The video at the bottom explains from 7:50 in. Gas pressure pressing on a spring holds a radiation shield in place. If the gas leaks the shield moves and the radiation can be detected.
MH-53E maintainer here. The system is IBIS (Inflight Blade Inspection System) and the little pods are BIM (blade inspection monitors). The box is pretty obvious sitting just behind the main rotor on the deck above the center engine. I found a random picture on the web:
And:
The IBIS devices provide in-flight warning of blade failure on various models of helicopters. One device is located on the root of each blade attached to the rotor. The IBIS detects a decrease in pressure in the blades of a CH-53 model helicopter. Any cracks in the blades will result in a decrease in air pressure in the blade. Such cracks could result in structural failure in the blade, thereby resulting in a potentially dangerous operational situation for the helicopter, such as a hard landing and/or injury to the craft and/or personnel up to, and including, death. Each device contains approximately 500 microcuries (18.5 MBq) of strontium-90 (Sr-90) in the form of a rolled metal foil, encased in a small stainless steel protective cylinder about the size of the press button on a ballpoint pen. The external radiation level of the IBIS is 0.8 mR/hr at 3 inches in the normal (shielded) mode, and 75 mR/hr at 12 inches in the failure (extended) mode.
Sr90 beta decays emitting an electron becoming daughter decay product Yttrium-90. Yttrium-90 then beta decays (after a half-life of a few days) emitting more energetic electrons than Sr90, and those electrons travel a mean distance through STP air of maybe meters and effectively don't travel further than say 10m.
At long distance through air the Bremsstrahlung xray radiation dominates? It is generated by the fast electrons curving when passing close to nuclei. Xrays through air are inverse square? I guess it depends on the absorbtion characteristics of air.
AFAIK the above is mostly correct but I haven't checked anything properly sorry.
Basically anything to do with radiation is usually deeply complex and you need to look at all the different factors before making assumptions.
Wireless systems aren’t used for opsec reasons.
Beta particles typically only travel a few meters in air before they lose most of their energy.
Was solved in Mi-6/Mi-8/Mi-26 blades without it.