MeV Positrons Research Articles

Peculiarities of channelling radiation spectra from 100 to 800 MeV electrons and positrons in (1 1 1) Si crystal

By means of numerical calculations, the channelling radiation spectra from relativistic electrons and positrons have been studied in detail for channelling along (1 1 1) double planes in a Si crystal. The evolution of the shape of radiation spectrum is studied under an increase of particles energies from 100 to 800 MeV and a change of the angle of incidence with respect to (1 1 1) double planes. It is shown that the (1 1 1) channelling radiation spectra have much more complicated structure and radiation yield is several times greater compared to the channelling along (1 0 0) or (1 1 0) planes.

Radiation damage studies of multipixel Geiger-mode avalanche photodiodes

Results on the radiation hardness of multipixel Geiger-mode avalanche photodiodes (G-APDs) are presented. Recently developed G-APDs from three manufacturers (Hamamatsu (Japan), CPTA(Russia) and Mikron/Dubna(Russia)) were exposed to 28 MeV positrons with fluences up to 8 × 10 10 positrons / cm 2 at the Paul Scherrer Institute. The effects of this radiation on many G-APD parameters such as gain, photon detection efficiency, dark current and count rate for these devices are shown and discussed.

The Physics of Positron Annihilation in the Solar Atmosphere

We compile the most recently measured and calculated cross sections for processes relevant to annihilation of positrons in solar flares and present the final evaluated cross sections. We calculate the 511 keV annihilation line spectrum, the number of 2γ line photons produced per positron, and the relative strength of the 3γ continuum for each of the annihilation processes, using a Monte Carlo code that simulates the thermalization of and positronium production by ~MeV positrons. We calculate the thermally averaged annihilation and positronium production rates and the positronium quenching rates. We apply the results to four specific environments (fully ionized, neutral, partially ionized, and a non-local thermodynamic equilibrium model of the quiet solar atmosphere), calculating the relative strengths of each process and the combined total annihilation line spectrum. The results are compared with data obtained recently from the high spectral resolution detectors of RHESSI. We find that positron annihilation in solar flares can occur in a wide variety of environments. The cross sections presented here are also useful for evaluation of positron annihilation in other environments such as the interstellar medium.

Crystal undulator as a new compact source of radiation

Crystalline undulators with periodically deformed crystallographic planes offer coherent electromagnetic fields on the order of 1000 T and provide undulator period $L$ in submillimeter range. We present an idea for creation of a crystalline undulator and report its realization. One face of a silicon crystal was given periodic microscratches (grooves) by means of a diamond blade, with a period ranging from 0.1 to 0.5 mm in different samples. The x-ray tests of the crystal deformation have shown that a sinusoidal-like shape of crystalline planes goes through the bulk of the crystals. This opens up the possibility for experiments with high-energy particles channeled in the crystalline undulator, a new compact source of radiation. The first experiments on photon emission in the crystal undulator are in preparation at IHEP (Protvino) with 2--15 GeV positrons and at LNF (Frascati) with 500--800 MeV positrons, aiming to produce undulator photons in the range of 50--500 keV. The results of Monte Carlo simulations for the planned experiments are presented as well.

Generation of MeV electrons and positrons with femtosecond pulses from a table-top laser system

In experiments, the feasibility was demonstrated of generating multi-MeV electrons in a form of a collimated beam utilizing a table-top laser system delivering 200 fs pulses with PL=1.2 TW and 10 Hz capability. The method uses the process of relativistic self-channeling in a high-density gas jet producing electron densities in the range of 3×1019–6×1020 cm−3. In a thorough investigation, angularly resolved and absolutely calibrated electron spectra were measured and their dependence on the plasma density, laser intensity, and gas medium was studied. For the optimum electron density of ne=2×1020 cm−3 the effective temperature of the electron energy distribution and the channel length exhibit a maximum of 5 MeV and 400 μm respectively. The laser-energyto-MeV-electron efficiency is estimated to be 5%. In a second step, utilizing the multi-MeV electron beam anti-particles, namely positrons, were successfully generated in a 2 mm Pb converter. The average intensity of this new source of positrons is estimated to be equivalent to a radioactivity of 2×108 Bq and it exhibits a very favorable scaling for higher laser intensities.

Theory of propagation of charged particles and soft X-rays in fullerites

We present the theory of multi-wave diffraction (quantum channeling) of MeV electrons and positrons, and soft X-rays in fullerite crystals. Using Doyle–Turner approximation to the atomic scattering factor and taking into account thermal vibrations, we have calculated the continuum potential and demonstrated that channeling in fullerites have some new features not available in ordinary crystals.

