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DIRECT MEASUREMENT OF THE SPEED OF 170-MeV $\gamma$ RADIATION
Until recently, direct measurements of the propagation speed of electromagnetic radiation had been limited to visible light. Some time ago, the development of radiolocation technology made it possible to supplement them with measurements in the microwave range; as was to be expected, values were obtained that are in excellent agreement with the measurements in visible light[^1]. At present, with the creation of fast-particle counters and coincidence circuits with high time resolution, it has become possible to advance far into the region of high frequencies and to carry out direct measurements of the speed of motion of individual $\gamma$ quanta.
The results of such measurements for $\gamma$ quanta with an energy of $0.5$ MeV have already been reported in our journal[^2]. The author of the note under review carried out analogous measurements with $\gamma$ quanta having an energy of 170 MeV, but used an entirely different method. The arrangement of the apparatus is shown in the figure.
The monoenergetic $\gamma$ rays used for the measurements were separated from the bremsstrahlung produced in a thin target 2 when electrons with an energy of 310 MeV struck it (the experiments were carried out on the Cornell synchrotron). The arrangement of the apparatus is shown in the figure.
The decelerated electrons were deflected by a magnetic field and entered a scintillation counter 6, positioned so that electrons with an energy of 140 MeV entered the counter. The bremsstrahlung corresponding to these electrons was selected from the total mass of $\gamma$ quanta entering the second scintillation counter 5 by means of a delayed-coincidence circuit. Spectrometric analysis of the energy spectrum of this $\gamma$ radiation showed that the peak maximum corresponded to an energy of 170 MeV, and its half-width did not exceed 20%.
By moving counter 5 and measuring the relative delay time as a function of the position of this counter, it was possible to determine directly the speed of the $\gamma$ quanta. The measurements were made for four positions of the counter, the extreme ones being 13 m apart. The resolving time of the coincidence circuit was $4 \cdot 10^{-9}$ sec. The error in determining the position of the peak did not exceed $2 \cdot 10^{-10}$ sec. The delay time due to the cables was determined with an accuracy of up to 0.5%. The speed was determined from the slope of the straight line on which lay
measured delay-time values plotted as a function of the distance to the counter.
Schematic of the apparatus for measuring the speed of bremsstrahlung γ-radiation.
1 — beam of electrons with energy 310 MeV;
2 — target;
3 — bremsstrahlung γ-radiation;
4 — trajectory of delayed electrons;
5 — movable stilbene counter of γ-quanta;
6 — stationary stilbene counter of delayed electrons.
As a result of the measurements, a value of the speed was obtained equal to \(2.974 \cdot 10^{10}\ \text{cm/sec}\), with a probable error of the order of 1%. Within the limits of error this value agrees with the value obtained\(^2\) for γ-quanta with energy 0.5 MeV: \((2.983 \pm 0.015)\,10^{10}\ \text{cm/sec}\), and also with the value of the speed of light in vacuum accepted at present, obtained as a result of critical treatment of the results of measurements in visible light and in the microwave radio-frequency range \((c = 2.998 \cdot 10^{10}\ \text{cm/sec})\)\(^4\).
Thus, direct measurements of the propagation speed of electromagnetic radiation now cover a very broad frequency range—from about \(10^3\) to \(10^{16}\) MHz (quantum energies from \(10^{-5}\) to \(10^8\) eV); moreover, throughout this range the propagation speed is the same, at any rate to an accuracy of the order of 1%. Although there is no reason to doubt the identity of the values of the speed of light in vacuum for all frequencies, direct experimental confirmation of this fact over so broad a range is of obvious interest.
References
- UFN 42, 458 (1950).
- M. R. Cleland and P. S. Jastram. Phys. Rev. 84, 271 (1951). See also UFN 46, 418 (1952).
- D. Luckey and J. W. Weil, Phys. Rev. 85, 1060 (1952).
- UFN 45, 458 (1951).