Initial Neutron Research Articles

Interfacial structure in semiconducting polymer devices

We discuss some recent findings relating to the structure of interfaces in semiconducting polymer devices. The structure of three different types of interface is characterized via neutron reflectivity and scanning force microscopy. In the first example we find that enrichment of dopant occurs at the surface of a doped polymeric conductor, and that this enriched layer penetrates several nanometres into the material. Secondly, we find that the interface between a semiconducting polymer and an insulating polymer is not molecularly sharp, but is rather broad with a width typically of the order of 1 nm. Finally we present some initial neutron reflectivity measurements on the interface between two different semiconducting polymers (one of which is deuterated). Again we find that this interface is diffuse, with a typical width of the order of a few nanometres.

A high resolution, low background fast neutron spectrometer

We discuss the possibility to create a spectrometer of full absorption based on liquid scintillator doped with enriched 6 Li. Of specific interest, the spectrometer will have energy resolution estimated to lie in the range 5–10% for 14 MeV neutrons. It will be sensitive to fluxes from 10 −4 to 10 6 cm −2 s −1 above a threshold of 1 MeV in a γ-background of up to 10 4 s −1 . The detector's efficiency will be determined by the volume of the scintillator only (∼3 l) and is estimated to be 0.2–10%. The main reason for the poor resolution of an organic scintillator based spectrometer of full absorption is a non-linear light-yield of the scintillator for recoil protons. The neutron energy is occasionally distributed among recoil protons, and due to non-linear light-yield the total amount of light from all recoil protons ambiguously determines the initial neutron energy. The high-energy resolution will be achieved by compensation of the non-linear light-yield of the scintillator through the use of optically separated sections. The applications of such a detector for low-background experiments in fundamental physics research, characterizations of neutron fluence in space, and the health physics community are discussed.

Velocity-induced collapses of stable neutron stars

The collapse of spherical neutron stars is studied in General Relativity. The initial state is a stable neutron star to which an inward radial kinetic energy has been added through some velocity profile. For two different equations of state and two different shapes of velocity profiles, it is found that neutron stars can collapse to black holes for high enough inward velocities, provided that their masses are higher than some minimal value, depending on the equation of state. For a polytropic equation of state of the form $p=K\rho^\gamma $, with $\gamma = 2$ it is found to be $1.16 (\frac{K}{0.1})^{0.5} \msol$, whereas for a more realistic one, it reads $0.36 \msol $. In some cases of collapse forming a black hole, part of the matter composing the initial neutron star can be ejected through a shock, leaving only a fraction of the initial mass to form a black hole. Therefore, black holes of very small masses can be formed and, in particular, the mass scaling relation, as a function of initial velocity, takes the form discovered by Choptuik for critical collapses.

Gallium arsenide fast photodetectors: Selection criteria and predicted response

The main characteristics (sensitivity, response time, dynamic range) of a photoconductive detector (PCD) depend on the intrinsic physical (lattice parameter, extended and local defects) and electrical properties (resistivity, mobility, carrier lifetime) of the material. Selection criteria for semi-insulating undoped gallium arsenide (GaAs) were thoroughly examined for their capacity to select the more suitable material for the fabrication of fast PCDs, used in gamma-ray pulse (E∼1.2 MeV, τFWHM=30 ns) metrology. An analytical model, which assumes that carrier recombination takes place through EL2 type recombination centers [M. O. Manasreh and B. C. Covington, Phys. Rev. B 35, 2524 (1987)], was developed. Taking into account the initial physical and electrical properties of the GaAs material, the model accurately predicts the response of the PCDs after a neutron preirradiation step (dose range: 5×1014 to 1×1016 neutrons/cm2), which was used to optimize the PCD characteristics and in particular to reduce the response time to values lower than 100 ps. This led to the identification and validation of key parameters of the initial properties of single crystal GaAs materials influencing the PCDs final characteristics. Thus, depending on the user defined specifications, GaAs material initial properties and neutron preirradiation dose can be fixed to fabricate GaAs PCDs exhibiting the desired response time, sensitivity, and dynamic range. The study established that the PCDs characteristics are controlled only by the initial GaAs material properties when neutron preirradiation doses remain below 1×1015 neutrons/cm2, whereas they are mainly controlled by neutron induced crystal defects above this dose.

