Die Energieverteilung der Neutronen aus einer Po-α-Be-Quelle

The neutrons from a Po-α-Be source were detected via the recoil protons in a stilbene scintillator. The pulse height spectrum of the scintillations produced in the stilbene crystal by the recoil protons is analysed to yield the incident neutron energy distribution. Theγ-ray background was eliminated through pulse shape discrimination. Separately the spectrum of neutrons coincident withγ-rays, i.e. of neutrons from the Be⁹(α,n)C^12* reaction, has been determined. The resulting spectra are compared to previous measurements. The structure of the spectra is not compatible with an isotropic angular distribution of the neutrons relative to theα-direction. Furthermore it could be shown that the fraction of reactions leading to the 7,65 MeV state of C¹² is only about 10^?3.     

Neutron and high-energy gamma-ray emission in 12C Induced reactions on 159Tb

The gamma-ray multiplicity associated with the emission of high-energy gamma rays and neutrons following ¹²C+¹⁵⁹Tb reactions at E_lab= 100 MeV has been measured. The spin decrease deduced for coincident gamma rays within the GDR energy region is found to be in agreement with an estimate based on a statistical model. Neutron spectra measured in coincidence with high-energy gamma rays are explained in terms of two components. The cooler component corresponds to neutrons evaporated from the compound nucleus. The hotter component emanates from a precompound source moving in the center-of-mass system along the beam direction. The multiplicity of gamma-ray cascades, coincident with neutrons from the two different sources, has been deduced. The results show that beside the GDR there appears to be a second source of gamma rays with E_γ≈10MeV. These gamma rays are emitted in more peripheral reactions and before neutron evaporation.     

In the wonderland of ultra-parallel neutron beams

Bragg reflections from single crystals yield angular widths of a few arcsec for thermal neutron beams. The Bonse-Hart proposal to attain a sharp, nearly rectangular profile by Bragg reflecting neutrons multiply from a channel-cut single crystal, was realized in its totality three and a half decades later by achieving the corresponding Darwin reflection curves for 5.23 Å neutrons. This facilitated SUSANS (Super USANS) measurements in the Q ～ 10^?5 Å^?1 range. The polarized neutron option was introduced into the SUSANS set-up by separating the up- and down-spin neutron beams by ～10 arcsec with a magnetic (air) prism. The neutron angular width has recently been reduced further by an order of magnitude to ～0.6 arcsec by diffracting 5.3 Å neutrons from a judiciously optimized Bragg prism. This constitutes the most parallel monochromatic neutron beam produced to date. I present the first SUSANS spectra probing the Q ～ 10^?6 Å^?1 domain, recorded with this beam.     

Study of Ξ− and Ω− production from K−n and K−p interactions at 4.93 GeV/c

Cross sections, mass spectra, angular distributions and several other features of Ξ^? and Ω^? production from photons and neutrons in K^?d interactions at 4.93 GeV/c are presented. A five standard deviation peak is observed in ΞKπ mass spectra near 2.6 GeV/c². A sharp rise in the Ξ^? three- and four-body cross sections from K^?n interactions suggests direct-channel Σ¹ (3.0–3.5) formation.     

High frequency waves in liquid metals

The inelastic scattering of long wavelength neutrons from coherently scattering metals has been used to examine the collective motions of atoms in the liquid state and to make comparisons with the polycrystalline solids. Well-defined peaks in the velocity spectra of the scattered neutrons have been found in both states for lead, rubidium, tin, bismuth and aluminium. These have been used to present parts of the dispersion curves for vibrational modes of motion in the frequency range 1 to 20 × 10¹² rad/sec and wave number range 1 to 3 Å^?1, and to estimate lifetimes of the modes of the order 0.4 × 10^?12 sec in the solid near the melting point and roughly half these values in the liquid.

In polycrystalline aluminium the major contribution to the observed intensity in the solid is due to transverse modes and the observed similarity of the liquid and solid spectra suggests that transverse motions are observed in the liquid also.     

