Radioactive_Manganese.avi

This 10-minute video follows a home experiment in which radioactive manganese-56 is produced from natural manganese-55 by neutron capture, and detected by gamma spectroscopy and decay-rate analysis. Neutron activation experiments can be accomplished without much difficulty by enterprising amateur scientists, perhaps surprising folks under the impression that access to a nuclear reactor or a major accelerator is required for this kind of thing.

The reactions:
Mn-55(n,g)Mn-56 (production)
Mn-56 = Fe-56 + beta + gamma (decay)

The video has four parts:

(0:00) Two similar manganese dioxide targets are prepared. One target will be irradiated for ~24 hours in a plastic flux trap that will also contain an AmBe source producing about 2000 n/s; the other is marked "B" for "background" and will not be irradiated.

(2:13) The detector and counting system are discussed. In this system, a sodium iodide (NaI:Tl) scintillator is the detector of gamma radiation. A Canberra 556 AIM module is used to gather counts from two sources simultaneously. On "ADC1" is a multichannel scaler that is set to a dwell time of 15 minutes per channel. On "ADC2" is an analog-to-digital converter that digitizes the amplitude of pulses from the detector and bins them accordingly. Both channels are fed by the detector through a common delay-line amplifier. The computer screen simultaneously displays both the time and energy spectra, and I show the calibration of the energy spectrum using check sources.

(5:20) It's now the day after the video started. The irradiated MnO2 target is removed from the neutron flux trap and placed under the scintillation detector. The energy and time spectra are recorded. Some raw data is shown five hours into the count, with telltale features of Mn-56 decay in evidence. Later, off camera, the "background" MnO2 target is counted. The background spectra will be subtracted from those for the irradiated target.

(7:05) Finally, the processed data (with backgrounds subtracted) are shown and discussed. The gamma energy spectrum has a very large and statistically-significant peak at 850 keV, consistent with Mn-56 decay. A smaller peak may be seen at 1800 keV; this is also associated with Mn-56, but is a lower-probability decay product. Also visible are a low-energy continuum from the Compton effect and other downscattering processes. The time spectrum has a decay half-life of 2.6 hours, consistent with Mn-56 decay.

Thanks for watching!