Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Observation of scattering and absorption centers in lead fluoride
  crystals   For the first time, lead fluoride is used as a fast and compact material in
electromagnetic calorimetry. Excellent optical and mechanical properties of the
pure Cherenkov crystals are necessary for the A4 collaboration to perform a
measurement of the nucleon's strange form factors. Visible scattering and
absorption centers as well as surface damages have been investigated to
characterize the quality of more than one thousand crystals. Besides,
transmittance measurements have been performed on all crystals to reveal
absorption bands produced by intrinsic or impurity related point-structure
defects. As a consequence, 89 crystals had to be replaced by the Chinese
manufacturer SICCAS.

25
The categories of this paper are: patt-sol
0
The categories of this paper are: hep-ph
30
The categories of this paper are: q-alg
4
The categories of this paper are: cond-mat
7
The categories of this paper are: hep-th
17
The categories of this paper are: q-fin
19
The categories of this paper are: eess
10
The categories of this paper are: q-bio
3
The categories of this paper are: physics
23
The categories of this paper are: solv-int
28
The categories of this paper are: alg-geom
16
The categories of this paper are: stat
35
The categories of this paper are: mtrl-th
20
The categories of this paper are: acc-phys
18
The categories of this paper are: econ
27
The categories of this paper are: comp-gas