Measurement of photon emission due to atomic rearrangement following nuclear decay

Abstract

An apparatus for the detection of single photons (2–5 eV) is described. Low noise cooled photomultipliers are used, the amplitude spectrum of which is measured with incident low intensity light (single electron peak); it is compared with the noise spectrum. A tunneldiode discriminator for the standardization of the photomultiplier pulses and the determination of its optimum threshold is described. The quantum efficiency of the photomultipliers used is about 25%, the following electronic circuitry has a detection efficiency of almost 100%. The effect of noise background is decreased by the application of the coincidence method. For this purpose an atomic relaxation process is used: Delayed single photons are emitted after atomic adaption to the new nuclear charge following a β-decay. The coincidence signals and their mutual delay (optical lifetime) are measured by means of a time to pulse height converter of the start-stop-principle and a multichannel analyzer. The time resolution of the fast-fast circuit alone is 6 nsec. The excitation of the daughter-ion is primarily due to the sudden change of nuclear charge; excitation by direct collision with charged particles can be neglected. The relative intensities of three emission lines and the lifetimes of the two 5 p( 1 2 1 2 0 - and 5 p( 3 2 case3 2) 0) − Rb + -states populated by the 85Kr-decay are measured as a function of krypton-gas pressure. The undisturbed lifetimes of these states are measured to be τ 0 = 8.0±0.6 nsec and τ 0 = 9.4 ± 0.4 nsec, respectively; the corresponding cross-sections for deexciting collisions ( Rb +∗− Kr) are σ ∗ = 5.5 ± 0.6 × × 10 −15 cm 2 and σ ∗ = 4.8 ± 0.5 × 10 −15 cm 2 .