The final beam stop on the high intensity beam line 1A produces neutrons from the spallation reaction of 100-150 µA of 400-450 MeV protons on an aluminum plate absorber surrounded by a water moderator. Neutron channels transport these neutrons through the shielding surrounding the TNF. Initially thermal neutrons were used for a neutron scattering experiment on one of these channels. In 2002 the energy spectrum of these neutrons was measured to determine the >1 MeV neutron component. Dosimetry is carried out using activation foils of nickel, aluminum, carbon and gold.
The flux of neutrons at the normal operating currents on BL1A is 6 x 106 neutron cm-2s-1 above 1 MeV, comparable with the neutron flux at the LANSCE neutron radiation effect line and has energies extending to about 400 MeV as shown in the neutron spectrum. Thermal neutrons are also present if desired. They can be easily removed using a cadmium shield.
Access to this neutron beam is available for 3000 hours per year as a parasitic operation on BL1A operation. At the present time the physical space for devices is limited to a cross section of 5 cm by 20 cm. However, studies are under way to determine if a shielding rearrangement can provide for a larger test space.
TNF Plan View
shows the layout of the testing area. The neutron beam is accessed from 5 m
above the beam through a vertical slot in the shielding. Devices are mounted on
a plate which is
lowered into the neutron beam on a cable. The neutron beam size obtained using
radiochromic film is shown in the neutron beam picture.
There is no neutron beam blocker. The DUT is lowered into the beam and then
raised out of the beam when the desired fluence is reached.
Neutrons can be produced in the present PIF area by stopping energetic protons from either BL2C or BL1B on a fully absorbing beam stop and setting up downstream of the beamstop. The neutron spectrum has 1/E falloff of flux with energies up to the proton energy.
The neutron flux and energy spectrum has been measured for different proton energies and a few geometries using a combination of Bonner spheres and carbon activation. These neutrons have already been used to test various types of neutron dosimeters and to check for neutron radiation effects in sensitive electronics.
The sea level neutron flux is approximated at 20 neutrons/cm2/hr
above 10 MeV. The neutron flux produced by stopping 3 nA of 110 MeV protons on
a lead absorber, at a location 1.4 m in downstream, is about 105
neutron cm-2s-1 or approximately
107 times higher than this sea level neutron flux.