For the efficiency vs E fit of the 90degree Xray detector I have used the following phenomenological funciton:

f(x) = a * (1-exp(-(x-c)/b)) * exp(-x/d)

Here the first exponent member is a saturation curve. This part describes the passing through of the two Be windows (chamber + before detector) and through the dead layer of Ge crystal. One needs a minimum energy to enter to the detecting Ge crystal = C parameter. b parameter = characteristic absorbtion E of these nondetecting layers.
The second exponent is an exponential decrease of the detector efficiency. Photons with higher energy are less detectable by the germaniums. The d parameter is the characteristic E for hard Xray and gamma (>40keV) detectability. 

https://www.amptek.com/internal-products/si-pin-vs-cdte-comparison

//Jan's comment: the tail of this function should more or less follow a linear trend a bit above than 40 keV.


In the attachment there is an example fit for 90 degree with combined 1. and 2. (before and after beamtime) calibration datasets. 
I made the fit with gnuplot:

degrees of freedom    (FIT_NDF)                        : 8
rms of residuals      (FIT_STDFIT) = sqrt(WSSR/ndf)    : 0.419915
variance of residuals (reduced chisquare) = WSSR/ndf   : 0.176329
p-value of the Chisq distribution (FIT_P)              : 0.994094

Final set of parameters            Asymptotic Standard Error
=======================            ==========================
a               = 0.00308376       +/- 0.0005144    (16.68%)
c               = 15.6259          +/- 2.035        (13.03%)
b               = 9.36888          +/- 3.16         (33.73%)
d               = 177.141          +/- 84.95        (47.95%)

Laszlo's out.