Counter Controlled Cloud Chamber Research Articles

On the electrical and anomalous scattering of mesotrons

Photographs obtained without a magnetic field in a large counter-controlled cloud chamber containing lead plates of two different thicknesses have been analyzed in regard to the scattering of the mesotrons in lead. The theory of scattering developed by E. J. Williams has been used. The mathematical form of this theory makes it possible to eliminate the energy distribution of the mesotrons in comparing the observed scattering intensity in two lead thicknesses. The scattering in a 5-cm lead plate into angles greater than 4\ifmmode^\circ\else\textdegree\fi{}, when compared to that in a 1-cm lead plate, is more than would have been expected on the basis of Williams' theory. The average cross section calculated for this anomalous large angle scattering agrees with the results obtained by Code but it is somewhat larger than that previously reported by Wilson. It is also noted that this cross section is in agreement with the interaction between mesotrons and nuclei computed by Marshak and Weisskopf when a spin of \textonehalf{} for the mesotrons and an energy distribution such as that computed by Hartree are assumed. Although these experiments are perhaps not yet able to distinguish between the scatterings to be expected from a mesotron spin of \textonehalf{} and that from a spin of 0, a spin of 1 seems to be definitely excluded since for this value of the spin the theory indicates a cross section of about 1000 times more than that found. It is noted that a possibility for an explanation of the observed anomaly still might be found in electromagnetic effects such as those leading to burst production by mesotrons or in attributing the anomaly to nuclear scattering of the proton component instead of mesotrons.

On the Electrical and Anomalous Scattering of Mesotrons

Photographs obtained without a magnetic field in a large counter-controlled cloud chamber containing lead plates of two different thicknesses have been analyzed in regard to the scattering of the mesotrons in lead. The theory of scattering developed by E. J. Williams has been used. The mathematical form of this theory makes it possible to eliminate the energy distribution of the mesotrons in comparing the observed scattering intensity in two lead thicknesses. The scattering in a 5-cm lead plate into angles greater than 4\ifmmode^\circ\else\textdegree\fi{}, when compared to that in a 1-cm lead plate, is more than would have been expected on the basis of Williams' theory. The average cross section calculated for this anomalous large angle scattering agrees with the results obtained by Code but it is somewhat larger than that previously reported by Wilson. It is also noted that this cross section is in agreement with the interaction between mesotrons and nuclei computed by Marshak and Weisskopf when a spin of \textonehalf{} for the mesotrons and an energy distribution such as that computed by Hartree are assumed. Although these experiments are perhaps not yet able to distinguish between the scatterings to be expected from a mesotron spin of \textonehalf{} and that from a spin of 0, a spin of 1 seems to be definitely excluded since for this value of the spin the theory indicates a cross section of about 1000 times more than that found. It is noted that a possibility for an explanation of the observed anomaly still might be found in electromagnetic effects such as those leading to burst production by mesotrons or in attributing the anomaly to nuclear scattering of the proton component instead of mesotrons.

The Scattering of Mesotrons in Tungsten

Measurements of the scattering of mesotrons have been made with a 30-cm counter-controlled cloud chamber in a 12,900-oersted magnetic field. The scattering block consisted of 3.8 cm of tungsten, and the lowest Geiger-Mueller counter was mounted directly over the center of the tungsten. Scattering angle and curvature measurements were made on 359 tracks. In addition, there were 92 high energy tracks with a deflection too small to be measurable. The scattering angles varied from 0 to 18.7\ifmmode^\circ\else\textdegree\fi{}, and the mean energies $\overline{E}$ were nearly all less than 2\ifmmode\times\else\texttimes\fi{}${10}^{9}$ ev. The values of the product $\overline{E}\ensuremath{\theta}$ varied from zero to a maximum of 13.7\ifmmode\times\else\texttimes\fi{}${10}^{9}$ ev degrees. These results confirm Williams' prediction of a Gaussian distribution of multiple scattering. A number of cases of anomalous large-angle scattering were obtained, as contrasted with the single case of large-angle scattering observed by Wilson, establishing a departure at these angles from the Gaussian law. This further substantiates Williams' theory with reference to large-angle scattering caused by nuclear forces. The mean value of the multiple electrical scattering calculated from Wilson's equation is 2.2\ifmmode\times\else\texttimes\fi{}${10}^{9}$ ev degrees for this experimental arrangement, which compares with the experimental value of 2.14\ifmmode\times\else\texttimes\fi{}${10}^{9}$. This very satisfactory agreement supports Williams' assumption that the main force responsible for the Gaussian part of the scattering is that arising from the electric charges of the mesotron and of the nucleus.

Specific Ionization of Cosmic Ray Particles

MEASUREMENTS of the specific ionization of cosmic ray electrons with energies between 0.1 × 106 ev. and 30 × 106 ev. have been made by Corson and Brode1 by counting droplets on photographs taken with a counter-controlled cloud chamber using a delayed expansion. They established the existence of a minimum ionization at about 1.5 × 106 ev. and a logarithmic increase in ionization with energy beyond this minimum. Using essentially the same method, I have measured the specific ionization of cosmic ray particles over the energy range 2 × 106–6 × 109ev.

Evidence for the Occurrence of Meson Pairs

WE have taken a number of photographs with a counter-controlled cloud chamber in a tunnel 30 m. under London clay. (1) 630 photographs were taken with a 1·4 cm. lead plate inside the chamber; (2) 948 photographs were taken with a 2·5 cm. lead plate inside the chamber; (3) 780 photographs were taken with a 2·5 cm. lead plate inside the chamber and 15 cm. of lead above the chamber.

