Regression analysis of coincidence measurements for determinating the neutron emission rate of <sup>252</sup>Cf spontaneous fission

Abstract

The <sup>252</sup>Cf isotope sources have a recommended standard neutron spectrum of spontaneous fission, and have been widely used in scientific researches, such as the detection efficiency calibration of neutron detectors, the characterization of neutron dose equivalent meters, the active analysis of special nuclear materials, etc. However, it is often necessary to correct the neutron emission rate due to its short half-life of 2.645 years. As the source age increases the contributions from <sup>250</sup>Cf and <sup>248</sup>Cm spontaneous fission become more significant, thus the neutron emission rate cannot be calculated simply according to the <sup>252</sup>Cf decay law. In addition, the indirect measurement method by manganese bath activation needs a long period more than 8 hours; and it will have a large uncertainty while the source strength is lower than 10<sup>4</sup> n/s. In order to develop a more portable measurement method for larger suitable dynamic range, the comprehensive algorithms based on the neutron multiplicity counting are studied in this paper. On the basis of the measurement equations under the point model assumption, the neutron coincidence counting rate is correlated with the total neutron counting rate, and then the regression analyses with different coincidence gates and different source locations in the counter are performed. On the assumption that the average neutron die-away time is constant in the sensitive range of detection system, therefore the characteristic coefficient from the changing process can be extracted, and two kinds of methods of measuring the neutron strength are established, which are independent of the efficiency variation. The verification experiments are carried out by the JCC-51 neutron coincidence counter. It is shown that the values measured by the two regression methods are consistent with the corrected results of the nominal value within 2% deviation. Furthermore, the detection efficiency is inversed by dividing the total neutron counting rate with the neutron emission rate when the source is placed at the central axis, which accords with the result of Monte Carlo simulation by using the MCNPX code well. It can provide an accurate method of determining the neutron emission rate of <sup>252</sup>Cf spontaneous fission, and also an approach to calibrating the detection efficiency of neutron coincidence counter while the source strength is unknown.