Silicon irradiation also known as Neutron Transmutation Doping (NTD), is conducted in the OPAL reactor.
Through this process thermal neutrons react with the silicon atoms, changing some of those atoms to phosphorus, which reduces the resistivity of the silicon and improves its ability to conduct electricity.
The NTD process is used when a high level of precision and uniformity in dopant (phosphorous) distribution is required. This allows the achievement of a better, more reliable performance across all electrical devices but is particularly critical for high power and very high power devices.
- High 'thermal to fast' neutron ratio (Cd ratio) - ensures minimal damage during irradiation
- High irradiation capacity - ensure a quick turnaround
- High accuracy - ensures the final resistivity within +/-5% of specified target
- High flexibility - we can accept a wide range of target resistivities and silicon sizes
- High quality - we maintain ISO 9001:2015 certification
- Six vertical irradiation facilities within the OPAL reactor
- Location: D20 reflector vessel
- Max sample size: Diameter - 203 mm (8"), length - 600 mm
- Min sample size: Diameter - 56 mm (24"), length - 550 mm
- Cd ratio (thermal to fast): > 800
- Thermal flux: 3.0E+12 to 1.4E+13 n/cm2 x sec
- Irradiation conditions: unsealed can in a rotating rig
- Cooling: downward water flow
- Loading/unloading: possible during reactor operation at nominal power
There are six specialised facilities in the OPAL reactor pool which are utilised for the irradiation of silicon.
Irradiation times required for different silicon ingots can vary from one hour (or less) to five days, depending on target resistivity requested by the silicon manufacturer. Initially, the irradiation canister containing fresh silicon is loaded by highly skilled utilisation operators into the irradiation facility. The can is then irradiated with neutrons. When the required parameters as specified by the customer are achieved, the irradiation canister with irradiated silicon inside is removed from the facility and transferred to the adjacent service pool to 'cool'.
The irradiated silicon “cools” by radioactive decay and stops emitting radiation approximately 48 hours after irradiation. It is then removed from the service pool using a dedicated unloading station and cleaned.
The final step is to ensure that the silicon ingots are no longer radioactive and ready for final inspection before shipping to the manufacturer.
ANSTO's silicon irradiation services operate in compliance with 9001 : 2015 Quality System, ensuring that a reliable and qualitative supply of NTD-silicon is provided to customers.