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2.3.3 Neutron flux measurements<br />

• Absolute calibration<br />

The integral flux is measured by activation of nickel foils. These foils are put against the<br />

GENEPI target. The dimensions of the foils are about 5 mm in radius and 0.5 mm in<br />

thickness. Six hours of irradiation with an intensity of 66 µA are enough to reach saturation.<br />

Following reactions are used: 58 Ni(n,2n) 57 Ni with a threshold 13 MeV, 58 Ni(n,np) 57 Co with a<br />

threshold 13 MeV. These activated foils are then counted in the low radioactive laboratory at<br />

ISN.<br />

•<br />

233 U fission detector<br />

The GENEPI neutron pulse, generated at time zero by the reaction T(d,n)α in the lead block<br />

centre region, gives at position a neutron flux φ(E,t, ) which is measured with a<br />

detection system using the exothermic reaction 233 U(n,fission). The reaction rate versus time<br />

is proportional to the quantity φ(E,t, )σ(E). Assuming that the cross-section σ(E) is<br />

known, the measurement of the reaction rate gives an experimental access to the quantity<br />

φ(E,t, ). The fission fragments produced in the reaction 233 U(n,fission) (Q = 180MeV) are<br />

collected by a silicon detector. The 233 U target of 200 µg/cm2 is pure electro-deposited 233 U<br />

on a 200 µm thick aluminium foil. This small detection device is enclosed in lead. Two small<br />

charge-preamplifier are connected to the detectors inside the steel cylinder [4].<br />

2.3.4 Capture rate reaction measurement<br />

A scintillator coupled with a photomultiplier is used for sample (n,γ) reaction rate measurements.<br />

The PM is a XP1911 type from Philips [5]. It was chosen for its reduced dimensions (φ = 19 mm).<br />

Teflon has been chosen for the embase material, to avoid hydrogen and subsequent neutron energy<br />

degradation. PM gain variation has been minimised by adequate decoupling capacitances. CeF 3<br />

scintillator has been chosen for its quick time response time (30 ns) and for its low neutron captures<br />

cross-section. The detection system and the sample embedded in a lead box in order to have a good<br />

reproducibility of the detection geometry. Every two samples, the background has been<br />

systematically measured in order to check the stability of the PM gain. The beam intensity was<br />

adjusted to have a low dead time for each sample (0.1 evts/pulse during the first 10 µs). This detector<br />

and its lead box are shown in Figure 4.<br />

Figure 4. Picture of the PM in its lead box<br />

701

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