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Low Energy Positron Source

#0767


RIKEN/Budker INP Low Energy Positron Source

Tech Area / Field

  • PHY-PFA/Particles, Fields and Accelerator Physics/Physics

Status
8 Project completed

Registration date
25.10.1996

Completion date
20.09.1999

Senior Project Manager
Malakhov Yu I

Leading Institute
Budker Institute of Nuclear Physics, Russia, Novosibirsk reg., Akademgorodok

Supporting institutes

  • VNIITF, Russia, Chelyabinsk reg., Snezhinsk

Project summary

The objective of this project is to create slow positron source by installing of superconducting wiggler with magnetic field of 8-10 Tesla on storage ring SPring-8 that will be open for other laboratories over the world. SPring- 8 storage ring has the highest beam energy among of the synchrotron light sources in the world and installation on this ring of the unique high field wiggler gives a possibility to create a unique low-energy positron source of high brightness. The Project will be done in collaboration between Budker Institute of Nuclear Physics, Russian Federal Nuclear Center - All Russian Scientific Research Institute of Technical Physics and RIKEN, Japan. RIKEN provides the financial support for this Proposal.

This proposal describes a two year research, engineering, development and test efforts which would be carried out by the employees and subcontractors of the Budker Institute of Nuclear Physics (Budker INP) in Novosibirsk, Russia and Russian Federal Nuclear Center - All Russian Scientific Research Institute of Technical Physics, Chelyabinsk, Russia in collaboration with the faculty and staff of RIKEN, Japan to provide the Japan and Russia with certain key components, subsystems and test data required for the development of low-energy positron source of high brightness by using of synchrotron radiation from superconducting wiggler installed on SPring-8 storage ring.

In general, there are two ways to obtain low-energy positrons. One is to moderate positrons produced by b-decay of isotopes, and the other is to moderate positrons produced via pair production process. In the first method, isotopes with different lifetimes are used:


i) Relatively long-lived isotopes, like 22Na (2.6 yr) or 58Co(71dy), are used in many laboratories.
ii) Relatively short-lived isotopes can also be used in some special research centers. These isotopes 64Сu (12.7 hr) are produced in nuclear reactors by irradiating thermal neutrons on copper, and 18F (110min) or 11C (20min) produced with cyclotrons. In the second method, different kinds of g-ray sources are available for producing positrons.
iii) In the nuclear reactor, Cd is used as an absorber of thermal neutrons, from which high-energy g-rays are emitted.
iv) By using electron linacs, one can inject high-energy electrons into a target and generate g-rays by Bremsstrahlung.
v) Superconducting wigglers installed in high-energy electron/positron storage rings produce an intense beam of synchrotron radiation (SR) with energies well above the threshold of pair production.

The linac method is adopted (or to be adopted) in many facilities for obtaining high intensity low energy positron beams. However, such a scheme of using linacs suffers from radiation hazard and induced radioactivity of a target-moderator system. It was pointed out [1-4] that this problem could be solved if one adopts the method of using SR for producing positrons, and with this method high intensity positron beams can be obtained.

The third generation light source SPring-8, an 8GeV electron storage ring under construction in Japan, will start commissioning in February 1997. In SPring-8 the project of producing low-energy positrons with SR has been nominated as one candidate of future plans of the facility. The goal of this project is to provide a high-intensity low-energy positron beamline in addition to usual SR beamlines and offer a unique method of probing materials.

In order to achieve this goal, however, there are some technical problems that must be solved. One problem is to fabricate a stable superconducting wiggler whose strength is around 8-10 Tesla or more.

It should be noted that even in the case of the 8 Т wiggler one can obtain a "rather intense" slow-positron beamline when it is installed in the SPring-8 storage ring. Then, one can have a unique facility where both SR and positron beamlines are available at the same time.

Using g-ray with energy well above 1 MeV, one can produce positrons in matter through the electron positron pair production process. These positrons can be collected and used as a positron beam, which is decelerated down to a few eV with a suitable moderator. The efficiency of moderation is typically 10-2 ё 10-4 depending on moderators. Installation of a superconducting wiggler with maximum magnetic field of 8-12 Tesla on SPring-8 storage ring gives a positron beam intensity of 1010 [slow e+/sec].

The low-energy positron beam can be used as


a) probe of Fermi surface. When a positron is incident on a substance, pair annihilation and subsequent two-photon emission occurs. By analyzing angular correlation of these photons, one can deduce information on the Fermi surface.
b) probe of defects. Since positrons have positive charge, they feel repulsive forces from ionized atoms and are captured selectively at defects like vacancy. This means that the positron beam can be used to study defects in metals, semiconductors, etc. It can also be used to detect impurities.
c) probe of surface and interface. By utilizing the process of losing energy, diffraction and reemission, one can obtain information on surfaces and interfaces. It should be noted that if the energy of the positron beam were tunable, it would become possible to study defects at a given depth from the surface.
d) micro-probe. Several kinds of microscopes using positrons will become available if high intensity positron beams are provided: transmission positron microscope, scanning positron microscope, positron reemission microscope and positron tunneling microscope have been proposed. For example, the positron reemission microscope can be used to analyze surface structures or surface defects with high resolutions.

Some examples with low-energy positron beams are the following:


- Two-dimensional Angular Correlation of Annihilation Radiation (2D-ACAR)
- Positron Spin Relaxation (PSR)
- Positron Reemission Microscopy (PRM)
- Positron Tunneling Microscopy (PTM)
- Positron Reemission Spectromicroscope (PRSM)
- Positron Induced Auger Emission Spectroscopy (PAES)
- Positron Induced Auger Emission Microscopy (РАЕМ)
- Low Energy Positron Diffraction (LEPD)

Our proposal is to develop high-field superconducting magnets and fabricate a prototype of a superconducting wiggler for the positron beamline project described above. Additional studies on the cryogenic system, power supply system, quench protecting system, etc. are also planned to be performed. Technologies developed and all information obtained through R&D works are to be used for the positron beamline project. On the basis of the field measurement of the prototype wiggler, suitable designs of the beamline including vacuum chamber, absorbers, radiation shield etc. are proposed.

The main results are as follows:


1. Development of stable high-field superconducting magnets. The goal of the maximum field strength is 8-10 Tesla.
2. Construction of a prototype of a high-field superconducting wiggler. The wiggler should be designed appropriately for installing in the Spring-8 storage ring.
3. Development of the cryogenic system, power supply system, quench protecting system, etc. for stable operation of the wiggler.
4. Measurement of field distributions of the prototype wiggler. By doing field measurements, magneticfield distributions, magnetic field integrals, multipole components, etc. are obtained. These values are then used for subsequent studies.
5. Simulation studies of SR spectrum and the effects on the electron beam on the basis of the fieldmeasurements.
6. Estimation of positron yield on the basis of the field measurements. Yield estimations of low-energy positrons are performed by using an existing simulation code on the basis of the field measurements.
7. Research and optimization of the target-moderator system for positron beamline. The researches will be performed by simulation using new version of the positron transport code and experiments employing isotope sources.
8. Designs of the beamline including vacuum chamber, absorbers, supporting systems etc.


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