EXPERIMENTAL STUDY OF THE BREMSSTRAHLUNG SPECTRUM

A necessary condition for the use of electron accelerators (betatron, synchrotron) as a source of γ-quanta in the investigation of various nuclear reactions (such as, for example, the nuclear photoeffect, photofission, etc.) is knowledge of the bremsstrahlung spectrum.

In the first studies carried out on betatrons, a simplified spectrum was used, close in form to the theoretical one calculated by

Fig. 1. Bremsstrahlung spectrum obtained on a betatron with electron energy 19.5 MeV in a body angle of 0.24°. Thickness of the platinum target—0.127 mm. Solid line—the experimental curve. Dashed line—the theoretical curve.

Bethe and Heitler¹. The results of investigations of the spectrum, first undertaken on a 2.3 MeV betatron and later on a 20 MeV betatron²˒³, were not convincing because of the low accuracy achieved in the measurements. In the work carried out on a 22 MeV betatron⁴˒⁵, the authors found the spectrum by measuring the energies of electron pairs formed in a Wilson chamber filled with air at \(p = 1.4\) atm. The collimated beam entered a small part of the chamber volume through a thin (0.04 mm Al) window. The thickness of the platinum target of the accelerator

Fig. 2. Dependence of the number of proton tracks \((N)\) on the gamma-quantum energy \(E_\gamma\). The solid line is the experimental curve. The dotted line is the theoretical curve.

Fig. 3. Intensity spectrum of a 10 MeV betatron.

equal to 0.125 mm. The photographs were taken synchronously with the beam every 30 seconds. As a result of the measurements, about 1300 pairs of electron and positron tracks were selected which could be measured with sufficient accuracy. By introducing correction coefficients, the authors took into account the fraction of scattered particles.

Figure 1 shows the bremsstrahlung spectrum obtained in the work and the theoretical spectrum calculated for small angular momenta on the basis of the Bethe–Heitler theory.

The discrepancy in the course of the curves in the region of intermediate energies apparently exceeds the possible experimental errors.

Another method of spectrum analysis was applied in work on a 10-MeV betatron⁶. The authors determined the photon energy by measuring the tracks of protons produced in the photodisintegration of deuterium. Ilford C-2 plates with an emulsion thickness of 100 μ were washed for 20 min in D₂O at \(t = 25^\circ\)C. The plates were then placed in the beam perpendicular to its axis, 1.5 m from the target. 312 tracks were selected in the angular interval \(\pm 45^\circ\). After corrections had been introduced for the dependence of the probability of recording a complete track on its length, the curve shown in Fig. 2 was obtained. The theoretical curve mentioned above is indicated by the dotted line (the number of proton tracks is plotted along the ordinate axis). Figure 3 presents the intensity spectrum obtained by the authors.

R.

CITED LITERATURE

1. H. Bethe and Geitler, Proc. Roy. Soc. 146, 83 (1934).
2. W. B. Lasich and L. Riddiford, J. Sci Instr. 24, 177 (1947).
3. Bosley, Craggs, Nach and Paync, Nature 161, 1022 (1948).
4. H. W. Koch and R. E. Carter, Phys. Rev. 75, 1950 (1949).
5. H. W. Koch and R. E. Carter, Phys. Rev. 77, 165 (1950).
6. P. K. S. Wang and M. Weiner, Phys. Rev. 76, 1724 1949).