If U use mid IR sensor (like the commonly used piroelectrical detectors for motion allarms), you will not need emmiter because the sensor detects the heat radiation of anything that have temperature sligtly different from the environment.
This is different from the transmitter/receiver pair which rely on a object xing the beam in order to detect it. In general there are ways for the intruder to detect the presense of an active allarm system, while there is very difficult to determine the presense of passive one.
If U decide to implement pyroelectrical sensors (those are cheap), you should bear in mind that the piyoelectrics react only on a RATE OF CHANGE in the temperature (it's derivative). It means that if some enters REALLY SLOW the field of view of such system, he may not be detected. To alleviate this problem, some should use optical chopper (presumably camertone type). Then the system senses the absolute value of the difference of the choper blade and whatever enters the field of view of the detector.