Light response of pure CsI calorimeter crystals painted with wavelength-shifting lacquer

We have measured scintillation properties of pure CsI crystals used in the shower calorimeter built for a precise determination of the π +→ π 0 e + ν e decay rate at the Paul Scherrer Institute (PSI). All 240 individual crystals painted with a special wavelength-shifting solution were examined in a custom-built detection apparatus (RASTA – radioactive source tomography apparatus) that uses a 137Cs radioactive gamma source, cosmic muons and a light-emitting diode as complementary probes of the scintillator light response. We have extracted the total light output, axial light collection nonuniformities and timing responses of the individual CsI crystals. These results predict improved performance of the 3 π sr PIBETA calorimeter due to the painted lateral surfaces of 240 CsI crystals. The wavelength-shifting paint treatment did not affect appreciably the total light output and timing resolution of our crystal sample. The predicted energy resolution for positrons and photons in the energy range of 10–100 MeV was nevertheless improved due to the more favorable axial light collection probability variation. We have compared simulated calorimeter ADC spectra due to 70 MeV positrons and photons with a Monte Carlo calculation of an ideal detector light response.

Improved source and transport of monoenergetic MeV positrons

A new monoenergetic MeV positron beam source was designed and constructed at the Lawrence Livermore National Laboratory (LLNL) positron facility. The positron source provides a low emittance beam with high energy resolution for channeling, scattering, and in-flight annihilation experiments. New beam optics for extracting slow positrons from a moderator mounted in a Pelletron electrostatic accelerator is presented. A beam line was designed and constructed to energy analyze the output of the accelerator and to provide adjustable size and angular divergence beams to a target station. A post target large angle acceptance magnet separates positrons transmitted through the target from forward gamma ray emission.

Preliminary results of simulating interaction of 0.01–3.0 MeV positrons with tungsten targets

Reflection and transmission of positrons falling on tungsten targets were simulated by the Monte-Carlo method. The calculations were made for a simplified approximation of a narrow beam of monoenergetic positrons on a grid of 15 points in the 0.01–3.0 MeV energy range and for positrons with the 62 Cu β-decay energy spectrum, for various incidence angles. Histograms by coordinates of moderated positron stop points in the target and by angle–energy distribution of positrons leaving the target, for cases of a semi-infinite target and a single foil 0.02 mm thick, are depicted.

Multiple scattering measurements of energetic positrons in a thin gold polycrystal

As a step towards developing fast positrons as a probe to measure electron density and spin magnetic density in crystals, an experiment has been performed to measure the multiple scattering distribution of MeV positrons penetrating through thin polycrystalline films in both random and channeling directions. Image plate detectors were used to determine the angular distribution of emergent positrons. Image plate sensitivity was measured for high-energy positrons and for the background radiation encountered in our experiment. The image plates showed good sensitivity and good linearity for MeV positrons. The multiple scattering results are compared with the theory for random direction and show disagreement due to crystal effects and quantum mechanical considerations.

Cosmic muon tomography of pure cesium iodide calorimeter crystals

Scintillation properties of pure CsI crystals used in the shower calorimeter being built for precise determination of the π +→π 0e +ν e decay rate are reported. Seventy-four individual crystals, polished and wrapped in Teflon foil, were examined in a multiwire drift chamber system specially designed for transmission cosmic muon tomography. Critical elements of the apparatus and reconstruction algorithms enabling measurement of spatial detector optical nonuniformities are described. Results are compared with a Monte Carlo simulation of the light response of an ideal detector. The deduced optical nonuniformity contributions to the FWHM energy resolution of the PIBETA CsI calorimeter for the π +→ e + ν 69.8 MeV positrons and the monoenergetic 70.8 MeV photons were 2.7% and 3.7%, respectively. The upper limit of optical nonuniformity correction to the 69.8 MeV positron low-energy tail between 5 and 55 MeV was +0.2%, as opposed to the +0.3% tail contribution for the photon of the equivalent total energy. Imposing the 5 MeV calorimeter veto cut to suppress the electromagnetic losses, GEANT-evaluated positron and photon lineshape tail fractions summed over all above-threshold ADCs were found to be 2.36±0.05 (stat) ±0.20 (sys)% and 4.68±0.07 (stat)±0.20 (sys)%, respectively.

On the role of classical and quantum notions in channeling and the development of fast positrons as a solid state probe of valence electron and spin densities

The subtleties of the interplay between classical and quantum pictures are revealed particularly clearly in connection with the effect of channeling. The role of this interplay is described and related to a new probe for valence electron and spin magnetic densities in thin films, which is based upon two-photon annihilation in flight of channeled MeV positrons. The channeling effect is used to steer positrons into the interstices of a crystal, where the valence electron density is high. Transmission measurements have been performed to test our quantitative understanding of the spatial and momentum distribution of channeled positrons. We calculate annihilation rates in a gold crystal and show that the probe is highly sensitive to valence electrons. Channeling states in iron are shown to be sensitive to different regions of the spin magnetic density. With a polarized beam of MeV positrons and the possibility for selective state population, we show that the spatial distribution of the magnetization density can be probed.

