Proposed new reactor-activated positron source for intense slow e+ beam production

A novel method is suggested for producing a new positron (e+) emitting isotope in a nuclear reactor with application to slow e+ beams. The initial radiated sample is 124Xe which is transformed to 126I by two neutron absorptions and an intermediate decay. Over 25 Ci of positrons with a specific activity of 25 Ci/gm can be produced by this technique, allowing the generation of a slow e+ beam of over 4 x 107 e+/cm2 -s. As discussed in the conclusion, specific activities approaching 200 Ci/gm should be for activation cells are presented, one with Xe in the gas phase, the other with solid Xe. Both designs allow the easy separation of the 126I from other contaminants, permitting the production of a pure, high specific activity source.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/27384/1/0000415.pd

Temperature dependence of positronium decay rates in gases

The decay rate of orthopositronium (o-Ps) formed and thermalized in eight different gases is systematically investigated as a function of temperature. The o-Ps collisional quenching rate is observed to increase with temperature, T , for He, Ne, Ar, N2 , ethane, methane, isobutane and neopentane. All of the gases except ethane and methane increase linearly over the investigated temperature range, 300 K<T <600 K. Recent theoretical work for noble gases at elevated temperatures suggests a linear increase in the collisional quenching rate with temperature at the low densities used herein where collective phenomena such as bubble formation and density fluctuations are not present. When comparisons can be made with previous experimental results the agreement is generally poor. Precision o-Ps decay rate measurements using gases are also discussed concerning systematic effects due to the energy dependence of the collisional quenching rate.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/48854/2/b00518.pd

A viable superluminal hypothesis: Tachyon emission from orthopositronium

Tachyons are hypothetical particles that travel faster than the vacuum speed of light. Previous experiments have searched for, but have not found evidence of tachyons. Long-standing, anomalous measurements of the orthopositronium (o-Ps) decay rate are interpreted as evidence for two tachyons being occasionally emitted when o-Ps decays. Restricting the coupling of tachyon pairs to a single photon (no tachyon coupling to matter) yields a new theory where tachyons are only observed in o-Ps decay and not in the previous tachyon experiments. Combining the single photon coupling theory with all previous experiments predicts that these tachyons must deposit energy while traversing scintillator detectors. A new tachyon search experiment will use this energy loss prediction to attempt to find tachyons passing through the apparatus or set limits disproving the original o-Ps to tachyon hypothesis. Viewing an intense o-Ps source, a time-of-flight spectrometer uses the superluminal property of tachyons for identification. Several months of continuous data acquisition will be necessary to completely eliminate the o-Ps to tachyon hypothesis. © 2000 American Institute of Physics.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87640/2/1119_1.pd

Search for C-violating, P-conserving interactions and observation of 23S1 to 21P1 transitions in positronium

Possible C-forbidden transitions between 23S1 and 21P1 states of positronium are investigated. Limits are placed on the CP-violating state-mixing matrix element:|1P1|HHCP| 23P1&gt;| 1P1 states and measurement of the 23S1 to 21P1 transition frequency, v0=11181 +/- 13 MHz, verifying QED calculations.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/30784/1/0000437.pd

Precise measurement of positronium hyperfine splitting using the Zeeman effect

Positronium is an ideal system for the research of the quantum electrodynamics (QED) in bound state. The hyperfine splitting (HFS) of positronium, $\Delta_{\mathrm{HFS}}$, gives a good test of the bound state calculations and probes new physics beyond the Standard Model. A new method of QED calculations has revealed the discrepancy by 15\,ppm (3.9$\sigma$) of $\Delta_{\mathrm{HFS}}$ between the QED prediction and the experimental average. There would be possibility of new physics or common systematic uncertainties in the previous all experiments. We describe a new experiment to reduce possible systematic uncertainties and will provide an independent check of the discrepancy. We are now taking data and the current result of $\Delta_{\mathrm{HFS}} = 203.395\,1 \pm 0.002\,4 (\mathrm{stat.}, 12\,\mathrm{ppm}) \pm 0.001\,9 (\mathrm{sys.}, 9.5\,\mathrm{ppm})\,\mathrm{GHz}$ has been obtained so far. A measurement with a precision of $O$(ppm) is expected within a year.Comment: 8 pages, 8 figures, 2 tables, proceeding of LEAP2011, accepted by Hyperfine Interaction

Search for an exotic three-body decay of orthopositronium

We report on a direct search for a three-body decay of the orthopositronium into a photon and two penetrating particles, o-Ps -> gamma + X1 + X2. The existence of this decay could explain the discrepancy between the measured and the predicted values of the orthopositronium decay rate. From the analysis of the collected data a single candidate event is found, consistent with the expected background. This allows to set an upper limit on the branching ratio < 4.4 \times 10^{-5} (at the 90% confidence level), for the photon energy in the range from 40 keV < E_gamma< 400 keV and for mass values in the kinematical range 0 gamma + X1 + X2 decay mode as the origin of the discrepancy.Comment: 9 pages, 3 figure