The scattering of cosmic ray particles in metal plates

Using an ordinary cloud chamber, Anderson (1933) measured the average angle of scattering of cosmic-ray particles of energy up to 3 x 10 8 e-volts in a lead plate of thickness 1·1 cm. Williams (1936) pointed out that, in such experiments, the scattering is multiple, and that the observed values agreed with the theoretical predictions, assuming the particles to be electrons. Recently Neddermeyer and Anderson (1937) have made some new measure­ments using a counter-controlled cloud chamber and a 1 cm. platinum plate. No numerical results are given, but the observed scattering of particles of energy up to 5 x 10 8 e-volts, which appears to be the limit of the energy measurements, seems in rough agreement with the earlier results. Using the counter-controlled cloud chamber already described by Blackett (1936) and by Blackett and Brode (1936), the multiple scattering of cosmic-ray particles of energy up 9 x 10 9 e-volts in lead and copper plates has been measured. In order to make such measurements possible, it is necessary to reduce as far as possible the distortion of the tracks in the chamber. The technique by which this can be achieved has been described by Blackett and Wilson (1937) in connexion with the measure­ment of the energy loss of rays in traversing metal plates. In fact, the photographs taken for the measurement of the energy loss were, amongst others, found suitable for the measurement of the scattering. The angle of scattering was measured by means of a goniometer eyepiece attached to a travelling microscope. The cross-wires were set tangentially to the track, first on one side and then on the other side of the plate. The mean of the two measured angles of deflexion, obtained from the two stereoscopic photographs, was taken as giving the projection on the plane of the chamber of the actual angle of scattering.

The Production of Cosmic-Ray Showers in Lead

Photographs were taken of cosmic-ray showers arising in lead plates of various thickness placed in turn inside a Geiger counter controlled cloud chamber. These showers are classified according to the number of particles in the shower and according to the manner of production, whether by an ionizing particle or by nonionizing radiation. The average size of showers was found to increase with the plate thickness. Studies of the frequencies of showers of different sizes as a function of plate thickness show that electron-produced and photon-produced showers occur in approximately equal numbers and are of the same character. These facts give support to the theories of Carlson and Oppenheimer and of Bhabha and Heitler. They consider the shower to be the result of repeated subdivision of incident radiation through pair production by gamma-rays and through radiative losses of high energy electrons giving rise to other gamma-rays. Eight percent of the electrons striking lead plates near seven millimeters thick gave rise to showers. About an equal number of showers from gamma-rays were present.

The energy loss of cosmic ray particles in metal plates

Measurements have been made of the energy loss of cosmic ray particles in metal plates, making use of a counter controlled cloud chamber in a magnetic field (Blackett 1936). A metal plate was placed across the centre of the chamber and the energy loss of a ray was deduced from the difference of the curvature of a track above and below the plate. Energy loss measurements by this method have been carried out by Anderson and Neddermeyer (1936) up to an energy of about 4 x 10 8 e-volts and recently by Crussard and Leprince-Ringuet (1937) up to an energy of 1·2 x 10 9 e-volts. The curvature measurements were made mainly by means of the optical null method recently described (Blackett 1937 a ) and this proved invaluable. It would have been hard to obtain so high an accuracy by the usual method of measuring coordinates. The curvature corrections to be applied to the measured curvatures were obtained by measurements on tracks in zero magnetic field (Blackett and Brode 1936). Two separate distortion curves were required, one for the top and one for the bottom of the chamber.

Further measurements of the cosmic-ray energy spectrum

The work already described (Blackett and Brode 1936) of determining the energy spectrum of cosmic-ray particles, by measuring the curvature of their tracks with a counter-controlled cloud chamber in a strong magnetic field, has been continued. A new set of 641 tracks in a magnetic field of 12,000-14,000 gauss have been measured, making with the 188 already reported a total of 829 tracks. An expansion ratio of about 1·08 was used, with an argon-oxygen mixture for the gas and with an ethyl alcohol and water mixture for the liquid. In order to avoid the inconveniently large distortions of the lenses previously used, Messrs Taylor, Taylor and Hobson supplied a new pair of 35 mm. f /2 ciné lenses, which were especially adjusted to have a very small distortion, when used with a 1·2 cm. glass plate as the roof of the chamber: fig. 1 shows the distortion curves, that is, the image curvature c as a function of the displacement x , both for the normal and for these special lenses. The initial slope of the curve for the new lens is about one-fifth of that for the normal lens, while the deviation from linearity is only reduced in much less degree. This confirms the statement (Blackett and Brode 1936, p. 578) of Mr A. Warmisham of Taylor, Taylor and Hobson, Ltd., that the coefficient a 1 of the linear term, which gives the initial slope of the curve c = f ( x ), is easily under the designer’s control, whereas the coefficient a 2 of the cubic term, which governs the departure from linearity, is roughly a constant for a given type of lens. The use of these lenses has greatly facilitated the work.

A Note on the Production of Cosmic-Ray Showers

The cosmic-ray showers, which are shown so beautifully in the counter controlled cloud chamber,(1) have been investigated by numerous observers. The method in general has consisted in putting three Geiger counters below a thin plate of heavy material. The counters are usually arranged so that three particles emerging simultaneously from the plate are required to register a coincidence. Hence a shower will consist of at least three particles associated in time.

On the technique of the counter controlled cloud chamber

The method by which very fast atomic particles are made to take their own could photographs has proved very useful for the investigation of cosmic rays. A short account of the method and a detailed account of the results obtained by its use have already been given by Blackett and Occhialini. Recently Locher and also Anderson, Millikan, Neddermeyer, and Pickering have used the same method and have obtained some beautiful results.