Angular Distribution of the High-Energy Component of Scattered Radiation from a 25 MeV Betatron

N. A. Tikhonov and B. M. Yakovlev

(Presented by the scientific seminar of the Faculty of Physics and Engineering)

Introduction

It is known that bremsstrahlung in a betatron arises as a result of the interaction of a beam of accelerated electrons with the target material; the direction of the radiation coincides predominantly with the direction of the electrons incident on the target. Theoretical and experimental works [1–3] devoted to the study of the interaction of the electron beam with the target material and to the angular distribution of the $\gamma$-radiation from the target give a sharply pronounced maximum of the radiation intensity and, in practice, an absence of radiation at angles greater than 15–20° to the direction of the main beam of $\gamma$-rays.

In addition to the main beam of $\gamma$-rays, the betatron also gives scattered radiation directed in all directions, especially in the plane of the electron orbit. Radiation at large angles to the direction of the main beam was assumed to be comparatively soft and was considered to be caused by bremsstrahlung at the chamber walls of electrons that had left, or had not entered, the accelerating regime.

However, in our study of the angular distribution of the radiation of a 25 MeV betatron, we observed not only soft scattered radiation, but also $\gamma$-rays with energies above 10–12 MeV at all angles to the direction of the main beam in the plane of the equilibrium orbit of the installation.

Experimental Method

The angular distribution of the radiation intensity was studied on a betatron with a maximum energy of accelerated electrons of 25 MeV.

Measurements of the integral intensity of the $\gamma$-rays were made by means of an ionization chamber with an active volume of 20 cm$^3$ and a variable thickness of the front wall. The chamber was moved in the plane of the equilibrium orbit of the installation along a circle of radius 1 m. The results of the measurements are presented in Fig. 1.

The curve of the angular distribution of the integral intensity of the $\gamma$-rays of the 25 MeV betatron is, in its character, similar to the angular distribution curves of radiation obtained by a number of authors for betatrons

with energies of 20–30 MeV. However, the half-intensity angle of the $\gamma$-radiation in our case is somewhat larger, which may be explained by the thickness of the target used (tungsten, 0.4 mm).

Fig. 1. Angular distribution of the integral intensity of the $\gamma$-radiation of a 25 MeV betatron at a distance of 1 m from the target.

The intensity of the high-energy part of the spectrum of the scattered radiation was measured by the method of induced activity in samples with different threshold energies for the $(\gamma, n)$ reaction. Samples of graphite and electrolytic copper were used, in the form of plates of specified geometric dimensions.

The samples under investigation were placed at a distance of 50 cm from the central axis of the apparatus, in the plane of the equilibrium orbit, perpendicular to the radius. The measurements were carried out on three different accelerating chambers; the chambers had different transverse cross sections, but the same injector and target. The maximum radiation intensity in all

in the three cases was different. For each series of measurements, the intensity of the radiation measured in the direction of the primary beam of $\gamma$ rays, as well as the irradiation time of the samples, remained strictly constant.

The activity of the samples, reduced to the beginning of the decay, was referred to a unit intensity of radiation in the direction of the primary beam and to one mole of substance. The observed decay curves of the induced activity of the samples corresponded to the decay of the isotopes $\mathrm{Cu}^{62}$ ($\tau = 9.8$–$10$ min) and $\mathrm{C}^{11}$ ($\tau = 20$–$21$ min).

The presence of other activities was not detected in either case.

To estimate possible errors, some measurements were carried out several times. The mean error was about 5%.

Results of the investigations

The results of measuring the activity of copper samples, approximately proportional to the intensity of the high-energy part of the spectrum of scattered radiation (above 11 MeV), are presented in Fig. 2 in the form of a “radiation rose.” Along the radius vector are plotted the values $\sqrt{N}$, where $N$ is the magnitude of the activity. The general form of the radiation rose is preserved for all accelerating chambers; moreover, for chambers No. 1 and No. 3, which had approximately equal working cross sections, the data practically coincide.

As is seen from Fig. 2, the relative intensity of the radiation in the direction opposite to the direction of the primary beam is about 25%. Such a distribution is quite unexpected and does not correspond to the distribution of the integral intensity recorded with an ionization chamber at a distance of 1 m. To clarify the reason for the anomalously large values of the intensity of scattered radiation near the accelerating chamber, a series of experiments is planned, including the taking of a spectrum and the separation of the electron flux.

