A disc of <a href="https://en.wikipedia.org/wiki/californium" class="extiw" title="w:californium">californium</a> metal (<sup>249</sup>Cf, 10 mg). The source implies that the disc has a diameter about twice the thickness of a typical pin, or on the order of 1 mm.
United States Department of Energy (see <a href="//commons.wikimedia.org/wiki/File:Einsteinium.jpg" title="File:Einsteinium.jpg">File:Einsteinium.jpg</a>)
<cite class="book" style="font-style:normal">"Californium" in   (2006) <i> THE CHEMISTRY OF THE ACTINIDE AND TRANSACTINIDE ELEMENTS</i>, III (3<sup>rd</sup> ed.), Springer, pp. 1,518  <small><a href="https://en.wikipedia.org/wiki/Digital_object_identifier" class="extiw" title="en:Digital object identifier">DOI</a>:  <a rel="nofollow" class="external text" href="https://dx.doi.org/10.1007/1-4020-3598-5_11">10.1007/1-4020-3598-5_11</a>. </small></cite>