How many TVL's in a linac head

At 1 m from the source, nonshielded vs shielded you get 0.1% (leakage through the head).

To get that think of half values 1/2=50 .......1/16=6.25.........1/1024=10 HVL'S

So 10 HVL's are required to get it to 0.1%

We know 1 TVL=3.3 HVL's so, so to get 10 HVL's we need about 3 TVL's.

The answer is then 3 TVL's.