Why does the movable fibre indicate a decrease in the charge on the dosimeter

I'm given a diagram of a pen-style dosimeter with four neutral gas molecules and a movable fibre. A particle of radiation has just entered the dosimeter and is about to strike one of the gas molecule. So what does the radiation do to the gas molecule and why does the movable fibre indicate a decrease in the charge on the dosimeter?

Inside the dosimeter is a charged insulated metal electrode. The fibre is also charged and is repelled away from the the electrode

"When a particle of ionizing radiation passes through the chamber, it collides with molecules of air, knocking electrons off them and creating positively and negatively charged atoms (ions) in the air. The ions of opposite charge are attracted to the electrode and neutralize some of the charge on it. The reduced charge on the electrode reduces the force on the fiber, causing it to move back toward the electrode."

The last t part is from the link.

The dosimeter works on static charge. At full charge you have full deflection of the fiber, this is your zero dose indication. As the radiation enters the dosimeter it ionizes the gas inside. The resulting positive ions are then attracted to the fiber where it picks up an electron and reduces the static charge. This in turn moves the fiber and provides you with an indication of your received dose.