Monoenergetic Positronium Beams and Laboratory Simulations of Galactic Positron Annihilation.
Abstract
A monoenergetic positronium (Ps) beam, useful for Ps-surface diffraction studies and scattering experiments, is introduced and demonstrated. Also a series of experiments simulating a neutral galactic positron annihilation medium is presented. Calculations show that the novel monoenergetic Ps beam created in a gas has an energy width limited only by current positron beam technology. Monoenergetic triplet positronium, created in He, is detected with an annihilation plate (or a channel plate) and two scintillation detectors. The observation of the singlet annihilation spectrum indicates that the beam is narrow in energy and that the angular distribution agrees with theory. The simulation of a galactic annihilation medium for positrons is effected by study of positrons slowing down in low-density gases. The annihilation linewidth was measured for Ps, formed by positrons slowing down from high energies in H_2 and He. The fraction of positrons which survive below the threshold for Ps formation was measured with a pulsed beam technique. The linewidth for positrons directly annihilating with bound electrons in H_2 was measured with another pulsing technique. The linewidth measurements agreed with theoretical predictions while the survival fraction did not agree. The combined measurements in H _2 are used to construct the annihilation spectrum expected for galactic positrons in neutral molecular hydrogen regions. The resulting lineshape is fit directly to the HEAO-3 satellite data. The fits indicate that neutral hydrogen is a possible annihilation medium, contrary to previous reports. Recent papers discussing positron annihilation data from the galactic center have used several incompatible definitions of "the positronium fraction." A consistent definition applied to the data shows that the observational data are fit best with a triplet Ps annihilation spectrum included at a level of nearly 100% Ps formation. This is also consistent with a neutral hydrogen medium and with other models such as an ionized or cool dusty ionized region.
- Publication:
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Ph.D. Thesis
- Pub Date:
- September 1987
- Bibcode:
- 1987PhDT.......116B
- Keywords:
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- Physics: Atomic