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Electron disintegration of helium between 45 and 100 MeV

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Title: Electron disintegration of helium between 45 and 100 MeV
Author(s): Murphy, James Jefferson
Doctoral Committee Chair(s): Hanson, A. O.
Department / Program: Physics
Discipline: Physics
Degree: Ph.D.
Genre: Dissertation
Subject(s): electron disintegration helium differential cross sections virtual photon spectra
Abstract: The differential cross section of 4He(e,efn)3He:,was measured at five angles over an energy range of 45-100 MeV. Electrons from the National Bureau of Standards 120 MeV, 40 kW linear electron accelerator struck a gaseous-He target. The charged reaction products were detected by 10 lithium drifted silicon detectors cooled to 770K located in the focal plane of the d~ub1e focusing 1200 wedge magnet of the positive ion spectrometer.The amplified pulses from the detectors, after conversion to digital form, were recorded and stored in an IDS 920 computer. The resulting pulse height spectra exhibited distinct peaks corresponding to tritons, deuterons, protons and 3He fs. The data were analyzed using virtual photon spectra to- yield photodisintegration, cross sections corresponding to the reaction 4He (y,h)3He . The resulting differential cross sections are consistent with the form go = A Sin2e(l + ~ cos e). The sin2e<term:corresponds to an El interaction; the Sin2ecose term to El-E2 interference. The total cross section decreases exponentially from a value of 360 \Jb at 45 MeV to 60\Jb at 100/MeV. This cross section is somewhat Smaller than that· reported by Gorbunovo The asymmetry coeffiCient, ~, changes sign only once in the energy region measured. It increases monotonically from -0.2 at 45 MeV to +1.0 at 100 MeV. This is in agreement with the findings of Gorbunov, but in d1sagreement with those of Arkatov. At the highest energies measured the asymmetry coefficient of the 4He (y,n)3He reaction 1s equal that of the 4He (y,p)3H reaction as measured by Gorbunov. A calculation of the-cross section is made in the long wave length limit negleoting meson effects and using Gaussian wave functions. Both El and E2 contributions to the cross section are considered. This calculation leads to the largely exponential energy dependence for the cross section and the dominant Sin2 e term in the angular distribution. But for reasonable choices of the Gaussian wave function parameters this calculation does not accurately describe the energy dependence of the cross section. It likewise fails to account for the observed behavior of the asymmetry coefficient, predicting instead that' the coefficient of the (y,n) reaction be one fifth that of the (y,p) reaction and of opposite sign. The experimental data are also compared to theoretical caloulations of Gunn and Irving and of Dzh1buti and Tagviashv1li. The Gunn and Irving calculation is similar to ours but includes only the El contribution and uselS Gunn-Irving wave functions. Their calculation does not accurately describe the energy dependence of the cross section and, of course, says nothing about trie asymmetric coefficient. Dzh1buti and agviashv11i do not expand the electromagnetic interaction in multipoles. Using the complete electromagnetic interaction and Gaussian wave functions, they are able to describe the energy dependence of the cross section correctly, but their values are 25% too small. Their differential cross section is not in a form in which the asymmetry coefficient is explicit. However their calculation predicts a backward asymmetry for the (y,n) reaction and a forward asymmetry for the (y,p) reaction. Therefore they too fail to explain the behavior of the asymmetry coefficient.
Issue Date: 1971
Genre: Dissertation / Thesis
Type: Text
Language: English
URI: http://hdl.handle.net/2142/25865
Rights Information: 1971 James Jefferson Murphy II
Date Available in IDEALS: 2011-07-28
Identifier in Online Catalog: 5975709
 

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