On the search for density isomers in interaction of relativistic12C nuclei with lead

A Pb target was bombarded with¹²C nuclei of an energy of 15 and 43 GeV. The target was periodically transported to the inside of the neutron multiplicity detector which allowed measuring time distributions for events of multiple emission of delayed neutrons. Abnormal neutron emitters have not been found. Upper limits of probabilities of their production were estimated for the half-life range 10–10⁵ s and for different mean multiplicities of neutrons¯n. The values obtained are within the interval from 1.4·10^?9 (for¯n=12)to 4·10^?6(for¯n=2).     

Excitation and neutron decay of xenon studied by forward-angle scattering of high-energy 17O ions

Excitation energy spectra and neutron decay of xenon have been measured in extreme forward angle scattering of 250A and 400A MeV ¹⁷O ions, using one quadrant of the CELSIUS storage ring as a magnetic spectrometer. The observed excitation energy spectrum, ranging from 12 to 36 MeV, has been compared with Coulomb cross section calculations. Neutron and missing energy spectra are compared with statistical decay calculations using the code CASCADE. In addition to statistical decay from giant resonances and other continuum excitations, a large fraction of forward peaked fast neutrons was observed throughout the entire excitation energy region. A possible reaction mechanism behind these fast neutrons is discussed.     

On spectral temperatures of negative pions produced in d 12C, 4He12C, and 12C12C collisions at 4.2 A GeV/c

The experimental transverse momentum distributions of negative pions produced in d ¹²C, ⁴He¹²C, and ¹²C¹²C collisions at 4.2 A GeV/c were analyzed in the framework of Hagedorn Thermodynamic Model. The spectral temperatures of π ^? mesons as well as their relative contributions to the total multiplicity of π ^? mesons were extracted from fitting the p _t spectra by two-temperature Hagedorn function. The results were compared systematically with the earlier results obtained from analysis of non-invariant center-of-mass energy spectra of negative pions produced in the analyzed collisions.     

Charged particle emission following the absorption of stopped π− in 12C, 59Co and 197Au

Energy spectra of charged particles (p, d, t, ³He, α) emitted following the absorption of stopped π^? in ¹²C, ⁵⁹Co and ¹⁹⁷Au have been measured. The results are compared with results from neutron spectroscopy and discussed in the context of the quasi-deuteron model of π^? absorption.     

Proton-neutron correlation in the deuteron breakup at 56 MeV and prior-form DWBA analysis

Proton-neutron angular correlations in the ¹²C, ⁵¹V and ¹¹⁸Sn(d, pn) reactions have been measured at 56 MeV to investigate the deuteron breakup process. The elastic breakup which leaves the target nucleus in its ground state dominates the coincident spectra. The elastic breakup cross sections are estimated to be 36–48% of the inclusive breakup yields at 15° or 17.5°. In the angular correlations the protons are emitted predominantly on the side of the beam opposite to the neutrons. The experimental data have been analyzed using the prior-form DWBA. For both nuclear and Coulomb breakup, sufficient convergence of the calculations is obtained by including the pn angular momenta up to l = 2. For the nuclear breakup calculations, the l = 0 and 2 contributions dominate the cross sections. For the Coulomb breakup the l = 1 contribution is predominant. In the calculations the effect of the Coulomb breakup is seen at forward angles of the angular correlation. The DWBA calculations reproduce fairly well the coincident energy spectra and the angular correlations in the angular region where the protons are emitted on the side of the beam opposite to the neutrons. On the other hand the calculations overestimate the break-up cross sections by a factor of 2 to 10 in the angular region where the protons are emitted on the same side of the beam as the neutrons. The distributions of deuteron c.m. angular momenta that contribute to the breakup amplitude are examined to obtain information on the region of space in which the breakup reactions takes place.     

