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The performance of a cognitive radio system depends on how well it can maximize the usage of a free frequency band without causing any interference to the primary user. Thus the abilities to detect a primary user as well as to avoid any false alarm are of paramount importance for such a system. For energy detector based spectrum sensing in cognitive radios, these abilities are influenced by the threshold set to distinguish signal from noise. Conventional energy detectors are based on a fixed threshold. This may not be optimum in low SNR conditions where the performance of fixed threshold based detector can vary from the targeted performance metrics substantially. In this paper, we propose a method to change the threshold based on the number of samples, to optimize the detection parameters, namely probability of detection and probability of false alarm. We also introduce a control parameter which is used to vary the set threshold and thereby obtain a response more suited to the operational requirements than that obtained by a fixed threshold based energy detector. Simulation results are presented to show a different perspective of the importance of the sensing time to the performance metrics of the energy detector.