  The absolute light yield of a scintillator, defined as the number of
scintillation photons produced per unit energy deposited, is a useful quantity
for scintillator development, research, and applications. Yet, literature data
on the absolute light yield of organic scintillators are limited. The goal of
this work is to assess the suitability of the EJ-204 plastic scintillator from
Eljen Technology to serve as a reference standard for measurements of the
absolute light yield of organic scintillators. Four EJ-204 samples were
examined: two manufactured approximately four months prior and stored in
high-purity nitrogen, and two aged approximately eleven years and stored in
ambient air. The scintillator response was measured using a large-area
avalanche photodiode calibrated using low energy $\gamma$-ray and X-ray
sources. The product of the quantum efficiency of the photodetector and light
collection efficiency of the housing was characterized using an
experimentally-benchmarked optical photon simulation. The average absolute
light yield of the fresh samples, 9100 $\pm$ 400 photons per MeV, is lower than
the manufacturer-reported value of 10400 photons per MeV. Moreover, the aged
samples demonstrated significantly lower light yields, deviating from the
manufacturer specification by as much as 26\%. These results are consistent
with recent work showcasing environmental aging in plastic scintillators and
suggest that experimenters should use caution when deploying plastic
scintillators in photon counting applications.
