function y=exp_decay(amptau,x_data)
%description:
%called by: change_ratio_v_original_plotter, mini_waveform_extractor, event_extractor_core
%calls: [no custom functions]
%notes:
%code confirmed date: feb 2, 2013

y=-1*amptau(1)*exp(-x_data/amptau(2));