An Experimental Investigation of the Parity Purity of the 110-keV Gamma-Decay of ^(19)F

Physics

Moline, Robert Alan ; Barnes, Charles A.

University:California Institute of Technology

Department:Physics, Mathematics and Astronomy

关键词: Physics;   

Others  :  https://thesis.library.caltech.edu/10107/1/Moline_RA_1969.pdf

美国|英语

来源: Caltech THESIS

【 摘 要 】

If either, or both, of the ground state (J^π = ½⁺) and firstexcited state (110 keV, J^π = ½^- of the ¹⁹F nucleus has impureparity, it follows that the de-excitation γ rays would have an angulardistribution of the form,

W(θ) = 1 + Pη cos(θ).

In this expression, θ is the angle between the chosen quantizationaxis and the photon momentum, and P is the polarization of the excitednuclei, with respect to the chosen quantization axis. By assumingthe existence of a parity-violating, weak-interaction, nucleon-nucleonforce, Maqueda has predicted that the asymmetry parameter, |η| inthe above expression, should be 4.3 X 10^-4.

The present experiment used the Coulomb excitation of37.5 MeV ¹⁹F projectiles in a gold target to produce ¹⁹F*(110 keV) nuclei with a large polarization perpendicular to the scatteringplane. After being scattered in the gold foil, the¹⁹F* nuclei recoiledinto an iron cylinder. In the geometry used, there was essentiallyazimuthal symmetry, and therefore no net polarization on any axisperpendicular to the beam direction, until the iron stopper wasmagnetized. The magnetization produced a net polarization of the¹⁹F* nuclei on a chosen axis perpendicular to the incident beam, byprecessing the ¹⁹F* nuclei in the hyper fine field in the iron.

From the asymmetry observed in the γ-ray counting ratewhen the magnetization was reversed, the quantity Pη was determinedto be (6.6 ± 5.9) X 10^-5.

Since the magnitude (and sign) of P is dependent on thehyperfine magnetic field, investigations were carried out to determinethe average hyperfine field of fluorine in iron by using thenuclear reaction ¹⁶O(³He,p) ¹⁸F* (937 keV).

A p-γ coincidence technique was used, and the averageprecession angle of the excited nuclei during their mean life of(68 ± 7) ps was observed, when the ¹⁸F* recoiled into an iron hostlattice. The results indicate an average hyperfine field of (88 ± 19)kG for fluorine in iron, if the g-factor of the 937-keV state isassumed to have the theoretical value 0.61.

There was evidence of serious radiation damage in the iron,produced by the ³He beam and by recoils from the target, during someof the magnetic field measurements. To investigate this effect, a thiniron foil was bombarded with a 100-keV ¹⁹F beam. At a radiationdosage of between 2 x 10¹⁶ and 7 x 10¹⁶ fluorine ions per cm² thepermeability of the iron changed dramatically, in such a way that aseveral times stronger external field was required to saturate theiron.

If it is assumed that the 110-keV level of ¹⁹F has ag-factor of -0.57 and a mean life of 0.88 ns, the most probablevalue for the net polarization is 0.109 ± 0.023, resulting in anasymmetry coefficient, η, of (6.1 ± 5.6)X 10^-4.