Thickness measurement of solid hydrogen thin film for muon catalyzed fusion via energy loss of alpha particles

• Main Author: Fujiwara, Makoto C.
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/5570

A novel target system for films of solid hydrogen isotopes has enabled unique experiments in muon catalyzed fusion. In order to understand the experimental data a knowledge of target thickness and uniformity is essential, but only indirect information was available. Conventional techniques for a thickness measurement do not apply, due to the limited available space and cryogenic requirements of the system. In this thesis, a method of thickness and uniformity measurement via the energy loss of alpha particles is presented. A critical review of the literature on the stopping powers of alpha particles was necessary, given no experimental data for solid hydrogen. An absolute precision of ~5% at optimal condition was obtained in the thickness determination. The uncertainty in the relative measurements can be less than 1%. The average target thickness per unit gas input, weighted by the Gaussian beam profile of FWHM 20-25 mm is determined to 3.29±0.16 μg/(cm²-torr-litre). A significant nonuniformity in the thickness distribution was observed with an average deviation of about 7%. The linearity of deposited hydrogen thickness upon gas input was confirmed within the accuracy. The cross contamination from the other side of the diffuser nozzle is found to be less than 0.8 x 10- 3 with 90% confidence level. The result is compared to a Monte Carlo study to understand deposition mechanism. The importance of the stopping process in the alpha-sticking problem in muon catalyzed D-T fusion is discussed in detail. The physical phase effect of the stopping power of hydrogen may partly explain the discrepancy in the sticking values between theory and experiment at high densities. The concept of a new experiment to measure directly the sticking probability at high density is proposed. This offers certain advantages over LAMPF/RAL measurements. A Monte Carlo simulation of the experiment is performed. A very preliminary result from a test run is presented.