Alpha-Alpha Scattering

Abstract

T HE differential scattering cross sections for 20.4±1.0-Mev alpha-particles by helium were determined at center-ofmass scattering angles of 60° and 90°. The experimental results are shown in Table I.The scattered particles were detected by a proportional counter. The amplified pulses from the counter went to a pulse-height discriminator circuit which rejected pulses of amplitude appreciably smaller than those desired. The unscattered particles were collected in a faraday cage which was connected to a one-^if condenser. The voltage across the condenser was measured by a null method using a Compton electrometer as the null device.There was an appreciable background counting rate resulting from the neutron flux accompanying the operation of the cyclotron. This background was reduced somewhat by shielding the counter with borax and paraffin. The background counting rate was determined before and after each set of scattering runs. Based on the internal consistency of the background data and the number of counts taken, a precision of db 10 percent was assigned to the background correction.The scattering chamber was filled to a pressure of about 2.2 cm of Hg with helium of 99.5 percent purity furnished in standard cylinders by the Ohio Chemical Company. The pressure was determined by a U-tube manometer filled with Apiezon oil B. Impurity scattering is not believed to be important in this work because the alpha-alpha scattering cross section is at least as great as the alpha-air scattering cross section for the angles studied, hence the possible 0.5 percent impurity in the scattering gas could contribute a maximum error of 0.5 percent. Furthermore, the pulses produced by impurity-scattered particles were smaller than the genuine pulses, and experimental tests showed that impurity-caused pulses were counted with poor efficiency. Outgassing of the metal surfaces of the scattering chamber or leaks in the system would have caused the pressure of the scattering gas to increase during a period of data taking. No increases in pressure greater than one percent were noted.No direct data on the cyclotron alpha-particle beam energy are available, so it has been estimated from measurements of the proton and deuteron beam energies. The probable errors quoted for the scattering cross section in Table I include the statistical probable error, the probable error in the background counts, and a collection of minor errors that are discussed in a paper on protonalpha scattering 1 in which the same apparatus was used. A more TABLE I. Results of the alpha-alpha scattering work for incident alphaparticles of 20.4 ±1.0 Mev. d is the scattering angle in the center-of-mass system, n is the number of counts observed, R is the ratio of the number of genuine counts to the number of background counts, and a is the differential scattering cross section in the center-of-mass system expressed in barns/sterad. 6 n R a 60° 900 19 0.086 (±5.5 percent) 90° 900 26 0.14 (±5.5 percent)detailed account of the apparatus and the procedure used in taking data ar...

Systematic Survey of theα−αInteraction

“…75', 45' Heydenberg and Temmer (1956) Russell, Phillips, and Reich (1956) Tombrello and Senhouse (1963) Jones, Phillips, and Miller (1960) Berk, Steigert, and Salinger (1960) Dunning, Smith, and Steigert (1961) Werner and Zimmerer (1964) Nilson, Jentschke, Briggs, Kerman, and Snyder (1958) Steigert and Sampson (1953) Mather (1951) Braden, Carter, and Ford (1951) Darriulat, Igo, Pugh, and Holmgren (1965) section from unity as a function of energy. ) Above 400 keV, nuclear interactions come into play; Bo decreases from approximately 180 at 400 keV to about 120 at 3 MeV.…”