Search

A multiline neutron source can be produced by using a metallic 232Th filter in conjunction with a white neutron source. The multiline spectrum consists of ∼20 relatively strong intensity lines ranging from 10 to 4000 eV. It is shown that the width of each neutron line of the spectrum is strongly dependent on the absorber thickness. This neutron source is useful for accurate cross section measurements with precise neutron energies. The optimum thickness of the 232Th absorber, which was found to yield a sharp multiline spectrum throughout the above energy range, was found to be ∼14 cm.

The STM (Scanning Tunneling Microscope) has been used for characterizing the microstructure of various steel samples by utilizing its surface imaging properties. The subgrain sizes of 15 different samples of steel (in the range 20 nm to 250 nm) were studied by the STM and compared with the results obtained using X-ray diffraction (XRD) by employing line profile analysis of the Bragg peaks. Good agreement was observed between the two sets of data of subgrain sizes. This work establishes STM as a useful characterization tool for studies in metallurgy and metal physics.

Nuclear resonance fluorescence excited with continuous electron bremsstrahlung from the 4.3 MV Stuttgart Dynamitron accelerator is used as a nondestructive method to determine the 13C content x of bulk isotopic diamonds (12C1−x13Cx). The smallest detectable amount of 13C in carbon or low Z matrices is on the order of 0.5 mg. The relative accuracy of absolute mass determinations is about ±7%. Errors are mainly due to uncertainties in the natural widths Γ of the 13C nuclear levels at 3089 and 3684 keV used in the measurements. The results confirm a previous calibration which is based on Raman scattering and the destructive determination of x by mass spectroscopy.

A search for an anomalous n‐p scattering from a polyethylene sample (CH2) at 8 final energies between 64 eV and 2.5 keV was carried out. The scattering intensities were compared to that from a graphite (C) sample. The results were found to confirm our previous n‐p results on H2O at a final energy of 24.3 keV where no n‐p scattering anomaly was observed. The present results refute all proposed models attempting to explain the occurrence of any n‐p scattering anomaly at keV neutron energies.

We measured the neutron scattering intensities from pure liquid H2O relative to that of pure D2O and also relative to H2O-D2O mixtures, at room temperature. This study is relevant to the problem of quantum entanglement. The neutrons were generated from an electron Linac and the final energy of the scattered neutrons was fixed at 24.3 keV using a 20 cm thick pure iron filter. The scattering intensity ratios were found to agree with expected values deduced from the tabulated total cross sections within an accuracy of 3%. Thus no anomaly was observed.

A review of recent studies using the nuclear resonance photon scattering (NRPS) from 15N occurring in several gases is presented. In particular, new results on the adsorption and molecular orientation of N2, N2O and NO2 on graphite are given and compared with theoretical calculations using molecular dynamics simulations (MDS). In another application, we studied the scattering of photons from NO2 occurring in solid NaNO2. The results were compared with those of adsorbed NO2 on graphite. A huge difference in the scattering cross section of the two systems was observed and explained.

The decay of /sup 80/Zn to levels in /sup 80/Ga has been studied from mass-separated activity produced in the thermal neutron fission of /sup 235/U. Based on ..gamma.. singles and coincidence measurements, 26 ..gamma.. rays were placed in a level scheme for /sup 80/Ga with 13 excited states up to 2655 keV. The emission of a ..gamma.. ray resulting from delayed neutron emission of /sup 80/Ga is also reported. The structure of the /sup 80/Ga level scheme is discussed. The properties of /sup 80/Zn and /sup 80/Ga reported here are of astrophysical interest for modeling the r process.

A half-life of 0.55 +- 0.02 s was measured for the neutron-rich fission-product nucleus /sub 30//sup 80/Zn/sub 50/. A Q/sub ..beta../ value for the decay of /sup 80/Zn of 7.15 +- 0.15 MeV and a level scheme for /sup 80/Ga have also been deduced. The properties of this N = 50 waiting-point nucleus are significant for the evaluation of different models of r-process environments and exposure times.

Resonant photon scattering from /sup 39/K has been studied with bremsstrahlung produced by electrons with end-point energies of 6.6, 8.5, and 9.4 MeV. Values of gGAMMA/sub 0//sup 2/GAMMA for 24 levels below 9 MeV were measured and the gGAMMA/sub 0/ values of 17 levels were deduced. The widths of some levels above 4 MeV were found to be much larger (on the average, more than a factor of 10) than those reported from experiments using the Doppler shift attenuation method. Three of these levels are known from studies of the (d,/sup 3/He) reaction to carry most of the d/sub 5/2/sup -1/ proton strength in /sup 39/K. The B(M1up-arrow) strength of J = (52/sup +/ levels below 7 MeV is compared with predictions based on simple single-particle shell-model wave functions coupled with free-nucleon and empirically renormalized M1 operators and weighted by single-proton pickup spectroscopic factors.

