I would not put new chicks outside even with a heat lamp. The least bit of draft can cause them to become very ill and even result in death. I feel very strongly that if you put new chicks out in an open air improperly secured brooder you will have very sick chicks in a very short time.
If you do plan to put them outside and not in your warm house they need to be in an enclosed area where the temp can be regulated at 95 degrees for the first week, 90 the second and decreasing by 5 degrees each week. You need to be very sure it is draft free from all sides. In about 8 to 12 weeks they will be nearly fully feathered and will be able to regulate their body temp better and keep themselves warm.
You will be able over that time to slowly wean them from the heat once you get them down to day temps of 70 degrees. In cooler night temps you will have to give them heat until they are completely feathered out.
Also be very sure the brooder is predator proof. There are many things that go bump in the night that would love to have a chick snack and won't stop until there are no chicks left to snack on.
