The low-energy part of the π⁰ spectrum associated with Mathematical expression annihilation at rest was measured in order to search for bound baryonium-like states. The upper limit for reaching such states via the emission of monochromatic π⁰’s was found to be 8% per annihilation in the mass region of 1650 MeV. The low-energy part of the η spectrum from Mathematical expression annihilations at rest was also observed.

The branching ratios of Mathematical expression annihilations into the neutral final states 2π⁰, π⁰γ, and 2γ are measured by stopping antiprotons in liquid hydrogen. They are Mathematical expression, Mathematical expression, and B_γγ<1.7×10^-6 (95% c.l.). © 1987 Springer-Verlag.

The [gamma]-spectrum originating from pp[combining macron] annihilations at rest in liquid hydrogen was measured with two BGO spectrometers. A total of 24 ï¿œ 106[gamma]’s were accumulated. No narrow peaks indicating exotic states such as baryonium were observed. The upper limit for the branching ratio with 1040 [less-than-or-equals, slant] mx [less-than-or-equals, slant] 1770 MeV/c2 and with [lambda]x [less-than-or-equals, slant] 25 MeV/c2 is less than 10-3 with more than 99.96% confidence.

After having found evidence for narrow exotic states in p annihilation, the production of such states on a nuclear target was studied with similar techniques. The γ spectrum associated with annihilation on ⁴He at rest was measured. The spectrum shows two peaks, at energies of 161.9 and 203.0 MeV, corresponding to intermediate narrow states of masses which seem to be related to states found in p annihilations. The confidence is better than 4 σ.

The inclusive η-momentum spectrum from Mathematical expression annihilations at rest in liquid hydrogen was measured at LEAR. Branching ratios were obtained for Mathematical expression, and ηη(8.1±3.1)×10^-5. An upper limit for Mathematical expression of 1.8×10^-4 at 95% CL was found. The ratio of the branching ratios is BR(ηρvariant)/BR(ηω)=0.51_-0.06^+0.20. For the ratio of branching ratios into two pseudoscalar mesons, we have BR(ηπ⁰)/BR(π⁰π⁰)=0.65±0.14, BR(ηη)/BR(π⁰π⁰), BR(ηη^′)/BR(π⁰π⁰) at 95% CL, and BR(ηη)/BR(ηπ⁰). © 1989 Springer-Verlag.

A spectrometer consisting of two sets of bismuth germanium oxide (BGO) crystals and a lead-glass array has been used to measure the [pi]0 and [eta] momentum spectra produced from proton-antiproton annihilations at rest. We describe the test of the BGO sets in electron beams of energies from 50 to 450 MeV. We discuss the method of construction and calibration of the lead-glass array, as well as procedures to extract the energy and position resolutions for detected photons. A momentum resolution ([sigma]) for [pi]0’s and [eta]’s of 4% and 3%, respectively has been achieved at momenta below 1 GeV/c.

Due to the clinical advantages of Intensity Modulated Radiation Therapy (IMRT) high flexibility and accuracy in intensity modulated dose delivery is desirable to really maximize treatment outcome. Although it is possible to deliver IMRT by using broad beams in combination with dynamic multileaf collimation the process is rather time consuming and inefficient. By using narrow scanned high energy photon beams the treatment outcome can be improved, the treatment time reduced and accurate 3D in vivo dose delivery monitoring is possible by PET-CT based dose delivery imaging of photo nuclear reactions in human tissues. Narrow photon beams can be produced by directing a low emittance high energy electron beam on a thin target, and then cleaning the therapeutic photon beam from transmitted high energy electrons, and photon generated charged leptons, with a dedicated purging magnet placed directly downstream of the target. To have an effective scanning and purging magnet system the purging magnet should be placed immediately after the bremsstrahlung target to deflect the transmitted electrons to an efficient electron stopper. In the static electron stopper the electrons should be safely collected independent of the desired direction of the therapeutic scanned photon beam. The SID (Source to Isocentre Distance) should preferably be short while retaining the ability to scan over a large area on the patient and consequently there are severe requirements both on the strength and the geometry of the scanning and purging magnets. In the present study an efficient magnet configuration with a purging and scanning magnet assembly is developed for electron energies in the 50-75 MeV range and a SID of 75 cm. For a bremsstrahlung target of 3mm Be these electron energies produce a photon beam of 25-17 mm FWHM (Full Width Half Maximum) at a SID of 75 cm. The magnet system was examined both in terms of the efficiency in scanning the narrow bremsstrahlung beam and the deflection of transmitted and photon generated electrons. The simulations show that its is possible to have a scan area on the patient of up to 43 x 40 cm² for an incident electron energy of 50 MeV and 28 x 40 cm² at 75 MeV, while at the same time adequately deflecting the transmitted electron beam.