Spectral temperatures of Δ0(1232) resonances produced in p 12C and d 12C collisions at 4.2 GeV/c per nucleon

The reconstructed experimental transverse momentum (p _t ) distributions of Δ⁰(1232) resonances produced in p ¹²C and d ¹²C collisions at 4.2 A GeV/c and the corresponding spectra calculated using Modified FRITIOF model were analyzed in the framework of Hagedorn Thermodynamic Model. The spectral temperatures of Δ⁰(1232) resonances were extracted from fitting their p _t spectra with one-temperature Hagedorn function. The extracted spectral temperatures of Δ⁰(1232) were compared with the corresponding temperatures of π ^? mesons in p ¹²C and d ¹²C collisions at 4.2 A GeV/c obtained similarly from fitting the p _t spectra of π ^? by one-temperature Hagedorn function. The spectral temperatures of Δ⁰(1232) resonances agreed within uncertainties with the corresponding temperatures of π ^? mesons produced in p ¹²C and d ¹²C collisions at 4.2 A GeV/c.     

Spin-flip probability in the inelastic scattering of 16.9 MeV neutrons from 12C

The neutron spin-flip probability S(θ) for inelastic scattering of unpolarized 16.9 MeV neutrons to the 4.44 MeV state of ¹²C has been determined by measuring the absolute directional correlation between the scattered neutrons and the subsequent E2 deexcitation γ- radiation emitted perpendicular to the scattering plane. Time-of-flight techniques with carbon recoil detection in a plastic scintillator were used to separate elastically and inelastically scattered neutrons. The neutron spin-flip data were found to be in close agreement with 20.0 MeV proton spin-flip results. Comparison of the measurement is made with the predictions of a microscopic antisymmetrized distorted wave calculation in which the direct reaction mechanism is supplemented by a two step resonance contribution. This theoretical analysis reveals the presence of a strong quadrupole resonance at 20.5 MeV excitation in ¹²C. The results are also influenced to a lesser extent by the E1 giant dipole resonance.     

Alpha induced reactions

A comprehensive model forα particle induced reactions, recently proposed by two of the authors, has been improved and utilised in the analysis of proton andα particle spectra, angular distributions, excitation functions, spectra of neutrons emitted in the decay of continuum states on nuclei with 50<A<230 atα energies varying from 50 to 160 MeV. A unique set of average parameters has been used in all the calculations. Most of them were the same utilised in previous analyses of nucleon andπ ^? induced reactions. The energy and mass dependence (which turns out to be quite weak) of the additional parameters which are needed to describe theα particle induced reactions has been determined. The comparison with experimental data shows that the model discussed allows one to reproduce successfully the cross sections ofα particle induced reactions with an accuracy comparable to the one already gained in the analysis of nucleon andπ ^? reactions with the Exciton Model.     

Absorption of stopped π− in 59Co, 75As, 197Au and 209Bi investigated by in-beam and activation γ-ray spectroscopy

In-beam pionic X-ray and γ-ray spectra and activation γ-ray spectra following π^? absorption at rest in ⁵⁹Co, ⁷⁵As, ¹⁹⁷Au and ²⁰⁹Bi have been measured. The isotopic yield distributions and thus the (π^?, xnγp) reaction yields were determined. The mean numbers of emitted neutrons and protons are, respectively, 4.5 ± 0.2 and 1.48 ± 0.16 for ⁵⁹Co, 5.1 ± 0.3 and 1.02 ± 0.14 for ⁷⁵As, 6.6 ± 0.4 and 0.32 ± 0.14 for ¹⁹⁷Au, and 6.8 ± 0.4 and 0.37 ± 0.16 for ²⁰⁹Bi. The experimental results are compared with a pre-equilibrium exciton model calculation and proton-induced spallation reactions. High spin states in the residual nuclei were observed. For pionic X-rays absolute intensities of various transitions are given.     

