These types of detectors are also sensitive to gamma and X rays. But I think it's not a wise way to calibrate them by gamma sources. You can use electron or beta sources in this range. The mechanism of interaction of charged particles differ from electromagnetic radiation. Although virtually every alpha or beta emitter is also a gamma emitter!

Gamma rays (and X rays) are electromagnetic radiation-i.e. made up of photons, which are electrically neutral. While these can ionize matter, this charged matter is background, not signal, for a charged particle detector!

(Alpha particles=He4 nuclei; beta particles=electrons. While these, as they travel through matter, can ionize it, too, this doesn't mean that the radiation is peaked at gamma ray frequencies, either...)

This is a homework problem...

Because of the rather low atomic number of Si, and the small sensitive volume of surface barrier detectors, the probability of full-energy absorption of gamma photons is low in these detectors, except perhaps for low-energy X-rays (the ~10 keV range). Thus almost no full-energy peak will be generated in the recorded spectrum.

Conversion electron reference sources can be recommended for calibration at energies < 1 MeV. 207Bi is a good example, but 137Cs and 109Cd are also used. They must be prepared as a thin deposit between thin foils (The same emitter prepared as a gamma source is not adequate).