Title :
A priori precision estimation for neutron Triples counting
Author :
Croft, Stephen ; Swinhoe, Martyn T. ; Henzl, Vladimir
Author_Institution :
Safeguards Sci. & Technol. Group (N-l), Los Alamos Nat. Lab., Los Alamos, NM, USA
Abstract :
The nondestructive assay of Plutonium bearing items for criticality, safety, security, safeguards, inventory balance, process control, waste management and compliance is often undertaken using correlated neutron counting. In particular Multiplicity Shift Register analysis allows one to extract autocorrelation parameters from the pulse train which can, within the framework of a simple interpretational model, be related to the effective 240Pu spontaneous fission mass present. The effective 240Pu mass is a weighted sum of the 238Pu, 240Pu and 242Pu masses so if the relative isotopic composition of the Pu can be established from the measured 240Pu effective mass one can estimate the total Pu mass and also the masses of the individual isotopes, example the fissile species 239Pu and 241Pu. In multiplicity counting three counting rates are obtained. These are the Singles, Doubles and Triples rates. The Singles rate is just the gross, totals or trigger rate. The Doubles and Triples rates are calculated from factorial moments of the observed signal triggered neutron multiplicity distributions following spontaneous fission in the item and can be thought of as the rate of observed coincident pairs and coincident triplets on the pulse train. Coincident events come about because the spontaneous fission and induced fission chains taking place in the item result in bursts of neutrons. These remain time correlated during the detection process and so retain information, through the burst size distribution, about the Pu content. In designing and assessing the performance of a detector system to meet a given goal it is necessary to make a priori estimates of the counting precision for all three kinds of rates. This is non-trivial because the counting does not obey the familiar rules of a Poissonian counting experiment because the pulse train has time correlated events on it and the train is sample- by event triggered gates that may overlap. For Singles and Doubles simple approximate analytical empirical rules for how to estimate the variance have been developed guided by theory and refined by experiment. However, for Triples no equivalent rules have been put forward and tested until now. In this work we propose an analytical expression, the CSH relation, for the variance on the Triples count and exercise it against experimental data gathered for Pu items measured in the Los Alamos National Laboratory´s Epithermal Neutron Multiplicity Counter (ENMC). Preliminary results are encouraging and reasonable agreement with observation, considered fit for scoping studies, is obtained. We have also looked at the behavior using Monte Carlo simulations.
Keywords :
Monte Carlo methods; coincidence techniques; neutron detection; nondestructive testing; nuclear electronics; radioisotopes; shift registers; spontaneous fission; trigger circuits; 238Pu weighted sum; 239Pu fissile species; 240Pu effective mass; 240Pu spontaneous fission mass present; 240Pu weighted sum; 241Pu fissile species; 242Pu weighted sum; CSH relation; Los Alamos National Laboratory; Monte Carlo simulations; Poissonian counting experiment; Pu content; a priori precision estimation; approximate analytical empirical rules; are factorial moments; autocorrelation parameters; burst size distribution; coincident events; coincident of triplets; coincident pairs; compliance; correlated neutron counting; counting precision; counting rates; criticality assement; detection process; detector system; doubles rate; epithermal neutron multiplicity counter; event triggered gates; fission chains; interpretational model; inventory balance; material safeguard; multiplicity counting; multiplicity shift register; neutron burst; neutron triples counting; nondestructive assay; plutonium bearing items; process control; pulse train; relative isotopic Pu composition; safety assement; security assement; signal triggered neutron multiplicity distributions; singles rate; spontaneous fission; trigger rate; triples rate; variance estimation; waste management; Correlation; History; Logic gates; Mathematical model; Neutrons; Radiation detectors; Uncertainty;
Conference_Titel :
Advancements in Nuclear Instrumentation Measurement Methods and their Applications (ANIMMA), 2011 2nd International Conference on
Conference_Location :
Ghent
Print_ISBN :
978-1-4577-0925-8
DOI :
10.1109/ANIMMA.2011.6172923