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NASA NTRS1973

A study of cosmic-ray positron and electron spectra in interplanetary and interstellar space and the solar modulation of cosmic rays

The differential energy spectra of cosmic-ray positrons and negatrons with energies between approximately 11 and 1500 MeV was measured during the period 1968-1971 using a balloon-borne magnetic spectrometer. These measurements fill a gap in the previously existing data and permit the determination of the interstellar spectra of cosmic-ray positrons and electrons. Knowledge of these spectra provides a crucial tool for studies of the distribution and density of matter and magnetic fields in the interstellar medium and the origin and dynamics of energetic particles contained in the fields. The differential energy spectrum of interstellar electrons may be represented as a power-law, j alpha T to the -1.8 power for 100 MeV approximately T approximately 2 GeV, but must flatten considerably at lower energies. From the measured electron charge composition, it is concluded that the majority of cosmic-ray electrons with energies above approximately 10 MeV originate in primary sources.

Cummings, A. C.↗

NASA NTRS1974

Relative abundances of positrons and antiprotons in the primary cosmic ray flux

Observations on the ratio of positrons to the electron-positron sum made in the 5 to 50 GeV range by Buffington et al. (1974) are used to put an upper limit on the ratio of antiprotons to protons at various energies. The calculation of the latter ratio is based on detailed measurements of the cross section of antiproton production up to intersecting storage ring energies.

Badhwar, G. D.↗

NASA NTRS1975

Energy loss of relativistic electrons and positrons traversing cosmic matter

Questions of adiabatic expansion are considered along with aspects of Compton scattering, bremsstrahlung, electronic excitation, synchrotron radiation, and electron-positron pair production. It is found that, unless the intergalactic magnetic field is very small, synchrotron radiation will dominate all other energy loss processes at ultrahigh electron and positron energies. The dependence of the loss rates on the cosmic epoch is also discussed.

Gould, R. J.↗

NASA NTRS1975

Cosmic ray antiproton and positron production in the interstellar medium

We have calculated the energy spectra of cosmic ray secondary antiprotons and positrons using the latest available data on inclusive reactions. Using the measured positron spectrum, we have found that the amount of matter traversed by the cosmic rays in the few GeV region to be 4.7 (+ or - 1.5) g/sq cm of interstellar hydrogen. The computed antiproton to proton ratio is about .0004 for energies 5-10 GeV. This is sufficient to make observations of antiprotons feasible from balloon flights. We have also pointed out the type of information that can be obtained if accurate information of the spectra of these two components becomes available.

Badhwar, G. D.↗

NASA NTRS1975

Interpretation of recent positron-electron measurements between 20 and 800 MeV

Recently measured positron and negatron spectra are discussed with regard to the problem of solar modulation. At energies above 180 MeV, the spherically symmetric Fokker-Planck equation with a diffusion coefficient proportional to particle rigidity provides reasonable fits to both the positron and total electron data. At energies below 180 MeV, the data are consistent with a continuation of the same diffusion coefficient and a local source of negatrons or with a change in the diffusion coefficient to a constant value.

Pellerin, C. J.↗

NASA NTRS1976

Differential cross section for positronium formation in positron-atomic-hydrogen collisions

The differential cross section for positronium formation in its ground state due to collision of a positron and a hydrogen atom is obtained on the basis of the combination of the L = 0 and 1 (L is the angular momentum quantum number) partial wave amplitudes with the L not less than 2 Born amplitudes. Minima at the scattering angles of 57 and 51 degrees were found for positron energies of 0.64 and 0.75 Ry. Total cross sections for positronium formation at low and intermediate impact energies are presented.

