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1. Introduction
Among the large number of topics relevant to the design of the n_TOF facility at CERN, a
question of primary importance from the experimental point of view is the determination and
definition of the characteristics of the installation. The time-of-flight (TOF) measurements require a
geometrically well-defined neutron beam at the sample position and the absence of backgrounds.
Moreover, the neutron beam has to be adapted to the size of the samples, which is limited by the
available amounts of high purity materials, by the target construction procedure and also by their
intrinsic radioactivity (in case of unstable isotopes). The neutron beam has to be compatible with the
requirements coming from the various proposed experimental techniques. The experimental
programme of the n_TOF project covers a wide range of measurements summarised as follows:
• (n,γ) cross-section measurements with C 6 D 6 detectors (in a first phase) and with a total
absorption 4π calorimeter (in the second phase).
• (n,f) cross-section measurements with Parallel Plate Avalanche Chambers (PPAC).
• (n,xn) cross-section measurements with Ge or Si detectors.
Even though many sources of background can be highly suppressed through the time-of-flight
tagging, those background events having a time correlation similar to the neutrons may not be rejected
and may severely distort the measurements. Such background sources can be classified in two
categories: i) neutron reactions at the sample without the proper time-energy relation and ii) signals in
the detectors (produced by photons, neutron recoils or other particles) not originating from the reaction
under study. In the first case, the background can be highly reduced by designing the optical and
shielding elements (beam tube, collimators and walls) of the neutron beam line. The second
background species are also reduced in this way, but in addition the number of secondary reactions (of
neutrons and charged particles) has to be minimised also at the experimental area and its vicinity. The
n_TOF collaboration has dedicated a big effort to the studies needed for the definition of the neutron
beam design. Such studies covered a large spectrum of issues that can be summarised as follows:
• Study of the neutronic properties of the spallation target. In particular, production rates, the
energy, the time, the spatial and the angular distribution of the neutrons.
• Study and design of the beam optics, i.e. beam tube and collimators.
• Design of the necessary shielding elements, which guarantee clean experimental conditions.
It should be emphasised that all these investigations were made under the scope of providing
optimal conditions to the measurements, according to the following directives:
• The neutron beam size should have a radius as small as 2 cm, given the availability of the
samples, its intrinsic radioactivity, the overall detection efficiencies and the experimental
requirements arising from the capture (both the C 6 D 6 detectors and the 4π calorimeter), the
fission and the (n, xn) measurements. However, further developments are considered in order
to adapt the beam characteristics to particular measurements by studying and designing
variable size collimators.
• Design the corresponding beam optics (beam tube and collimators) such as to achieve neutron
and gamma backgrounds by order of magnitudes lower than the beam flux.
• Define the shielding elements such, as to improve the background attenuation and respect at
the same time the conditions imposed by the CERN safety rules, which impose the
accessibility and emergency escape paths of the installation.
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