A two-dimensional neutron spin modulation technique for inelastic and elastic neutron scattering

Abstract Based on the independent modulation of the neutron spin evolution of both incoming and scattered neutrons, a two-dimensional data set of intensities of initial versus scattered neutron wavelengths can be obtained, that yields the scattering function S( K , ω) after a two-dimensional Fourier transform. Such an experiment uses a polarized neutron beam and two independent spin precession coils.

Scattering of neutrons on a Bose condensate

The spectrum of noncondensate excitations in neutron scattering on bosons is obtained in the framework of the Bogoliubov models both for liquid 4He and a dilute gas. The problem is solved using a path-integral representation of the partition function of the system. We describe the influence of scattering of neutrons on a Bose condensate in a stationary (time-independent) picture in the Gibbs equilibrium ensemble. This influence is a stationary boson response, and it depends on the initial neutron momentum k, transfer momentum p, and the neutron-boson interaction λ, which is related to the scattering length. The contribution of the neutrons to the initial Bogoliubov spectrum is found to be important for “quasi-elastic” scattering on the noncondensate, while the contribution of deep inelastic scattering is small; no contribution from elastic scattering on the Bose condensate is found. In the case of liquid Helium, the response is unlikely to be observable for all values p. On the other hand, for a gas one may expect a visible effect, in particular for a small momentum transfer p and a small density of the Bose condensate ϱ.

Scattering of neutrons on a Bose condensate

AbstractThe spectrum of noncondensate excitations in neutron scattering on bosons is obtained in the framework of the Bogoliubov models both for liquid 4He and a dilute gas. The problem is solved using a path‐integral representation of the partition function of the system. We describe the influence of scattering of neutrons on a Bose condensate in a stationary (time‐independent) picture in the Gibbs equilibrium ensemble. This influence is a stationary boson response, and it depends on the initial neutron momentum k, transfer momentum p, and the neutron‐boson interaction λ, which is related to the scattering length. The contribution of the neutrons to the initial Bogoliubov spectrum is found to be important for “quasi‐elastic” scattering on the noncondensate, while the contribution of deep inelastic scattering is small; no contribution from elastic scattering on the Bose condensate is found. In the case of liquid Helium, the response is unlikely to be observable for all values p. On the other hand, for a gas one may expect a visible effect, in particular for a small momentum transfer p and a small density of the Bose condensate ϱ.

Analysis of isospin dependence of nuclear collective flow in an isospin-dependent quantum molecular dynamics model

Within the framework of an isospin-dependent quantum molecular dynamics model in which the initial neutron and proton densities are sampled according to the densities calculated from the Skyrme-Hartree-Fock method and the initial Fermi momenta of neutrons and protons are calculated from the Fermi gas model, we study systematically the transverse collective flow of different fragment types at an energy of 55 MeV/nucleon and the balance energy in the reactions ${}^{58}\mathrm{Fe}{+}^{58}\mathrm{Fe}$ and ${}^{58}\mathrm{Ni}{+}^{58}\mathrm{Ni}.$ The results from the present calculations indicate that the neutron-rich system ${(}^{58}\mathrm{Fe}{+}^{58}\mathrm{Fe})$ displays stronger negative deflection and has a higher balance energy, which are qualitatively in agreement with the experimental data. Furthermore, the effects of the isospin-dependent symmetry energy and nucleon-nucleon cross sections on collective flow are studied.

