Nuclear Physics – AFRODITE
Large -ray detector arrays are usually inefficient for the detection of photons below 20 keV. AFRODITE is a medium size array that has the unique capability of detecting both high and low energy photons with a reasonably high efficiency by combining large volume escape suppressed HPGe detectors (CLOVERS) with Low Energy Photon Spectrometer (LEPS) detectors.The AFRODITE frame is a small rhombicuboctahedron with 16 detector positions. The target chamber has the same geometry and has thin (75 micron) Kapton windows on the 16 square faces. AFRODITE comprises 8 Clover detectors similar to those of EUROBALL III and 8 LEPS for photons between 30 and 300 keV. Each Clover consists of four 50 x 70 mm HPGe crystals and the 8 Clovers subtend 11% of 4pi. The LEPS consist of four segmented planar Ge detectors of 2800 mm2 x 10 mm and they subtend 12.5% of 4pi. An ancillary detector for the detection of fission fragments can be mounted inside the target chamber. It comprises 32 photovoltaic cells. To view the array in the target chamber click here.
Under normal operation, the Afrodite detectors need to be maintained at temperatures of approximately -180 °C. For this reason, an automated liquid nitrogen cooling system was designed and built, in order to reliably keep cool and monitor the individual detectors.
Each detector is constructed with an integral 2.5 litre liquid nitrogen dewar, which in turn is coupled to the sensitive crystals by means of a copper rod for continuous cooling. The detectors are filled via insulated PTFE tubes and solenoid-operated valves, from a 180 litre liquid nitrogen supply tank. The filling sequences and times are controlled by a programmable logic controller while the supply tank volume is monitored by means of a load-cell scale with a digital readout unit.
The detector temperatures are monitored by an electronic alarm system, as well as a graphic temperature display and temperature history on a dedicated PC. If the electronic alarm is activated, a blinking indicator on the PC’s monitor will appear. The PC also monitors an output from the scale-readout unit, for checking that there is always an adequate supply of liquid nitrogen available during filling cycles.