Measuring the NSRL Beam Energy using the Bragg Curve
When charged particles pass through matter, then at the energies of interest for NSRL Users, the charged particles lose energy primarily through ionization of the target material. The energy loss depends on the kinetic energy of the incident ion, having a minimum near 2-3 GeV per nucleon in the ion. For energies less than this, the energy loss per unit length increases as the ion slows down, with the greatest energy density occurring as the ion stops. Measuring this energy loss profile for a monoenergetic beam of incident ions shows a characteristic Bragg Peak with a width determined by the statistical nature of the ionization process and factors having to do with the fragmentation of the incident beam particles, scattering of target particles, and production of secondary particles due to interaction in the target. Figure 1 shows a typical Bragg Curve for 1000 MeV/nucleon Fe-56 taken at NSRL.
Looking at the Bragg Curve in detail, it is possible to distinguish some interesting features. For the iron beam, the effects of fragmentation dominate at the beginning. Fragments of lower Z produce less energy loss than the unfragmented ion. Depending on the Z of the beam and target, some Bragg Curves initially rise indicating that the effect of secondary particle production and scattering outweighs the effect of fragmentation. Obviously, in the case of proton beams, there is no initial drop in the energy loss distribution. The tail of the distribution beyond the Bragg Peak is due in part to the statistical fluctuations in the energy loss mechanism, and due to the longer range of lower Z fragments and secondaries in the particle shower that develops when the beam particle interacts in the target.
Additional Bragg Curves are shown for different beam energies and ions.
Last Modified: February 1, 2008