A measurement of the proton structure function F-2(x, Q(2)) is presented in
the kinematic range 0.045 GeV2 < Q(2) < 0.65 GeV2 and 6 . 10(-7) < x < 1 .
10(-3). The results were obtained using a data sample corresponding to an
integrated luminosity of 3.9 pb(-1) in e(+)p reactions recorded with the ZE
US detector at HERA. Information from a silicon-strip tracking detector, in
stalled in front of the small electromagnetic calorimeter used to measure t
he energy of the final-state positron at small scattering angles, together
with an enhanced simulation of the hadronic final state, has permitted the
extension of the kinematic range beyond that of previous measurements. The
uncertainties in F-2 are typically less than 4%. At the low Q(2) values of
the present measurement, the rise of F-2 at low x is slower than observed i
n HERA data at higher Q(2) and can be described by Regge theory with a cons
tant logarithmic slope partial derivative lnF(2)/partial derivative ln(1/x)
. The dependence of F-2 on Q(2) is stronger than at higher Q(2) values, app
roaching, at the lowest Q(2) values of this measurement, a region where F-2
becomes nearly proportional to Q(2). (C) 2000 Elsevier Science B.V. All ri
ghts reserved.