  The dose distribution of a monoenergetic pencil beam in water consists of an
electromagnetic "core", a "halo" from charged nuclear secondaries, and a much
larger "aura" from neutral secondaries. These regions overlap, but each has
distinct spatial characteristics.
  We have measured the core/halo using a 177MeV test beam offset in a water
tank. The beam monitor was a fluence calibrated plane parallel ionization
chamber (IC) and the field chamber, a dose calibrated Exradin T1, so the dose
measurements are absolute (MeV/g/p). We performed depth-dose scans at ten
displacements from the beam axis ranging from 0 to 10cm. The dose spans five
orders of magnitude, and the transition from halo to aura is clearly visible.
  We have performed model-dependent (MD) and model-independent (MI) fits to the
data. The MD fit separates the dose into core, elastic/inelastic nuclear,
nonelastic nuclear and aura terms, and achieves a global rms measurement/fit
ratio of 15%. The MI fit uses cubic splines and the same ratio is 9%.
  We review the literature, in particular the use of Pedroni's parametrization
of the core/halo. Several papers improve on his Gaussian transverse
distribution of the halo, but all retain his T(w), the radial integral of the
depth-dose multiplying both the core and halo terms and motivating measurements
with large "Bragg peak chambers" (BPCs).
  We argue that this use of T(w), which by its definition includes energy
deposition by nuclear secondaries, is incorrect. T(w) should be replaced in the
core term, and in at least part of the halo, by a purely electromagnetic mass
stopping power. BPC measurements are unnecessary, and irrelevant to
parameterizing the pencil beam.