Thermalization of Positronium in Gases

The thermalization of positronium ( Ps) formed at a few eV in gases is investigated using timeresolved, Doppler broadening measurements of the annihilation photons. Magnetic quenching permits energy measurements about 40 ns after Ps is formed in H 2 , N 2 , He, Ne, Ar, isobutane, and neopentane. The thermalization rate is measured by changing the gas density, and a classical elastic scattering cross section and a Ps formation energy are determined. The impact of Ps thermalization on decay rate experiments using gases is also discussed. [S0031-9007(98) PACS numbers: 36.10. Dr, 34.50.Bw, 78.70.Bj Collisions between normal gas atoms and the exotic atom positronium (Ps, positron-electron bound state) are interesting and unique because Ps is so light relative to its target. Hence Ps, formed at typically a few eV in most gases, will thermalize very slowly if elastic scattering is the only available energy loss mechanism. In the elastic case, the fractional energy loss per collision is only of order m͞M ϳ 10 24 (m is the Ps mass, M is the atomic/molecular mass.) The low energy Ps-atom collision is also inherently quantum mechanical in nature since the de Broglie wavelength of Ps below 1 eV is greater than 9 Å, larger than the classical geometric atomic size. Moreover, it was recognized early [1] that cross section calculations must include the polarization/Van der Waals interaction and electron exchange. These features apparently complicate the calculations of cross sections, done presently including the exchange interaction for only oneand two-electron systems scattering Ps (H: [2]; H 2 : [3]; He: Positronium as a scattering probe offers a unique experimental advantage since its annihilation into two photons provides a mechanism for determining its velocity and hence the rate of thermalization and the momentum transfer cross section ͑s m ͒ for gas targets. Previous measurements of Ps thermalization in gases In this Letter, we report the measurement of Ps thermalization rates, formation energies, and momentum transfer cross sections in purely gaseous target using time-resolved Doppler Broadening Spectroscopy (DBS). In this complementary technique to ACAR, the Doppler broadening of the back-to-back annihilation photons observed in a single high-resolution Ge detector is a measure of the longitudinal momentum of the annihilating Ps. Timing information as well as DBS is derived from the Ge detector signal, enabling the direct correlation between age and energy of the Ps to be determined. The rate of thermalization and the average formation energy of Ps can then be determined. The gases used in this investigation include He, H 2 , and Ar for comparison to the theoretical calculations The thermalization rate of Ps in a noble gas was calculated 30 years ago by Sauder [14], under the assumption of classical elastic scattering, i.e., an energy-independent cross section for energy loss (momentum transfer) s m which was interpreted as the classical geometrical atomic cross section. If Ps is formed at only a few eV, below the 5.1 eV threshold for excitation of the Ps or the noble gas ͑.10 eV͒ and slow enough to avoid collisional dissociation, Sauder&apos;s elastic model may be appropriate. The Ps kinetic energy E͑t͒, as it asymptotically approaches thermal energy, E th , is given by where b is related to the average initial energy E 0 of Ps that can eventually thermalize: coth 2 b E 0 ͞E th . G is 0031-9007͞98͞80(17)͞3727(4)$15.0

An overview of the Michigan Positron Microscope Program

An overview of the Michigan Positron Microscope Program is presented with particular emphasis on the second generation microscope that is presently near completion. The design and intended applications of this microscope will be summarized.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/87602/2/391_1.pd

Low-energy quenching of positronium by helium

Very low-energy scattering of orthopositronium by helium has been investigated for simultaneous study of elastic cross section and pick-off quenching rate using a model exchange potential. The present calculational scheme, while agrees with the measured cross section of Skalsey et al, reproduces successfully the parameter ^ 1Z_{\makebox{eff}}, the effective number of electrons per atom in a singlet state relative to the positron. Together with the fact that this model potential also leads to an agreement with measured medium energy cross sections of this system, this study seems to resolve the long-standing discrepancy at low energies among different theoretical calculations and experimental measurements.Comment: 4 latex pages, 3 postscript figure

Electron exchange model potential: Application to positronium-helium scattering

The formulation of a suitable nonlocal model potential for electron exchange is presented, checked with electron-hydrogen and electron-helium scattering, and applied to the study of elastic and inelastic scattering and ionization of ortho positronium (Ps) by helium. The elastic scattering and the $n = 2$ excitations of Ps are investigated using a three-Ps-state close-coupling approximation. The higher ($n\ge 3$) excitations and ionization of Ps atom are treated in the framework of Born approximation with present exchange. Calculations are reported of phase shifts, and elastic, Ps-excitation, and total cross sections. The present target elastic total cross section agrees well with experimental results at thermal to medium energies.Comment: 16 latex pages, 7 postscript figure