Isotropic Angular Distribution Research Articles

A new method of absolute neutron flux determination using an associated gamma-ray technique

The 7Li(p,n 1γ) 7Be reaction is developed as a primary neutron standard for the determination of absolute neutron flux and neutron detector efficienty over a neutron energy range 0.350 ≤ E n≤2.8 MeV. A new associated γ-ray method is used, in conjunction with a neutron time-of-flight spectrometer, whose essential feature is that the associated γ-ray to neutron ratio is known. For the 7Li(p,n 1γ) 7Be reaction the ratio of the 431 keV γ-rays to n 1 neutrons is unity. It simplifies the method that the 431 keV γ-rays associated with the n 1 neutrons have an isotropic angular distribution. In this case, an accurate measurement of the 431 keV γ-rays emitted at any angle determines the total number of n 1 neutrons emitted independent of the 7Li(p,n 1) 7Be cross section, Li target thickness, and beam current. After the n 1 angular distributions are determined as a function of proton energy, the 7Li(p,n 1γ) 7Be reaction can be used to determine the absolute n 1 neutron flux at any angle by measuring the 431 keV γ-ray production, for this purpose, twelve n 1 neutron angular distributions have been measured between 3.1≲ E p≲4.9 MeV.

An Improved Response Matrix Method and Its Verification by One-Dimensional Analysis on Fast Reactors

Abstract The current response matrix method based on isotropic (P0) neutron angular distribution has been improved by effectively including a linearly anisotropic (P1 ) angular distribution. The proposed method does not take into account the P1 component directly, but it includes the effect of this component through consideration of the relation between the P0 and P1 components. The accuracy of the proposed method has been verified by comparing with exact calculation the diffusion length obtained by the present method in a uniform and infinite system: The diffusion length can be evaluated within 1% error. The present method has been further applied to one-dimensional fast reactor systems: The power distribution can be evaluated within 1%, and the neutron multiplication factor within 0.1%∇K/K, in reference to S16 calculations. This accuracy is comparable to that of a response matrix method in which the Legendre expansion about the angle is performed with components taken up to P1 The proposed method adopts the same number of unknown components of neutron current as in the current response matrix method (order P0 ). The computing time is thus of comparable order between these two methods, which is much shorter than for the response matrix method using the P1 angular distribution. The proposed method can hence be considered useful for one-dimensional transport problems, and should also have good prospects of proving equally effective for multidimensional problems.

An automatic system for the recognition and counting of traces in fast neutron film-badges

This paper describes the design of a special-purpose electronic unit for the recognition and counting of rectilinear traces present in film-badges for fast neutrons and having an isotropic angular distribution. The unit has been especially designed for measurements in the presence of high X- or γ-ray background. The system consists of a microscope, a servo-mechanism which controls the movements of the microscope stage for the mechanical scanning of the film, a TV camera for the optical-electronic scanning of the images, a unit for the identification and counting of the traces, and a data printer. Identification is based on a numerical type logic and is performed on the electrical signals emitted by the camera which are sampled, digitalized and memorized. Suitable filtering operations are performed to eliminate signals due to X or y background or to spots present in the film. The design was tested by means of an IBM 360/75 computer and gave excellent results.

Auroral electron fluxes parallel to the geomagnetic field lines

Auroral electron flux characteristics from ∼300 eV to 30 keV were measured with twelve detectors launched on the ESRO Skylark rocket S46/1 from Kiruna, Sweden, on November 10, 1969, at ∼2346 magnetic local time (MLT) into a breakup aurora. The energetic electrons (E ≳ 5 keV) displayed isotropic angular distributions during the entire flight, whereas the lowest-energy electrons (E ≲ 5 keV) were characterized by very anisotropic angular distributions, maximum along geomagnetic field lines when the precipitation was enhanced, ≳108 (cm² s sr keV)−1. The highest value of the ratio between the fluxes measured at 0° and 30° pitch angles was about 40. The energetic and angular distributions thus observed must result from the action of a selective energy mechanism acting mainly on the parallel component of the electron velocity. Experimental results are in agreement with the presence of a parallel electric field along the field lines in the upper ionosphere.

Measurements of the atmospheric neutron leakage rate

The atmospheric neutron leakage rate in the energy range from 0.01 to 10,000,000 eV has been measured as a function of latitude, altitude, and time with a neutron detector on board the Ogo 6 satellite. The latitude dependence of the neutron leakage is in reasonable agreement with that predicted by Lingenfelter (1963) and Light et al. (1973) if the neutron energy spectrum has the shape calculated by Newkirk (1963). The change in the neutron latitude dependence with the cosmic ray modulation agrees with the predictions of Lingenfelter and Light et al. For several solar proton events enhancements were observed in the neutron counting rates at lambda greater than or equal to 70 deg. Such events, however, provide an insignificant injection of protons at E less than or equal to 20 MeV into the radiation belts. An isotropic angular distribution of the neutron leakage in the energy range from 0.1 keV to 10 MeV best fits the observed altitude dependence of the neutron leakage flux.