The mass of the neutron star in the binary millisecond pulsar PSR J1012 + 5307

We have measured the radial velocity variation of the white dwarf secondary in the binary system containing the millisecond pulsar PSR J 1012 + 5307. Combined with the orbital parameters of the radio pulsar, we infer a mass ratio q (≡M1/M2) = 10.5 ± 0.5. Our optical spectroscopy has also allowed us to determine the mass of the white dwarf companion by fitting the spectrum to a grid of DA model atmospheres: we estimate M2 = 0.16 ± 0.02 M⊙, and hence the mass of the neutron star is 1.64 ± 0.22 M⊙, where the error is dominated by that of M2. The orbital inclination is 52 ± 4°. For an initial neutron star mass of ∼ 1.4 M⊙, only a few tenths of a solar mass at most has been successfully accreted over the lifetime of the progenitor low-mass X-ray binary. If the initial mass of the secondary was ∼ 1 M⊙, our result suggests that the mass transfer may have been non-conservative.

Theory of Quasi‐Equilibrium Nucleosynthesis and Applications to Matter Expanding from High Temperature and Density

Our first purpose is construction of a formal theory of quasi-equilibrium. We define quasi-equilibrium, in its simplest form, as statistical equilibrium in the face of an extra constraint on the nuclear populations. We show that the extra constraint introduces a uniform translation of the chemical potentials for the heavy nuclei and derive the abundances in terms of it. We then generalize this theory to accommodate any number of constraints. For nucleosynthesis, the most important constraint occurs when the total number of heavy nuclei Yh within a system of nuclei differs from the number that would exist in nuclear statistical equilibrium (NSE) under the same conditions of density and temperature. Three situations of high relevance are (1) silicon burning, wherein the total number of nuclei exceeds but asymptotically approaches the NSE number; (2) alpha-rich freezeout expansions of high entropy, wherein Yh is less than the NSE number; and (3) expansions from high temperature of low-entropy matter, in which Yh exceeds the NSE number. These are of importance, respectively, within (1) supernova shells, (2) Type II supernova cores modestly outside the mass cut, and (3) Type Ia supernova cores in near-Chandrasekhar-mass events. Our next goal is the detailed analysis of situation (2), the high-entropy alpha-rich neutron-rich freezeout. We employ a nuclear reaction network, which we integrate, to compare the actual abundances with those obtained at the same thermal conditions by the quasi-equilibrium (QSE) theory and by the NSE theory. For this detailed comparison, we choose a high-entropy photon-to-nucleon ratio = 6.8, for which we conduct expansions at initial bulk neutron excess η0 = 0.10. We demonstrate that the abundance populations, as they begin expansion and cooling from temperature 10 × 109 K, are characterized by three distinct phases: (1) NSE, (2) QSE having Yh smaller than the NSE value, and (3) final reaction rate-dependent freezeout modifications of the QSE. We demonstrate that the true final abundances are well approximated by the QSE distribution near the freezeout temperature T9f = 4.0. During the expansion, the QSE distribution changes shape continuously in ways that are independent of the reaction cross sections of the heavy nuclei with free light particles. It is this changing shape, rather than nuclear flows, that establish the abundance pattern. The abundance pattern is actually determined by the parameter Yh and the degree to which it differs from the NSE value owing to the slowness with which light particles can be assembled into heavy nuclei (A ≥ 12). We also detail the nature and magnitude of the freezeout corrections to the QSE distribution. The entire distribution depends less upon the values of heavy-element cross sections than has been heretofore thought. Our third goal is to survey the alpha-rich freezeout. We do this by less complete analysis of nine different expansions determined by the matrix of three distinct entropies ( = 1.7, 6.8, and 17) and three distinct initial neutron excesses (η0 = 0.003, 0.10, and 0.1667). The trends are easily comprehended in terms of the concept of quasi-equilibrium, whereas they are not understandable in terms of either NSE or in terms of reaction rates. This secures for the QSE concept a major diagnostic capability within nucleosynthesis theory. We delineate the key trends and also remark on the ways that order arises from disorder in this complex system. We conclude with a discussion of how such systems assemble heavy nuclei.

