A systematic study of the pulse height deficit in propane-filled gas ionization detectors

The response of gas ionization detectors to heavy ions is species dependent. Relative to a linear calibration based on the response to light ions, heavy ions show a pulse height deficit. Detailed knowledge of this deficit is essential for accurate energy spectroscopy of heavy ions. Precise measurements of the response of a propane-filled gas ionization detector have been performed for a large selection of ions in the ranges Z=6–79 and E=0.4–2.6 MeV/amu. Pulse height deficits were determined with respect to the response to carbon ions, and found to be as large as 25% of the incident energy. The deficit was found to depend on ion energy and atomic number. Isotopic effects were also observed. Care was taken to obtain a reliable estimate of the energy loss of the ions in the detector window. This included measurements of the window thickness and shape, and measurements of heavy ion stopping powers for Mylar. Monte-Carlo simulations were carried out using the code TRIM to evaluate the contribution of elastic collisions of the ions in the detector gas. It was found that after accounting for both window energy loss and elastic collisions, a significant residual pulse height deficit remains. An empirical formula was developed to predict the magnitude of the pulse height deficit, and aid in the accurate energy calibration of gas ionization detectors.