-228A, NE-228, NE-224, and NE-102 scintillators to protons from 2.43 to 19.55 MeV has been measured relative to electrons. The NE-228A scintillator has the same high hydrogen content (CH 2.11 ) as NE-228; but it has a 30% higher light output. Proton recoils in this energy range were obtained by elastically scattering (20.76±0.13) and (26.08±0.06) MeV neutrons from protons in the scintillators through angles of 20°, 30°, 45°, and 60°. The neutrons were obtained from the T(d, n)α reaction. The measured response (in units of electron energy) for protons above 5 MeV in NE- 102 is about 8% higher than that assumed in popular computer programs for calculating the neutron detection efficiency of plastic scintillators. The response of NE-228A is equivalent to that of NE-228. The response of NE-224 differs from that of either NE-102 or NE-228.

Thermodynamics of gas adsorption on fractal surfaces of heterogeneous microporous solids

Thermodynamic equations are derived for physical adsorption of gases and vapors on fractal surfaces of heterogeneous microporous solids. The influence of surface geometry on gas adsorption on a microporous solid is studied by analyzing the dependence of the differential molar enthalpy ΔH, the immersion enthalpy ΔHim, and the differential molar entropy ΔS on the fractal dimension D, which is used to characterize the surface irregularity. It is shown that −ΔH, −ΔHim, −ΔS, and the average adsorption potential A increase as the fractal dimension of the surface accessible for adsorption increases. The dependence of these thermodynamic quantities on the fractal dimension D is attributed to the fact that the fraction of small micropores in microporous solids increases as the fractal dimension D increases.

Gas-Solid Chromatography of Methane-Helium Mixtures: Transmission of a Step Increase in the Concentration of Methane through an Activated Carbon Adsorber Bed at 25°C

Abstract Time-dependent transmission or “breakthrough” curves of methane in helium flowing through an activated carbon adsorber bed were measured for methane concentrations between 34 and 105 ppm, and for mixture flow rates between 0.69 and 6.64 cm/s. The transmission is the ratio of the outlet concentration to the inlet concentration. The experimental transmission curves for a step-function increase in the methane concentration are compared with the predictions from a model which assumes a linear adsorption isotherm and equilibrium between the gas and solid phases. These two basic assumptions are discussed in detail. The data show that the two assumptions hold within the concentration and flow rate regions of this study. Effective diffusion coefficients of methane were calculated from the transmission data and found to increase with increasing flow rates.

Large volume neutron detectors with subnanosecond time dispersions

Abstract Design criteria and performance characteristics are described for large-volume (13.1 1 to 118 1) mean-timed plastic-scintillator detectors for neutrons from ≈2 MeV to 2 GeV. Detectors were constructed in six sizes (all 0.102 m thick) from 0.126 m × 1.016 m × 0.102 m to 0.762 m × 1.524 m × 0.102 m. Overall energy resolutions of 230 keV for 80 MeV neutrons, 320 keV for 133 MeV neutrons, and 440 keV for 157 MeV neutrons were achieved in time-of-flight experiments with flight paths of 76, 68, and 91 m, respectively. The detectors have pulse-height responses uniform to ±5% except close to the ends, position resolutions less than 5 cm, and intrinsic time dispersions less than 400 ps. Typical neutron detection efficiencies are 20% for 20 MeV neutrons at a pulse-height threshold of 2 MeV equivalent-electron energy (MeV ee) and 3.5% for 200 MeV neutrons at a pulse-height threshold of 50 MeV ee.

A time-of-flight spectrometer for neutrons from 1 to 500 MeV☆

Abstract A self-contained time-of-flight spectrometer has been developed for neutrons from 1 to about 500 MeV. The spectrometer consists basically of two scintillation counters. The principle of the spectrometer requires an incident neutron to scatter elastically from a proton in the first scintillator, travel over a known flight-path, and interact in the second scintillator. The time interval between the scintillation pulses in the two detectors is the time-of-flight of the scattered neutron. Kinematically, the kinetic energy of the incident neutron is specified in terms of the scattering angle, the flight-path, and the flight-time of the scattered neutron. The intrinsic time dispersion of the system is 2.2 ns (fwhm) with a 2 1 2 ″ diam. by 2 1 2 ″ high first scintillator and a 9″ diam. by 8″ thick second scintillator. For a mean flight-path of 4 m between these two scintillators, the energy resolution of the spectrometer varies from about 7 to 20% and the spectrometer efficiency varies from about 2×10−5 to 8×10−7 over the range of energies from 10 to 500 MeV. The background from interactions of neutrons above about 15 MeV with carbon nuclei in the first scintillator is subtracted by measuring the spectrum twice with first scintillators differing significantly in their relative hydrogen-carbon composition. Measurements with monoenergetic neutrons at 14 MeV confirm the resolution and efficiency of the spectrometer at low energies. Measurements with monoenergetic neutrons at 220 MeV verify the resolution of the spectrometer at this energy and demonstrate the technique for subtracting the carbon background. Recently, we have measured neutron spectra from 740 MeV proton bombardment of a 30 cm thick uranium target over the energy range from 1 to 500 MeV. In each of these measurements, the count rate of real events was typically a count/s, and the chance coincidence count rate was of the same order of magnitude.

Induced Proton Polarization for pi0 Electroproduction at Q2 = 0.126 GeV2/c2 Around the Delta(1232) Resonance

We present a measurement of the induced proton polarization P{sub n} in {pi}{sup 0} electroproduction on the proton around the {Delta} resonance. The measurement was made at a central invariant mass and a squared four-momentum transfer of W = 1231 MeV and Q{sup 2} = 0.126 GeV{sup 2}/c{sup 2}, respectively. We measured a large induced polarization, P{sub n} = -0.397 {+-} 0.055 {+-} 0.009. The data suggest that the scalar background is larger than expected from a recent effective Hamiltonian model.

Gamow-Teller and spin-dipole strength in the 40,48Ca(p,n) reactions at 135 MeV

Abstract Spin-flip probabilities for 48Ca( p n )48Sc reveal that at 0° the apparent continuum under and adjacent to the Gamow-Teller giant resonance is also primarily 1+ strength. A comparison of 40Ca[ p n )40Sc and 48Ca( p n )48Sc shows no discernable signature of Gamow-Teller strength in the region −30 > Q(MeV) >−45. The spin-flip component of the dipole resonance for 40Ca is broader than the non-spin-flip component.