Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The HERMES Polarized Hydrogen and Deuterium Gas Target in the HERA
  Electron Storage Ring   The HERMES hydrogen and deuterium nuclear-polarized gas targets have been in
use since 1996 with the polarized electron beam of HERA at DESY to study the
spin structure of the nucleon. Polarized atoms from a Stern-Gerlach Atomic Beam
Source are injected into a storage cell internal to the HERA electron ring.
Atoms diffusing from the center of the storage cell into a side tube are
analyzed to determine the atomic fraction and the atomic polarizations. The
atoms have a nuclear polarization, the axis of which is defined by an external
magnetic holding field. The holding field was longitudinal during 1996-2000,
and was changed to transverse in 2001. The design of the target is described,
the method for analyzing the target polarization is outlined, and the
performance of the target in the various running periods is presented.

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The categories of this paper are: econ
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The categories of this paper are: adap-org
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The categories of this paper are: nlin
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The categories of this paper are: ao-sci
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The categories of this paper are: cs
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The categories of this paper are: hep-lat
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The categories of this paper are: astro-ph
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The categories of this paper are: alg-geom
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The categories of this paper are: funct-an
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The categories of this paper are: cond-mat
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The categories of this paper are: mtrl-th
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The categories of this paper are: hep-th
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The categories of this paper are: math
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The categories of this paper are: math-ph
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The categories of this paper are: hep-ph
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The categories of this paper are: hep-ex