I found a similar question asked on the Internet. Here is the answer provided by an experienced person.

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Given the initiative, time, and money we can convert most forms of nonelectrical energy into electricity. This includes gamma rays. A direct conversion, though inefficient, is possible through interaction of the gamma rays with an appropriate material that will suffer a loss of electrons through the gamma interactions with it; the electrons can be collected in an electric field and represent a current flow. In fact, most radiation detectors operate on this principle. Gases and solids have been used in this regard. Various semiconductors undergo electron ejection in response to ionizing radiation and have had many applications in radiation detection and measurement. Photovoltaic cells, commonly used in the visible and ultraviolet portion of the electromagnetic spectrum, can also convert gamma ray energy to electron current, but the process is very inefficient in terms of the fraction of gamma energy that ends up as collectible electrical charge.

The process of ionization, which gamma rays are capable of inducing in all materials, is almost always accompanied by secondary processes of excitation in which electrons are excited within atoms, but the electrons are not removed from the atoms. In general, most of the energy transferred to electrons by ionization ends up as kinetic energy of the electron and is lost mostly and ultimately in the form of heat, as is much of the energy of excitation. Thus the direct conversion of gamma energy to electric current is very inefficient. However, it is possible to convert heat to electricity. In fact, when nuclear power plants operate, a small, but not insignificant, portion of the power generated comes from heat generated by the decay of the radioactive fission products that build up in the fuel. The decay radiations include primarily beta particles and gamma rays, both of which produce heat. This heat adds to the fission heat that is used to convert water to steam to drive the turbine/generators. Also, electricity can be generated from heat through the use of thermoelectric generators (e.g., a series of thermocouples, called a thermopile, in contact with a heat source), which have been around for decades and have been used with a variety of heat sources.

Gamma radiation is generally not a good choice for thermoelectric applications because the gamma radiation is very penetrating, and large masses of material may be necessary to absorb most of the energy, likely making for an inefficient arrangement for concentrated heat production and thermoelectric generation. In addition, large-intensity gamma sources may require more engineering safeguards and controls in their preparation and handling than do other radiation sources. Many radionuclide-fueled thermoelectric generators have been designed and used over the years, and the most common radionuclide choices have been those that emit particulate radiations (beta particles or alpha particles) that can be easily stopped with small thicknesses of materials. You may be familiar with SNAP (acronym for systems nuclear auxiliary power) thermoelectric generators, fueled commonly with plutonium-238, an alpha emitter, and used in many space missions. Also, in the 1970s and 1980s, plutonium-fueled thermoelectric generators were used to power pacemakers intended to stimulate the heart to beat with a regular rhythm. You can find considerable information on thermoelectric generators, including SNAP systems and pacemakers, by doing a simple search on the Internet.
In summary, gamma radiation energy can be converted to electricity, but not very practically or efficiently, and practical nuclear electric generators use other radiation types to make reasonably efficient devices. I hope this answer is sufficient for your needs.
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http://www.hps.org/publicinformation/ate/q4853.html

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