For the determination of the bound-electron g factor in hydrogen-like heavy ions the mass of the ion is needed at a relative uncertainty of at least 1 ppb. With the SMILETRAP Penning trap mass spectrometer at the Manne Siegbahn Laboratory in Stockholm several mass measurements of ions with even-even nuclei at this level of precision have been performed so far, exploiting the fact that the mass precision increases linearly with the ion charge. Measurements of masses of the hydrogen-like ions of the two Mg-isotopes ²⁴Mg and ²⁶Mg are reported. The masses of the hydrogen-like ions are 23.979011054(14) u and 25.976562354(34) u, corresponding to the atomic masses 23.985041690(14) u and 25.982592986(34) u, respectively. The possibility to use these two isotopes for the first observation of an isotope effect in the bound-electron g factor in hydrogen-like heavy ions is discussed.

A detector consisting of 54 NaI (Tl) modules is described. The detector has been optimized for the detection of 20-1000 MeV photons. An energy resolution (fwhm) of 5.5% at 130 MeV could be attained, and the stability has been better than 1% over several months.

In this paper we report the results of a study on the dissociative recombination (DR) of the diacetylene cation, C₄D₂⁺, which has been carried out at the ion storage ring CRYRING in Stockholm, Sweden. The energy-dependent absolute DR cross-section as well as the branching fractions at 0 eV collision energy were measured. The DR cross-section was best fitted using the expression σ(E) = (7.5 ± 1.5) × 10⁻¹⁶ × E^{−(1.29±0.03)} cm² over the collision energy range 1–100 meV. The thermal rate coefficient was deduced from the cross-section to be α(T) = (1.10 ± 0.15) × 10⁻⁶ × (T/300)^{−(0.79±0.03)} cm³/s. The reported branching fractions for C₄D₂⁺ agree with previous experiments on the DR of C₄H₂⁺ performed at the ASTRID storage ring in Aarhus, Denmark, and furthermore, indicate that the DR of C₄D₂⁺ possesses only two channels leading to the following products: C₄D + D (75%) and C₂D + C₂D (25%).

Charge (time) evolution and the angular dependence of incident electrons in the range 300-1030 eV through a single macroscopic glass capillary was studied. Charge measurements were done at a sample tilt angle of psi = 2' for observation angles 0= O' and 05 (both psi and 0 were measured with respect to the incident beam direction) at incident energies of 520.7 and 824.5 eV using a parallel-plate spectrometer. After equilibrium of transmission, electrons had lower average centroid (mean) energies than the respective primary beam values. Centroid energies of transmitted electrons at the centroid of the angular distribution (where the observation angle 0 is nearly equal to tilt angle of the sample psi) were found to decrease exponentially with increasing sample tilt angles for all the measured electron energies. This energy loss is attributed to inelastic scattering of electrons with the inner wall of the sample close to the capillary entrance. Furthermore, the centroid energies of the transmitted electron angular distributions at 520.7 eV were found to lose energy for angular positions away from the capillary axis (angular centroid position) for all tilt angles, indicating a higher degree of inelastic scattering at the edges of the angular distributions.

The transmission of electrons through an insulating single cylindrically shaped glass capillary of macroscopic dimensions has been investigated for electron energies from 300 to 1000 eV using a high-resolution electrostatic parallel-plate analyzer (spectrometer). The transmitted intensity decreased with increasing sample tilt angle relative to the beam direction, and had two regions: direct, where there is no interaction of the beam with the inner capillary wall, and indirect, where it does interact. From the full-width-at-half-maximum of the angular distributions, the indirect region was found to reveal a further two distinct areas of characteristics versus tilt angle with respect to elasticity/inelasticity of transmitted electrons. Electron transmission for the case of no tilt of the sample was found to be time dependent, due to charge-up of the capillary inner surface. The new results are compared with previous experimental data obtained using a 10x lower resolution spectrometer.

