An official website of the United States government
We outline an experimental technique for measuring the degree of polarization of a positron beam using an optically pumped, spin-polarized Rb target. The technique is based on the production and measurement of the ortho- and para-positronium fractions through positron collisions with the Rb atoms as a function of their polarization. Using realistic estimates for the cross sections and experimental parameters involved, we estimate that a polarization measurement with an uncertainty of 3% of the measured value can be achieved in an hour.
more »
« less
Conservation of longitudinal spin polarization of positrons emitted from a thin Ni(100) foil
We have discovered that 5 keV bursts of 5 × 107 positrons with an initial longitudinal spin polarization of (28.8 ± 0.7)%, when implanted into a thin Ni(100) crystal, are emitted with 20% efficiency at thermal energies from its surface with (30.9 ± 0.5)% polarization. We conclude that the positron spin polarization is preserved while interacting with the Ni, despite the 0.61 T average transverse magnetization of the Ni at room temperature. The resulting polarized beam has been focused to a 0.025-mm mean-diameter spot when accelerated to 5 keV and will be uniquely suited for experiments on a neutral spin aligned e+-e − plasma, spin- and angle-resolved positronium emission spectroscopy, and critical for producing a triplet positronium Bose-Einstein condensate.
more »
« less
- PAR ID:
- 10421988
- Publisher / Repository:
- American Physical Society: Physical Review A
- Date Published:
- Journal Name:
- Physical Review A
- Volume:
- 107
- Issue:
- 6
- ISSN:
- 2469-9926
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
We find a zero in the positronium formation scattering amplitude and a deep minimum in the logarithm of the corresponding differential cross section for positron–helium collisions for an energy just above the positronium formation threshold. Corresponding to the zero, there is a vortex in the extended velocity field that is associated with this amplitude when one treats both the magnitude of the momentum of the incident positron and the angle of the scattered positronium as independent variables. Using the complex Kohn variational method, we determine accurately two-channel K-matrices for positron–helium collisions in the Ore gap. We fit these K-matrices using both polynomials and the Watanabe and Greene’s multichannel effective range theory taking into account explicitly the polarization potential in the Ps-He+ channel. Using the fitted K-matrices we determine the extended velocity field and show that it rotates anticlockwise around the zero in the positronium formation scattering amplitude. We find that there is a valley in the logarithm of the positronium formation differential cross section that includes the deep minimum and also a minimum in the forward direction.more » « less
-
null (Ed.)Missing mass spectroscopy with the (e,e′K+) reaction was performed at Jefferson Laboratory's Hall C for the neutron rich Λ hypernucleus 9ΛLi. The ground state energy was obtained to be Bg.s.Λ=8.84±0.17stat.±0.15sys. MeV by using shell model calculations of a cross section ratio and an energy separation of the spin doublet states (3/2+1 and 5/2+1). In addition, peaks that are considered to be states of [8Li(3+)⊗sΛ=3/2+2,1/2+] and [8Li(1+)⊗sΛ=5/2+2,7/2+] were observed at EΛ(no. 2)=1.74±0.27stat.±0.11sys. MeV and EΛ(no. 3)=3.30±0.24stat.±0.11sys. MeV, respectively. The EΛ(no. 3) is larger than shell model predictions by a few hundred keV, and the difference would indicate that a 5He+t structure is more developed for the 3+ state than those for the 2+ and 1+ states in a core nucleus 8Li as a cluster model calculation suggests.more » « less
-
Abstract It was recently proposed that exotic particles can trigger a new stellar instability that is analogous to thee−e+pair instability if they are produced and reach equilibrium in the stellar plasma. In this study, we construct axion instability supernova (AISN) models caused by the new instability to predict their observational signatures. We focus on heavy axion-like particles (ALPs) with masses of ∼400 keV–2 MeV and coupling with photons ofgaγ∼ 10−5GeV−1. It is found that the56Ni mass and the explosion energy are significantly increased by ALPs for a fixed stellar mass. As a result, the peak times of the light curves of AISNe occur earlier than those of standard pair-instability supernovae by 10–20 days when the ALP mass is equal to the electron mass. Also, the event rate of AISNe is 1.7–2.6 times higher than that of pair-instability supernovae, depending on the high mass cutoff of the initial mass function.more » « less
-
Grinstein, B (Ed.)We calculate hard spin-independent contributions to energy levels in muonium and positronium which are due to radiatively corrected electron factor insertion in two-photon exchange diagrams. Calculation of these corrections is motivated by the new round of precise measurements of spin-independent transition frequencies in muonium and positronium.more » « less