Recoil distance lifetime measurements in82Kr

The lifetimes τ=124±12, 6 _?2 ⁺⁴ and 380±100 ps of theE _x (I ^π )=3.46(8⁺), 2.92(6⁺) and 3.04(6^?) MeV states, respectively, populated by the reaction⁷⁶Ge(¹²C,α2n) were measured with the recoil distance method. In addition upper lifetime limits were obtained for nine states. The measured lifetimes and energies indicate a band crossing at aboutI ^π =8⁺, probably arising from the alignment of twog _9/2 neutrons. For the 3.04 MeV 6^? state as a second member of a band built on the 2.65 MeV 4^? state the measured lifetime points to a two-quasiparticle configuration. The positive-parity states have been discussed in the frame of the interacting boson approximation, nuclear field theory and the cranked shell model.     

Removal of 1p and 1s nucleons from 6,7Li

Deuteron spectra have been measured for the reactions ^6,7Li(p,d) in the separation energy range E_s= 0 ? 50 MeV. Two groups of peaks appeared which were identified as due to pickup of 1s and 1p neutrons on the basis of the extracted spectroscopic factors and the comparison made with (p, 2p) data. The measured (1s)^?1 spectral functions were found to change with the momentum transfer of the reaction.     

Pion production in 3He-nucleus interactions at 910 MeV

The (³He, π^±X) reaction has been studied at 910 MeV using the ³He⁺⁺ beam of the CERN SC. Complete π^? spectra on CH₂, CD₂, ⁹Be, ¹²C, ²⁷Al, Cd and Pb, and π⁺ spectra on CH₂, CD₂ and ¹²C targets have been measured at 0°, up or close to the kinematic limit x_F = 1. The results are compared to various theoretical calculations.     

Tunable laser Stark-difference spectra of CD3OH

We present Doppler resolution limited spectra of the P(J) and R(J) multiplets for J ≦ 10 of the 10-μm CO stretch band of ¹²CD₃¹⁶OH using a tunable diode laser. Relative frequencies within the multiplets accurate to ±0.0002–0.0005 cm^?1 are obtained, but no absolute frequencies are given. We are able to assign most of the hindered rotation and K substructure in these multiplets. The assignments are based on analyses of Stark-difference spectra combined with the ground-state microwave data and the intensity variations which are expected theoretically. The ground and excited state A, K = 1 asymmetry splitting parameters are measured to be δ₁″ = (8.5450 ± 0.0080) × 10^?3cm^?1 and δ₁′ = (9.7706 ± 0.0080) × 10^?3cm^?1, respectively. The ground-state value agrees well with the microwave results. A rapid-scan system for recording data and a computer-aided technique for calibrating and plotting the spectra are described.     

The compound nucleus reaction 12C(11B, 2α) 15N

Energy spectra and differential cross sections of nitrogen products formed in the reaction 28 MeV ¹¹B + ¹²C have been measured using a ΔE?E counter telescope. The energy spectra are smooth and therefore indicate that the nitrogen products were formed by a compound nucleus mechanism, via the formation and decay of the compound nucleus ²³Na. The experimental results are compared with statistical model calculations and good agreement is obtained. This result provides further evidence for the importance of the compound nucleus mechanism in heavy ion reactions with light nuclei and also gives added validity to the statistical model for light compound systems.     

Prompt-fission-neutrons spectra of <Superscript>238</Superscript>U

The spectra of prompt fission neutrons from the reaction ²³⁸U(n, F) for neutrons of energy in the range E _n ≤ 20 MeV are interpreted within the statistical model. It is shown that exclusive spectra of prefission neutrons emitted in (n, xnf) reactions play a decisive role in describing the observed promptfission-neutron spectra and determine the average energies of prompt-fission neutrons. The dependence of the effect of prefission neutrons on the fissility of a target nucleus is demonstrated for the reactions ²³²Th(n, F), ²³⁵U(n, F), and ²³⁹Pu(n, F).