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The London Edinburgh and Dublin Philosophical Magazine and Journal of Science

XXXIV. An experimental study of the quantum nature of X-rays

M. Parker Givens

Published May 1, 1946• 8 citations

Abstract

A study has been made of the X-rays symmetrically diffracted by the (130) and (10) planes of NaCl. The object of the study was to determine whether or not X-rays are diffracted in both directions simultaneously. Two Geiger Muller counters were used to detect the X-rays. A coincidence circuit was connected to the two counters. Over 20 million quanta were counted by each counter, yet only 12 thousand apparent coincidences were observed. A detailed study of the data reveals that this small number of coincidences is best interpreted by assuming no coherence between the two diffracted X-ray beams and attributing the coincidence counts to the chance arrival of two quanta, one at each counter, separated in time by less than the resolving time of the coincidence circuit. This lack of coincidence is in accord with the quantum picture of radiation.

DOI: 10.1080/14786444608561338

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A study has been made of the X-rays symmetrically diffracted by the (130) and (10) planes of NaCl. The object of the study was to determine whether or not X-rays are diffracted in both directions simultaneously. Two Geiger Muller counters were used to detect the X-rays. A coincidence circuit was connected to the two counters. Over 20 million quanta were counted by each counter, yet only 12 thousand apparent coincidences were observed. A detailed study of the data reveals that this small number of coincidences is best interpreted by assuming no coherence between the two diffracted X-ray beams and attributing the coincidence counts to the chance arrival of two quanta, one at each counter, separated in time by less than the resolving time of the coincidence circuit. This lack of coincidence is in accord with the quantum picture of radiation.

The existence of a correlation between the arrival times of photons has been confirmed by measurements with a coincidence counter having a resolving time of 3·5 X 10-9 sec in three different experiments. In the first experiment it was found that the number of coincidence counts recorded from two photomultipliers, the apertures of which were optically superimposed, was significantly greater than when the light beams were incoherent. Furthermore, the number of these correlated counts was in satisfactory agreement with that predicted by theory. In the second experiment the change in the number of excess coincidences was measured as the degree of coherence of the light was altered by increasing the apparent separation of the photocathodes, and in this case also there was reasonable agreement between theory and experiment. In the final experiment it was shown that there was a significant difference between the number of coincidences observed when the light beams were in identical as opposed to orthogonal polarizations, and this last result especially makes it extremely improbable that the correlation could be caused by some spurious effect, such as plasma oscillations in the source, since the light source itself was found to be completely unpolarized.

A test has been made of the photon theory of the scattering of high frequency radiation. The pairs of scattered photons and recoil electrons predicted by this theory have been looked for by means of specially designed Geiger-M\"uller counters. Coincident discharges in the electron and photon counters were recorded by means of a vacuum tube amplifying and adding circuit. The scatterers used were air, aluminum, beryllium, filter paper and paraffin. The radiation was the gamma-rays from radium C. Experiments were performed with the counters set at various angles, some where the photon theory predicts coincidences, and others where coincidences should not be expected. The experiments uniformly gave fewer coincidences in the correct positions than were expected, and those observed could in every case be accounted for as chance coincidences due to the finite resolving time of the apparatus. It has not been found possible to bring the results of these experiments into accord with the photon theory of scattering. The wave-mechanical theory of the scattering process has not yet been extended to include the gamma-ray region so that it is impossible to compare this theory with the present experiments. Unless it is shown that the two theories disagree in the gamma-ray region it does not seem possible to reconcile the present experiment with the Bothe-Geiger and Compton-Simon experiments.

Synopsis Methods which are widely used in the mathematical analysis of random noise are used here to obtain expressions for the coincidence-counting rates which could be obtained in studies of the intensity correlations between plane-polarized, parallel beams of nearly monochromatic light. It is shown that, with presently available circuit techniques, delayed coincidence measurements could provide information about the breadths of lines radiated by atomic beam light sources, and in prompt coincidence experiments it should be possible to observe interference between beams of incoherent light from sources whose line widths are comparable with those of the Hg198 electrodeless discharge.

Interference experiments were carried out in which the source was so faint that effects could be attributed to separate quanta of radiation. The energy radiated in the helium line 4471A from a glow discharge was measured by comparison with a black body, and from the known decay constant of the atomic radiation process, the separation of atomic radiations in time was deduced. With an echelon diffraction grating the characteristic double order pattern was photographed when each quantum of light passed separately through the instrument. Under these conditions it gave rise to a pattern due to simultaneous passage through several steps. With an air film between two parallel plates, interference fringes were photographed in 4471, when the quanta emitted from the volume of the source used were completely separated in time, showing that a single quantum obeys the classical laws of partial transmission and reflection at a half silvered mirror and of subsequent combination with the phase difference required by the wave theory of light.