Development of the solar flare plasma density investigation method based on the 2.2 MeV gamma-line time profile analysis

We propose the method of studying the flare plasma density in different layers of the solar atmosphere, in particular, in the deep layers, which are inaccessible for optical observations. The method is based on the analysis of the time profile of 2.223 MeV gamma-rays, produced by neutron captures by hydrogen. We present the model calculations of the 2.223 MeV gamma-ray production time profiles and depth distribution densities at initial neutron energies 0.1–100 MeV. Both monoenergetic neutrons and power low spectra neutrons are considered at 5 solar plasma density altitude models. The dependence of the resultant curves on the initial neutron energy spectrum and on the altitude profile of the solar atmospheric and subphotospheric density is analysed. Applicability of the method to experimental data is discussed.

Influence of oxygen content on the structural, magnetotransport, and magnetic properties ofLaMnO3+δ

A systematic study of the effect of oxygen content on the structural, magnetotransport, and magnetic properties has been undertaken on a series of ${\mathrm{LaMnO}}_{3+\mathrm{\ensuremath{\delta}}}$ samples, with $\ensuremath{\delta}=0,$ 0.025, 0.07, 0.1, and 0.15. Measurements of the ac initial magnetic susceptibility, magnetization, magnetoresistance, and neutron diffraction, including small-angle neutron scattering (SANS), were performed in the temperature range 1--320 K using high magnetic fields up to 12 T. The antiferromagnetic order found in ${\mathrm{LaMnO}}_{3}$ evolves towards a ferromagnetic order as \ensuremath{\delta} increases. This behavior is accompanied by a drastic reduction of the static Jahn-Teller distortion of the ${\mathrm{MnO}}_{6}$ octahedra. The ferromagnetic coupling weakens for $\ensuremath{\delta}g~0.1.$ The magnetic behavior is interpreted by taking into account two effects caused by the increase in \ensuremath{\delta}: cation vacancies and ${\mathrm{Mn}}^{4+}{/\mathrm{M}\mathrm{n}}^{3+}$ ratio enhancement. The orthorhombic crystallographic structure becomes unstable at room temperature for $\ensuremath{\delta}g~0.1.$ The sample $\ensuremath{\delta}=0.1$ shows a structural transition from rhombohedral to orthorhombic below ${T}_{S}\ensuremath{\approx}300\mathrm{K}$ with a huge change in the cell volume. All the studied compounds were found to be insulating at low temperatures with no appreciable magnetoresistance, except for $\ensuremath{\delta}=0.15,$ in which we observed a large value for the magnetoresistance. The SANS results indicate that magnetic clustering effects are important below ${T}_{C}$ for $\ensuremath{\delta}g~0.07,$ which could explain the intriguing ferromagnetic insulator state. In the $\ensuremath{\delta}=0.07$ and $\ensuremath{\delta}=0.10$ samples we found at temperatures below ${T}_{C}$ magnetic and structural anomalies that are characteristic of charge ordering.

Clustered and neutron-rich low density “neck” region produced in heavy-ion collisions

The creation of a clustered and neutron-rich midvelocity (``neck'' or ``hot spot'') region in intermediate-energy heavy-ion collisions is discussed. Reaction simulations suggest that the preponderance of the neutron-rich species in isotopic and isobaric ratios results primarily from the amplification of the initial neutron excess due to $d$ (and by inference $\ensuremath{\alpha}$-particle) cluster formation.

Isospin dependence of collective flow in heavy-ion collisions at intermediate energies.

Within the framework of an isospin-dependent Boltzmann-Uehling-Uhlenbeck (BUU) model using initial proton and neutron densities calculated from the nonlinear relativistic mean-field (RMF) theory, we compare the strength of transverse collective flow in reactions $^{48}Ca+^{58}Fe$ and $^{48}Cr+^{58}Ni$, which have the same mass number but different neutron/proton ratios. The neutron-rich system ($^{48}Ca+^{58}Fe$) is found to show significantly stronger negative deflection and consequently has a higher balance energy, especially in peripheral collisions. NOTE ADDED IN PROOF: The new phenomenon predicted in this work has just been confirmed by an experiment done by G.D. Westfall et al. using the NSCL/MSU radioactive beam facility and a spartan soccer. A paper by R. Pak et al. is submitted to PRL to report the experimental result.

