Proton Scanner

Abstract

The scanner is based on the nuclear scattering of high energy protons by the nucleons (protons and neutrons) included in the atomic nuclei. Since it has been described elsewhere (1, 2, 3) we simply recall that

• a/ because of the wide scattering angle; three coordinates in space of the interaction point can be computed, giving directly three dimensional radiographs. Volumic resolution is of about a few cubic-millimeters,

• b/ because the base interaction is the strong nuclear force, the atomic dependence of the information obtained is different from that of the X-ray scanner, for which the base interaction is electro-magnetic force. For a volume element (vel) expressed in cc, with density d and atomic composition the number of scattered protons is :

  $${{N}_{S}} = 0.6 {{N}_{l}} \times \Delta {V} \times d\frac{{\Sigma {{ }^{a}}i{{ }^{o}}i}}{{\Sigma {{ }^{a}}i{{ }^{A}}i}}$$

  ((1))

  where N l is the intensity of the incident proton beam (particles/ cm2) and o i are the nuclear scattering cross-section of the elements A i expressed in barns (10 −24 cm2). Because the localisation of only one scattered particle is involved, the radiographs obtained from the variations of N S are called simple or S radiographs in this paper. To illustrate the difference between the proton scanner and the X-ray scanner, we recall that calcium (d = 1.53) gives, to within 10%, the same N S as water. In the case of a 60 keV-X ray, the absorption of calcium is higher by a factor of about 5 as compared with water.

• c/ By detecting not only the scattered proton but also the recoil nucleon (2) , one is able to separate simultaneously the nuclear scattering by the hydrogen contained in objects. The number of protons scattered by hydrogen is :

  $${{N}_{H}} = 0.6 \times {{N}_{I}} \times \Delta V \times dx\frac{{^{a}H{{ }^{o}}H}}{{\Sigma {{ }^{a}}i{{ }^{A}}i}}$$

  ((2))

  with the same units as in formula 1.