Annihilation In Flight Of Positrons Research Articles

The 9Be Photoneutron Cross Section from 17—25 MeV

We measured the 9Be photoneutron production cross section σ(γ, T, n) = σ(γ, n) + 2 σ(γ, 2n) + σ(γ, pn) in the energy range from 17 -25 MeV with quasimonoenergetic photons from positron annihilation in flight. In contrast to experiments of Thomas, Crawford and Thies (1972) and Hughes (1973), performed with bremsstrahlung photon sources we found no pronounced structure in this energy range.

Search for Two-Quantum Annihilation of Positrons in Flight withK-Shell Electrons

The two-quantum annihilation of positrons in flight with the $K$-shell electrons of silver has been investigated using 300-keV positrons. The experimental result (7.7 \ifmmode\pm\else\textpm\fi{} 6.4) \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}27}$ ${\mathrm{cm}}^{2}$/${\mathrm{sr}}^{2}$ has been obtained for the double-differential angular cross section of this process, at 30 and - 100\ifmmode^\circ\else\textdegree\fi{} for each annihilation photon with respect to the incident positron direction.

Annihilation photon beam with angular discrimination

Abstract We describe a monochromatic photon beam with peak energy in the range 250–500 MeV and a constant fwhm energy resolution of about 0.07. The photons come from annihilation in flight of positrons in a HLi target. They are collected in a very small solid angle at a relatively large mean angle. The prediction of the beam as well as the calculations of the photon energy spectrum are discussed. The calculations are then compared to measurements of photoproduction of deuterium and photoproduction on hydrogen made with multiwire proportional chambers in a magnetic spectrometer.

Photoneutron Cross Sections forNa23andMg25

Nearly monochromatic photons from positron annihilation in flight have been used to measure photoneutron cross sections for $^{23}\mathrm{Na}$ and $^{25}\mathrm{Mg}$ in the giant-resonance region. The shape of the giant resonance of each of these nuclei is rather broad, with less striking intermediate structure than in most neighboring $\mathrm{sd}$-shell nuclei. In $^{23}\mathrm{Na}$ the total cross section rises to approximately 10 mb at 17.5 MeV and remains essentially constant up to 25 MeV. In $^{25}\mathrm{Mg}$ the total cross section reaches a peak of 28 mb at 23.1 MeV and falls to approximately half that magnitude at the upper energy limit of the experiment (29 MeV). In both nuclei the ($\ensuremath{\gamma},2n$) cross section appears to rise slowly above threshold but does not exceed 1 mb within the energy limits of the experiment. The integrated cross sections for $^{23}\mathrm{Na}$ and $^{25}\mathrm{Mg}$ are 119 and 249 MeV mb, respectively; the bremsstrahlung-weighted integrated cross sections are 5.7 mb for $^{23}\mathrm{Na}$ and 11.7 mb for $^{25}\mathrm{Mg}$.

Subject index

Absolute (γ, xn) cross sections for 10B and 11B were measured up to 35 MeV using a quasi-monoenergetic photon beam from positron annihilation in flight. The experimental cross sections were compared with many-particle shell-model calculations and data from previous bremsstrahlung experiments. The 11B photoneutron cross section is discussed with respect to possible isospin splitting. A systematic review of the photoneutron cross sections of the p-shell nuclei integrated up to about 30 MeV seems to show a strong influence of the α-cluster structure of the target nuclei.

On the possibility of obtaining monochromatic ?-quanta by the annihilation of positrons in flight in cylindrical accelerators

On the possibility of obtaining monochromatic ?-quanta by the annihilation of positrons in flight in cylindrical accelerators

The photon spectrum from positron annihilation in flight

Figures are presented which enable the target parameters of a positron annihilation in flight system to be chosen to obtain the maximum photon yield consistent with the required photon energy spread in the annihilation “spike”.

Monoenergetic photon beams by positron annihilation in flight, between 80 and 300 MeV

The photon spectra from positron annihilation in hydrogen for different angles were calculated between 100 and 300 MeV. A particular beam handling system for the positrons to improve the photon resolution was also proposed.

