The operation of accelerator-driven systems or spallation sources requires the monitoring of intense neutron fluxes, which may be billions-fold more intense than the fluxes obtained with usual radioactive sources. If a neutron detector is placed near a very intense source, it can become saturated because of detector dead time. On the contrary, if it is placed far away from the source, it will lose counting statistics. For this reason, there must exist an optimal position for placing the detector. The optimal position is defined as the one with the minimal relative uncertainty in the counting rate. In this work, we review the techniques to determine the detector dead time that can be applied with an accelerator-driven subcritical system or a spallation source. For the case of a spallation source, counting rates do not follow Poisson's statistics because of the multiplicity of the number of neutrons emitted by incident proton. It has been found a simple expression that relates the optimal counting rate with the source multiplicity and the uncertainty in the determination of the dead time. 1. Introduction A strong interest in intense accelerator-driven neutron sources for different applications has grown in the last decades. Although a number of reactions is available for neutron production with accelerated particles (photoneutrons, D-Be, and others), preferred technology for these neutron sources is spallation, because it provides the highest neutron yield per unit of energy of the incident particles. Neutron spallation sources consist of a high-energy proton accelerator coupled to a high Z target. Typically, a 1000？MeV incident proton beam could produce ~50？nucleons per incident proton, approximately half of them neutrons [1]. In this way, very intense neutron fluxes can be obtained (over 1015？n/cm2？s). Although current mode detectors are generally preferred for monitoring such high fluxes, pulse mode detectors are also of interest because of their ability to record information from the individual detector signals. However, if a pulsed mode detector is used to monitor such high fluxes, it can become saturated because of dead time effects. An important application of spallation sources is to drive accelerator-driven systems (ADSs). An ADS consists of a subcritical reactor driven by an external spallation neutron source. The interest in these systems has risen since the 1990s [2–6] as it has been recognized their ability to reduce the volume and radiotoxicity of high-level nuclear waste. Operation of ADSs requires the monitoring of the neutron source
References
[1]
A. Letourneau, J. Galin, F. Goldenbaum et al., “Neutron production in bombardments of thin and thick W, Hg, Pb targets by 0.4, 0.8, 1.2, 1.8 and 2.5 GeV protons,” Nuclear Instruments and Methods in Physics Research, Section B, vol. 170, no. 3, pp. 299–322, 2000.
[2]
H. Nifenecker, S. David, J. M. Loiseaux, and A. Giorni, “Hybrid nuclear reactors,” Progress in Particle and Nuclear Physics, vol. 43, pp. 683–827, 1999.
[3]
A. Gandini and M. Salvatores, “The physics of subcritical multiplying systems,” Journal of Nuclear Science and Technology, vol. 39, no. 6, pp. 673–686, 2002.
[4]
OECD-NEA, “Accelerator-Driven Systems (ADS) and fast reactors (FR) in advanced nuclear fuel cycles. A comparative study,” Tech. Rep. NEA-3109, 2002.
[5]
OECD-NEA, “Physics and safety of transmutation systems–A status report,” Tech. Rep. NEA-6090, 2006.
[6]
W. von Lensa, R. Nabbi, and M. Rossbach, “RED-IMPACT—impact of partitioning, transmutation and waste reduction technologies on the final nuclear waste disposal—synthesis report,” in Schriften des Forschungszentrums Juelich—Reihe Energie & Umwelt, vol. 15, 2008.
[7]
P. Baeten and H. A. Abderrahim, “Reactivity monitoring in ADS, application to the Myrrha ADS project,” Progress in Nuclear Energy, vol. 43, no. 1–4, pp. 413–419, 2003.
[8]
E. González-Romero, D. Villamarín, M. Embid et al., “The MUSE4 pulsed Neutron source experiments,” in The Physics of Fuel Cycles and Advanced Nuclear Systems—Global Developments (PHYSOR '04), pp. 1399–1407, Chicago, Ill, USA, April 2004.
[9]
R. Soule, W. Assal, P. Chaussonnet et al., “Neutronic studies in support of accelerator-driven systems: the MUSE experiments in the MASURCA facility,” Nuclear Science and Engineering, vol. 148, no. 1, pp. 124–152, 2004.
[10]
V. Bournos, et al., “YALINA-Booster benchmark specifications for the IAEA coordinated research projects on analytical and experimental benchmark analysis on accelerator driven systems, and low enriched uranium fuel utilization in accelerator driven sub-critical assembly systems,” IAEA Document, 2007.
[11]
H. Kiyavitskaya, et al., “Experimental investigations at sub-critical facilities of Joint Institute for Power and Nuclear Research—Sosny of the National Academy of Sciences of Belarus,” in Technical Meeting on Use of Low Enriched Uranium in Accelerator Driven Sub—Critical Assemblies, Vienna, Austria, October 2005.
[12]
European Commission, Integrated Project EUROTRANS. Annex I—Description of Work. Contract no. FI6W-CT-2004-516520, 2005.
[13]
G. F. Knoll, Radiation Detection and Measurement, John Wiley & Sons, 3rd edition, 1999.
[14]
International Comission on Radiation Units & Measurements, “Particle counting in radioactivity measurements,” ICRU Report 52, 1994.
[15]
A. E. Profio, Experimental Reactor Physics, John Wiley & Sons, 1976.
[16]
G. R. Keepin, Physics of Nuclear Kinetics, Addison-Wesley, 1965.
[17]
K. O. Ott and R. J. Neuhold, Introductory Nuclear Reactor Dynamics, American Nuclear Society, 1985.
[18]
D. L. Hetrick, Dynamics of Nuclear Reactors, American Nuclear Society, 1993.
[19]
H. D. Choi, “Counting statistics distorted by two dead times in series which end with an extended type dead time,” Nuclear Instruments and Methods in Physics Research, Section A, vol. 599, no. 2-3, pp. 251–259, 2009.
[20]
H. D. Choi, “Counting statistics modified by two dead times in series,” Nuclear Engineering and Technology, vol. 43, no. 3, pp. 287–300, 2011.
[21]
Y. Yamane and I. Pázsit, “Heuristic derivation of Rossi-alpha formula with delayed neutrons and correlated source,” Annals of Nuclear Energy, vol. 25, no. 17, pp. 1373–1382, 1998.
