You can use the find script command to obtain the indices of all elements in R0 that have values greater than 0.9.
ind = find(R0>0.9);
However, this will return single index values of the desired elements. You would have to convert these single index values into multiple index (i,j) values, which is quite straightforward for a 2D matrix.
More information on this conversion can be found on the following link:
Once you have the (i,j) values, you can determine the required period and fill factor as period(i) and fillfactor(j) or vice versa depending upon the way you have defined the parameter sweep.