PRODUCTION OF POSITRONS BY THE METHOD OF ELECTRON CONVERSION AT THE MICROTRON OF the Research Institute of Nuclear Physics, TPI

S. A. Vorob’ev

Positrons and the annihilation radiation of positrons are finding ever broader application in experimental nuclear physics [1—3]. A method that makes it possible to obtain intense positron beams with an energy variable over a wide range of values is electron–positron conversion in a target irradiated by an electron beam [4—6].

Electron–positron conversion consists in the generation of positrons by bremsstrahlung quanta that arise when fast electrons pass through a target—the converter. The yield of positrons from the target depends on the energy of the primary electrons, and on the material and thickness of the target.

In the present work, the dependence of the energy distributions of positrons emitted in the forward direction on the converter thickness was investigated.

As the electron source in our work we used a microtron with a maximum accelerated-electron energy of 6.51 MeV. The electron beam extracted from the microtron was formed by means of three quadrupole lenses and directed into a vacuum chamber, where a tungsten target—the converter—was mounted perpendicular to the beam. The energy distribution of the positrons emerging from the target in the forward direction was studied using a sector magnetic beta spectrometer with a beam deflection angle of 90° and a central-ray radius of curvature of 200 mm. The energy resolution of the beta spectrometer was 3%. The positrons selected by the magnetic beta spectrometer were recorded with a scintillation counter with a thin stilbene scintillator.

Figure 1 gives the normalized energy distributions of positrons emitted in the forward direction from tungsten targets of different thicknesses. As the converter thickness increases, there is a noticeable shift of the maximum of the distribution toward lower energies and a relative decrease in the yield of positrons of higher energies.

Figure 2 shows the dependence of the positron energy at the maximum of the distribution on the thickness of the tungsten target. Within the experimental error, the dependence obtained is described fairly well by a linear law.

The author expresses gratitude to Doctor of Physical and Mathematical Sciences A. N. Didenko for supervising the work.

Fig. 1. Normalized energy distributions of positrons emitted forward for different converter thicknesses: \(0.75\ \mathrm{g/cm^2}\) (1), \(1.25\ \mathrm{g/cm^2}\) (2), \(1.84\ \mathrm{g/cm^2}\) (3), \(2.34\ \mathrm{g/cm^2}\) (4).
\(T\) is the kinetic energy of the positrons.

Fig. 2. Dependence of the energy at the maximum of the positron energy distribution \(T_{\max}\) on the target thickness \(t\), \(\mathrm{g/cm^2}\).

REFERENCES

1. Annihilation of Positrons in Solids, collection of articles, Izd. inostr. lit., Moscow, 1960.

2. Proceedings at the Conference on Positron Annihilation held at Wayne State University, Detroit, Mich., USA, July 26–30, 1965, Edited by Stewart A. T. Roelling, Academic Press, New York—London, 1967.

3. V. I. Gol’danskii, Physical Chemistry of the Positron and Positronium, Nauka, Moscow, 1968.

4. Milles L. et al., J. Phys. Radium, 21, 296, 1960.

5. K. A. Belovintsev et al., Proceedings of the International Conference on Accelerators, Dubna, 1963. Atomizdat, p. 1061, 1964.

6. I. A. Grishaev et al., UFZh, 10, 260, 1964.