Is there a pulse height defect for methane?

The spontaneous fission of 252Cf has been studied using a twin ionization chamber with Frisch grids. Due to the discrepant results in literature on the value of the pulse height defect (PHD) in pure methane (CH4), measurements with two different counter gases, 90%Ar+10%CH4 and CH4 have been conducted at gas pressures of 0.6 × 105 Pa and 1.2 × 105 Pa. The higher pressure enabled an absolute energy calibration using the α-activity of the 252Cf-sample. The measurement with 90%Ar+10%CH4, where the PHD is better known, served as a reference. For both counter gases a similar PHD (ca. 4 MeV for light and ca. 5 MeV for heavy fragments) had to be taken into account to achieve comparable values of the mean fission fragment masses. However, for CH4 the absolute calibration yielded a mean TKE of only 176.8 MeV, which is some 7 MeV too low as compared to the recommended value of 184.1 MeV.