Levels at 6605 keV in 48Ti and 7362 keV in 68Zn were photoexcited using neutron-capture gamma rays emitted by V(n, gamma ) and Cr(N, gamma ) reactions respectively. The widths and other spectroscopic properties of these levels were measured. The possibility of utilising these resonance scattering events for solid-state studies are discussed in detail.

Differential cross sections for elastic and Raman scattering of photons from 232 Th and 238 U targets were measured. Eight photon energies in the range 7.9–11.4 MeV were used and were obtained from thermal neutron capture in Fe, Cr, Cu and Ni. The angular distribution of the elastic and Raman scattered radiation from 232 Th was measured. The results are compared with calculations of the simple rotator model and the dynamical collective model of the giant dipole resonance after incorporating the effect of Delbruck scattering.

The widths of nine energy levels in $^{11}\mathrm{B}$ at 2.125, 4.445, 5.021, 6.743, 6.793, 7.286, 7.978, 8.559, and 8.920 MeV were determined by carrying out self-absorption and photon scattering measurements. The temperature variation of the scattering cross section was measured at $T=298$ K and $T=730$ K to study in some detail the effective temperature ${T}_{e}$ of $^{11}\mathrm{B}$ in ${\mathrm{B}}_{4}$C, BN, and elemental B. The influence of ${T}_{e}$ on the deduced values of the radiative widths of $^{11}\mathrm{B}$ is discussed.

Nuclear resonance scattering of photons from 15 N has been used to study the orientation of the HNO 3 intercalant molecule with respect to the graphite planes. In the second-stage compound C 10 (HNO 3 ) and the temperature range T = 15°K − 300°, no orientational phase transition was observed and the NO 3 molecular plane was found to be oriented at an angle θ ∽ 82 ± 8° with respect to the graphite planes.

Resonance fluorescence has been used for measuring the widths of over 42 levels in /sup 40/Ar below 11 MeV by employing Ge(Li) detectors and bremsstrahlung beams with end point energies of 8.5, 10.3, and 11.8 MeV. The gGAMMA/sub 0//sup 2/GAMMA of these levels were measured, and several J = 1 levels were identified. A strong photon strength was found, extending between 8 and 11 MeV which complements a bump observed between 10.4 and 12.4 MeV using the (..gamma..,n) reaction. This behavior is theoretically explained using the open-shell-linear-response method.

Resonance fluorescence is used for measuring the widths of 13 levels in /sup 40/Ca below 10.4 MeV among which nine E2 transitions were identified. The total E2 strength of those nine levels together with some other known 2/sup +/ levels below 10 MeV exhausts approx.22% of the energy weighted sum rule. The partial and total widths of the strong M1 state at 10.32 MeV are compared with theory.

A high-resolution gamma monochromator having a resolution of ≁10 −6 is described. The variable-energy γ source is obtained by nuclear resonance scattering, of neutron-capture γ rays, through various scattering angles. Several possible examples of combinations of γ sources and resonance scatterers are discussed. In particular, a lead target was employed to scatter the 7.28 MeV γ line of neutron-capture γ rays of iron. Variation of the angle of the resonantly scattered 7.28 MeV photons between 60°–150° permits an energy scan of 400 eV in any absorber. Thus, nuclear energy levels in some absorbers were photoexcited and the corresponding ground-state widths were extracted from the measured absorption spectrum. The results for the case of a Ce absorber are discussed in detail.

Nuclear resonance fluorescence from energy levels in /sup 27/Al lying between 2 and 10 MeV excitation energy has been measured with bremsstrahlung beams of end-point energies of 7.66 and 10.37 MeV. Values of gGAMMA/sub 0/ and gGAMMA/sub 0/GAMMA/sub ..gamma..//GAMMA for 39 levels below 10 MeV were obtained from scattering cross section and self-absorption measurements. The measurements give an upper bound of total B(M1up-arrow) below 7.7 MeV excitation which is about 25% less than the predictions of a large-scale shell model calculation based on the free-nucleon M1 operator. The same shell model wave functions combined with an effective M1 operator based on moments and transitions between low-lying states is in good agreement with the present experimental results.

