Dead-time correction for any multiplicity using list mode neutron multiplicity counters: A new approach e Low and medium count-rates Ludwig Holzleitner a, * , Martyn T. Swinhoe b a European Commission, Joint Research Center (JRC), Institute for the Protection and the Security of the Citizens (IPSC), Nuclear Security Unit, Via E. Fermi 2749, 21027 Ispra (VA), Italy b N-1, Safeguards Science and Technology Group, Los Alamos National Laboratory, Los Alamos, NM 87545, USA article info Article history: Received 4 March 2010 Accepted 2 January 2011 Keywords: Neutron multiplicity counting Dead-time correction List mode counters Pulse train analysis abstract In the field of neutron multiplicity counting, modern list mode counters provide increased possibilities for neutron data analysis. Here a new method to correct dead-time using a multichannel list mode neutron counter is described. As it will become clear in this article, the data analysis can be done “on the fly” without further data storage. This will allow an instrument to be built having this method imple- mented, which will give directly dead-time corrected results. In practice, a classical theory of dead-time correction is applied to the final Totals, Reals (or Doubles) and possibly Triples, whereas for higher multiplicities no dead-time correction has been implemented so far. In contrast to that, this approach directly corrects the multiplicity distributions of the Reals plus Accidentals (R þ A) and Accidentals (A) obtained by multiplicity counting. Hence this dead-time correction holds for any kind of multiplicity (Totals, Doubles, Triples, Quadruples, Quintuples, etc.) because the calculation of these values can then be derived directly from the corrected multiplicity distribution of R þ A and A. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction A neutron multiplicity counter usually consists of a big tube of polyethylene with a cavity in the middle. Into this cavity the sample is put for measurement. Gas proportional counters, often 3 He tubes are embedded in the polyethylene moderator see Fig. 1 If a neutron is emitted from the source to be measured, it has a high probability to collide with the hydrogen in the polyethylene. It loses energy until it reaches thermal equilibrium with the material. Then it diffuses around the moderator until it is lost or captured. During this process of collisions with other atoms the neutron loses its information about its point of origin and original direction. A fraction of these thermalised neutrons eventually hits an atom of the gas contained in the embedded tubes within the polyethylene and results in a digital pulse. Due to the fact that the neutron is bounced around within the polyethylene, its path is sufficiently random and for each neutron the destination tube is random and independently distributed, although the probability of receiving a neutron is not equally distributed among the gas tubes. This is an important feature and we will in the following make extensively use of it in order to correct for dead-time. This can be visualized with the game of balls running through many rows of nails, each row shifted by half the distance to the previous one, see Fig. 2. The final position of the ball is random but not equally distributed. If a neutron hits an atom in the detector gas, a charge is released and collected at the electrodes of the tube. The preamplifier receives a pulse, processes it and sends out a digital pulse. These pulses are usually ”ORed” (summed) onto a single line and sent to a multiplicity counter for further analysis, see Fig. 1 . This results in a single pulse train, see Fig. 3. Although the time of the pulse train is continuous, the multiplicity counter needs to break it up into discrete time intervals, hence discrete time intervals are used here for any explanation. Let us in the following call such a discrete time interval a “TIC”. Some modern instruments however, provide also the possibility of recording the signal from each preamplifier on a separate line, see Fig. 4. These multiple pulse trains are time-synchronized and can, in case of a list mode electronics, be transferred to a computer for more detailed analysis, see Fig. 5. Although the time of the pulse train is continuous, the list mode electronics need to break it up into discrete time intervals, hence discrete time intervals are used here for any explanation. Let us in the following call such a discrete time interval a “TIC”. After the detection of a neutron, the 3 He tube needs some time to recover (usually about a microsecond), in which no further pulse * Corresponding author. Tel.: þ39033786647; fax: þ390332786082. E-mail addresses: ludwig.holzleitner@ec.europa.eu (L. Holzleitner), swinhoe@ lanl.gov (M.T. Swinhoe). Contents lists available at ScienceDirect Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas 1350-4487/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.radmeas.2011.01.001 Radiation Measurements 46 (2011) 340e356