Neutron beam

The neutron beam from the H22 guide is collimated to a halo-free pencil beam of 1.5cm diameter. The neutron beam intensity at the target position is of 10⁸ n.cm^-2.s^-1.
The collimation consists of a sequence of 5 circular apertures (10mm, 4x12mm) made from high neutron absorbing materials. To avoid gamma  ray background all apertures are doubled with 5 cm thick lead absorbers downstream the neutron beam direction. The sequence of apertures is inserted in a round vacuum tube system, the inside walls of which are completely covered by 1 cm thick borated plastics. The total length of the aperture sequence is about 2.7 meters and is kept under primary vaccum in order to minimize neutron scattering in  air.

The FIPPS HPGe array consists of a central ring holding 8 clovers, each composed of four n-type HPGe crystals (diameter 50mm, length 80mm, front tapered), mounted around the target position.  The FIPPS setup is designed to be modular so that additional equipment and detectors can be added to improve the sensitivity with respect to specific nuclear physics observables.

The signals from the detectors are treated with digital electronics (CAEN V1724, 100 MHz sampling) and data are acquired in triggerless mode. The validated count rate is 8 kHz (instrument commissioning, December 2016, n,gamma on Ti target).

Target to detector distance (without AC shields) = 9cm.

Performance:

• Full array energy resolution at 1.4 MeV = 2.3 keV (152Eu source, 3kHz count rate per detector)
• Full array energy resolution at 121 keV = 1.7 keV (152Eu source, 3kHz count rate per detector)
• Energy range: ~40 keV - 8 MeV
• Efficiency at 1.4 MeV = 3% (from singles 152Eu).
• Add-back factor at 1.4 MeV = 1.17

The ambitious goal of FIPPS is to combine a high resolution gamma-ray spectroscopy together with a large acceptance recoil spectrometer based on a gas-filled magnet (GFM) device. Such a device will allow the identification of a given fragment and will open unprecedented possibilities to study thermal-neutron induced fission and fission products at ILL.

The GFM-spectrometer is now under development. It will be designed such that it can be transported to other facilities where beams of fast neutrons, light charged particles and photons are available to induce fission reactions of targets ranging from the heaviest elements down to the rare earth region.