Compound-Nucleus Contribution for (p,α) Reaction from Sn and Cd Isotopes

A clean separation of compound-nucleus and direct-reaction processes in ($p,\ensuremath{\alpha}$) reaction was made for 12- and 17-MeV protons bombarding on Cd and Sn isotopes. Results indicated the compound-nucleus contributions for $^{112}\mathrm{Sn}$ give isotropic angular distribution for the $\ensuremath{\alpha}$ particles. Shapes of the energy spectra of emitted $\ensuremath{\alpha}$ particles were in good agreement with statistical theory calculation, but absolute cross sections were larger than predicted by a factor of 2 to 7.

The circular polarization of sources of synchrotron radiation

The degree of circular polarizationpc is calculated for two models of a source of synchrotron radiation: (1) A source with an inhomogeneous magnetic field and isotropic angular distribution of the electrons with respect to the magnetic field; (2) A source with a homogeneous magnetic field and anisotropic angular distribution of the electrons in which the anisotropy of angular distribution substantially increases with the electron energy.

Particle influx and the ‘winter anomaly’ in the mid-latitude ( L = 2.5–3.5) lower ionosphere

The two explanations for the ‘winter anomaly’ of radio wave absorption: 1. (1) Changes in the neutral and ionised components of the atmosphere because of meteorological influences. 2. (2) Enhancements in, or the introduction of, an ionising corpuscular flux of precipitated particles, are briefly reviewed. There is now convincing evidence that the first explanation is involved in the anomaly. The second explanation is investigated using mid-latitude ( L = 2.5–3.5 particle precipitation data and electron density measurements below 100 km. It is first shown that the effect of the ‘anomaly’ is a maximum above 80 km, where ionisation due to particle influx is a maximum. Ionisation rates and electron densities due to various measured fluxes of precipitated electrons are then calculated, assuming an isotropic angular distribution. Effective recombination rates of 10 −5 and ∼-5 × 10 −7 cm 3/ sec near 80 km are used. An average flux ( N(>40 keV) = 5 × 10 3 electrons/ cm 2 sec ster) of electrons, and a few high flux ‘events’ could contribute 3 and 9 db respectively to the monthly mean noon absorption at 2.4 MHz. This compares with the total ‘anomaly’ of ≈ 15 db. The effect of seasonal changes in the parameters of the neutral atmosphere is shown to be significant in the calculation of ionisation below 100 km. Mean noon electron density profiles due to photoionisation and particle ionisation for summer and winter atmospheres, are similar to measured profiles. Both sets of profiles reveal that electron densities between 80 and 88 km are comparable in winter and summer. It is concluded that two explanations for the winter ‘anomaly’, presented in order of importance, are: changes in the neutral and ionised components of the atmosphere because of meteorological influences, and a seasonally constant flux of precipitated electrons. The two processes are linked through the neutral density and pressure changes.

Evaluation of the imaginary part of the optical potential with a Monte-Carlo method

A study of the use of a Monte-Carlo method, in evaluating multi-dimensional integrals, where the domain of integration is restricted by the Pauli exclusion principle, is made. This method is applied to the evaluation of the imaginary part of the optical potential using the Fermi-gas model and experimental nucleon-nucleon cross sections. The results show that the discrepancy reported by Greenlees et al., between the calculated and phenomenological imaginary potentials is not due to the assumption of an isotropic angular distribution in the nucleon-nucleon differential cross sections.

The influence of dielectric properties on the emission of a metal-dielectric-metal system: Angular distribution of emitted electrons

In two previous papers [1,2] the emission current from a sandwich cathode was calculated under various assumptions about the properties of the dielectric layer. The angular distribution of electrons emitted from this cathode is calculated in the present paper. The derived characteristics for the case of ideal dielectrics, dielectrics with phonons and traps are then compared with the experimental results from papers [3–5] and some conclusions are drawn about the dielectric layer. It is shown that isotropic angular distribution requires extraordinarily short mean free paths for elastic interactions (equal to several A) and large mean free paths for collisions with traps (at least larger than several tens A).

