What it shows:
This block of uranium is of great historical significance -- it is a remnant of the WWII German Atomic Bomb Project. It was brought to Harvard by Prof. Edwin C. Kemble, Physics Dept. Chairman and also Deputy Science Director of the ALSOS mission in 1945. The American ALSOS mission was an intelligence effort to discover the extent of German progress toward atomic weapon development and its ultimate purpose was to secure all the uranium ore the Germans had confiscated during the war and finally close the books on the German program to build an atom bomb. In recent years, Carl F. von Weizsäcker has confirmed that the block is indeed one of the pieces confiscated by ALSOS in April of 1945. 1
The uranium block is a cube, with its corners truncated, and measures 5 cm on a side. It weighs 2.28 kg. Uranium is the heaviest naturally occurring element found in nature (density = 18.7 g/cm3). For comparison, we have fabricated a replica out of aluminum. With a density of only 2.6 g/cm3, the Al block weighs a paltry 356 g. U-238 is an alpha emitter (half life of 4.5×109 years) that decays into Th-234, a radioactive beta emitter. The decays continue as follows:
Called the 238U-Radium series, a long series involving alpha and beta decays, it ends with the stable isotope Pb-206. 2 Conversion electrons are also emitted in many of the decays as well as gamma rays, which accompany beta decays. The total spectrum of radiation emitted by the uranium block is therefore extremely complex. It is dominated by a continuum of betas with gamma ray and alpha peaks superimposed on this overwhelming background radiation. At low energies (<150 keV), uranium K and L x-rays can also be observed. 3 (We won't worry about the presence of 0.72% U-235 (half life = 7.1×108 yr) and its subsequent decays and radiations, known as the 235U-Actinium series, which further complicates the radiation spectrum.)
Setting it up:
Although the radiation level is low (≈1 mR/hr at 30 cm) standard radiation safety precautions should be observed.
The block of uranium is a rarity with a fascinating history.
1 Professor Owen Gingerich has provided us with this background information. Details about the ALSOS mission can be read in The Making of the Atomic Bomb by Richard Rhodes, (Simon & Schuster, NY, 1986) pp 605-10, 612-13.
2 Chart of the Nuclides, (General Electric Co., CA, 1984), revised 1983 by F.W. Walker, D.G. Miller, and F. Feiner, pp14-15. The U-238 decay chain is one of three such chains found in nature. The other two are (1) the actinium series which starts with U-235 and ends with Pb-207 and (2) the thorium series, which goes from Th-232 to Pb-208. See Thoron Decay for details on the thorium series.
3 We use a NaI detector for the high energies and a Ge detector for energies <200 keV.