  Development of ultra low background gas proportional counters has made the
contribution from naturally occurring radioactive isotopes -- primarily
$\alpha$ and $\beta$ activity in the uranium and thorium decay chains --
inconsequential to instrumental sensitivity levels when measurements are
performed in above ground surface laboratories. Simple lead shielding is enough
to mitigate against gamma rays as gas proportional counters are already
relatively insensitive to naturally occurring gamma radiation. The dominant
background in these surface laboratory measurements using ultra low background
gas proportional counters is due to cosmic ray generated muons, neutrons, and
protons. Studies of measurements with ultra low background gas proportional
counters in surface and underground laboratories as well as radiation transport
Monte Carlo simulations suggest a preferred conceptual design to achieve the
highest possible sensitivity from an array of low background gas proportional
counters when operated in a surface laboratory. The basis for a low background
gas proportional counter array and the preferred shielding configuration is
reported, especially in relation to measurements of radioactive gases having
low energy decays such as $^{37}$Ar.
