Detector assembly

Claim 1

In a detector assembly adapted to be disposed adjacent a sheet of material to determine a characteristic of the sheet of material by receiVing radiation, including beta particles, which has passed through the sheet of material, a housing having a hole extending longitudinally therethrough a grid structure mounted in said housing in said hole and having passages therein with their longitudinal axes being generally parallel to the axis of the hole extending through said housing, said grid structure facing in a direction so that radiation passing through said sheet can pass through said grid structure, a scintillator disposed in said housing in said hole on the side of said grid structure opposite the side facing said sheet material and producing light pulses when struck by radiation passing through said grid structure, a light pipe disposed in said housing adjacent said scintillator for conveying said light pulses in said housing, a single photomultiplier tube disposed in said housing and receiving light pulses from said light pipe and converting said light pulses to electrical signals, a conductive metal layer encircling said photomultiplier tube, an insulating layer encircling the conductive metal layer, and a radiation permeable film impervious to ambient light pulses to which said photomultiplier tube is sensitive located substantially adjacent said scintillator between said scintillator and said grid structure, said grid structure being formed of a material having a density sufficiently high so that thin foils having a thickness ranging from 1 to 5 mils are capable of stopping or appreciably attenuating beta particles, the grid structure having a depth ranging from 10 to 5 times the maximum width of the passages in the grid structure, said grid structure serving to prevent or minimize reception of radiation having angles of incidence greater than 11* from the axes of the holes in the grid structure to thereby limit the angular field of view of the detector assembly so that sheet movements within the range of 1/2 inch can occur without changing the number of counts of light pulses detected by the photomultiplier tube by appreciably more than 0.1 per cent.