Anticobalt

It is believed that the same experiment with anticobalt nuclei would have the exact opposite result.

Source: wiktionary

Of course, we have not been able to test the validity of the CP conservation using anticobalt, although other experiments have been conducted to show that it is preserved to a high degree.

Source: wiktionary

For example, in the well-known Wu experiment arguing from the observed asymmetry of Co⁶⁰-decay electrons (without the experiments on anticobalt nuclei!) it followed that charge parity was not conserved.

Source: wiktionary