Intense Positron Beam Research Articles

An apparatus for high-resolution positron-annihilation induced Auger-electron spectroscopy using a time-of-flight technique

An apparatus for high-resolution positron-annihilation induced Auger electron spectroscopy (PAES) has been constructed at the intense slow positron beam line of the Electrotechnical Laboratory. The system uses a short pulsed positron beam (∼ ns width) and the time-of-flight (TOF) method for the energy analysis of the Auger electrons. In order to improve the energy resolution, an electrically isolated flight-tube is installed between a sample and a detector, and the electrons are retarded by biasing the flight tube. This method makes it possible to measure high-resolution PAES spectra with high efficiency.

Control and measurement system for positron experiments at the ETL linac facility

A control and measurement system for positron experiments in the Electrotechnical Laboratory (ETL) slow positron facility has been developed. The system acquires both monitor signals from the ETL electron linac and time spectrum data from the positron experimental apparatus. Furthermore, this system enables us to control both the linac and positron experimental apparatus remotely and semi-automatically. Using this system we can perform positron lifetime measurements as well as positron annihilation-induced-Auger electron spectroscopy with an intense pulsed positron beam with minimum labor.

Conceptual design of the low energy positron storage ring

A method for the generation of intense antihydrogen and positronium streams using a low energy positron storage ring is considered. A longitudinal magnetic field for the focusing system and electron cooling of the positrons are essential features of the ring. The general limitations of the storage ring parameters are determined by the relation between scattering and cooling processes. The positron beam energy is determined by physics processes which are to be studied; it lies in the range from 5 up to 30 keV. The positron lifetime is about 100 s at a vacuum pressure of 100 pTorr. The generation of slow antihydrogen atoms by using electrostatic deceleration of positrons and antiprotons is described. The lowest antihydrogen energy in such a scheme is about a few eV and the generation rate is about 1 atom per second. The positron storage ring design is described. The general parameters of the positron storage ring, which is under design in JINR, are presented. The main purpose of the proposed machine construction is the performance of experimental studies of intense positron beam storing, electron cooling and deceleration in a straight section.

Very high intensity positron beam generated by LEP2 synchrotron radiation

This year (1996), the CERN large electron-positron collider (LEP) will produce electron positron collisions from accelerated beams with energies up to 80 GeV per beam, thus providing the highest energy, intense electron beam in the world. At LEP, a 4 T superconducting wiggler could be used to produce synchrotron radiation [1]. For an electron beam with an intensity of 9 mA, this unique γ-beam would contain photons in the energy range 1–100 MeV at an intensity up to 10 16 γ/s mrad MeV. The laminar shape of the synchrotron beam can be used to build a converter that preserves some of the initial (at the production) forward collimation of the positron beam. The converter consists of 200 targets with individual thicknesses of 2 · 10 −3 X 0, where X 0 is the radiation length, spaced by 10 cm for a total length of 20 m. The 20 m target is contained in a solenoid to focus the positron beam. The e + beam is transported by a solenoid and is energy dispersed by a pair of dipole magnets. The positron yield in the energy interval 1–2 MeV is N e + ∼ 10 13 positron/s MeV at the production level. The flux of positrons that can be obtained at LEP2 is two orders of magnitude larger than that predicted for future projects [2].

Of atomic collisions with positrons and positronium

Recent progress in the study of atomic collisions using positronic projectiles is outlined and advances which might ensue from the production of more intense positron beams are indicated. Material in this paper was contained in a lecture delivered at the “Workshop on Intense Positron Beams in Europe” held at UCL on the 18th and 19th September, 1995.

A Technique for Generating Potent Positron Beams

This paper reports on a novel scheme that has the potential to generate intense positron beams. It is based on the modified betatron accelerator, a compact high-current device that has been developed in the last few years. Briefly, the proposed accelerator consists of two modified betatron accelerators that are stacked together and share the same core. The electrons are accelerated in the upper torus during the first half of the flux waveform when the time rate of the magnetic flux (varphi) is positive. After completion of the acceleration, the electrons are extracted and guided to a high-Z target producing a positron beam that is accelerated in the lower torus during the second half of the waveform when dvarphi/dt is negative.

