Abstract: A radiation detector assembly employing an enlarged volume scintillation element and a photomultiplier tube. The scintillation element is surrounded by a reflective material and may be enclosed within a rigid shield for protection against shock and/or humidity. A Sidewall Axial Restraint and Compliance Assembly (SARCA) is positioned radially outside the reflective tape. Radial springs are placed outside the SARCA to provide stiff restraint in the axial and radial directions, The SARCA and springs provide for thermal expansion of the element. A two-stage axial biasing means is positioned at an end of the element, and an optical window may be positioned at the other end. Non-cylindrical portions may be formed along the circumference of the element. These non-cylindrical portions enable a larger element to be placed within the shield. In an alternative embodiment, the element is positioned apart from the photomultiplier tube with no window therebetween.