Title: Method and means for helium/hydrogen ratio measurement by alpha scattering

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Abstract: Apparatus and method for determining helium to hydrogen ratios in a gaseous sample by measurement of forward scattering products due to alpha particle collisions with helium and hydrogen contained within the gaseous sample. More specifically, an apparatus is disclosed in which a gaseous sample, contained within an enclosure, is bombarded by alpha particles created by a self contained radioactive source. Baffles are positioned in the enclosure so that only scattering products falling within a predetermined forward scattering angular range can impact a detector assembly. In an embodiment scattered particles are detected by two detectors mounted in tandem, the first completely blocking the second detector with respect to incident scattering products. This embodiment is based on the principle that scattering products have a forward scattering angle .theta. greater than 15 degrees due to alpha particle/hydrogen collisions comprise only recoil protons. For a given kinetic energy, recoil protons will penetrate farther into silicon detector material than will scattering products from alpha particle/helium collisions. Thus, an apparatus according to the teachings of the invention identifies alpha particle/hydrogen or alpha particle/helium collisions primarily by whether scattering product impacts occur simultaneously in both the first and second detectors or occur only in the first detector. Relative magnitudes of the two pulses can be used to further discriminate against other effects such as noise and cosmic ray events.


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