The character of the induced activity indicates only reactions of the $(\gamma,n)$ type. The influence of reactions of the $(n,2n)$ type, leading to the same activities, may be disregarded, since they have the same threshold energies, while the number of neutrons emitted by the betatron with such energies is negligible and, according to the literature data [7], amounts to only hundreds of particles per roentgen of radiation in the primary beam.

In Fig. 3 is presented the dependence of the activity of graphite on the angle at which the irradiation was carried out. The rapid decrease in activity with increasing angle indicates that at angles greater than 50–60° there are practically no

Fig. 2. “Rose” of the intensity of the high-energy part of the scattered-radiation spectrum, taken at a distance of 0.5 m from the central axis of the installation. —O—O—O— curve for chambers with a large working cross section. —X—X—X— the same for a chamber with a small working cross section.

…rays with energy above 19 MeV. For comparison, the same graph gives the averaged values of the activity in copper samples.

The intensity of the scattered radiation varied in proportion to the intensity of the primary beam of γ-rays, from which it may be concluded that the high-energy component of the scattered radiation is associated with the electron beam incident on the target. Fast electrons that have passed through the target and have lost part of their energy thereby, under the action of the magnetic field of the betatron, strike the inner wall of the accelerating chamber, as a result of which radiation is produced that may also contain high-energy quanta.

An approximate calculation shows that the minimum energy loss at which electrons, after passing through the target without a change in the direction of motion, strike the inner wall of the chamber under the action of the magnetic field of the installation is, for our betatron, about 2.5 MeV.

The presence of a high-energy component of the scattered radiation is of interest from several points of view.

Since samples placed in the plane of the equilibrium orbit at angles of 90 and 270° were located near the poles of the betatron electromagnet, it should be expected that radioactivity is constantly induced in the steel of the electromagnet. Since the iron isotope Fe\(^{55}\) has a half-life of about 4 years, the induced activity will accumulate gradually.

Fig. 5. Angular distribution of the radiation according to the induced activity in the samples:
—●—●— copper, threshold energy 10.65 MeV;
—○—○— carbon, threshold energy 18.7 MeV.

Although the total operating time of the betatron is small, \(\sim 150\) hours, measurements carried out in the apparatus room show an increase in the background by approximately 50–60%. Measurements made immediately after a half-hour operation of the betatron, with a counter installed in the air gap between the electromagnet coils, showed considerable activity with a half-life of about 9 min. This activity may be attributed to the decay of the iron isotope Fe\(^{53}\) (\(\tau = 8.9\) min) and the copper isotope Cu\(^{62}\) (\(\tau = 10\) min). The similar half-lives of these isotopes make separation of the activities difficult. A small, slowly decreasing activity of a very complex character, with an average period of about 20–40 min, was also observed; it may be attributed to activation of the zinc white, textolite, and other parts of the electromagnet lining.

From the work of a number of authors [4, 5] and from our preliminary investigations it follows that betatrons with energies of 20–30 MeV are sources of neutron fluxes of appreciable magnitude. Therefore activation…

of some materials may also be due to other reactions, in particular \((n,\gamma)\)-reactions.

Conclusions

The measurements carried out on the scattered radiation of the betatron make it possible to draw the following conclusions:

1. A betatron with a radiation energy of \(25\) MeV has, in the spectrum of the scattered radiation, a high-energy component (with energy up to \(19\) MeV).

2. The intensity of the high-energy component is considerable and leads to activation of structural parts of the installation. We observed activation of copper samples located at an angle of \(90^\circ\) to the beam behind the magnet uprights, as well as behind a lead shield \(30\) mm thick.

3. The presence of high-energy scattered radiation should be taken into account in calculations and in the design of shielding for betatrons, especially for mobile installations of the enclosed type.

4. Special consideration should be given to the question of the possible use of scattered radiation. For example, simultaneous radiography of a series of specimens is possible, which may be of some interest for the purposes of flaw detection.

References

1. Schiff H. Phys. Rev., 70, 87, 1946. Phys. Rev., 83, 252, 1951.
2. Lawson J. D. Proc. Phys. Soc., 63 A, 653, 1950.
3. Phillips K. Proc. Phys. Soc., 65 A, 57, 1952.
4. Clark S. H. Nucleonics, 11, No. 9, 38, 1953.
5. Price G. A. and Kerst D. W.—Phys. Rev., 77, 806, 1950.
6. Wäffler H. Collection of articles, “Das Brown Boveri Betatron,” p. 26, 1953.