PIR sensor based LED light controller
As Passerby is saying the PIR sensors are designed to trigger when the rate of change of IR signal it receives is higher than the threshold set by the sensitivity adjustment. To aid this most have a dimpled lens or multi-faced reflector so that as a person walks across the field of view the detector gets a series of pulses. This in turn makes them more sensitive to traffic across the field of view than traffic walking towards the sensor.

One of the advantages of this method is that it self-compensates for slowly changing background IR radiation. In your case you seem to need to trigger and hold when the IR exceeds some absolute (fixed) level of radiation. If your sensor can see material that warms or cools - say due to sunlight - then you will probably have trouble with false triggering when it gets warm or no triggering when the background level reduces the total IR, even with a target present, below the threshold.

If you want to try threshold triggering you would need to see if you can get the analog signal from the sensor inside the PIR unit and process that somehow. A simple voltage comparitor circuit may do the trick.

http://learn.adafruit.com/downloads/pdf/pir-passive-infrared-proximity-motion-sensor.pdf has some very good explanation and diagrams.