RADIOACTIVITY

Film Loop: Radioactive Decay

Length(min.):4:55, Color: No Sound: No

1. Assembly of scintillation detector. A 2-inch diameter NaI (T1) crystal mounted on a photomultiplier tube is placed in a cylindrical steel shield which has an aluminum window. The window absorbs beta particles, but allows gamma ray photons to penetrate to the crystal.

2. Samples of Cu64 and Mn56 are placed in position. The sources have been prepared by irradiating 17 mg of MnO and 130 mg of Cu metal for 30 minutes in the Ohio State University reactor at an indicated power of 20 watts. The reactions are Mn55 + n = Mn56 and Cu63 + n = Cu64. 60% of the disintegrations of Mn56 are by beta emission of maximum energy 2.86 MeV, followed by a gamma-ray photon of 0.845 MeV as the Fe56 nucleus falls to its ground state. The other 6 gamma rays from the decay of Mn56 have energies greater than 1 MeV and do not appear on the screen of the analyzer. The nuclide Cu64 can decay to Ni64 in several ways. 19% of the time the mass difference 1.68 MeV gives rise to a positron. When the positrons combine with electrons in the scintillation crystal, they form two photons of annihilation radiation, each having energy 0.51 MeV (equal to the rest energy of an electron).

3. Gamma ray spectra are displayed. Considering both sources together, only two gamma rays are in the range 0-1 MeV selected by the spectrometer. The strength of the samples is adjusted so that at the beginning of the run (3 hours after the end of irradiation) the composite spectrum shows two peaks of equal height. Because the Cu64 annihilation gamma ray is superposed on the Compton edge from the Mn56 peak, its initial strength at t= 0 is 0.75 unit, and the initial strength of the Mn56 sample is 1.0 unit.

4. Radioactive decay of Cu64 (half life 12.84 hr) and Mn56 (half life 2.56 hr). Time-lapse photography of the 400-channel analyzer display is performed as follows: (1) Counts are accumulated and displayed after 20 sec of "live time"; this requires more than 20 sec of real time because the "dead time" during the sorting and storing operations in the spectrometer is automatically ignored. (2) The display of 20 seconds-worth of counts is photographed twice, on two successive frames of film. (3) The storage registers are erased, and exactly 40 sec after the start of (1) a new accumulation is initiated and photographed on the next two frames. This process is continued for 12.84 hours, exposing at the rate of 2 frames per 40 sec. When projected at 18 frames/sec, the half life of Cu64 is compressed by a factor of 360, and 12.84 hr becomes 2.09 minutes.

5. Elapsed time 12.84 hr. Cu64 (1 half life): 1/2 X 0.75 = 0.38; Mn56 (5 half lives): 1/32 X 1.00 = 0.03. It happens that the half life of Cu64 is almost exactly 5 times that of Mn56. In 5 half lives, Mn56 decays to 1/32 of its initial value. The Cu64 in this same time has decayed to 1/2 of its initial value.