Introduction
In a simple cubic lattice, the ratio between the unit cell length and the separation of two adjacent parallel crystal planes is given by Miller indices. The Miller indices are a set of three integers that define a plane in a crystal lattice. The ratio of the unit cell length and the separation of two adjacent parallel crystal planes cannot have a value of some particular numbers, as discussed below.

Explanation
The Miller indices for the simple cubic lattice are (hkl), where h, k, and l are integers. The separation between two adjacent parallel crystal planes is given by the formula d = a/√(h^2+k^2+l^2), where a is the unit cell length. Therefore, the ratio of the unit cell length to the separation between two adjacent parallel crystal planes is given by a/d = √(h^2+k^2+l^2).

We need to find the values of h, k, and l for which the ratio a/d cannot have a value of 5^1/2, 7^1/2, 11^1/2, or 13^1/2.

- For a/d to be equal to 5^1/2, h^2+k^2+l^2 must be equal to 5. This is not possible for integers h, k, and l.
- For a/d to be equal to 7^1/2, h^2+k^2+l^2 must be equal to 7. This is not possible for integers h, k, and l.
- For a/d to be equal to 11^1/2, h^2+k^2+l^2 must be equal to 11. This is not possible for integers h, k, and l.
- For a/d to be equal to 13^1/2, h^2+k^2+l^2 must be equal to 13. This is possible for integers h=2, k=3, and l=2. Therefore, the ratio a/d can have a value of 13^1/2.

Conclusion
Therefore, the ratio of the unit cell length to the separation between two adjacent parallel crystal planes in a simple cubic lattice cannot have a value of 5^1/2 or 7^1/2 or 11^1/2. However, it can have a value of 13^1/2 for a specific set of Miller indices.