Innocent neutron alarms caused by cosmic rays.

The increasing threat of the illicit proliferation of nuclear and radioactive materials in the territories of different states and across their borders with the possible use of these materials for terrorist attacks has within the world community necessitated the creation of effective counteractions. Among various radiation detection equipment used for the control of illicit traffic, the fixed installed gamma-neutron monitor is the most common because of its superior sensitivity for detection of radiation which exceeds the natural background. Each of the alarms of the monitor requires performing the appointed procedure by the services in charge. Practically all countries have different response procedures to be performed in the event of the alarm of the gamma detector and neutron detector. The existence of natural gamma radiation sources in the environment and their presence in the materials transported through the controlled areas cause the so called “innocent” alarms of the gamma channel of the monitor. So, not every alarm of the gamma detector of the portal monitor means that the hazardous radiation source appeared in the controlled area. But it is quite another matter when the alarm occurs in the neutron detector. Up till now it has been considered that the transported material has neutron source inside, since natural neutron sources do not exists in nature. However, recently some information appeared at some sites of installation of the gammaneutron monitors the real alarms of the neutron detectors have occurred even though the transported materials did not contain any neutron sources. The events of such alarms make it necessary to reconsider the response procedures and develop the appropriate algorithms which would enable the services in charge to discern the real neutron sources from the ones which can be called the “innocent” sources. This task is of current importance in the field of creation of the effective equipment for combating the nuclear and radiation terrorism. In this paper the reasons of innocent neutron alarms caused by the interaction of the cosmic rays with the materials placed in the vehicles crossing the controlled zone are considered. It shows the results of calculation of the effects of moderation of the secondary neutron radiation, generation of additional neutrons resulted in the nuclear reactions of various components of the cosmic rays with the kernels of the light, medium and heavy elements which cause the increase of the count rate of the neutron channels and alarm. The recommendations of how to discern the innocent alarms from the real neutron sources are given.