NeuLAND - FAIR

Since, as discussed above, several hits in the detector occur per neutron, it is essential
to find not only the correct number of neutrons but also to reconstruct their momentum
from the first interaction point.
In the following we describe the methods developed for the NeuLAND neutron reconstruction,
as of today. It turns out that a generalized tracking mechanism alone does
not lead to satisfactory results with respect to the multi-neutron identification. Instead,
we use a method which combines tracking with calorimetric information in order to
resolve multi-neutron events. We first introduce the definition of a valid cluster in Neu-
LAND, then detail the calorimetric resolving power and finally, by a combination of
both, determine the number of neutrons and their momentum.
Clusters
Within one event a hit in NeuLAND is defined as a coincident observation of a signal,
above threshold, at both ends of a NeuLAND submodule. The hits are sorted time-wise,
and then, in a first step, neighbouring hits are identified, starting from the first hit in
time. Hits belonging to a cluster are thus defined by certain distance in space (≤ 7.5 cm
in x, y and z each) and a certain time difference (≤ 1 ns) to at least one other member
of the cluster. After all hits have been assigned to clusters, for each cluster only the first
and last hit in time are stored. 3 Again, the clusters are sorted according to their time of
the first hit. The first cluster is treated as occurring from a first interaction of an incident
neutron. For all following clusters a procedure with kinematical conditions is applied
in order to check whether these clusters could stem from elastically scattered neutrons
from a prior incident point with an earlier cluster. That takes into account the scattering
angle of the particle of the earlier cluster (most likely a proton), which we define from
the first and last hits of the cluster. If one of the later clusters fulfills the criteria of
an elastic scattering process of the incident neutron, this cluster is eliminated from the
further analysis. This procedure is iterated for all clusters until no more correlated
secondary clusters can be found.
The number of remaining clusters still is substantially higher than the number of incident
neutrons and depends on the energy of impinging neutrons. Exemplarily, for 200 MeV
neutrons, a mean value of about 2.7 clusters is observed for one-neutron events, at
600 MeV the value increases to about 6.3 and for 1000 MeV we observe on average
10.5 clusters per neutron. In figure 4.13 the number of clusters is displayed for one- to
six-neutron events at 600 MeV. The overlap of the number of clusters for the different
neutron channels is large. Therefore, in the following we detail the use of the calorimetric
properties of NeuLAND for the assignment of proper neutron multiplicities.
3 As special case hits without any neighbour are defined as clusters by themselves.
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