NeuLAND - FAIR

We observe a very high percentage of proper neutron recognition, i.e. the neutron
number is correctly derived from the algorithm, indicated in bold in the tables. Even
for the lowest neutron energies the 4-neutron recognition still stays close to 50%, for
the highest beam energies values of more than 60% are obtained. In this example the
calorimetric cuts were set in manner favouring too low neutron multiplicities over too
high ones. So, more events are erroneously assigned to a lower neutron multiplicity than
to a higher neutron multiplicity. The separation matrices derived in this simulation,
explore a combined efficiency, taking both the neutron tracking capabilities and the
geometrical acceptance into account. NeuLAND is located in the full-acceptance distance
for the relative energy example of Erel = 500 keV discussed here. However, a certain
fraction of neutrons does not reach the detector volume, due to scattering in the air along
their flight path from the target to the detector at a distance of 15.5 m. Exemplarily,
for 600 MeV neutrons approximately 1.5% of the neutrons undergo such an interaction.
For events with a neutron multiplicity of four, in approximately 6% of the events at least
one neutron will not arrive at the detector volume. These losses are contributing to the
values derived in the separation matrices.
The high values for correct multiplicities for 3 and 4 neutron cases are of extreme importance
for the envisaged physics programme with NeuLAND, since the investigation
of more and more neutron-rich nuclei is accompanied by higher neutron multiplicities.
The detector depth of 3 m plays a major role for the multi-neutron recognition, since
the calorimetric properties depend significantly on the detector volume. Exemplarily, we
study the one- to five-neutron recognition of a detector, same as NeuLAND, but with
a reduced depth of 2 m only. The neutron separation matrix for 600 MeV neutrons is
shown in table 4.6.
detected
600 MeV, 2m depth
generated
% 1n 2n 3n 4n 5n
1n 83 30 7 1 0
2n 7 63 45 17 5
3n 0 5 39 36 18
4n 0 0 8 42 54
5n 0 0 0 3 22
6n 0 0 0 0 2
Table 4.6.: Neutron separation matrix for 600 MeV neutrons, as in the middle panel of
table 4.5, but for a reduced depth of NeuLAND, i.e. 2 m instead of 3 m.
While the one-neutron recognition is affected mildly, decreasing from 92% (3 m) to 83%
(2 m), the impact on the multi-neutron recognition is more drastic. For the detector
depth of 2 m, four-neutron events are detected with the correct multiplicity in 42%
of all cases, which is a decrease by 26% compared to the 57% detected with the 3 m
depth of NeuLAND. Correspondingly the fraction of misidentified neutron multiplicities
is enlarged.
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