Absolute Pair Production Cross Section of Lead at 2.76 MEV.

Standil, S.; Moore, R.

doi:10.1139/p56-126

Can. J. Phys.

1956

DOI: 10.1139/p56-126

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Absolute Pair Production Cross Section of Lead at 2.76 MEV.

S. Standil¹,

R. Moore²

Abstract: A value has been obtained for the absolute pair production cross section of lead for the 2.76 Mev. gamma rays of Na24. A collimated beam of these gamma rays was made to fall on a specially constructed target and the positrons produced in the target were detected by counting two-quanta annihilation events by means of two scintillation spectrometers in coincidence. The strength of the Na24 source was measured by a coincidence counting technique and the counter detection efficiency for annihilation radiation was … Show more

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Cited by 3 publications

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References 8 publications

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“…The principal advantage of this method over an earlier one (Standil and Moore 1956) is that experimental errors are reduced by about a factor of four.…”

mentioning

confidence: 99%

Determination of an Absolute Pair Production Cross Section by Relative Measurements

Shkolnik¹,

Standil²

1957

Can. J. Phys.

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“…The principal advantage of this method over an earlier one (Standil and Moore 1956) is that experimental errors are reduced by about a factor of four.…”

mentioning

confidence: 99%

Determination of an Absolute Pair Production Cross Section by Relative Measurements

Shkolnik¹,

Standil²

1957

Can. J. Phys.

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Absolute Pair Cross Sections at 1.119 MeV

1963

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Screening Effects in Atomic Pair Production below 5 MeV

Tseng

Pratt

1972

Phys. Rev. A

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Quantitative predictions for atomic-electron screening effects in low-energy pair production follow from the knowledge that the small-distance shape of screened-electron and positron continuum wave functions is close to that of point-Coulomb wave functions of shifted energy. These predictions are verified by making exact numerical calculations in representative cases. The energy-shift normalization theory is then used in conjunction with the point-Coulomb results of @verb/ to obtain predictions for atomic-pair-production energy distributions and total cross sections for photon energies from threshold to 5 MeV. Atomicelectron screening effects cause appreciable modifications of the total cross sections for photon energies below 1.5 MeV and continue to have a major effect on some portions of the energy distribution at higher photon energies. Results are also compared with Bethe-Heitler predictions and with experiments.With the continuing improvements in computers it is becoming feasible to make fairly accurate theoretical calculations of atomic-pair-production cross sections in the low-energy region where the Bethe-Maximon~high-energy results and Born approximation (the well-known Bethe -Heitler formula) need not be valid. Relativistic calculations of pair production in a point-Coulomb-potential model have now been reported by Qhrerbg Mork, and Olsen (QlMO); more extensive results have been given by (|)verbg'. This use of a point-Coulomb model relies on the expectation, based on form-factor estimates, 4 that the effects of atomic-electron screening would be unimportant in this energy region. Such an estimate is obtained because the maximum impact parameter r~discussed by Heitler, 5 equal to q~, with q &, = k' -P, p, is of the order of the electron Compton wavelength and is quite small compared to the radius of the atom. However, we subsequently performed the lengthy relativistic calculations~of pair-production cross sections in screened potentials and found that, near threshold, atomic-electron screening effects are important. At electron-Comptonwavelength distances an electron sees a point-Coulomb potential corresponding to the nuclear charge Z. The electron wave function has a hydrogenlike shape; the only effect of atomic-electron screening, as described by a central potential V deviating from the point-Coulomb form, is to modify the normalization.For a very-low-energy continuum wave function (but not for higher energies) this normalization is indeed sensitive to the screening. We showed in fact that we could roughly obtain screened pair-production cross sections from point-Coulomb cross sections simply by using a multiplicative normalization factor.We have recently examined~in greater detail the

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Absolute Pair Production Cross Section of Lead at 2.76 MEV.

Cited by 3 publications

References 8 publications

Determination of an Absolute Pair Production Cross Section by Relative Measurements

Determination of an Absolute Pair Production Cross Section by Relative Measurements

Absolute Pair Cross Sections at 1.119 MeV

Screening Effects in Atomic Pair Production below 5 MeV

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