A bremsstrahlung photon beam from electrons with end-point energies of 7.66 and 10.37 MeV was used for photoexciting levels in /sup 23/Na. Values of gGAMMA/sub 0//sup 2//GAMMA and gGAMMA/sub 0/ of 22 levels below 10 MeV were obtained using scattering and self-absorption measurements. The values for 7 levels are reported for the first time. The temperature variation of the scattering cross section was measured at T = 300 K and T = 78 K to study in some detail the effective temperature T/sub e/ of metallic /sup 23/Na. The results of the total B(M1up-arrow) strength of T = 1/2 levels below 9 MeV were found to be in excellent agreement with the large-scale shell-model calculations.

Strong correlations between the measured strengths of excitations in the (γ, γ′) and (d, τ) reactions to the d 5 2 −1 proton states in 39K and to the p 3 2 −1 proton states in 15N were observed. The results are discussed in terms of the analogies between the two reactions.

The ground-state radiative widths of nine levels below 10.2 MeV in /sup 15/N were measured using resonance fluorescence. The results are compared with various theoretical predictions and are found to best agree with very recent unpublished calculations in which the full 1h..omega.. configurational basis is included in constructing the wave functions of the /sup 15/N levels.

A nuclear resonance photon scattering (NRPS) study of the out-of-plane orientation of nitric oxide (NO) physisorbed onto basal planes of graphite was made as a function of temperature T and the coverage n. At T = 12 K and n ⩽ 1, no configuration was observed in which all the NO molecules are oriented parallel to the graphite surface. Oblique or rectangular dimers, and monomers in which the average tilt angle of the NO molecular axes is at least 21° were found to yield a good agreement with the measured data. The precise nature of the configurations or the arrangement of the adsorbed NO molecules remain unknown.

A Compton polarimeter for measuring the linear polarization of resonantly scattered neutron-capture γ-rays in the energy region 4–9 MeV is described. The performance of this system as a function of photon energy for perfectly polarized radiation is given. It is shown that reliable information regarding the parities of the resonance levels and the quadrupole-dipole mixing ratios of high energy transitions may be obtained from these measurements.

The gamma radiation following the decay of 23 Ne produced by 22 Ne(n, γ) reaction has been studied using Ge(Li) detectors. No indications were found for levels at 2.285, 2.405 and 2.870 MeV excitation reported by Lancman et al .

The elastic scattering cross section of 7.9 MeV photons by U and Th was measured in the angular range 25°–140°. It is shown that at this energy the forward elastic scattering is due almost entirely to Delbruck scattering. The results are systematically lower by > 50% than the calculated values. A good agreement with theory was obtained only after excluding the contribution of the real part of the Delbruck scattering amplitude.

Gamma transitions and branching ratios are calculated for 18 O excited levels using different sets of shell-model wave functions having deformed state admixtures reported by Federman-Talmi, by Benson-Irvine and by Federman. The neutron and proton effective charges are allowed to vary and the results are compared with recent experimental measurements on the decay of 18 O levels. The results obtained using Benson-Irvine wave functions were found to give better agreement with experiment.

The capture $\ensuremath{\gamma}$-ray spectrum resulting from thermal-neutron capture in $^{46}\mathrm{Ti}$ has been measured using a 30-cc Ge(Li) detector. Some 19 new $\ensuremath{\gamma}$ lines corresponding to transitions between levels of $^{47}\mathrm{Ti}$ were observed. The neutron separation energy of $^{47}\mathrm{Ti}$ was found to be 8884\ifmmode\pm\else\textpm\fi{}7 keV. Angular correlation measurements were carried out, using NaI(Tl) detectors, and the spins of the following levels in $^{47}\mathrm{Ti}$ were obtained: $1.556 (\frac{3}{2})$, $1.798 (\frac{3}{2})$, $2.556 (\frac{3}{2})$, $3.685 (\frac{1}{2} or \frac{3}{2})$, and 3.925 MeV ($\frac{3}{2}$ or \textonehalf{}). Some of these results disagree with those obtained by Lee and Schiffer, who established the spins according to the $J$-dependence rule for the angular distributions in ($d,p$) reactions. Some indication of a direct capture mechanism for the ($n,\ensuremath{\gamma}$) reaction process is presented. The role of the direct capture mechanism in the $^{6}\mathrm{Ti}(n,\ensuremath{\gamma})$ reaction is discussed.

