The question is likely to be not easily answered, particularly if the spectrum is wide.
Most light sensing devices have different responses depending on the wavelenght.
At this article you can see that it is not a trivial answer, but perhaps should be obtained from experimentations; the typical frequency response of both cyntilators an photodetectors are strongly dependent on not only the material composition, but varies from one manufacturer to another. Furthermore, each spectrum is not represented by simple equations.