We report on the first experimental study of the lifetime of a bound excited state of a negative ion. A new experimental technique was developed and used to measure the radiative lifetime of the 5p^5 2P_1/2 level of Te^-. The experiment was performed in a magnetic storage ring, where a laser beam was applied along one of the straight sections. In the experiment the population of the excited J=1/2 level was probed each time the Te^- ions passed through the laser field. A decay curve was built up by sampling the population of the excited level of the Te^- ions as a function of time after injection into the ring. A multiconfiguration Dirac-Hartree-Fock calculation was performed in conjunction with the experiment. The calculation yielded a radiative lifetime of 0.45 s, in excellent agreement with the measured value of 0.42(5) s.

We report experimental angular differential cross sections for nonradiative single-electron capture in p-He collisions (p + He -> H + He+) with a separate peak at the 0.47 mrad Thomas scattering angle for energies in the 1.3-12.5 MeV range. We find that the intensity of this peak scales with the projectile velocity as v(P)(-11). This constitutes the first experimental test of the prediction from 1927 by L. H. Thomas [Proc. R. Soc. 114, 561 (1927)]. At our highest energy, the peak at the Thomas angle contributes with 13.5% to the total integrated nonradiative single-electron capture cross section.

Nuclear spectroscopy studies including determination of spatial and time distributions of nuclear electromagnetic radiations have been performed in off- and on-line experiments. The following eight nuclei have been the subject of these studies ⁹⁰Zr, ¹¹⁹Sb, ¹²⁶Te, ¹⁶⁰Dy, ¹⁸⁰Hf, ¹⁸⁴Pt and ^{195, 197}T1. Interpretations of the results in terms of current nuclear models are presented.

We have studied the stability of C₅₉ anions as a function of time, from their formation on femtosecond timescales to their stabilization on second timescales and beyond, using a combination of theory and experiments. The C₅₉ fragments were produced in collisions between C₆₀ fullerene anions and neutral helium gas at a velocity of 90 km/s (corresponding to a collision energy of 166 eV in the center-of-mass frame). The fragments were then stored in a cryogenic ion-beam storage ring at the DESIREE facility where they were followed for up to one minute. Classical molecular dynamics simulations were used to determine the reaction cross section and the excitation energy distributions of the products formed in these collisions. We found that about 15 percent of the C₅₉ ions initially stored in the ring are intact after about 100 ms, and that this population then remains intact indefinitely. This means that C₆₀ fullerenes exposed to energetic atoms and ions, such as stellar winds and shock waves, will produce stable, highly reactive products, like C₅₉, that are fed into interstellar chemical reaction networks.

The branching ratios of the different reaction pathways and the overall rate coefficients of the dissociative recombination reactions of CH₃OH₂⁺ and CD₃OD₂⁺ have been measured at the CRYRING storage ring located in Stockholm, Sweden. Analysis of the data yielded the result that formation of methanol or deuterated methanol accounted for only 3 and 6% of the total rate in CH₃OH₂⁺ and CD₃OD₂⁺, respectively. Dissociative recombination of both isotopomeres mainly involves fragmentation of the C–O bond, the major process being the three-body break-up forming CH₃, OH and H (CD₃, OD and D). The overall cross sections are best fitted by s = 1.2 ± 0.1 × 10^-15 E^-1.15±0.02 cm² and s = 9.6 ± 0.9 × 10^-16 E^-1.20±0.02 cm² for CH₃OH₂⁺ and CD₃OD₂⁺, respectively. From these values thermal reaction rate coefficients of k(T) = 8.9 ± 0.9 ×10^-7 (T/300)^-0.59±0.02 cm³ s^-1 (CH₃OH₂⁺) and k(T) = 9.1 ± 0.9 × 10^-7 (T/300)^-0.63±0.02 cm³ s^-1(CD₃OD₂⁺) can be calculated. A non-negligible formation of interstellar methanol by the previously proposed mechanism via radiative association of CH₃⁺ and H₂O and subsequent dissociative recombination of the resulting CH₃OH₂⁺ ion to yield methanol and hydrogen atoms is therefore very unlikely.

