Design and performance of an actively collimated phoswich system for X-ray astronomy.

The design and performance of a phoswich type scintillation detector system having 34 sq cm effective collecting area and a sensitivity of 3 x 10 to the -5th power photons/sq cm/s/keV for balloon-borne cosmic X-ray source observations are discussed. The various shield anticoincidence systems are evaluated for effectiveness in reduction of the system background. The total background of nominally 4 x 10 to the -4th power counts/sq cm/s/keV is analyzed and found to consist of aperture flux, K-escape X-rays from the shielding, leakage, and degradation of gamma-rays through the shielding, and several particle induced effects. The background analysis suggests an ultimate practical upper limit of about 8 x 10 to the -5th power counts/sq cm/s/keV might be attainable beyond which further sensitivity improvements will require increased collecting area or a basically differing design approach.