Calculation of differential and total neutron cross sections in superfluid helium 4 / Berechnung von differentiellen und totalen Neutronenwirkungsquerschnitten in supraflüssigem Helium 4.

A calculation of the differential neutron cross sections per unit intervals of energy and solid angle as well as the total neutron scattering cross sections in superfluid helium 4 at a temperature of about 1 K and saturated vapor pressure is presented. One-phonon and multiphonon down-scattering processes are considered. An equation for the cross section per unit interval of energy is derived and applied to neutrons with initial energies of 1.02x10 -3 and 2.23x100 -3 eV. Around these initial neutron energies, the neutrons are scattered down to less than 10 -5 eV by one-phonon and multiphonon transitions. Upscattering is neglected.

157 Neutrons therapy for inoperable or recurrent pelvic chordomas (results on 13 patients)

Between 1981 and 1994,13 patients were referred to the Neutron Therapy Department for inoperable or recurrent pelvic chordoma. <h3>Patient recruitment</h3> Among the 13 patients, 3 were females and 10 were males; their mean age was 62 years. Two patients presented with a primary tumour and 11 with a recurrence. Three were referred for palliative treatment after previous surgery and radiation therapy (50 Gy or more). For one of the patients, the neutron treatment was interrupted after 3 fractions of 2 Gy. Among the 12 patients suitable for evaluation, 10 had previous surgery. They underwent 1 to 5 (men 1. 9) surgical operations. The delay between initial diagnosis and neutron therapy was 46 months (median) and 45 months (average), and the delay between the last surgical operation and neutron therapy was 13 months (median). <h3>Treatment technique</h3> Neutrons were used alone or as boost depending on the tumour volume or treatment purpose:<ul><li>•7 patients with large tumours (mean diameter: 16.8cm) received a photon dose of 40 Gy followed by a neutron boost of 10 to 25 Gy (photon equivalent);</li><li>•5 patients with smaller tumours (mean diameter 8cm) were treated with neutrons alone; 2 were given 10 Gy in 12 fractions over 21 days with a palliative intention; 3 were given 17.6 Gy in 12 fractions over 28 days with a curative intention.</li></ul> <h3>Results</h3> At three years, the crude survival according to Kaplan-Meier is 61%. The local control probability is 54%. Two patients presented metastatic evolution, but one was cured by surgery. At the time of the evaluation, none of the patients treated with neutrons has grade 3 complications. <h3>Conclusions</h3> Although this series is rather small, it suggests that fast neutron therapy can provide a good alternative for the treatment of inoperable sacral chordomas.

Ethylene-propylene diene rubber/low density polyethylene/boron carbide composites as neutron shields

Composites of ethylene-propylene diene rubber and low density polyethylene were prepared with two different concentrations of boron carbide powder, namely, 47 and 57 wt%. The composites were investigated for their gamma and slow neutron radiation shielding properties. In addition, the temperature dependence of the electrical and thermal properties of the composites in the temperature range 30–150 °C, has been studied. The initial 1.5 cm thickness of the composite sample containing 57 wt% boron carbide sharply reduced the initial direct slow neutron flux by about 85%. The total macroscopic cross-section of this sample is 0.215 cm −1.