Photoneutron Cross Sections forZr90,Zr91,Zr92,Zr94, andY89

Photoneutron cross sections, including σ[(γ, n)+(γ, pn)] and σ(γ, 2n) for Zr90, Zr91, Zr92, Zr94, and Y89, and σ(γ, 3n) for Zr94, have been measured as a function of photon energy up to 30 MeV with a monoenergetic photon beam from positron annihilation in flight. The photon energy resolution varied from less than 300 keV at the lowest to 400 keV at the highest energies. The average neutron energy was determined simultaneously with the cross-section data. Nuclear information extracted from the data includes giant-resonance parameters, integrated cross sections, level densities, and threshold (and hence mass) values. In particular, it is found that a Lorentz-shaped curve gives an accurate representation of the total photoneutron cross section near the peak of the giant resonance, although in several cases there is important structure about 5 MeV above the giant resonance. The variation with neutron number and atomic weight of these quantities is studied for these nuclei near the N=50 closed neutron shell.

Photoneutron Cross Sections ofPr141andI127from Threshold to 33 MeV

The photoneutron cross sections for ${\mathrm{I}}^{127}$ and ${\mathrm{Pr}}^{141}$ have been measured with about 170- and 320-keV photon energy resolution, using monoenergetic photons obtained from annihilation in flight of positrons. The ($\ensuremath{\gamma}, n$), ($\ensuremath{\gamma}, 2n$), and ($\ensuremath{\gamma}, 3n$) cross sections were determined experimentally by a neutron-counting technique; hence, the nuclear photon-absorption cross sections were obtained without calculated multiplicity corrections. The photon-absorption cross section obtained for ${\mathrm{Pr}}^{141}$ is a single-peaked, giant dipole resonance, well described by a Lorentz-shaped curve with a maximum of 320 mb at 15.2 MeV and a width of 4.5 MeV. The ${\mathrm{I}}^{127}$ giant resonance is broadened slightly by its vibrational deformation. Although a single Lorentz curve is in fair agreement with the data, better agreement is obtained by the sum of two Lorentz curves, when adjusted to fit the data with the restrictions implied by the hydrodynamic model. The parameters of the two curves are: resonance energies, 15.2 and 15.8 MeV; peak cross sections, 180 and 50 mb; and widths, 4.1 and 8 MeV. The integrated nuclear photon-absorption cross sections of ${\mathrm{I}}^{127}$ and ${\mathrm{Pr}}^{141}$, obtained from the areas under the Lorentz curves which fit the data, are 1.8\ifmmode\pm\else\textpm\fi{}0.2 and 2.3\ifmmode\pm\else\textpm\fi{}0.2 MeV b, respectively. These integrated cross sections are in good agreement with the dipole sum rule uncorrected for exchange effects, which predicts 1.9 and 2.1 MeV b. The ${\mathrm{Pr}}^{141}(\ensuremath{\gamma}, 3n)$ threshold, which was measured to be 28\ifmmode\pm\else\textpm\fi{}1 MeV, determines the mass of ${\mathrm{Pr}}^{138}$.

Photoneutron Cross-Section Measurements on Gold Using nearly Monochromatic Photons

Continuously variable monochromatic gamma rays with an energy spread of 3% have been produced using the annihilation in flight of positrons accelerated in a linear electron accelerator. These gamma rays have been employed to measure the ($\ensuremath{\gamma}, n$) and ($\ensuremath{\gamma}, 2n$) cross sections for gold. A neutron counting method was used to determine the neutron multiplicity. The photoneutron yield for Au has a maximum of 535\ifmmode\pm\else\textpm\fi{}37 mb at 13.7\ifmmode\pm\else\textpm\fi{}0.3 MeV. The ($\ensuremath{\gamma}, 2n$) cross section was observed to have a threshold at 14.5\ifmmode\pm\else\textpm\fi{}0.5 MeV and a maximum of approximately 100 mb at 18 MeV. The integrated ($\ensuremath{\gamma}, n$) and ($\ensuremath{\gamma}, 2n$) cross sections, up to 25 MeV, were found to be 2.14\ifmmode\pm\else\textpm\fi{}0.15 and 0.83\ifmmode\pm\else\textpm\fi{}0.16 MeV-b, respectively. The value of $\ensuremath{\int}{0}^{\ensuremath{\infty}}\ensuremath{\sigma}{E}^{\ensuremath{-}2}\mathrm{dE}$ was determined to be 15.3\ifmmode\pm\else\textpm\fi{}1.5 mb/MeV. The level density parameter $a$ was found to be 17.1\ifmmode\pm\else\textpm\fi{}0.5 Me${\mathrm{V}}^{\ensuremath{-}1}$. The total cross section $\ensuremath{\sigma}(\ensuremath{\gamma}, n)+\ensuremath{\sigma}(\ensuremath{\gamma}, 2n)+\ensuremath{\sigma}(\ensuremath{\gamma}, pn)$ can be fitted with a single Lorentz curve having a peak cross section of 535 mb at 13.90 MeV and a width of 4.2 MeV.

