Using a previously determined detective quantum efficiency for a detector I am doing a radiation damage survey on a few different materials and will need the Detective Quantum Efficiency (DQE) when calculating the dose. I will be using the same detector for each sample and was wondering if it would be sufficient to acquire the DQE once, or even use a previously determined DQE, rather than recalculating it for each sample. My thinking is that since the detector is the same for each sample the DQE shouldn't fluctuate appreciably.
 A: Short version: It's better to measure it in situ and shortly before (or better before and after) taking data.

Explanation
Quantum efficiency can be a function of various operating parameters, so you would need to know that the previous measurement was done using the same parameter you plan to use.
Various classes of detectors can experience changes in their actual quantum efficiency for various reasons: radiation damage; current erosion; mechanical, chemical, or thermal damage at a minimum.
It is also possible that the previous measurement intentionally or unintentionally folded some acceptance features into their reported QE.
For these reasons it is better to measure the QE 


*

*explicitly for the data taking you plan

*in the configuration you will be using

*under the operating parameters you will be using

*in close temporal proximity to the data taking


But, as always, you have to balance the risk against and the costs. If you know that the device was previous testing in a configuration compatible with the one you intend under operating condition compatible with those you intend, and has been stored with due care to protect if from things that could damage it (which could include humidity, bright light, intense radiation fields, mechanical shocks, and others depending on the nature of the detector), and you don't have a lot of money/time then you might decide to chance it.