Impulsive emission of ∼40-kev electrons from the Sun

Using data from a system of three thin-window Geiger tubes and one thin silicon surface barrier detector on the Mars-bound spacecraft Mariner 4, we have observed three interplanetary particle events, on May 25–28, June 5–7, and June 13–14, 1965. The particles are identified conclusively as electrons of energy Ee ≳ 40 kev with steeply falling energy spectrums and with approximately isotropic angular distributions. The maximum unidirectional intensities are 80, 58, and 5 cm−2 sec−1 ster−1, respectively. The time profiles of intensity exhibit abrupt onsets, increases to maximum values in times of the order of several hours, and gradual declines over periods of the order of a day. It appears that these bursts of interplanetary electrons are emitted impulsively from the sun at approximately the same time as bursts of radio noise and X rays observed by others with terrestrial equipment. The interplanetary diffusion of electrons is discussed, and it is estimated that about 1034 electrons Ee > 40 kev were emitted on May 25 and on June 5 and about 1033 on June 13. The solar emission of nonthermal electrons has been suggested previously but not observed so clearly. Such electrons constitute a new tool for study of the structure of the interplanetary magnetic field and for discussion of the physics of solar flares.

The reaction Ni 58(n, p)Co 58 at different neutron energies

The energy and angular distributions of the protons from the reaction Ni 58(n, p)Co 58 at incident neutron energies E n = 8.4, 14.1 and 22.0 MeV have been studied The measurements at E n = 8.4 MeV yield an isotropic angular distribution, while especially at E n = 22 MeV the angular distribution of the high-energy protons is strongly peaked forward. Parts of the energy spectra have been used to evaluate level density parameters for the residual nucleus Co 58. At E n = 22 MeV deuteron spectra have also been recorded.

Ionization in the Earth's atmosphere by aurorally associated Bremsstrahlung X-rays

The production of bremsstrahlung X-rays associated with energetic auroral electron streams is computed, and the ionization rate due to the X-rays is deduced. The incident electron stream is assumed to have an isotropic angular distribution over the downward hemisphere; energy distribution functions which closely predict luminosity profiles of N 2 + radiation are adopted. Bremsstrahlung ionization production peaks at altitudes between 40 and 74 km for electron streams yielding auroral height-luminosity profiles which have a maximum in the range 100 to 120 km, but the ionization rate due to bremsstrahlung is several powers of ten below that obtained from direct ionization by electron impact. The spectral distribution and flux of X-rays associated with auroral bombardment are predicted at various altitudes, particularly in the height range reached by balloons.

Energieverteilung und Wirkungsquerschnitt der Photoprotonen aus N15

The energy spectra of photo protons from highly enriched N15 were investigated with the bremsstrahlung from the Heidelberg betatron running at six different endpoint energies between 19 and 30.5 MeV. The protons were detected at 90° to theγ-beam by means of a CsJ-spectrometer with pulse shape discrimination. The energy spectra show pronounced maxima atE p =3.2; 4.6; 9.5 and 13.3 MeV. Proton yields are given as a function of endpoint energy, the yield value atE 0=30.5 MeV being (7.0±0.8) μb/MeV · ster. Because the first excited state in the daughter nucleus C14 lies 6.09 MeV above the groundstate, the cross section for groundstate transitions of the process N15(γ, p)C14 could be derived from the upper 6 MeV of the single proton spectra. The main contribution to the cross section comes from the region between 18 and 26 MeV excitation energy with maxima at 19.5; 20.4; 22.7 and 24.5 MeV. A “pygmy resonance” occurs at 15.2 MeV with further less pronounced structures at 13.6 and 17.0 MeV. The integrated cross section for groundstate transitions up to 30.5 MeV is (22±3) MeVmb assuming isotropic angular distribution. The ratios of protons from transitions to excited states and from the (γ, n p)-reaction to those of groundstate transitions rise from 0.45 atE 0=24.5 MeV to 0.70 atE 0=30.5 MeV.

Scattering and Polarization of Neutrons from Al, Si, Fe, and Co at 2 MeV

Angular distributions of elastic scattering and polarization from $\ensuremath{\theta}=30 \mathrm{to} 120\ifmmode^\circ\else\textdegree\fi{}$, have been obtained for Al, Si, Fe, and Co at ${\mathrm{E}}_{n}=1.95\ifmmode\pm\else\textpm\fi{}0.09$ MeV, using polarized neutrons from the $\mathrm{T}(p, n){\mathrm{He}}^{3}$ reaction. Scattering and polarization from the 0.845---MeV level of ${\mathrm{Fe}}^{56}$ were also measured; an isotropic angular distribution and zero polarization were found for this level. The angular distributions of the elastic polarization for Si, Fe, and Co were negative at forward and backward angles with two zero cross-over points; the first zero of the polarization occurs approximately at the minimum of the elastic scattering for these three elements. In distinct contrast, the angular distribution of the polarization for Al is positive and constant at forward angles, becoming negative at backward angles. Absolute cross sections were obtained by scattering from polyethylene and an optical-model analysis of the experimental results was made.

