Neutron Detector

A Neutron Detector is placed just under the S4 counter with the aim to increase the electromagnetic and hadronic discrimination capability of the Pamela instrument and to expand the energy range of the detected primary protons and electrons up to 1011-1013 eV. It consists of 36 proportional counters, filled with 3He, placed in two planes surrounded by a polyethylene moderator ~ 9 cm thick.
To suppress the outside thermal neutron background, the neutron detector is shielded by a thin Cd layer from the bottom and side directions.

The size of the Neutron Detector is 60x55x15 cm3, the total weight is 30 kg, power consuming is 10 W. When a high-energy hadron interacts inside the calorimeter, a large number of neutrons from the decay of excited nucleus are produced, while if a primary particle is a lepton the number of neutrons generated in the photonuclear interactions, mainly from the giant resonance, is 10-20 times lower. A part of these neutrons is thermalized by the polyethylene moderator and detected by the 3He counters.

To reach energies at level of several TeV, because the very low number of expected events it is necessary to detect the omni directional flux of particles impinging the calorimeter, except from the angles screened by the PAMELA magnet and the satellite. In this configuration the geometry factor increases up to about 450 cm2 sr. A special trigger from the S4 counter is developed indicating the passage of a primary particle with initial energy greater than 200 GeV.