An Automated Irradiation Facility for Neutron Doping of Large Silicon Ingots

Abstract

The tendency of the NTD silicon producers to increase their capacities and the extending range of devices in which NTD silicon is being used, calls for special reactor irradiation facilities. This is increasingly important at low resistivity values where the neutron dose requires a high neutron flux to keep within acceptable values the irradiation time. In addition an automated charging- discharging device for the silicon ingots is desirable to optimize the dead handling times with respect to the irradiation time.

The JRC heavy water moderated ESSOR reactor presents large irradiation volumes in large diameter experimental channels with neutron flux characteristics especially suited to the above requirements. The described facility has been conceived for installation in one of these channels; its main characteristics are indicated below.

The irradiation volume consists of a cylinder 50 cm long, 87 mm in diameter; the future extension of the diameter to 112 mm is feasible; the thermal neutron flux is equal to 2.7xl0^1.4n cm^-2s^-1 with a thermal to fast (> 100 keV) neutron flux ratio equal to about 400. The irradiation position in the channel is optimized by displacement during the reactor operation cycle so that the axial spread of the neutron flux is maintained within ± 4% over the total length.

The silicon ingots are irradiated in bored plastic material capsules immersed in a D₂O flux to ensure efficient cooling; a slowrotation of the ingots is induced by a suitable shaping of the capsule. The D₂O flux from the moderator carries the capsules from a valving arrangement (SAS) to the irradiation position and keeps them for the required irradiation time determined from a calibrated three collectrons system with associated electronics where the integrated neutron dose can be achieved with a precision of 3%.

At the end of the irradiation time, the capsule is discharged by gravity into the SAS where the remaining D₂O is evaporated. The capsules are introduced into the SAS and extracted by a suitable system (TAU) which is manually loaded and unloaded with the capsules also during the reactor operation; the TAU system holds up to 100 capsules, and advances them automatically to the correct position for injection into the reactor.

Preliminary experimental production of NTD-Si in ESSOR has shown the outstanding advantages of this reactor for such work.