High-latitude geophysical studies with satellite Injun 3: 4. Auroras and their excitation

Photometric observations of auroras were made with the satellite Injun 3 at altitudes of 250 to about 600 km over North America early in 1963. The magnetically oriented satellite simultaneously observed the precipitated electrons which caused the aurora and trapped electrons. The auroral zone, as defined on some fifty latitude surveys of the brightness of the N2+ emission at 3914 A, showed an average maximum brightness of about two kilo-rayleighs at an invariant latitude (Λ) around 69° on the magnetic shell L = 7.8. Several visible auroras are discussed in detail. It is suggested that the auroras delineated the outer edge of the region of durable trapping of Van Allen electrons, and that the electrons causing the auroras and having an isotropic angular distribution over the upper hemisphere at the satellite altitude were mostly freshly accelerated rather than old electrons of the Van Allen belts with similar energy. In two auroras measured by the satellite near perigee, less than 0.1 per cent of the oxygen 5577 A emission came from altitudes above 250 km. Hence the flux of electrons of energy ∼10 ev was no larger than the flux of those with ∼10 kev.

Unstable fragment emission from 25 GeV/c proton stars in nuclear emulsion

The heavy prongs from 8000 25 GeV proton interactions in Ilford G5 emulsions giving 4 or more heavy prongs have been followed in the pellicle containing the primary interaction and all secondary interactions recorded. 379 secondary interactions were found including 129 hammer tracks, 6 mesonic hyperfragments, and 5, definite nonmesonic hyperfragments. The range spectrum of connecting tracks of other secondary stars shows a marked peaking below 50 microns. Statistical arguments are used to show that there are 26 unidentified nonmesonic hyperfragments with connecting track less than 50 microns. The hyperfragment production rate is ∼ 0.5% and the hammer track production rate is R~1.6% for the star sample scanned. An attempt is made to interpret the rise from lower bombarding energies of the former rate. An isotropic angular distribution of the hammer tracks is found and the energy spectrum fits the evaporation model with parametersT = 12 andV = 0 MeV. Possible explanations of these features are discussed.

Die Energieverteilung der Neutronen aus einer Po-?-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)C12* 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.

The alpha spectrum from (γ, α) on V 51

The alpha -spectra from V/sup 51/( gamma , alpha )Sc/sup 47/ are shown for 21 and 30 Mev end point betatron energies. Assuming an isotropic angular distribution for emitted alphas, total integrated cross sections of 2,4(1 plus or minus 0.3) and 6(1 plus or minus 0.3) mb Mev were obtained for 21 and 30 Mev energies, respectively. The magnitude of the cross sections as well as the alpha spectra for the reaction agree with the statistical model. (H.D.R.) The mass of single charged particles emitted from stars produced by absorption of slow pi - mesons stopped in emulsion were measured in the range 20 to 100 Mev. Capture phenomena on light (C, N, and O) and heavy (Ag and Br) nuclei were differentiated on the basis of the standard criteria of Coulomb barrier and Auger electrons. The basic results were obtained by measuring the multiple Coulomb scattering by the constant sagitta method, and the measurements were repeated by counting the number of delta electrons versus residual range-and by measuring ionization versus residual range. (H.D.R.)

Charge Asymmetries in the Angular Distribution ofπandKMesons from Antiproton Annihilations in Flight

A difference in the angular distributions of positive and negative pi and K mesons produced in p-barp annihilations was sought for the center of mass momentum 657 plus or minus 16 Mev/c (1.6 Bev/c in the lab). The analysis was done on two samples of events: 1620 events of the type p-bar + p yields n pi with n =4to 8 and 287 events of the type p-bar+ p yields K-bar + K + n pi , n = 1 to 4 and at least one of the K mesons was observed to decay in the chamber. These events were observed inside a central volume of a 72-inch bubble chamber. The average antiproton momentum at the point of interaction was 1.61 approximately 0.04 Bev/c (925 Mev kinetic energy) for the first sample mentioned and for 86% of the second. The remainder of the second sample was at 1.99 plus or minus 0.05 Bev/c. The angular distribution obtained indicates a possibility that two different amihilation mechanisms are operative: the statistical one, which is dominant and produces an isotropic angular distribution; and a dynamical mechanism. (L.N.N.)