Disabling the field Equipment number in Change mode of Notification

Dear Friends,

                            I am creating a Notification using IW21 and entering the Equipment number in this screen. I release this Notification and save.I would be converting this Notification into Order.

Once my notification is saved i donot want anyone (including myself) to change the equipment number field. I should be able to edit the rest of the fields in IW22(Change Notification) mode.Can you suggest me the settings required for making this equipment field not editable(initially it should allow me to enter the equipment number & restrict me after releasing the Notification).

Regards

Rama Krishna