[en] ²⁴¹Pu has the shortest half-life of the abundant plutonium isotopes present in reprocessed irradiated nuclear fuel with a value of approximately 14.3 years. It is important to know the half-life of ²⁴¹Pu with a higher fractional accuracy than that of the other plutonium isotopes because the half-life of ²⁴¹Pu and its associated uncertainty affects the estimation by decay calculation of both the total amount of separated plutonium in storage and the determination of the total plutonium mass by non-destructive assay. This paper addresses the determination of the ²⁴¹Pu half-life using nuclear calorimetry by the measurement of the thermal power as ²⁴¹Pu evolves in time from a sealed plutonium source, ideally initially rich in ²⁴¹Pu and chemically stripped of ²⁴¹Am. The absolute accuracy of nuclear calorimeters can be ensured over long periods of time (many years) using long-lived nuclear reference materials and/or traceable electrical heat standards. One can, therefore, expect nuclear calorimetry to offer an accurate way to determine the half-life of ²⁴1Pu, which is comparable in quality and independent, yet complementary, to other approaches. Temporal analysis of the power-versus-time data also yields an estimate of the specific power of ²⁴¹Pu, which other methods do not. After describing the principle of the method and developing the pertinent mathematical expressions, we outline the approach by drawing on some unpublished notes of Kenneth C. Jordan who carried out such experiments at the Mound Laboratory over 40 years ago. Today, Jordan’s work remains possibly the most significant experiment of its type to the ²⁴¹Pu nuclear data evaluator. However, objectively assigning confidence to his results is problematic because the details of the experiments and data reduction have never been adequately reported. This work goes some way to that end but, without the raw data and first-hand knowledge, cannot provide a complete record. We conclude that a new high-accuracy nuclear calorimetry campaign to re-measure the ²⁴¹Pu half-life and specific ²⁴¹Pu has the shortest half-life of the abundant plutonium isotopes present in reprocessed irradiated nuclear fuel with a value of approximately 14.3 years. It is important to know the half-life of ²⁴¹Pu with a higher fractional accuracy than that of the other plutonium isotopes because the half-life of ²⁴¹Pu and its associated uncertainty affects the estimation by decay calculation of both the total amount of separated plutonium in storage and the determination of the total plutonium mass by non-destructive assay. This paper addresses the determination of the ²⁴¹Pu half-life using nuclear calorimetry by the measurement of the thermal power as ²⁴¹Pu evolves in time from a sealed plutonium source, ideally initially rich in ²⁴¹Pu and chemically stripped of ²⁴¹Am. The absolute accuracy of nuclear calorimeters can be ensured over long periods of time (many years) using long-lived nuclear reference materials and/or traceable electrical heat standards. One can, therefore, expect nuclear calorimetry to offer an accurate way to determine the half-life of ²⁴¹Pu, which is comparable in quality and independent, yet complementary, to other approaches. Temporal analysis of the power-versus-time data also yields an estimate of the specific power of ²⁴¹Pu, which other methods do not. After describing the principle of the method and developing the pertinent mathematical expressions, we outline the approach by drawing on some unpublished notes of Kenneth C. Jordan who carried out such experiments at the Mound Laboratory over 40 years ago. Today, Jordan’s work remains possibly the most significant experiment of its type to the ²⁴¹Pu nuclear data evaluator. However, objectively assigning confidence to his results is problematic because the details of the experiments and data reduction have never been adequately reported. This work goes some way to that end but, without the raw data and first-hand knowledge, cannot provide a complete record. We conclude that a new high-accuracy nuclear calorimetry campaign to re-measure the ²⁴¹Pu half-life and specific

[en] We present that neutron time correlation analysis is one of the main technical nuclear safeguards techniques used to verify declarations of, or to independently assay, special nuclear materials. Quantitative information is generally extracted from the neutron-event pulse train, collected from moderated assemblies of "3He proportional counters, in the form of correlated count rates that are derived from event-triggered coincidence gates. These count rates, most commonly referred to as singles, doubles and triples rates etc., when extracted using shift-register autocorrelation logic, are related to the reduced factorial moments of the time correlated clusters of neutrons emerging from the measurement items. Correcting these various rates for dead time losses has received considerable attention recently. The dead time losses for the higher moments in particular, and especially for large mass (high rate and highly multiplying) items, can be significant. Consequently, even in thoughtfully designed systems, accurate dead time treatments are needed if biased mass determinations are to be avoided. In support of this effort, in this paper we discuss a new approach to experimentally estimate the effective system dead time of neutron coincidence counting systems. It involves counting a random neutron source (e.g. AmLi is a good approximation to a source without correlated emission) and relating the second and higher moments of the neutron number distribution recorded in random triggered interrogation coincidence gates to the effective value of dead time parameter. We develop the theoretical basis of the method and apply it to the Oak Ridge Large Volume Active Well Coincidence Counter using sealed AmLi radionuclide neutron sources and standard multiplicity shift register electronics. The method is simple to apply compared to the predominant present approach which involves using a set of "2"5"2Cf sources of wide emission rate, it gives excellent precision in a conveniently short time, and it yields consistent results as a function of the order of the moment used to extract the dead time parameter. In addition, this latter observation is reassuring in that it suggests the assumptions underpinning the theoretical analysis are fit for practical application purposes. However, we found that the effective dead time parameter obtained is not constant, as might be expected for a parameter that in the dead time model is characteristic of the detector system, but rather, varies systematically with gate width.

[en] Correlated neutron counting using multiplicity shift register logic extracts the first three factorial moments from the detected neutron pulse train. The descriptive properties of the measurement item (mass, the ratio of (α,n) to spontaneous fission neutron production, and leakage self-multiplication) are related to the observed singles (S), doubles (D) and triples (T) rates, and this is the basis of the widely used multiplicity counting assay method. The factorial moments required to interpret and invert the measurement data in the framework of the point kinetics model may be calculated from the spontaneous fission prompt neutron multiplicity distribution P(ν). In the case of "2"3"8U very few measurements of P(ν) are available and the derived values, especially for the higher factorial moments, are not known with high accuracy. In this work, we report the measurement of the triples rate per gram of "2"3"8U based on the analysis of a set of measurements in which a collection of 10 cylinders of UO_2F_2, each containing about 230 g of compound, were measured individually and in groups. Special care was taken to understand and compensate the recorded multiplicity histograms for the effect of random cosmic-ray induced background neutrons, which, because they also come in bursts and mimic fissions but with a different and harder multiplicity distribution. We compare our fully corrected (deadtime, background, efficiency, multiplication) experimental results with first principles expectations based on evaluated nuclear data. Based on our results we suspect that the current evaluated nuclear data is biased, which points to a need to undertake new basic measurements of the "2"3"8U prompt neutron multiplicity distribution