N ACIONAL

3.4.1 Description of the Neutron Irradiation Facility

Neutron irradiations were performed at a monoenergetic fast neutron facility located at the Lucas Heights Research Laboratories of ANSTO in Sydney, Australia.

The facility consists of a positive ion accelerator, a series of beam lines and a number of experimental stations. One of these stations was equipped with a neutron production target. The accelerator is a 3 MV horizontal Van de Graaff (type KN-3000) which is capable of accelerating singly charged positive ions up to energies of 3 MeV. The machine is installed in a heavily shielded cell adjacent to the experimental area. Control is facilitated from a dedicated room also adjacent to the experimental area. The accelerated ion beams leave the accelerator and through a series of analysers and switching magnets can be deflected into the appropriate beam line to terminate at the required experimental target. The experimental area in which the target is located is bounded by heavily shielding concrete blocks which rise to a height of 6 m as

measured from the floor. Access to this area during beam on periods is not possible due to an interlocking mechanism between the entry gates and the beam control.

The target used for this work was a thin lithium metal target. The neutrons are produced by bombarding the target with energetic protons according to the reaction:

p + Li7→ Be7 + n (3.17)

The reaction has a Q value of -1.646 MeV. The corresponding threshold is 1.8811 MeV (for protons measured in the laboratory frame of reference [194]). For protons of energy 2.7 MeV, neutrons with energies of 0.995 MeV are produced in the forward direction. At other angles the neutron energy is less as a result of the anisotropy of the reaction. The neutron yield is also anisotropic. A second reaction channel with a threshold of 2.378 MeV also exists and is associated with the Li7(p,n)Be7* reaction. The contribution of this second reaction to the total neutron yield is in most cases negligible.

The lithium targets are produced regularly for each major experiment. The preparation involves the evaporation of a thin film of high purity lithium metal onto the inner surface of a copper flange. This is done in a conventional tungsten filament low vacuum metal evaporator. The target is then transferred and attached to a dedicated accelerator beam line under a continuously purged atmosphere of argon. This is to prevent oxidation of the lithium metal. Target thickness can be estimated from the quantity of lithium metal evaporated and the solid angle that the copper flange presents to the filament. A more accurate measurement of target thickness can be made when the target is in use on the beam line. This involves the measurement of the neutron yield as a function of proton energy. In the laboratory frame of reference the neutron yield in the forward direction is sharply peaked about the reaction threshold. By measuring the neutron yield from the reaction threshold and up until the proton energy at which the neutron yield first starts to decrease, the target thickness can be obtained in terms of the proton energy lost in the target. For targets produced here typical thicknesses of

100 - 250 keV could be obtained in one evaporation step. Thicker targets can be produced by multiple evaporation steps.

In the neutron irradiations undertaken in this work proton energies of between 2.6 and 2.7 MeV with typical beam currents of 24.0 µA were used. Due to this relatively high target current cooling was required in order to reduce the loss of target caused by excessive heating. This involved the application of a fine water spray directed towards the target in addition to continuous rotation of the target cap to reduce any localised heating.

The closest scattering surface from the point of neutron production is the floor at a distance of 1.4 m. To reduce the flux of backscattered neutrons the floor has been covered with 12 cm thick paraffin blocks. The next closest scattering surface is more than 3 metres away from the neutron producing target.

The neutron flux could be monitored during irradiation using a calibrated neutron long counter which was placed several metres from the neutron source. Such a detector is characterised by an almost uniform detection efficiency for neutrons of energies about 1 MeV. The beam current on the target could also be measured using a Faraday cup and the accumulated charge determined using a current to frequency converter and a counter/timer unit. This allows measurement of the neutron flux as a function of target current and not as a function of time. This removes the effect of beam current fluctuation on measurement of the neutron yield.

3.4.2 Neutron Irradiation of the Detector Test Structures

A neutron energy of 1 MeV was desired requiring a proton energy of 2.7 MeV. Due to machine instability at 2.7 MV the voltage was reduced to a more stable operating voltage of 2.6 MV. The corresponding 2.6 MeV protons produce neutrons of energy 0.891 MeV in the forward direction [194]. The target thickness in terms of proton energy loss was measured to be 200 keV. This proton energy loss in the target results in a range of neutron energies from the maximum of 0.8898 MeV to a minimum of 0.679 MeV. The minimum neutron energy corresponds to the minimum proton energy of 2.4 MeV.

The neutron flux was monitored by the neutron long counter which was positioned at a distance of 3.038 m from the target in the zero degree beam line direction. The measured neutron fluence was estimated to be accurate to 20 %.

The detectors were mounted at a distance of 18.9 mm from the target. They were positioned orthogonally to the beam line axis in order to achieve a variation in neutron fluence across the two substrates. The irradiation was performed over a period of two days in two separate 10 hr sequences. A contour map of the neutron fluence across the two substrates is shown in Figure 3.15.

Figure 3.15: Neutron fluence contour map. Detector U4a was exposed to the highest neutron

fluence. U5a U5b U5c U4c U4b U4a 11.0 10.6 9.50 7.05 5.80 4.76×1012 n⋅cm-2