2D Angular Correlation of Annihilation Radiation: Current Trends and Future Possibilities

Recent improvements in two-dimensional angular correlation measurement and analysis techniques have greatly increased the power of the method to image the Fermi surfaces of electronically complex systems. That, and the further and dramatic improvement that must result when the present radioisotope positron sources are replaced by intense monoenergetic positron beams are discussed and illustrated. PACS numbers: 78.70.B.1, 71.28.+d, 71.25.Ηc

Generation of an intense pulsed positron beam for positron lifetime and TOF experiments

Recent advances in the intense slow positron beam line at the Electrotechnical Laboratory are reported. Several improvements on the beam line have been made in order to carry out high count-rate positron lifetime spectroscopy and TOF experiments with a wide spectral range. We report the present status of the intense positron beam, high count-rate (∼10 5 cps) time-spectrum measurement system, and design of a new linear storage section which generates an intense beam of low energy spread.

Production of an intense slow positron beam by using an electron LINAC and its applications

An image of reemitted positrons has been produced in a proto-type positron transmission-type reemission microscope by using the electrostatic intense slow positron beam from an electron LINAC. RHEPD experiments have then been performed on H-terminated Si(111).

Positron Facilities in Europe

Positron annihilation is a well established method in solid state physics and material science. The positron being a very sensitive probe, can give very precise information on the momentum distribution of electrons in metals and alloys as well as on lattice defects in crystals. Starting with the energy distribution of positrons from a radioactive decay, the current development is directed more to monoenergetic positrons of variable energy and of high intensity. Pulsed and continuous beams of moderated positrons have been designed and developed to study very efficiently surface and near surface regions in metals and semiconductors. Especially a pulsed positron beam with narrow beam pulses (∼ 150 ps) enables positron lifetime experiments as a function of the positron energy. The impact of intense positron beams is straightforward: a decrease of the counting time. There are various possibilities and approaches to realize intense positron beams. Developments at several institutes in Europe are under way. The aim is to obtain a beam intensity in the order of ∼ 10 10 positrons/sec. Parallel to the installment of intense positron beams, the development of positron microscopes is pursued. Two types of positron microscopes are being set up: a scanning positron microscope with a pulsed beam of 100 ps and a beam diameter of 1 μm and a positron reemission microscope with about the same beam diameter

Production of an intense slow positron beam using a compact cyclotron

An intense slow positron beam has been produced for the first time using a compact cyclotron. Used in an ondashline mode, an intense positron source has been produced by making use of the 27Al(p,n) 27Si reaction. A transmission type target system has been developed in order to avoid decreases in the slow positron beam intensity due to the damage induced in the moderator by proton irradiation. In the near future beam intensities of the order of 10 7–10 9 slow e +/s are expected from this system.

Slow positron beam extraction from high magnetic fields

The high intensity positron beam at PSI will be produced in a high magnetic field region (∼ 1 T). On the other hand, micro-beam formation (through brightness enhancement operations) has to be carried out in field-free space. This requires the extraction of the positron beam into field-free space, an operation which becomes possible when symmetry breaking elements are introduced. In this contribution a method is presented, based on B-field termination through a magnetic shield with an aperture furnished by a magnetically conductive internal structure. The positron beam is first guided to a low field region where the shield is placed and then extracted into field-free space by letting it pass through the aperture. The designed aperture has a diameter of 120 mm, an 80% open area and a thickness of 10 mm. The simulated results of the field distributions show that a factor of nearly two orders of magnitude reduction of the field across the aperture and a spread of B ⊥ in the plane of the aperture opening can be achieved. This reduction factor has been confirmed experimentally. Measurements of transmission efficiency and divergency will be discussed. Two parameters can be varied in order to achieve reasonably small transverse energy of the extracted positrons which makes the following focusing performance highly efficient.

An intense pulsed positron beam

A positron pulsing system for an intense positron beam generated by an electron linac is reported. The pulsing system generates an intense pulsed positron beam of variable energy and variable pulse period. The pulsed positron beam is used as a non-destructive probe for various material research. In this paper, we also discuss applications of the pulsed positron beam: positron lifetime spectroscopy, age-momentum correlation spectroscopy, positronium time-of-flight measurement, and positron annihilation-induced Auger electron spectroscopy with a time-of-flight technique.

