No its probably energy.

Radiation as used in physics, is energy in the form of waves or moving subatomic particles.

Actually, 343 said that they all died when he was first explaining to MC how the rings work.

"After exhausting every other strategic option, my creators activated the rings. They, and all additional sentient life in three radii of the galatic center, died as planned. Would you like to see the relevent data?"

"Would you like to see the relevent data?"

"In Halo: First Strike it is revealed that the weapons are based on Forerunner technology. Halo's plasma weapons contain a battery that generates plasma and discharges it at a target.[22] Frank O'Connor of the Halo Message Boards claims there may be something more to the Covenant's weaponry. He has been quoted saying: "The actual technology is not plasma as we know it, but something far more dangerous, arcane, and destructive."[23]"

"Technologically the Covenant are shown to be mostly imitative rather than innovative — most of the time they weaponize technologies discovered rather than technologies researched.[21] The technology the player is exposed to in-game and while reading mostly centers around the military weapons and deployment (such as guns, ships, and vehicles), while little is known of any commercial- or domestic-use commodities.

It has been noted both in game and in the books that the Covenant, not having researched the technologies relied upon, are incapable of utilizing them to their full potential. Repeatedly, UNSC artificial intelligence units (specifically Cortana), have been able to boost the efficiency and output of much of the Covenant's weaponry when given access to it. Because the Covenant only incorporate technologies into their armies, as opposed to actually researching them, their knowledge of the fundamental functions of weapons is minimal."

From Wikipedia:

hi, new here.

This is just speculation but what if the covenant species (brutes, elites, prophets, etc.) were put in the Ark along with humans to shield them from Halo and afterwards the covenant species boarded a ship in the Ark, sending it inadvertently into space, where they made a society with the forerunners as fabled gods and imitated technology off that ship to build a city around it (high charity) and their own weapons, ships etc? After stumbling around looking for clues about their so called gods, they found they had activated the sacred rings (halos) and assumed they had to do the same to be with them (Great Journey). The Covenant don"t even know the full power of their own weapons (see below wiki quotes). They more or less utilize rather than research technology, unlike innovative humans, which the forerunners may have found them unique in that aspect.

Okay...the only reason I mentioned the Brutes, Humans, Elites, and Prophets in the same paragraph on how the Forerunners might have advanced them was because they had the proper biomass for Floods to take them over (I was just guestimating for Prophets). This doesn't make them "The Four Races" like from Stargate.(Sorry, Stagate SG1 was the only thing I could think of to reference four races. x_x)
Okay for a real response now.
One: the Covenant was formed over time.
And two: They didn't activate the rings. We wouldn't be here if they did. Only "Reclaimers" can activate the rings. Hence the "Libaray" part of Halo: Combat Evolved.

I believe he meant the Forerunner when he said "they". Sure, it's improperly linked, but it would make more sense that way. Your point about the gradual formation of the Covenant is correct though.

Keep in mind that Sentience is not the same thing as Sapience.

Sentient: http://en.wikipedia.org/wiki/Sentience
Sapient:http://en.wikipedia.org/wiki/Sapience

Dogs are sentient. Humans are sapient. Elites are sapient. Cows are sentient. Fish are sentient. Halos care nothing about sapience; only sentience (and biomass).

Sentience refers to utilization of sensory organs, the ability to feel or perceive subjectively, not necessarily including the faculty of self-awareness. The possession of sapience is not a necessity. The word sentient is often confused with the word sapient, which can connote knowledge, consciousness, or apperception. The root of the confusion is that the word conscious has a number of different usages in English. The two words can be distinguished by looking at their Latin roots: sentire, "to feel"; and sapere, "to know".