A new detection system for the measurement of beta-delayed neutron emission probabilities has been characterized using fissionproducts with well known β-delayed neutron emission properties. The setup consists of BELEN-20, a 4π neutron counter withtwenty 3He proportional tubes arranged inside a large polyethylene neutron moderator, a thin Si detector for β counting and a selftriggeringdigital data acquisition system. The use of delayed-neutron precursors with different neutron emission windows allowedthe study of the effect of energy dependency on neutron, β and β-neutron rates. The observed effect is well reproduced by MonteCarlo simulations. The impact of this dependency on the accuracy of neutron emission probabilities is discussed. A new accuratevalue of the neutron emission probability for the important delayed-neutron precursor 137I was obtained, Pn = 7.76(14)%.
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