Intense low energy positron beams

Intense positron beams are under development or being considered at several laboratories. Already today a few accelerator based high intensity, low brightness e+ beams exist producing of the order of 108–109 e+/s. Several laboratories are aiming at high intensity, high brightness e+ beams with intensities greater than 109 e+/s and current densities of the order of 1013–1014 e+ s−1 cm−2. Intense e+ beams can be realized in two ways (or in a combination thereof) either through a development of more efficient Β+ moderators or by increasing the available activity of Β+ particles. In this review we shall mainly concentrate on the latter approach. In atomic physics the main trust for these developments is to be able to measure differential and high energy cross-sections in e+ collisions with atoms and molecules. Within solid state physics high intensity, high brightness e+ beams are in demand in areas such as the re-emission e+ microscope, two-dimensional angular correlation of annihilation radiation, low energy e+ diffraction and other fields. Intense e+ beams are also important for the development of positronium beams, as well as exotic experiments such as Bose condensation and Ps liquid studies.

Intense sources for positron research

Positron annihilation is a well-established method in solid state physics and material science. The positron being a very sensitive probe, can give very precise information on the momentum distribution of electrons in metals and alloys as well as on lattice defects in crystals. Starting with the energy distribution of positrons from a radioactive decay, the current development is directed more to monoenergetic positrons of variable energy and of high intensity. The impact of intense positron beams is straightforward: a decrease of the counting time. There are various possibilities and approaches to realize intense positron beams. The aim is to obtain a beam intensity in the order of ∼ 1010 positrons/s. Parallel to the instalment of intense positron beams, the development of positron microscopes is pursued.

Results of the Bielefeld-Brookhaven e+-H experiment

Positronium formation (Ps) cross sections for positrons impinging on atomic hydrogen were measured in the impact energy range from 13 to 255 eV at the High Intensity Positron (HIP) beam at Brookhaven National Laboratory (BNL). The Ps-formation cross section was found to rise rapidly from the threshold at 6.8 eV to a maximum value of (2.98±0.18) ×10−16 cm2 for ∼15 eV positrons. By 75 eV it drops below the detection limit of 0.17×10−16 cm2 which is the present level of statistical uncertainty. A more elaborate data analysis was performed leading to somewhat smaller cross sections than originally published [1]. During the course of this experiment further relative data on impact-ionization cross sections were accumulated.

High Intensity Positron Beam Generation by Synchrotron Radiation

High Intensity Positron Beam Generation by Synchrotron Radiation

An intense monoenergetic positron beam with an adjustable energy between 0.5 and 3.0 MeV

Abstract An adjustable 0.5 to 3.0 MeV monoenergetic positron beam has been developed at Brookhaven National Laboratory. Positrons leaving a 22Na β+ source with a ∼ 6000 A thick W(100) transmission moderator are accelerated to higher energies in a 3.0 MeV Dynamitron. At a target position, located 12 m away from the exit of the accelerator, the beam has a maximum intensity of 5 × 105 e+/s inside a 1.1 mm full width at half maximum (FWHM) spot and is characterized by an energy spread of

Experiments at the Giessen source TEPOS with low energy positrons

The TEPOS facility at the Giessen LINAC delivers intense positron beams in the energy range between some eV and 6 keV; with postacceleration up to 80 keV. Results for remoderation and positron storage will be discussed. Further the energy-loss of positrons in thin aluminium foils at incident energies of 6–20 keV was measured. Cross sections for K- and L-shell ionization of thin silver and gold targets by positron and electron impact were determined at projectile energies of 30–70 keV. The experimental results are presented in detail; they are confirmed by calculations in plane wave Born approximation (PWBA) which include an electron exchange term and take into account the deceleration or acceleration of the incident projectile in the nuclear field of the target atom.

Slow positron production using an 18 MeV electron linac

Intense slow positron beams have been produced using an 18 MeV electron linac. A NaI(Tl) detector with well-known detection efficiency has been used to measure the 511 keV annihilation γ-rays. Slow positrons detected are about 7.7 × 10 5 per second at 0.9 μA electron beams and a moderator potential, V mod, pf 7 V. The conversion efficiency of slow-positron production for 18 MeV electrons is as high as 1.35 × 10 −7. The following results are obtained using a Ceratron detector: 1 the energy spectrum has a peak around 2 eV and the FWHM is 1.5 eV, 2 the lifetime of the slow positron beam is about 0.5 ms at 2×10 −7 Torr, and 3 the positron intensity at V mod > 20 V is eight times as high as the intensity at V mod = 0.