Rapid cooling is counter intuitive, cooling at a moderate rate would result in better results. This is because when your tissue comes in contact with temperature too cold, your blood vessels contract, reducing blood flow that aids in heat transfer. As I recall the temperature was roughly 60 degrees Fahrenheit, or approximately 15.56 degrees Celsius.
In one of the popular science episodes, they feature a device for cooling down your core temperature rapidly. It was one of the superhuman episodes.