Need to better understand the relationship between Be as used in:
Small scale neutron production, ie for detector demonstration
He-4 + Be-9 ⇒ C-12 + n
PANDA: interaction with matter
Typically either He-3 or BF3 as the main fill gas. In more detail: “Gas-filled detectors typically employ He-3 He-4, BF3, or CH4 as the primary constituent, at pressures less than 1 to about 20 atm…A gas such as argon can be used to … [improve] the output pulse-height resolution…” [PANDA 384]
“A polyatomic gas may also be added to proportional counters to serve as a quench gas. …Gases such as BF3 and CH4 are already polyatomic gases and require no additional quench gas. Tubes filled with He-3 and He-4 often have a small quantity of CH4 or CO2 added.” [PANDA 384]
“If little or no voltage is applied to the tube, most of the ions will recombine and no electrical output signal is produced. If a positive voltage is applied to the central wire (anode) the electronics will move toward it and the positively charged ions will move toward the tube wall (cathode). An electrical output signal will be produced whose magnitude depends on the applied voltage, the geometry of the counter, and the fill gas.” [PANDA 381]
Why can't you switch positive and negative? Wouldn't the electrons just hit the walls instead?
Important point above: the strikes cause create charge, canceling out some of the electric field. This results in a decrease in current (negative pulse). If the applied voltage was very high though it would breakdown and increase current?