  The dose distribution of a pencil beam in water consists of a core, a halo,
an aura and (possibly) spray. The core is due to primary protons which suffer
multiple Coulomb scattering (MCS) and slow down by multiple collisions with
atomic electrons (Bethe-Bloch theory). The halo is due to charged secondaries,
many of them protons, from elastic interactions with H, elastic and inelastic
interactions with O, and nonelastic interactions with O. We show that the halo
radius is roughly one third of the beam range. The aura is due to neutral
secondaries (neutrons and gamma rays). Spray denotes dose, avoidable in
principle, coming in from the beam line.
  We have measured the absolute dose at 177 MeV using a test beam in a water
tank. The beam monitor was a PPIC 'proton counter' and the field IC a dose
calibrated Exradin T1. We took depth-dose scans at ten displacements from the
beam axis ranging from 0 to 10 cm. The dose spans five orders of magnitude, and
the transition from halo to aura is obvious.
  We present model-dependent (MD) and model-independent (MI) fits to these
data. The MD fit has 25 parameters, and the goodness of fit (rms
(measurement/fit) - 1) is 15%. The MI fit uses cubic spline fits in depth and
radius. The goodness of fit is 9%. This fit is more portable and conceptually
simpler.
  We discuss the prevalent parameterization of the core/halo originated by the
PSI group. We argue that its use of T(w), a mass stopping power which includes
energy deposited by nuclear secondaries, is incorrect. The electromagnetic
(Bethe-Bloch) mass stopping power should be used instead. In consequence,
'Bragg peak chamber' measurements and associated corrections are, in our view,
irrelevant. Furthermore, using T(w) leads to spurious excess dose on the axis
of the core around midrange, which may be significant in fields involving
relatively few pencil beams.