Secondary electron emission from channeled MeV positrons and electrons

A new positron and electron channeling effect, channeling secondary electron emission or “channeling Auger”, is proposed. To discuss the possibility of observing “channeling Auger” electrons, the channeling Auger cross section is calculated as a function of the energy and the emergence angle of secondary electrons.

Planar Channeling of Electrons and Positrons in Crystals. Local‐Density Calculation of Planar Averaged Potential and Channeling Radiation Spectrum

AbstractResults of calculation of channeling potentials in crystals using the local density theory are given. The linear muffin‐tin orbitals method is used in the atomic spheres approximation (LMTO‐ASA) to calculate the planar‐averaged potential seen by MeV electrons and positrons channeled along (110) planes of diamond, Si, and Ge. The calculations thus include the spherical charge relaxation effects (“solid state effects”) in the solid but omits the non‐spherical part. The calculated potentials are in excellent agreement with the empirical potentials derived from channeling radiation experiments, but are significantly different near the atomic planes from the potentials obtained from superposition of the atomic Thomas‐Fermi statistical (Molière) potentials. Agreement of the potentials with the ones obtained from superposition of atomic Doyle‐Turner potentials is reasonably good. Frequencies of emitted radiation from the channeled electrons and positrons are also calculated and compared to experiments. For the crystals considered, it is concluded that the solid state effects on the planar‐averaged potential are moderate to small. It is expected that these effects should be more important for channeling in ionic solids. The deviation of the observed positron channeling radiation from the calculated values for Si and C indicates that the non‐spherical part of the charge relaxation might be relatively important for these solids.

Effects of dislocations on axial channeling radiation from positrons

The effects of a moderate concentration of noninteracting dislocations on the axial channeling radiation due to MeV positrons are analytically calculated. The axial potential due to all the six strings surrounding a 〈110〉 axial channel in diamond structure is first obtained which is exact up to quartic terms in distance from the central axis of the channel. The particle oscillation period and hence the radiation frequency depends upon the oscillation amplitude. The effects of dislocations are manifested through the modification in the oscillation amplitude of the channeled particle. We have suggested some suitable experiments to test the results.

Measurement of frequencies in phonon-photon conversion by 1035 MeV positrons in diamond

Measurement of frequencies in phonon-photon conversion by 1035 MeV positrons in diamond

Continuous Slowing down Approximation Range of 50 keV to 1000 MeV Positrons

The Wilson theory has been modified for the calculation of the continuous slowing down approximation (CSDA) range of positrons. An empirical correction factor P+ has been introduced which depends upon energy and atomic number Z. Calculation for CSDA range of 50 keV to 1000 MeV positrons has been made for elements Z=1 to 92, detector materials, minerals, Organic & Inorganic compounds. The values obtained by the present approach have been shown in good agreement with theoretical values of Berger & Seltzer.

Electron and positron channeling radiation from type-Ia and type-IIa diamonds

Planar channeling radiation spectra have been obtained from type-IIa and type-Ia diamonds with 54.5 and 30.5 MeV electrons and with 54.4 MeV positrons. Type-IIa diamonds are relatively free of impurities while Type-Ia diamonds contain “platelet defects” [N atoms bonded in the (100) plane]. For platelet free (type-IIa) diamond, theoretical calculations are found to fit experimental results quite well for electron channeling radiation. However, significant discrepancies (~ 5%) exist between the theoretical predictions and the positron data. Measurements on a type-Ia diamond show large differences in the planar channeling radiation spectra which are anisotropic and which depend upon whether electrons or positrons are used as a probe.

Photon intensity and polarization of channeling radiation emitted by 56 MeV positrons in a silicon single crystal

The channeling radiation and its polarization emitted by 56 MeV positrons in a Si crystal have been studied by computer simulation. The photon spectrum of the (111) planar channeling was found to have two peaks which correspond to the wide (111) channel and the narrow (111) channel of an Si crystal with the diamond structure, respectively. The photon spectrum of the 〈110〉 axial channeling has three peaks. The first peak is the superposition of the (100), (110), and (111) planar channeling radiations, the second peak corresponds to the narrow (111) planar channeling, and the third peak is due to the radiation from the 〈110〉 axial channeling.

Electron and positron planar channeling radiation from diamond

Channeling-radiation spectra have been obtained from a diamond with ≈ 54.5 MeV electrons and positrons. Theoretical calculations are found to fit the experimental results well for electrons, but significant discrepancies exist between theoretical predictions and the positron data.