• Reactor

Ljubljana Neutron Irradiation Facility

The reactor research centre which is a part of Jozef Stefan Institute, has an experimental nuclear reactor of type TRIGA.

Irradiation of a sample is done by placing it into the core through an irradiation tube which occupies a fuel rod position.
ATLAS group uses two such irradiation tubes which are located at the edge of the core. The tubes have different dimensions:
the smal tube has a circular cross section with 2.2cm diameter and  the large tube has an elliptic cross section with axis lengths 7 and 5 cm.

The irradiation tubes enter the core from above and are about 5m long because the core is covered with 5m of water. The tubes are curved in a chicane just above the core to prevent radiation from the core escaping through the tube. The chicane limits the length of the sample to be irradiated to about 15cm.

The neutron energy spectrum in the tube was determined  by measuring the activation of foils of different materials. The spectrum is shown in the figure below and one can compare measurements with simulation of the spectrum.

                           Neutron energy spectrum in the irradiation tube
 

Using the measured energy spectrum and damage functions one can determine the flux of 1MeV NIEL equivalent neutrons. NIEL equivalent flux was also measured using the leakage current method (see M. Moll, E. Frewurst, G. Lindstroem, Leakage current of hadron irradiated silicon detectors - material dependence, Nucl. Instr. & Meth. in Phys. Res. A426(1999) 87-93)

Since the flux is linearly proportional to the reactor operating power the flux of 1MeV NIEL equivalent neutrons in our reactor is:

• 8.8x10⁹ n/kWcm²s  in the small tube
• 18x10⁹ n/kWcm²s  in the large tube

The reactor can be run with large span of operating powers (few W to 250 kW), enabling irradiations with various neutron fluxes. The irradiation of ATLAS electronics was done in the big tube at the reactor power of 25 kW which means that the fluence of 1x10¹³ n/cm² was reached in 22 seconds.