Steve Ashman said the man's information led to the conviction of 17 men and a woman for abusing girls in Newcastle.
The informant was paid £10,000, and headlines have focused on criticism of that, rather than on the case.
The paedophile had himself served a prison sentence for rape.
The last of four trials, spread across two years, ended on Wednesday and in total 18 people, mostly of Asian backgrounds, were convicted of sexual exploitation and drugs offences.
Most of them were men from Pakistani, Bangladeshi or Indian backgrounds.
The court heard how vulnerable girls and women were abused at parties or "sessions" in Newcastle.
Northumbria Police asked a previously convicted child rapist if he could get them more information about the parties.
The NSPCC said that using him in this way was a decision that "crossed the line".
Jim Gamble, former head of the Child Exploitation and Online Protection Centre, said: "Personally, I can't envisage circumstances where I would have authorised payment to someone convicted of rape.
"I can't imagine how you could have control mechanisms in place with an informant of that type that you were going to task that would give reassurance that they still didn't represent a risk to young and vulnerable women."
But Northumbria Police's Chief Constable Steve Ashman rejected suggestions that the informant was encouraged to go to the parties in order to report back to them.
In fact, Mr Ashman said the informant was specifically told he would be arrested if he attended the parties himself.
He said: "In this specific instance, we used an informant in very controlled circumstances.
"The recruitment and registering of any informant is carefully risk assessed against what are the potential gains we can get from using that individual."
The defendants were convicted of abusing girls who were plied with alcohol and drugs before being forced to have sex.
A court heard that the vulnerable victims, some as young as 14, were exploited by a "cynical organisation".
Over the course of the four trials, 20 young women gave evidence covering a period from 2011 to 2014.
The police investigation, Operation Sanctuary, is continuing.
There have been about 100 convictions so far.
Northumbria's chief constable has firmly rejected claims that paying a child rapist to help secure convictions of sexual exploitation may have placed some victims at greater risk.