Keywords: pulsed neutron activation, PNA flowmeters, pipe flow, water flow, collimation, neutron radiation
Collimation and background in the pulsed neutron activation method for determination of water flow in pipes
In a Pulsed Neutron Activation (PNA) flowmeter, the water in a pipe is bombarded with neutron pulses, which introduce activity into the pipe. The activity is mixed with the flow, and gamma radiation emitted from the activity is measured with a detector downstream. The average velocity of the water is then calculated using the time-resolved detector signal. In this paper, the effect of collimation of the neutron radiation has been investigated. Such a collimation would increase the accuracy of the measurement, but the use of a collimator requires the distance between the pipe and the neutron source to be increased, with a corresponding loss in count rate. The results show that this loss makes neutron collimators in PNA of little use. In addition, a method to identify and subtract the background originating from stationary sources in a PNA measurement was developed.