Drachman, R. J.↗

NASA NTRS1975

Positron emission reconstruction tomography for the assessment of regional myocardial metabolism by the administration of substrates labeled with cyclotron produced radionuclides

A positron emission transverse tomograph device was developed which provides transaxial sectional images of the distribution of positron-emitting radionuclides in the heart. The images provide a quantitative three-dimensional map of the distribution of activity unencumbered by the superimposition of activity originating from regions overlying and underlying the plane of interest. PETT is used primarily with the cyclotron-produced radionuclides oxygen-15, nitrogen-13 and carbon-11. Because of the participation of these atoms in metabolism, they can be used to label metabolic substrates and intermediary molecules incorporated in myocardial metabolism.

Ter-Pogossian, M. M.↗

NASA NTRS1978

The annihilation of galactic positrons

The probabilities of various channels of galactic positron annihilation were evaluated and the spectrum of the resulting radiation was calculated. The narrow width (FWHM less than 3.2 keV) of the 0.511 MeV line observed from the galactic center implies that a large fraction of positrons should annihilate in a medium of temperature less than 100,000 K and ionization fraction greater than 0.05. HII regions at the galactic center could be possible sites of annihilation.

Bussard, R.↗

NASA NTRS1978

Detection of 511 keV positron annihilation radiation from the galactic center direction

A balloon-borne gamma ray telescope with an approximately 130 cu cm high purity germanium detector was flown over Australia to detect sharp spectral features from the galactic center direction. A 511 keV positron annihilation line was observed at a flux level of (1.21 plus or minus 0.22) x (10/cu cm) photons/sec/sp cm. Suggestive evidence for the detection of the three-photon positronium continuum is presented. The possible origin of the positrons is discussed.

Leventhal, M.↗

NASA NTRS1978

Wave production in an ultrarelativistic electron-positron plasma

In this paper we calculate the eigenmodes associated with an ultrarelativistic electron-positron beam traversing a low-energy electron-positron plasma under physical conditions that may exist along open magnetic field lines above pulsar polar caps. We assume that both beam and plasma are cold and charge neutral, and that magnetic field strength and particle density decrease as the cube of (1/R). In the superstrong magnetic fields near the stellar surface where cyclotron frequencies exceed plasma frequencies, electrostatic waves and associated transverse low-frequency waves are driven unstable by the two-stream type particle distribution. At larger distances above the stellar surface the slow beam cyclotron wave is driven unstable. At even greater radial distance (approximately when particle energy density exceeds magnetic energy density) both the Ordinary and Alfven modes become unstable. All of these instabilities can lead to generation of radiation. If the particle distribution as generated at the stellar surface, assumed to consist of two streams, is modified only by plasma effects, then electrostatic and associated transverse waves may only be unstable near the stellar surface. The beam cyclotron mode which is driven by a two-stream or an inverted particle distribution may be stable.

Hardee, P. E.↗

NASA NTRS1979

The annihilation of galactic positrons

The annihilation of galactic positrons is studied in order to evaluate the probabilities of various channels of annihilation and to calculate the spectrum of the resulting radiation. The narrow width (FWHM less than 3.2 keV) of the 0.511-MeV line observed from the galactic center by Leventhal, McCallum, and Stang (1978) implies that a large fraction of positrons should annihilate in a medium of temperature less than 100,000 K and ionization fraction greater than 0.05. H II regions at the galactic center could be possible sites of annihilation.

Bussard, R. W.↗

NASA NTRS1978

Secondary positrons and electrons in the cosmic radiation

An improved calculation of the secondary production and equilibrium spectrum of positrons and electrons in the Galaxy in the energy range from 1 MeV to 100 GeV has been performed. This has been done by obtaining an analytic representation of the accelerator data which describes accurately the invariant cross-section of pions, kaons, and their antiparticles from threshold energy to about 1500 GeV. This calculation takes into account the correct angular distribution of electrons in the decay of muons and the effect of nuclei-nuclei collisions. The contributions of beta-decay positrons and knock-on electrons have been included. A comparison of the present calculations with earlier calculations and experiment is presented.

