Mullard Space Science Laboratory

I. M. Mason
The environmental background in detectors for X-ray astronomy

1981 (supervisor: J. L. Culhane)

X-ray detectors in space register photons from astronomical sources against a background counting rate which is generally referred to as "the environmental background" (to distinguish is from the diffuse cosmic X-ray background). Caused mainly by photon and particle radiation from local and cosmic sources, the environmental background often limits the sensitivity of observations in X-ray astronomy. In gas-filled detectors covering the photon energy range ~ 0.1 to ~30 keV, however, attempts to predict and reduce the background have so far proceeded in a largely empirical manner. This thesis describes a detailed investigation of the sources of background and the ways in which they produce detectable events in these detectors; this should benefit the design and evaluation of low background X-ray detectors.

A brief review of background-limited counting in X-ray astronomy is first presented. In order to study the sources of background in detail a number of useful geometric formulae are derived, and various measured background count rates are assembled and presented. Possible background sources are then assessed in turn to account for these data. Limits to their contributions are found, and the prediction of background count rates is discussed. The distribution of the background events in path length and pulse height is then considered, particularly at low values, where an analysis of the spectral shape is given. The reduction of background is discussed and analysed in the light of the results obtained, and methods of evaluating low background detectors are investigated, particularly the use of gamma-ray simulation sources. Finally the design and development of some low background detectors for X-ray astronomy is described, including a novel mechanism for background rejection.


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