neutron — cosmic rays.

INTRODUCTION
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The [*Fermi*]{} Gamma-Ray Space Telescope has detected $\gamma$-ray pulsed emissions from more than twenty millisecond pulsars (MSPs) [@Ab11], which have a rotation angular frequency $\Omega\sim 10^3$s$^{-1}$ and a stellar surface magnetic field $B_s\sim 10^{8.5}$G. The detection of the GeV emissions from a pulsar magnetosphere means that electrons and positrons are accelerated to more than $\sim$ TeV by the electric field parallel to the magnetic field, which arises in a depleted region of the Goldreich-Julian (GJ) charge density [@GJ69]. The $\gamma$-ray light curve is an important tool for probing the particle acceleration process in the pulsar magnetosphere. Therefore, the $\gamma$-ray emission region has been explored by comparing theoretical models such as polar cap [@DH96], outer gap [@CHR86] and slot gap models [@MH04] with the observed light curve (e.g., Venter, Harding & Guillemot 2009; Romani & Watters 2010; Kisaka & Kojima 2011).

@VHG09 fitted the pulse profiles of the [*Fermi*]{} detected MSPs with the geometries of $\gamma$-ray emission region predicted by different theoretical models. They found that the pulse profiles of six of eight MSPs could be fitted by the geometries of either the outer gap or the slot gap model, as was the case of 