Introduction:
The element used in the production of photocells and solar cells, as well as in Xerox machines, is selenium.

Explanation:
1. Photocells:
- Photocells, also known as photoelectric cells or solar cells, are devices that convert light energy into electrical energy.
- These cells are composed of semiconductor materials that can absorb photons (particles of light) and release electrons.
- Selenium is a semiconductor material that has the ability to absorb light and release electrons when exposed to photons.
- When light strikes the surface of a selenium photocell, the energy from the photons causes the release of electrons, which generates an electric current.

2. Solar Cells:
- Solar cells are similar to photocells and are used to convert sunlight into electricity.
- Selenium is commonly used in the production of solar cells due to its unique properties.
- Selenium has a bandgap energy that allows it to absorb a wide range of wavelengths, including visible and infrared light.
- When sunlight strikes the surface of a selenium solar cell, the energy from the photons is absorbed by the selenium material, causing the release of electrons and the generation of an electric current.

3. Xerox Machines:
- Selenium is also used in the production of Xerox machines, specifically in the photoreceptor drum.
- In a Xerox machine, the photoreceptor drum is a key component responsible for capturing the image to be reproduced.
- Selenium is used as a coating on the surface of the photoreceptor drum.
- When light from the original document shines onto the drum, the selenium coating becomes conductive in areas where light strikes, allowing for the formation of an electrostatic image.
- This electrostatic image is then transferred to paper and fused to create a copy of the original document.

Conclusion:
Selenium is the element used in the production of photocells and solar cells, as well as in Xerox machines. It is a semiconductor material that can absorb light and release electrons, making it suitable for converting light energy into electrical energy. In Xerox machines, selenium is used as a coating on the photoreceptor drum to capture and reproduce images.