Electronic elimination of noise pulses in scintillation counting

Abstract

This paper describes the principle of operation and the performances of a circuit designed for electronic elimination of noise pulses in a scintillation spectrometer. The circuit makes use of the fact that the photoelectron which are released at the cathode of the photomultiplier due to a single scintillation flash are not exactly coincident in time, since the respective photons are emitted according to the transient decay characteristics of the involved scintillator. Because more than one noise pulse (single electron pulse) within the decay time τ is highly improbable to occur, the pulses consisting of two or more photoelectrons within τ, are signal pulses originating from a scintillation flash. These pulses are accepted, whereas noise pulses are discriminated by means of the fast photomultiplier able to resolve a signal pulse into its sequential single electron pulses and by means of a difference amplifier which compares the amplitudes of an integrated dynode output and a differentiated anode output of the photomultiplier. The limits of this elimination principle and the resulting design features are discussed. The spectrometer presented makes use of the high speed RCA C 70045 D photomultiplier. Both slow and fast scintillators can be used. The elimination circuit decreases the noise count rate to about 5% of its original value. The experimental results are illustrated with pulse height spectra obtained with various scintillators and radioactive sources. The measurements show that a further decrease of the residual noise count rate to less than 0.1% of its original value can be achieved if the arrangement is to operate with fast scintillators only.