How to make CellTable Header always on top

In my application, I have a scrollpanel and a celltable inside the scrollPanel, most of the column in the celltable is sortable, the problem is that when the record in the celltable is large and when the user scroll down, the celltable header will be hide. is it possible to make the celltable header always on top even we scroll down the content (in current situation, I feel it is impossible)?

currently, the header is created by

celltable.addColumn(column, "headername");

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