FROM CURRENT LITERATURE

VISIBLE RADIATION IN A SYNCHROTRON

An electron rotating in a circle in a magnetic field must radiate. Thus, in accelerators of the synchrotron or betatron type, electromagnetic radiation should be observed, as Ivanenko and Pomeranchuk drew attention to. This radiation adiabatically reduces the equilibrium radius of the betatron. Along the trajectories shown by them, electrons succeed in striking the inner wall of the accelerator chamber.[^2] In a synchrotron, radiation acts differently. If the energy loss due to radiation in one revolution is less than the maximum energy that an electron can acquire in the accelerating interval, then the radiation does not disturb the synchronous mode of operation.[^3][^4]

Artsimovich and Pomeranchuk[^2] investigated in detail the field structure of the incoherent radiation of electrons*). Their main results are as follows. An electron rotating at radius \(R\) with frequency \(\omega\) radiates per revolution

\[ \sim 6.03 \cdot 10^{-7}\,\frac{1}{R}\left(\frac{\varepsilon}{\varepsilon_0}\right)^4 \]

electron-volts**). Here \(\varepsilon\) is the total energy of the electron, and \(\varepsilon_0\) is the rest energy. The radiation is concentrated in a narrow cone whose axis is the direction of the instantaneous velocity, and whose angular aperture is, in order of magnitude, equal to \(\frac{\varepsilon_0}{\varepsilon}\) radians. The radiation spectrum of the electron is very complex and consists of many lines corresponding to harmonics of the fundamental frequency \(\omega, 2\omega, 3\omega,\ldots n\omega\), etc. The figure shows the distribution of radiation intensity over harmonics. The maximum of the radiation lies near the \(n_0\)-th harmonic, where

\[ n_0=\left(\frac{\varepsilon}{\varepsilon_0}\right)^3 . \]

Thus, for \(\varepsilon>\varepsilon_0\), the radiation maximum shifts toward short wavelengths and, under certain conditions, may overlap the visible region.

The first experiments to detect radiation in a 100 MeV betatron were carried out by Blewett.[^6] However, he was unable to observe the electromagnetic radiation directly. He succeeded only in showing that under

*) The solution of a similar problem is also contained in Schott’s book.[^5]

**) All formulas refer to the relativistic case \(\left(\frac{v}{c}\sim 1\right)\).

under the action of radiation the betatron radius is in fact reduced to 3.2 cm.

Recently, a group of researchers⁷ observed visible radiation in the General Electric synchrotron at 70 MeV. The radius of the electron orbit in this accelerator was 29.3 cm. The radiation was visible as a small white luminous spot if one looked in the tangential direction to the orbit, toward the approaching electron. The spot was very bright when the intensity of the x-rays produced in the accelerator was 50 roentgens per minute at a distance of 1 m from the target, and it could still be observed in daylight at an intensity of 0.1 roentgen. The synchrotron had two targets—an outer and an inner one. If the electric field of the synchrotron was switched off before the magnetic field increased to its maximum value, the radius of the orbit decreased, and the electrons struck the inner target at a given energy depending on the moment at which the accelerating electric field was switched off. Conversely, switching off the electric field while the magnetic field was decreasing increased the radius of the orbit, and the electrons struck the outer target. In the second case the energy of the electrons rose to the maximum value of 70 MeV and then decreased to the specified energy.

When the maximum energy was increased in the first case (inner target), the intensity increased rapidly. If the electrons struck the inner target before their energy reached 30 MeV, the visible radiation disappeared.

In the second case (outer target), the intensity did not depend on the energy at which acceleration ended. The authors explain this phenomenon by the sharp dependence of the radiation on the energy (\(\sim \mathcal{E}^4\)). Indeed, in this case the intensity is almost entirely determined by the maximum energy. Radiation at lower energies gives only a small correction. Observing the radiation through a slotted disk rotating synchronously with the change of the magnetic field, the authors found that in the second case the radiation is visible in the region \(90\)—\(100^\circ\) with respect to the magnetic field.

If the accelerating electric field was switched off somewhat before the magnetic field reached its maximum, then the radius of the orbit at first decreased, but before the electrons struck the inner target the magnetic field began to decrease, and therefore the radius of the orbit increased, and the electrons struck the outer target. In this case, instead of a spot, a luminous line was observed in the plane of the orbit.

It was found that the electric vector of the radiation lies in the plane of the orbit. At present the spectral composition of the radiation is being investigated.

In the popular-science journal Science News Letters an explanation is given⁸ of why no such radiation was observed in the 100 MeV betatron. The betatron chamber was silvered, and observation could not be carried out through it, whereas in the synchrotron a transparent covering was used. During operation of the synchrotron it could not be approached because of the strong x-radiation; therefore the observations were made from behind a protective concrete wall with the aid of mirrors.

M. Rabinovich

CITED LITERATURE

1. D. D. Ivanenko and I. Ya. Pomeranchuk, DAN 44, 343 (1944).
2. L. Artsimovich and I. Ya. Pomeranchuk, ZhETF 16, 379 (1946).
3. M. S. Rabinovich, Journ. of Phys. 10, 523 (1946).
4. L. Foldy and D. Bohm, Phys. Rev. 70, 249 (1946).

1. G. A. Schott, Electromagnetic Radiation (Cambridge University Press, Cambridge, 1912).
2. J. P. Blewett, Phys. Rev. 67, 87 (1946).
3. F. R. Elder et al., Phys. Rev. 71, 829 (1947).
4. Science News Letters 51, 339 (1947), No. 22, May 31, 1947.