With password, how to find the passphrase for full disk encryption after a clone

  1. I first installed Xubuntu 18.04 with full disk encryption with
    default option; I remember the password, but I lost the passphrase
    accidentally.
  2. I cloned my original disc with clonezilla on a new disc;

Now I want to retrieve the secret phrase of the new disc and find out if it is possible.