Badhwar, G. D.↗

NASA NTRS1979

Annihilation in positron-atom collisions - A new approach

Two identities are derived for the exact scattering wave function by using the method of Hiller, Sucher, and Feinberg. It is shown that the resulting identities can be used to improve the calculation of positron annihilation cross sections whenever an approximate positron-atom scattering wave function is known. In addition, the advantages of the method are illustrated with a static model for e(+)-H scattering and then with a more realistic polarized orbital approximation.

Drachman, R. J.↗

NASA NTRS1980

Moisture determination in composite materials using positron lifetime techniques

A technique was developed which has the potential of providing information on the moisture content as well as its depth in the specimen. This technique was based on the dependence of positron lifetime on the moisture content of the composite specimen. The positron lifetime technique of moisture determination and the results of the initial studies are described.

Singh, J. J.↗

NASA NTRS1979

Electron-positron pair production in hot unsaturated Compton accretion models around black holes

Electron-positron pairs photoproduced in conventional unsaturated Compton disk and spherical accretion models with electron temperature greater than 10 to the 9 K are found to dominate over the background electrons. This results in an equilibrium temperature lower than previously found and severely constrains the model parameters in some cases. Electron-positron pair production does not alleviate the thermal instability of the optically thin two-temperature disk model.

Liang, E. P. T.↗

NASA NTRS1980

Interstellar propagation and the relative spectra of cosmic ray electrons and positrons

The interstellar origin and propagation of cosmic ray electrons and positrons are discussed on the basis of radio observations and direct measurements of cosmic ray spectra. Data on the galactic nonthermal radio spectrum are indicated which imply an exponent of -2.2 for the interstellar electron spectrum below 2 GeV and suggest, together with direct cosmic ray evidence, that the spectrum steepens to -3.2 at higher energies. Comparison of the radio data for higher energies with earth-based measurements reveals that the position of the break in the spectrum is dependent on the strength of the interstellar magnetic field and thus all measured intensity values are equally valid. Analysis of the homogeneous model of cosmic ray electron and positron propagation reveals that the limits on propagation parameters are independent of the set of cosmic ray measurements considered, and predict an injection spectral index for electrons of -2.24, an energy loss parameter of 1.5 + or - 0.5 x 10 to the -16th/GeV per sec, a path length of 7.5 times the 0.33 power of the ratio of initial to measured energies (in g/sq cm), average interstellar hydrogen density of 0.22/cu cm and a cosmic ray age of 25 million years. The absence of short cosmic ray lifetimes is shown to affect the interstellar electron spectrum above 100 GeV.

Webber, W. R.↗

NASA NTRS1981

Variable positron annihilation radiation from the galactic center region

HEAO 3 Cosmic Gamma-Ray Spectrometer evidence is presented for the existence of a time-varying, unshifted, narrow 511 keV line emission from the vicinity of the galactic center. Although uncertainties exist regarding the spatial extent of the features as well as its centroid, all data are consistent with emission from a single point source located at the galactic center. This interpretation would require a source luminosity of 2 x 10 to the 37th ergs/sec, and a positron annihilation rate of about 10 to the 43rd/sec. It is concluded that a variable source of positrons which could generate such an annihilation figure might be a massive black hole at the galactic center, as has been suggested by IR observations.

Riegler, G. R.↗

NASA NTRS1982

Positron lifetime studies in thermoplastic polyimide test specimens

Positron lifetime measurements were made in two thermoplastic polyimide materials recently developed at Langley. The long component lifetime values in polyimidesulfone samples are 847 + or - 81 Ps (dry) and 764 + or - 91 Ps (saturated). The corresponding values in LARC thermoplastic imides are 1080 + or - 139 Ps (dry) and 711 + or - 96 Ps (saturated). Clearly, the presence of moisture has greater effect on positron lifetime in LARC thermoplastic imides than in the case of polyimidesulfones. This result is consistent with the photomicrographic observations made on frozen water saturated specimens of these materials.

Singh, J. J.↗