The energy and angular distributions of $\ensuremath{\alpha}$ particles emitted in thermal-neutron fission of $^{235}\mathrm{U}$ have been studied in a three-parameter correlation experiment for $\ensuremath{\alpha}$ angles between 50 and 130\ifmmode^\circ\else\textdegree\fi{}. These distributions are compared with the results of trajectory calculations to get a "best fit" set of initial scission conditions. The derived initial fragment separation of 24 fm and initial average kinetic energy of 31 MeV are in good agreement with liquid-drop calculations, while the initial most probable $\ensuremath{\alpha}$ energy was found to be 3.4 MeV. The results indicate a shift in the scission point of 1-2 fm towards the light fragment, for a variation in mass ratio between 1.1 and 2. The effect of $\ensuremath{\alpha}$ recoil on the experimental distributions is discussed in detail.

A comparative study of the Li6(d, p)Li7 and Li6(d, n)Be7 reactions has been made in order to serve as a critical test of the principle of charge symmetry in nuclear reactions. The ground and first excited states of Li7 and Be7 are believed to be pure isobaric spin doublets. The need has been eliminated for substantial Coulomb and phase space corrections, since the ratios of the ground to first excited state cross-sections are compared. The observed equality of the neutron and proton results over the bombarding energy range 0.4-3.2 MeV may be taken as a striking and model independent confirmation of the principle of charge symmetry. An interesting anomaly in the angular distribution results was observed which does not seem explainable in ordinary distorted wave stripping theory.

By using tritons from pile irradiated Li 6 , the reaction O 17 (t, p)O 19 was observed. The thick target yield at E t ≦2.73 MeV was measured. The effective cross section of the reaction was evaluated and found to be 34±7mb.

Elastic and inelastic nuclear resonant scattering of monochromatic photons from $^{75}\mathrm{As}$ have been studied using a 47-cc Ge (Li) detector. The $\ensuremath{\gamma}$ source was provided by thermal-neutron capture in iron. The energy of the resonance level in $^{75}\mathrm{As}$ was found to be 7.646 MeV. Assuming the high-energy lines to be primary transitions deexciting the resonance level, 25 energy levels were found from the ground state up to 2.6 MeV, seven of which may be identified with recently reported levels. By measuring the angular distribution of the scattered radiation, the spin of the scattering level was determined to be \textonehalf{} and the spins of 14 low-lying levels were thus found to be either \textonehalf{} or $\frac{3}{2}$. The total radiative width of the resonance level was determined and was found to be $\ensuremath{\Gamma}=0.36\ifmmode\pm\else\textpm\fi{}0.10$ eV and $\frac{{\ensuremath{\Gamma}}_{0}}{\ensuremath{\Gamma}}=0.11$. The $^{75}\mathrm{As}$ levels are compared with the predictions of the Coriolis coupling model.

A new reaction, 59Ni(n,γ)Ni60, producing the highest energy of all known useful capture γ-rays is utilised for photon scattering experiments in the energy region of the giant dipole resonance. The γ line energies of the ground state transitions of the 59Ni(n, γ), 61Ni(n,γ) and 63Ni(n, γ) reactions were measured and found to be 11387.5 ± 1.7 keV, 10596.2 ± 3.7 keV and 9657.4 ± 2.8 keV respectively. A comparison is made of the intensity of the 11387.5 keV γ line with that obtained from other (n, γ) sources.

Quasi-monochromatic photons in the range E γ = 14.4−16.6 MeV have been used to study inelastic scattering to the 2 + , γ -vibrational band head in 166 Er. The results for transitions to this branch are drastically smaller (by a factor ≈3–5) than the predictions of the dynamic collective model (DCM).

The dependence of resonance scattered photon intensities on the molecular orientation of the target was observed using bremsstrahlung photons. This was achieved in a self-absorption measurement by employing a 11 B target in the form of pyrolytic boron nitride (PBN).

The radiative widths of two $^{16}\mathrm{O}$ levels were determined using photon scattering and self-absorption measurements. The results were ${\ensuremath{\Gamma}}_{0}$(6.917)=0.094\ifmmode\pm\else\textpm\fi{}0.004 eV and ${\ensuremath{\Gamma}}_{0}$(7.117)=0.054\ifmmode\pm\else\textpm\fi{}0.004 eV. The effective temperature ${T}_{e}$ of the O atom in a water target is calculated by accounting for the effect of the zero-point molecular vibrations of the atom. The influence of this new value of ${T}_{e}$ on the Doppler broadening of the nuclear level and hence on the resulting value of ${\ensuremath{\Gamma}}_{0}$ as deduced from a self-absorption measurement is discussed.

Neutron angular distributions around θ = 90° from the 206 Pb (γ, n ) reaction were measured using n-capture γ-rays, E γ = 8.5–11.4 MeV, and a high-resolution 3 He spectrometer. Pronounced asymmetry around θ = 90° was observed indicating the existence of E1-E2 and possibly E1-M1 interference effects.