The effects of strong interaction on the 3 –> 2 X-ray transition in antiprotonic 6Li/7Li atoms have been measured at the low energy -beam at CERN. For the shifts and widths of the levels the values [epsilon]2p = (-230 ï¿œ 72) eV, [Gamma]2p = (443 ï¿œ 210) eV, and [Gamma]3d = (0.130 ï¿œ 0.045) eV for 6Li, and [epsilon]2p = (-336 ï¿œ 60) eV, [Gamma]2p = (456 ï¿œ 190) eV, and [Gamma]3d = (0.210 ï¿œ 0.062) eV for 7Li were found. The data are compared with optical model predictions.

Determination of dissociative recombination processes of H¹³CO⁺ using merged ion-electron beam methods has been performed at the heavy storage ring CRYRING, Stockholm, Sweden. We have measured the branching fractions at ~0 eV as: CO+H 87±2%, OH+C 9±2% and O+CH 4±2%. The channels leading to CO+H have the following branching fractions between the accessible electronic states of CO(X¹S⁺)+H 46±3%, CO(a³P_g)+H 20±1% and CO(a’³S⁺)+H 34±3% respectively. The reaction cross section was fitted between 1-300 meV and followed the expression σ = 1.2±0.25×10^-16 E^-1.32±0.02 cm² and the corresponding thermal rate constant was determined to k(T) = 2.0±0.4×10⁻⁷(T/300)^{−0.82±0.02} cm³s⁻¹. The cross sections between ~2-50 000 meV were investigated showing resonant structures between 3-15 eV.

Aims:  Determination of branching fractions, cross sections and thermal rate constants for the dissociative recombination of CD₃CDOD⁺ and CH₃CH₂OH₂⁺ at the low relative kinetic energies encountered in the interstellar medium.

Methods: The experiments were carried out by merging an ion and electron beam at the heavy ion storage ring CRYRING, Stockholm, Sweden.

Results: Break-up of the CCO structure into three heavy fragments is not found for either of the ions. Instead the CCO structure is retained in 23 ± 3% of the DR reactions of CD₃CDOD⁺ and 7 ± 3% in the DR of CH₃CH₂OH₂⁺, whereas rupture into two heavy fragments occurs in 77 ± 3% and 93 ± 3% of the DR events of the respective ions. The measured cross sections were fitted between 1-200 meV yielding the following thermal rate constants and cross-section dependencies on the relative kinetic energy: σ(E_cm[eV]) = 1.7 ± 0.3 × 10⁻¹⁵(E_cm[eV])^{−1.23±0.02} cm² and k(T) = 1.9 ± 0.4 × 10⁻⁶(T/300)^{−0.73±0.02} cm³s⁻¹ for CH₃CH₂OH₂⁺  as well as k(T) = 1.1 ± 0.4 × 10⁻⁶(T/300)^{−0.74±0.05} cm³s⁻¹ and σ(E_cm[eV]) = 9.2 ± 4 × 10⁻¹⁶(E_cm[eV])^{−1.24±0.05} cm² for CD₃CDOD⁺.

Aims: Determination of branching fractions, cross sections and thermal rate coefficients for the dissociative recombination of CD₃OCD₂⁺ (0-0.3 eV) and (CD₃)₂OD+ (0-0.2 eV) at the low relative kinetic energies encountered in the interstellar medium.

Methods: The measurements were carried out using merged electron and ion beams at the CRYRING storage ring, Stockholm, Sweden.

Results: For (CD₃)₂OD+ we have experimentally determined the branching fraction for ejection of a single hydrogen atom in the DR process to be maximally 7% whereas 49% of the reactions involve the break up of the COC chain into two heavy fragments and 44% ruptures both C-O bonds. The DR of CD₃OCD₂⁺ is dominated by fragmentation of the COC chain into two heavy fragments. The measured thermal rate constants and cross sections are k(T) =1.7 ± 0.5 × 10⁻⁶(T/300)^−0.77±0.01 cm3s−1,  σ= 1.2 ± 0.4 × 10⁻¹⁵(Ecm[eV])^{−1.27 ± 0.01} cm² and k(T) = 1.7 ± 0.6 × 10⁻⁶(T/300)^{−0.70±0.02} cm³s⁻¹,σ= 1.7 ± 0.6 × 10⁻¹⁵(Ecm[eV])^{−1.20±0.02} cm² for CD₃OCD₂⁺ and (CD₃)₂OD⁺, respectively.