The scattering of 9-5 MeV protons by He, C, N, 0, F, Ne, A, Kr and Xe nuclei By W. M. Gibson Physics Department, Queen's University of Belfast J. D. Prowse H. H. Wills Physical Laboratory, University of Bristol J. and R otblat Physics Department, Medical College of St Bartholomew's Hospital, London (Communicated by C. F. Powell, F.R.S.—Received 26 June 1957) A nuclear emulsion method for studying scattering and nuclear reactions of particles from the Nuffield cyclotron of the University of Birmingham, in gaseous targets, is presented. This method has been developed from that described by Burrows, Powell & Rotblat (1951).
238 W. M. Gibson, D. J. Prowse and J. Rotblat a-particles, and to the p + p, p + d and processes. Results for the elastic scattering of a-particles in helium have already been published (Burcham, Gibson, Prowse & Rotblat 1957).
240 W. M. Gibson, D. J. Prowse and J. Rotblat plates; and thence to reduce the number of incomplete tracks due to the scattering of particles out through the surface of the emulsion. With the increased angles of dip, it has sometimes been necessary to use emulsions 400 or even 600 p, thick, to prevent excessive loss of the longer tracks at the bottom surface, through their entry into the glass.
The scattering 9of-5 protons 241 Up to July 1956, the current from the Faraday cup was fed to an integrator with an input resistance which was either 5 or 40 MQ. The voltage so produced was amplified and converted to an alternating voltage, which after further amplifica tion was integrated by a watt-hour meter. The circuit was calibrated by measuring the input resistors and by recording the response of the meter to standard voltages applied across the input resistor. With the 5 MQ resistor the wheel of the watt-hour meter made 32*0 rev/pU, and with 40 257 rev//xC. The balancing of the circuit gave uncertainties in the current of about 8 x 10-u A, and hence 0-3 /xC in the charge during an exposure lasting 1 h.
242 W. M. Gibson, D. J. Prowse and J. Rotblat a given angle was corrected for the foreshortening due to the dip of the tracks, and the corresponding energy obtained from the range-energy relation of Gibson, Prowse & Rotblat (1954). To this was added the energy lost by the protons in traversing the gas between the target and the plate (normally estimated from an extrapolation of the stopping powers quoted by Brolley & Elbe (1955)), and from the dynamics of the collision the energy of the incident beam was calculated. This was usually accurate to ± 0*03 MeV, the limit set by the range-energy rela tion, but differences, e.g. those quoted in § 3(6), are normally accurate to 0-01 or 0 02 MeV.
The scattering of9*5 MeV jprotons 243 occasionally a few reaction products of characteristic range may aid the identifica tion. Separate measurements have been made with oxygen, nitrogen and hydrogen as target gases, and the differential cross-sections obtained from these allow the proportion present as impurity in another gas to be calculated from the intensity of the spurious group. A correction for the partial pressure of the impurity may thus be made. Sometimes, when the impurity is identified as oxygen or nitrogen, it may partly result from leakage of air into the camera: a slight increase of pressure during the particular exposure confirms this.
244 W. M. Gibson, D. J. Prowse and J. Rotblat Oo o 60 <p 90 Figure 7. C.m.s. angular distributions of elastically scattered protons, with optical model curves. Ordinates are ratios to Rutherford cross-section.
The scattering of9-5 MeV protons 245 (1954). Melkanoff, Nodvik, Saxon & Woods (1957) and Glassgold, Cheston, Stein, Schuldt & Erickson (1957) have used a potential well with a diffuse edge. The latter authors have made use of the results of Hintz (1957) at an energy of 9-8 MeV, very close to our own. For the two elements studied by both Hintz and ourselves (nitrogen and argon), the results are in good agreement, and show no more than slight dependence on bombarding energy.
246 W. M. Gibson, D. J. Prowse and J. Rotblat communication). There is a very marked variation with energy, especially at backward angles. This cannot be explained by the optical model, which can give only slight changes over such a small range of energies. It is necessary to assume that the process is controlled by the properties of the particular levels excited in the compound nucleus 13N (see § 4 (a)).