The proton pack was designed by Dr. Egon Spengler & Dr. Ray Stanz, and is the standard issue 'Ghostbusting' tool of the Ghostbuster team.
The handheld "Proton Gun" of the weapon is connected to what Dr. Peter Venkman refers to as, an "unlicensed nuclear accelerator," small enough to be worn on your back, very much like a backpack.
The proton pack delivers a powerful stream of positively charged energy capable of destroying object in its path. This stream of Proton Energy is immediately attracted to the Neutron composition of Ecotplasic material, allowing the wielder to essentially lasso a ghost and guide it into the Ghost Trap.
"It Would Be Bad..."
The one rule one must always follow when using multiple proton packs in close proximity is, "don't cross the streams." Crossing the proton streams can cause "total protonic reversal", which as described by Dr. Spengler would be like, " ...all life as your know it stopping instantaneously, and every molecule in your body exploding at the speed of light."
Extreme Ghostbusters Proton Pack:
The original version of the Proton Pack used in the first two movies was eventually replaced by a more power and compact model, designed by Dr. Spengler, years after the original team broke up.
This new improved Proton Pack used "Proton Canisters" attached directly onto the Proton Gun itself. This would allow for safer, more legal storage and portability while packing a bigger punch.
The male members of the "Extreme" Ghostbuster team, (Eduardo, Garrett, and Roland) carried a Proton Pack with additional Proton Canisters to provide more power to their guns, while Kyle simply carried a Proton Gun with 1 Proton Cannister attached to it on her belt. This meant that the male members of the team had stronger Proton Streams than Kyle, but the lighter weight of Kyles gear allowed her to maintain her speed and flexibility.