Nuclear Instruments and Methods in Physics Research A 489 (2002) 346–356 On the figure of merit in neutron time-of-flight measurements C. Coceva a,1 , M. Frisoni b , M. Magnani a , A. Mengoni a, * ,2 a ENEA, Applied Physics Division, Via Don Fiammelli 2, I-40129 Bologna, Italy b Athena s.a.s., Via Battiferno 3, I-40129 Bologna, Italy Received 24 October 2001; received in revised form 26 February 2002; accepted 4 April 2002 Abstract We address the task of evaluating the basic performances of neutron time-of-flight spectrometers, by means of an appropriate figure of merit. Calculations of neutron flux, energy resolution, and figure of merit are presented with a critical discussion on their definitions. The results of Monte Carlo simulations for two presently operating facilities, CERN n TOF and GELINA, are commented and compared. r 2002 Published by Elsevier Science B.V. PACS: 29.25.Dz; 28.20.G Keywords: Neutron sources; Time-of-flight measurements 1. Introduction Several features determine the performances of a neutron time-of-flight (t.o.f.) facility. The most important of them are: * time-averaged neutron flux in the flight-path direction, as a function of neutron energy, * resolution as a function of neutron energy, * neutron and gamma background at the detector station position. As for the first two items, complete reviews of the existing or planned high-intensity neutron sources were published in 1987 [1] and in 1990 [2]. As suggested in Ref. [3], the quality of different t.o.f. facilities was synthetically evaluated by means of an energy-dependent figure of merit simply defined as the average neutron flux at the end of the flight distance needed to obtain a prefixed energy resolution. However, we have found that unreliable values of the figure of merit are obtained when trying to apply the same rules to neutron generators based on completely differ- ent production methods. As an example of the pitfalls that can be met, we examine here two existing facilities, one based on a 100 MeV electron accelerator and another one based on a 20 GeV proton accelerator. Another comparison of different t.o.f. facilities, including a new facility lately set up at CERN, described below as the n TOF facility, was recently published [4]. The aim of that analysis, however, is less general than ours. In fact, it was focused on measurements for specific applications in nuclear *Corresponding author. Fax: +39-051-6098-359. E-mail address: mengoni@bologna.enea.it (A. Mengoni). 1 Present address: Via San Pier Grisologo 44, I-40126 Imola, Italy. 2 Present address: CERN, CH-1211 Geneva 23, Switzerland. E-mail: alberto.mengoni@cern.ch 0168-9002/02/$ - see front matter r 2002 Published by Elsevier Science B.V. PII:S0168-9002(02)00903-8