There are several macros for tof peak shapes for different instruments on the user macros page.

For powgen the following peak shape seems to work reasonably (though only tested on lowish quality data):

'{{{ instrumental peak shape for this bank here
   push_peak
   TOF_Exponential(a0_1_bank5, 25.09796`, a1_1_bank5, 58.58890`, 1, difc_bank5, +)
   bring_2nd_peak_to_top   
   TOF_Exponential(a0_2_bank5, 40.39882`, a1_2_bank5, 1260.50074`, 4, difc_bank5, +)
   scale_top_peak @  25.2088988` min 0.0 max 200
   add_pop_1st_2nd_peak 
'}}}