Die Vernichtung von Positronen im Fluge

The differential cross section at 0° for annihilation in flight of positrons has been measured with foils of lucite, Al, Cu, Ag, Sn, Au, and Pb. Positrons from Cu64 and Co56 with energies of 400, 600, 800, and 1000 keV were selected by means of a magnetic spectrometer of the Kofoed-Hansen type, and the annihilation was detected in a telescope consisting of three scintillation counters. The experimental results were compared with theoretical values computed from the two-quantum cross section by taking into account the effect of multiple scattering of the positrons. The agreement is good for light elements within a statistical accuracy of 10%. For heavy elements the experimental value was found to be 15% greater than the theoretical value which perhaps could be attributed to the one-quantum annihilation.

Measurement of electron polarization

As we have heard from Dr Grace, a great variety of techniques have been developed in the past 9 months or so for studying the polarization of electrons. To summarize these methods briefly, they include Mott, or nuclear scattering, Moller or electronelectron scattering, the circular polarization of bremsstrahlung, and a number of methods which depend on the annihilation of positrons in flight, or the formation and annihilation of positronium. All except the Mott scattering are essentially new and the results need to be taken with a grain of salt, since in most cases there has been no assessment of systematic errors in the observations. This needs bearing in mind especially in estimating the resultant error when the observations from a number of different laboratories are collated

Annihilation of Positrons in Flight

The cross section for annihilation of positrons in flight has been measured for positron energies of 0.765, 1.02, 2.2, and 3.33 Mev. Positron energies were selected by means of a magnetic lens monochromator. A differential energy interval of about 0.1 Mev was defined by the pulse height from an anthracene crystal in which annihilation occurred. The annihilation radiation was detected in a scintillation counter biased to record only quanta above 0.51 Mev. The angular distribution of the annihilation radiation and the total annihilation cross section, at all energies, agrees with the theoretical values within the experimental accuracy, estimated to be about \ifmmode\pm\else\textpm\fi{}5%.

Annihilation of Positrons in Flight

The weak continuous spectrum of hard $\ensuremath{\gamma}$ rays resulting from the stopping of positrons in various materials has been investigated with a NaI(Tl) scintillation spectrometer. Measurements were made chiefly in the $\ensuremath{\gamma}$-ray energy interval from 0.511 Mev to 1.3 Mev, for the continuous $\ensuremath{\beta}$ spectra of ${\mathrm{Ne}}^{19}$ and ${\mathrm{A}}^{35}$ (maximum positron energies of 2.2 Mev and 4.4 Mev, respectively). The experimental geometry was chosen to ensure averaging of the annihilation spectrum over all angles. Theoretical spectra were calculated for Lucite, brass, and lead as stopping materials. Comparison with experiment on an absolute basis was made possible by an experimental determination of the spectrometer efficiency as a function of $\ensuremath{\gamma}$-ray energy. The calculations included bremsstrahlung and single-quantum annihilation as well as two-quantum annihilation. In the case of ${\mathrm{Ne}}^{19}$ positrons annihilating in brass, the data are in satisfactory agreement with the calculations. However, the calculated spectra show a conspicuously stronger dependence on the atomic number of the stopping material than do the observed spectra. The experimental spectra from ${\mathrm{Ne}}^{19}$ positrons stopping in Lucite and brass are indistinguishable, while the calculated spectra show a brass to Lucite ratio of 1.3. In addition, the calculated intensities tend to be lower than the observed ones, particularly for ${\mathrm{A}}^{35}$ positrons and for stopping materials of low atomic number.