IS THERE A PULSE-HEIGHT DEFECT FOR METHANE

The spontaneous fission of Cf-252 has been studied using a twin ioniza tion chamber with Frisch grids. Due to the discrepant results in liter ature on the value of the pulse height defect (PHD) in pure methane (C H4), measurements with two different counter gases, 90%Ar+10%CH4 and C H4 have been conducted at gas pressures of 0.6 x 10(5) Pa and 1.2 x 10 (5) Pa. The higher pressure enabled an absolute energy calibration usi ng the alpha-activity of the Cf-252-sample. The measurement with 90%Ar + 10%CH4, where the PHD is better known, served as a reference. For bo th 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 va lues of the mean fission fragment masses. However, for CH4 the absolut e calibration yielded a mean TKE of only 176.8 MeV, which is some 7 Me V too low as compared to the recommended value of 184.1 MeV.