This backpack contains a portable nuclear reactor. I know that sounds a bit unsafe, but... well, it is. It's a terrible idea. It has blinky lights, though. You'll need to wear a Neutron Pack to power a Neutron Gun.
Neutron Pack price in Growtopia: sells for about 1 WL (median of 108 offers from community Discords, past 30 days). Live price chart and the latest buy and sell offers.
Recently sold at: KFGO, ZREY, LKARD, WOHEE, CIHIMA