Shred (Secure Delete) a file in NTFS

I need a secure way to delete a file.
I've gone through the standard procedure of overwriting 6 times with 0 and 255 bytes and then with random data, but when I scan the disk, I see a copy of the information is there.
I've used:

  public static void secureDelete(File file, byte[] b) throws IOException {
    if (file.exists()) {
      long length = file.length();
      SecureRandom random = new SecureRandom();
      RandomAccessFile raf = new RandomAccessFile(file, "rws");
      raf.seek(0);
      raf.getFilePointer();
      byte[] data = new byte[50];
      int pos = 0;
      while (pos < length) {
        random.nextBytes(data);
        raf.write(data);
        pos += data.length;
      }
      raf.close();
      file.delete();
    }
  }

using as 'b' zeros, 255 and random numbers.
It works for FAT but not for NTFS