Merging neutron stars. 1. Initial results for coalescence of noncorotating systems

We present three-dimensional Newtonian simulations of the coalescence of two neutron stars, using a smoothed particle hydrodynamics (SPH) code. We begin the simulations with the two stars in a hard, circular binary, and have them spiral together as angular momentum is lost through gravitational radiation at the rate predicted by modeling the system as two point masses. We model the neutron stars as hard polytropes (gamma = 2.4) of equal mass, and investigate the effect of the initial spin of the two stars on the coalescence. The process of coalescence, from initial contact to the formation of an axially symmetric object, takes only a few orbital periods. Some of the material from the two neutron stars is shed, forming a thick disk around the central, coalesced object. The mass of this disk is dependent on the initial neutron star spins; higher spin rates result in greater mass loss and thus more massive disks. For spin rates that are most likely to be applicable to real systems, the central coalesced object has a mass of 2.4 solar mass, which is tantalizingly close to the maximum mass allowed by any neutron star equation of state for an object that is supported in part by rotation. Using a realistic nuclear equation of state, we estimate the temperature of the material after the coalescence. We find that the central object is at a temperature of approximately 10 MeV, while the disk is heated by shocks to a temperature of 2 to 4 MeV.

The distribution of old neutron stars in the galaxy

The distribution of old galactic neutron stars is computed for model galactic potential and initial neutron star velocity distribution following from evolution scenario of binary systems. Neutron stars are supposed to get their initial velocities during (asymmetric) supernova explosion of a single massive (M,, > 10 Ma) star or a massive component of a binary system. Old neutron star population is shown to be capable of filling a torus-like area extending to a few tens kiloparsecs above the galactic plane. Statistical tests related to the spatial distribution of gamma-ray bursts are carried out for the distribution obtained. Recent results from the BATSE device onboard the GRO observatory have shown a full isotropy of gamma-ray burst locations the sky and their inhomogeneous spatial density (( V/V,,,) = 0.33 f 0.02, (sin2 6) = 0.31 f 0.02, (cos 0) = 0.008 f 0.035) (Meegan et al., 1992), where 0 is the angle between the source and the galactic center, b is the galactic latitude and ( ) denotes averaging over the whole sample. In the case of a purely homogeneous, isotropic distribution these quantities would be, respectively 1/2, 1/3 and 0. These observed statistical features of gamma-bursts greatly reinforced interest to the nature of their possible progenitors. An attractive possibility of old galactic neutron stars looses its credibility, especially in view of negative results of searching for any spectral feature in the gamma-burst spectra obtained by BATSE. Some indications appear that gamma-ray bursts can be divided into at least two types: the strong bursts having hard spectra, with (V/Vm,,) > 1/2 and the weaker ones with softer spectra and (V/V,,,) < 1/2 (APEX experiment results, Mitrofanov et al., 1992; see also Lingenfelter & Higdon, 1992). These results might indicate that two populations of gamma-ray burst progenitors having different spatial distributions actually exist, viz. that producing weak bursts and lying outside the galactic disc, and that producing stronger bursts and residing in the galactic disc. Brainerd (1992) has shown that the BATSE results can be reconciliated with the old neutron star paradigm if one considers an extended (2100 kpc) halo consisting of old neutron stars. The question arises, whether there is any mechanism of natural populating such a halo by old neturon stars? The spatial distribution of old galactic neutron stars as putative gamma-ray burst progenitors has been studied earlier (e.g. Paczynski, 1990; Hartmann et al.,

Three-body K+d reactions.

A relativistic Faddeev calculation to describe simultaneously the reactions {ital K}{sup +}{ital d}{r arrow}{ital K}{sup +}{ital d}, {ital K}{sup +}{ital d}{r arrow}{ital K}{sup +}{ital pn}, and {ital K}{sup +}{ital d}{r arrow}{ital K}{sup 0}{ital pp} for incident kaon momenta between 250 and 600 MeV/{ital c} has been performed. It was found that while the elastic and charge-exchange reactions are well described by the impulse approximation, that is not true in the case of the breakup reaction. However, using the distorted-wave impulse approximation suffices to describe well also this reaction. We point out the importance of the orthogonality condition between initial and final neutron proton wave functions in the breakup process. This condition needs to be taken into account when extracting {ital K}{sup +}{ital N} phase shifts from {ital K}{sup +}{ital d} data. We conclude therefore that the existing isospin-zero {ital K}{sup +}{ital N} amplitudes extracted from {ital K}{sup +}{ital d} data are not reliable.